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A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims Robert L Chatburn MHHS RRT-NPS FAARC, Mohamad El Khatib PhD MD RRT FAARC, and Eduardo Mireles-Cabodevila MD Introduction What Is a Mode of Mechanical Ventilation? The 10 Maxims Application of the Taxonomy Discussion The Problem of Growing Complexity The Problem of Identifying Unique Modes The Problem of Teaching Mechanical Ventilation The Problem of Implementation Conclusions The American Association for Respiratory Care has declared a benchmark for competency in mechanical ventilation that includes the ability to “apply to practice all ventilation modes currently available on all invasive and noninvasive mechanical ventilators.” This level of competency pre- supposes the ability to identify, classify, compare, and contrast all modes of ventilation. Unfortu- nately, current educational paradigms do not supply the tools to achieve such goals. To fill this gap, we expand and refine a previously described taxonomy for classifying modes of ventilation and explain how it can be understood in terms of 10 fundamental constructs of ventilator technology: (1) defining a breath, (2) defining an assisted breath, (3) specifying the means of assisting breaths based on control variables specified by the equation of motion, (4) classifying breaths in terms of how inspiration is started and stopped, (5) identifying ventilator-initiated versus patient-initiated start and stop events, (6) defining spontaneous and mandatory breaths, (7) defining breath se- quences (8), combining control variables and breath sequences into ventilatory patterns, (9) de- scribing targeting schemes, and (10) constructing a formal taxonomy for modes of ventilation composed of control variable, breath sequence, and targeting schemes. Having established the theoretical basis of the taxonomy, we demonstrate a step-by-step procedure to classify any mode on any mechanical ventilator. Key words: taxonomy; ontology; mechanical ventilation; mechanical ven- tilator; modes of ventilation; classification; ventilator; survey; standardized nomenclature; controlled vocabulary. [Respir Care 2014;59(11):1–•. © 2014 Daedalus Enterprises] Introduction The American Association for Respiratory Care (AARC) has sponsored a number of conferences to outline the com- petencies of the registered respiratory therapist (RRT) of the future. 1-3 One of the competencies in the area of crit- ical care was declared as the ability to “apply to practice Mr Chatburn and Dr Mireles-Cabodevila are affiliated with the Respira- tory Institute, Cleveland Clinic, Cleveland, Ohio and the Lerner College of Medicine of Case Western Reserve University, Cleveland, Ohio. Dr El Khatib is affiliated with the Department of Anesthesiology, American University of Beirut Medical Center, Beirut, Lebanon. Supplementary material related to this paper is available at http:// www.rcjournal.com. RESPIRATORY CARE NOVEMBER 2014 VOL 59 NO 11 1 RESPIRATORY CARE Paper in Press. Published on August 12, 2014 as DOI: 10.4187/respcare.03057 Copyright (C) 2014 Daedalus Enterprises ePub ahead of print papers have been peer-reviewed, accepted for publication, copy edited and proofread. However, this version may differ from the final published version in the online and print editions of RESPIRATORY CARE

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Page 1: A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims

A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims

Robert L Chatburn MHHS RRT-NPS FAARC, Mohamad El Khatib PhD MD RRT FAARC,and Eduardo Mireles-Cabodevila MD

IntroductionWhat Is a Mode of Mechanical Ventilation?The 10 MaximsApplication of the TaxonomyDiscussion

The Problem of Growing ComplexityThe Problem of Identifying Unique ModesThe Problem of Teaching Mechanical VentilationThe Problem of Implementation

Conclusions

The American Association for Respiratory Care has declared a benchmark for competency inmechanical ventilation that includes the ability to “apply to practice all ventilation modes currentlyavailable on all invasive and noninvasive mechanical ventilators.” This level of competency pre-supposes the ability to identify, classify, compare, and contrast all modes of ventilation. Unfortu-nately, current educational paradigms do not supply the tools to achieve such goals. To fill this gap,we expand and refine a previously described taxonomy for classifying modes of ventilation andexplain how it can be understood in terms of 10 fundamental constructs of ventilator technology:(1) defining a breath, (2) defining an assisted breath, (3) specifying the means of assisting breathsbased on control variables specified by the equation of motion, (4) classifying breaths in terms ofhow inspiration is started and stopped, (5) identifying ventilator-initiated versus patient-initiatedstart and stop events, (6) defining spontaneous and mandatory breaths, (7) defining breath se-quences (8), combining control variables and breath sequences into ventilatory patterns, (9) de-scribing targeting schemes, and (10) constructing a formal taxonomy for modes of ventilationcomposed of control variable, breath sequence, and targeting schemes. Having established thetheoretical basis of the taxonomy, we demonstrate a step-by-step procedure to classify any mode onany mechanical ventilator. Key words: taxonomy; ontology; mechanical ventilation; mechanical ven-tilator; modes of ventilation; classification; ventilator; survey; standardized nomenclature; controlledvocabulary. [Respir Care 2014;59(11):1–•. © 2014 Daedalus Enterprises]

Introduction

The American Association for Respiratory Care (AARC)has sponsored a number of conferences to outline the com-

petencies of the registered respiratory therapist (RRT) ofthe future.1-3 One of the competencies in the area of crit-ical care was declared as the ability to “apply to practice

Mr Chatburn and Dr Mireles-Cabodevila are affiliated with the Respira-tory Institute, Cleveland Clinic, Cleveland, Ohio and the Lerner Collegeof Medicine of Case Western Reserve University, Cleveland, Ohio. Dr El

Khatib is affiliated with the Department of Anesthesiology, AmericanUniversity of Beirut Medical Center, Beirut, Lebanon.

Supplementary material related to this paper is available at http://www.rcjournal.com.

RESPIRATORY CARE • NOVEMBER 2014 VOL 59 NO 11 1

RESPIRATORY CARE Paper in Press. Published on August 12, 2014 as DOI: 10.4187/respcare.03057

Copyright (C) 2014 Daedalus Enterprises ePub ahead of print papers have been peer-reviewed, accepted for publication, copy edited and proofread. However, this version may differ from the final published version in the online and print editions of RESPIRATORY CARE

Page 2: A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims

all ventilation modes currently available on all invasiveand noninvasive mechanical ventilators, as well as all ad-juncts to the operation of modes.”2 Kacmarek4 recentlypublished a paper discussing the expectations of this futureRRT regarding mechanical ventilation competencies. (Ofcourse, these competencies apply to any clinician respon-sible for managing ventilated patients, as many countriesdo not have RRTs.) He states that:

The RT of 2015 and beyond must be a technicalexpert on every aspect of the mechanical ventilator.They should be able to discuss all of the technicalnuances of the mechanical ventilator. They shouldbe able to compare the capabilities of one ventilatorto the other. They should be able to discuss in detailthe mechanism of action of all of the modes andadjuncts that exist on the mechanical ventilator.

He further says that “The RT of 2015 and beyond shouldbe capable of defining the operational differences betweeneach of these modes.” These statements seem reasonableat first glance, but further consideration reveals some ma-jor challenges.

The number of modes of ventilation has grown expo-nentially in the last 3 decades. Consider just one populartextbook on respiratory care equipment5 that includes 174unique names of modes on 34 different ventilators. Thelevel of complexity in terms of the real number of uniquemodes is much greater: most ICU ventilators allow theoperator to activate various features that modify a givenmode and actually transform it into anther mode withoutany naming convention to signify the transition. The resultis that there are many more unique modes (in terms ofdifferent patterns of patient-ventilator interaction) thanthere are names indicated on the ventilators, in operators’manuals, or in textbooks. This growing complexity hasgenerated an urgent need for a classification system (tax-onomy) for modes of mechanical ventilation to facilitatethe identification and comparison of the technical capabil-ities of ventilators.

The purpose of this article is to describe a formal tax-onomy for modes of mechanical ventilation (ie, a classi-fication of modes into groups based on similar character-

istics) using a simple structured approach to teaching andlearning the fundamental principles of ventilator opera-tion. This taxonomy has recently been adopted by theECRI (formerly the Emergency Care Research Institute)for describing and comparing ventilators.6 We do not dis-cuss clinical application, but rather the technology that isthe foundation for clinical application. This is a topic thatwe believe is not sufficiently discussed in current text-books. We have developed this system over many years ofclinical experience and instruction of medical students,physicians, and respiratory therapists in both the hospitaland university environments. It is based on what we con-sider to be 10 fundamental theoretical constructs or max-ims (Table 1) that are recognizable to most people familiarwith mechanical ventilation.7 We demonstrate how these10 maxims form the basis of the taxonomy. We also showhow the taxonomy is a practical tool for dealing with thecomplexity represented by the many mode names men-tioned above. Figure 1 illustrates a hierarchy of skills webelieve must be mastered before one is fully able to usemechanical ventilation technology as suggested by theAARC competency statements. Note that this hierarchy isconsistent with Bloom’s revised taxonomy of learning ob-jectives (a classification of levels of intellectual behaviorimportant in learning).8

Mr Chatburn is a paid consultant for Philips Respironics, Covidien, Drager,Hamilton Medical, and ResMed. The other authors have disclosed noconflicts of interest.

Correspondence: Robert L Chatburn MHHS RRT-NPS FAARC, Cleve-land Clinic, M-56, 9500 Euclid Avenue, Cleveland, OH 44195. E-mail:[email protected].

DOI: 10.4187/respcare.03057

Table 1. Ten Basic Maxims for Understanding Ventilator Operation

(1) A breath is one cycle of positive flow (inspiration) and negativeflow (expiration) defined in terms of the flow vs time curve.

(2) A breath is assisted if the ventilator provides some or all of thework of breathing.

(3) A ventilator assists breathing using either pressure control orvolume control based on the equation of motion for therespiratory system.

(4) Breaths are classified according to the criteria that trigger (start)and cycle (stop) inspiration.

(5) Trigger and cycle events can be either patient-initiated orventilator-initiated.

(6) Breaths are classified as spontaneous or mandatory based on boththe trigger and cycle events.

(7) Ventilators deliver 3 basic breath sequences: CMV, IMV, andCSV.

(8) Ventilators deliver 5 basic ventilatory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV.

(9) Within each ventilatory pattern, there are several types that canbe distinguished by their targeting schemes (set-point, dual, bio-variable, servo, adaptive, optimal, and intelligent).

(10) A mode of ventilation is classified according to its controlvariable, breath sequence, and targeting schemes.

CMV � continuous mandatory ventilationIMV � intermittent mandatory ventilationCSV � continuous spontaneous ventilationVC � volume controlPC � pressure control

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What Is a Mode of Mechanical Ventilation?

A mode of mechanical ventilation may be defined, in gen-eral, as a predetermined pattern of patient-ventilator interac-tion. It is constructed using 3 basic components: (1) theventilator breath control variable, (2) the breath sequence,and (3) the targeting scheme (Fig. 2) To understand eachof these components, we use the maxims that form thebasis for the taxonomy of mechanical ventilation. These10 maxims describe, in a progressive manner, the rationaleof how we classify modes by understanding what a modedoes. Maxims 1–3 explain the ventilator breath controlvariable. Maxims 4–8 explain the breath sequence. Maxim9 explains the targeting schemes. Maxim 10 pulls togetherthe previous maxims to formulate the complete taxonomy.

The 10 Maxims

The following sections describe the 10 theoretical con-structs that we believe form the basis of a practical sylla-bus for learning mechanical ventilation technology. Theyalso provide the context for some basic definitions of termsused to construct a standardized vocabulary (see the sup-plementary materials at http://www.rcjournal.com). Westart with very simple, intuitively obvious ideas and thenbuild on these concepts to form a theoretical frameworkfor understanding and using ventilators.

(1) A breath is one cycle of positive flow (inspiration)and negative flow (expiration) defined in terms of the flow-time curve. A breath is defined in terms of the flow-timecurve (Fig. 3). By convention, positive flow (ie, values offlow above zero) is designated as inspiration. Negativeflow (values below zero) indicates expiration. Inspiratorytime is defined as the period from the start of positive flowto the start of negative flow. Expiratory time is defined as

the period from the start of negative flow to the start ofpositive flow. Total cycle time (also called the ventilatoryperiod) is the sum of inspiratory and expiratory times. It isalso equal to the inverse of breathing frequency (total cy-cle time � 1/frequency, usually expressed as 60 s/breaths/min). The inspiratory-expiratory ratio is defined as theratio of inspiratory time to expiratory time. The duty cycle(or percent inspiration) is defined as the ratio of inspira-tory time to total cycle time. The tidal volume (VT) is theintegral of flow with respect to time. For constant flowinspiration, this simply reduces to the product of flow andinspiratory time.

(2) A breath is assisted if the ventilator provides someor all of the work of breathing. An assisted breath is onefor which the ventilator does some portion of the work ofbreathing. This work may be defined, for example, as theintegral of inspiratory transrespiratory pressure with respectto inspired volume. Graphically, this corresponds to airwaypressure increasing above baseline during inspiration. In-creased work of breathing per breath, as a result of increasedresistive and/or elastic work, is characterized by increasedtransrespiratory pressure (for a definition of transrespiratorypressure, see the supplementary materials at http://www.rcjournal.com). In contrast, a loaded breath is one for whichtransrespiratory pressure decreases below baseline during in-spiration9 and is interpreted as the patient doing work on theventilator (eg, to start inspiration).

A ventilator provides all of the mechanical work ofinspiration (ie, full support) only if the patient’s inspira-tory muscles are inactive (eg, drug-induced neuromuscularblockade). An unassisted breath is one for which the ven-tilator simply provides flow at the rate required by thepatient’s inspiratory effort, and transrespiratory systempressure stays constant throughout the breath. An exampleof this would be CPAP delivered with a demand valve. Aventilator can assist expiration by making the transrespi-ratory pressure fall below baseline during expiration. Anexample of this is automatic tube compensation on theEvita XL ventilator (Drager, Lubeck, Germany). Whentube compensation is activated, the ventilation pressure inthe breathing circuit is increased during inspiration or de-creased during expiration. The airway pressure is adjustedto the tracheal level if 100% compensation of the tuberesistance has been selected. Another example is the use ofa cough-assist device (eg, CoughAssist mechanical insuf-flator-exsufflator, Philips Respironics, Murrysville, Penn-sylvania). In this case, transrespiratory pressure goes neg-ative during expiration because pressure on the body surfaceis increased while pressure at the mouth remains at atmo-spheric pressure.

(3) A ventilator assists breathing using either pressurecontrol or volume control based on the equation of motionfor the respiratory system. The theoretical framework forunderstanding control variables is the equation of motion

Fig. 1. Pyramid of skills required to master ventilator technology.The terms in green are from Bloom’s revised taxonomy of learningobjectives.

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for the passive respiratory system: P(t) � EV(t) � RV(t).This equation relates pressure (P), volume (V), and flow(V) as continuous functions of time (t) with the parametersof elastance (E) and resistance (R). If any one of the func-tions (P, V, or V) is predetermined, the other two may bederived. The control variable refers to the function that iscontrolled (predetermined) during a breath (inspiration).This form of the equation assumes that the patient makesno inspiratory effort and that expiration is complete (noauto-PEEP).

Volume control (VC) means that both volume and floware pre-set prior to inspiration. Setting the VT is a neces-sary but not sufficient criterion for declaring volume con-trol because some modes of pressure control allow theoperator to set a target VT but allow the ventilator todetermine the flow (see adaptive targeting scheme below).Similarly, setting flow is also a necessary but not sufficientcriterion. Some pressure control modes allow the operator

to set the maximum inspiratory flow, but the VT dependson the inspiratory pressure target and respiratory systemmechanics.

Pressure control (PC) means that inspiratory pressure asa function of time is predetermined. In practice, this cur-rently means pre-setting a particular pressure waveform(eg, P(t) � constant), or inspiratory pressure is set to beproportional to patient inspiratory effort, measured by var-ious means. For example, P(t) � NAVA level � EAdi(t),where NAVA stands for neurally adjusted ventilatory as-sist, and EAdi stands for electrical activity of the dia-phragm (see servo targeting scheme below). In a passivepatient, after setting the form of the pressure function (ie,the waveform), volume and flow depend on elastance andresistance.10

Time control is a general category of ventilator modesfor which inspiratory flow, inspiratory volume, and in-spiratory pressure are all dependent on respiratory systemmechanics. As no parameters of the pressure, volume, orflow waveforms are pre-set, the only control of the breathis the timing (ie, inspiratory and expiratory times). Exam-ples of this are high-frequency oscillatory ventilation (3100ventilator, CareFusion, San Diego, California) and volu-metric diffusive respiration (Percussionaire, Sagle, Idaho).

(4) Breaths are classified according to the criteria thattrigger (start) and cycle (stop) inspiration. Inspiration starts(or is triggered) when a monitored variable (trigger vari-able) achieves a pre-set threshold (the trigger event). Thesimplest trigger variable is time, as in the case of a pre-setbreathing frequency (recall that the period between breathsis 1/frequency). Other trigger variables include a minimumlevel of minute ventilation, a pre-set apnea interval, orvarious indicators of inspiratory effort (eg, changes in base-line pressure or flow or electrical signals derived fromdiaphragm movement).

Inspiration stops (or is cycled off) when a monitoredvariable (cycle variable) achieves a pre-set threshold (cy-cle event). The simplest cycle variable is a pre-set inspira-tory time. Other cycle variables include pressure (eg, peakairway pressure), volume (eg, VT), flow (eg, percent ofpeak inspiratory flow), and electrical signals derived fromdiaphragm movement.

Fig. 2. Building blocks for constructing a mode. CMV � continuous mandatory ventilation; IMV � intermittent mandatory ventilation;CSV � continuous spontaneous ventilation.

Fig. 3. A breath is defined in terms of the flow-time curve. Impor-tant timing parameters related to ventilator settings are labeled.

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(5) Trigger and cycle events can be either patient-ini-tiated or ventilator-initiated. Inspiration can be patient-triggered or patient-cycled by a signal representing in-spiratory effort (eg, changes in baseline airway pressure,changes in baseline bias flow, or electrical signals derivedfrom diaphragm activity, as with neurally adjusted venti-latory assist11 or a calculated estimate of muscle pres-sure12). Furthermore, the ventilator can be triggered andcycled solely by the patient’s passive respiratory systemmechanics (elastance and resistance).13 For example, anincrease in elastance or resistance in some modes willincrease airway pressure beyond the alarm threshold andcycle inspiration. Inspiration may be ventilator-triggeredor ventilator-cycled by pre-set thresholds.

Patient triggering means starting inspiration based on apatient signal, independent of a ventilator-generated trig-ger signal. Ventilator triggering means starting inspiratoryflow based on a signal (usually time) from the ventilator,independent of a patient-triggered signal. Patient cyclingmeans ending inspiratory time based on signals represent-ing the patient-determined components of the equation ofmotion (ie, elastance or resistance and including effectsdue to inspiratory effort). Flow cycling is a form of patientcycling because the rate of flow decay to the cycle thresh-old (and hence, the inspiratory time) is determined bypatient mechanics (ie, the time constant and effort). Ven-tilator cycling means ending inspiratory time independentof signals representing the patient-determined componentsof the equation of motion.

As a further refinement, patient triggering can be de-fined as starting inspiration based on a patient signal oc-curring in a trigger window, independent of a ventilator-generated trigger signal. A trigger window is the periodcomposed of the entire expiratory time minus a short re-fractory period required to reduce the risk of triggering abreath before exhalation is complete (Fig. 4). If a signalfrom the patient (ie, some measured variable indicating aninspiratory effort) occurs within this trigger window, in-spiration starts and is defined as a patient-triggered event.

A synchronization window is a short period, at the endof a pre-set expiratory or inspiratory time, during which apatient signal may be used to synchronize the beginning orending of inspiration to the patient’s actions. If the patientsignal occurs during an expiratory time synchronizationwindow, inspiration starts and is defined as a ventilator-triggered event initiating a mandatory breath. This is be-cause the mandatory breath would have been time-trig-gered regardless of whether the patient signal had appearedor not and because the distinction is necessary to avoidlogical inconsistencies in defining mandatory and sponta-neous breaths (see below), which are the foundation of themode taxonomy. Trigger and synchronization windowsare another way to distinguish between continuous man-datory ventilation (CMV) and intermittent mandatory ven-

tilation (IMV) (see below). Sometimes a synchronizationwindow is used at the end of the inspiratory time of apressure control, time-cycled breath. If the patient signaloccurs during such an inspiratory time synchronizationwindow, expiration starts and is defined as a ventilator-cycled event, ending a mandatory breath.

Some ventilators offer the mode called airway pressurerelease ventilation (or something similar with a differentname), which may use both expiratory and inspiratorysynchronization windows. This mode is an example of theimportance of distinguishing between trigger/cycle win-dows (allowing for patient-triggered breaths) and synchro-nization windows (allowing for patient-synchronized,ventilator-triggered breaths). Airway pressure release ven-tilation is intended to provide a set number of so-calledreleases or drops from a high-pressure level to a low-pressure level. Spontaneous breaths are possible at thehigh-pressure and low-pressure levels (although there maynot be enough time to accomplish this if the duration of thelow pressure is too short). Using the standardized vocab-ulary we have been discussing, these releases (paired withtheir respective rises) are actually mandatory breaths be-cause they are time-triggered and time-cycled. On someventilators, synchronization windows were added to boththe expiratory time (to synchronize the transition to highpressure with a patient inspiratory effort) and the inspira-tory time (to synchronize cycling with the expiratory phaseof a spontaneous breath taken during the high-pressurelevel). If both triggering and cycling occurred with patientsignals in the synchronization window, and if we called

Fig. 4. Trigger and synchronization windows. If a patient signaloccurs within the trigger window, inspiration is patient-triggered. Ifa patient signal occurs within a synchronization window, inspira-tion is ventilator-triggered (or cycled if at the end of inspiration)and patient-synchronized. Note that, in general, a trigger windowis used with continuous mandatory ventilation, a synchronizationwindow is used with intermittent mandatory ventilation.

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these events patient-triggered and patient cycled, then wewould end up with the ambiguous possibility of havingspontaneous breaths (ie, synchronized) occurring duringspontaneous breaths (unsynchronized breaths during thehigh-pressure level). Another example occurs with a ven-tilator such as the CareFusion Avea, which allows theoperator to set a flow cycle criterion for pressure controlPC-IMV. Thus, every inspiration is patient-cycled, and ifwe said that any synchronized breaths (synchronized IMV)were patient-triggered, we would be implying that thesemandatory breaths were really spontaneous breaths. Thiswould be misleading because the pre-set mandatory breath-ing frequency would then be larger than what we count as

mandatory breaths when observing the patient. On modesthat are classified as forms of IMV (such as airway pres-sure release ventilation), we need to distinguish betweenthe mandatory minute ventilation and the spontaneous min-ute ventilation (to gauge the level of mechanical support),and we cannot do this if the definitions of mandatory andspontaneous breaths are in any way ambiguous. Figure 5shows the decision rubric for classifying trigger and cycleevents.

(6) Breaths are classified as spontaneous or mandatorybased on both the trigger and cycle events. A spontaneousbreath is a breath for which the patient retains control overtiming. This means that the start and end of inspiration are

Fig. 5. Rubric for classifying trigger and cycle events. Courtesy Mandu Press.

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determined by the patient, independent of any ventilatorsettings for inspiratory and expiratory times. That is, thepatient both triggers and cycles the breath. A spontaneousbreath may occur during a mandatory breath (eg, airwaypressure release ventilation). A spontaneous breath may beassisted or unassisted. Indeed, the definition of a sponta-neous breath applies to normal breathing as well as me-chanical ventilation. Some authors use the term spontane-ous breath to refer only to unassisted breaths, but that is anunnecessary limitation that prevents the word from beingused as a key term in the mode taxonomy.

A mandatory breath is a breath for which the patient haslost control over timing (ie, frequency or inspiratory time).This is a breath for which the start or end of inspiration (orboth) is determined by the ventilator, independent of thepatient: the ventilator triggers and/or cycles the breath. Amandatory breath can occur during a spontaneous breath(eg, high-frequency jet ventilation). A mandatory breathis, by definition, assisted.

(7) Ventilators deliver 3 basic breath sequences: CMV,IMV, and continuous spontaneous ventilation CSV. Abreath sequence is a particular pattern of spontaneous and/ormandatory breaths. The 3 possible breath sequences areCMV, IMV, and CSV. CMV, commonly known as assistcontrol, is a breath sequence for which spontaneous breathsare not possible between mandatory breaths because everypatient-triggered signal in the trigger window produces aventilator-cycled inspiration (ie, a mandatory breath). IMVis a breath sequence for which spontaneous breaths arepossible between mandatory breaths. Ventilator-triggeredmandatory breaths may be delivered at a pre-set frequency.The mandatory breathing frequency for CMV may be higherthan the set frequency but never below it (ie, the set fre-quency is a minimum value). In some pressure controlmodes on ventilators with an active exhalation valve, spon-taneous breaths may occur during mandatory breaths, butthe defining characteristic of CMV is that spontaneousbreaths are not permitted between mandatory breaths. Incontrast, the set frequency of mandatory breaths for IMVis the maximum value because every patient signal be-tween mandatory breaths initiates a spontaneous breath.

There are 3 variations of IMV. (1) Mandatory breathsare always delivered at the set frequency (eg, SIMV vol-ume control mode on the PB840 ventilator, Covidien,Mansfield, Massachusetts). In general, if a synchroniza-tion window is used, the actual ventilatory period for amandatory breath may be shorter than the set period. Someventilators will add the difference to the next mandatoryperiod to maintain the set mandatory breathing frequency(eg, Drager Evita XL ventilator). (2) Mandatory breathsare delivered only when the spontaneous breathing fre-quency falls below the set frequency (eg, BiPAP [bi-levelpositive airway pressure] S/T mode on the Philips Respi-ronics V60 ventilator). In other words, spontaneous breaths

may suppress mandatory breaths. (3) Mandatory breathsare delivered only when the measured minute ventilation(ie, product of breathing frequency and VT) drops below apre-set threshold (examples include Drager’s mandatoryminute volume ventilation mode and Hamilton Medical’sadaptive support ventilation mode). Again, in this form ofIMV, spontaneous breaths may suppress mandatorybreaths.

(8) Ventilators deliver 5 basic ventilatory patterns: vol-ume control VC-CMV, VC-IMV, PC-CMV, PC-IMV, andPC-CSV. A ventilatory pattern is a sequence of breaths(CMV, IMV, or CSV) with a designated control variable(volume or pressure) for the mandatory breaths (or thespontaneous breaths for CSV). Thus, with 2 control vari-ables and 3 breath sequences, there are 5 possible venti-latory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV,and PC-CSV. The VC-CSV combination is not possiblebecause volume control implies ventilator cycling, andventilator cycling makes every breath mandatory, not spon-taneous (maxim 6). For completeness, we should also in-clude the possibility of a time control ventilatory patternsuch as time control IMV. Although this is uncommon andnonconventional, it is possible, as demonstrated by modessuch as high-frequency oscillatory ventilation and intrapul-monary percussive ventilation. Because any mode of ven-tilation can be associated with one and only one ventila-tory pattern, the ventilatory pattern serves as a simplemode classification system.

(9) Within each ventilatory pattern, there are severaltypes that can be distinguished by their targeting schemes(set-point, dual, bio-variable, servo, adaptive, optimal, andintelligent). A targeting scheme is a model14 of the rela-tionship between operator inputs and ventilator outputs toachieve a specific ventilatory pattern, usually in the formof a feedback control system. A target is a predeterminedgoal of ventilator output. Targets can be viewed as thegoals of the targeting scheme. Targets can be set for pa-rameters during a breath (within-breath targets). These pa-rameters relate to the pressure, volume, and flow wave-forms. Examples of within-breath targets include peakinspiratory flow and VT or inspiratory pressure and risetime (set-point targeting); pressure, volume, and flow (dualtargeting); and constant of proportionality between inspira-tory pressure and patient effort (servo targeting).

Targets can be set between breaths to modify the with-in-breath targets and/or the overall ventilatory pattern (be-tween-breath targets). These are used with more advancedtargeting schemes, where targets act over multiple breaths.Examples of between-breath targets and targeting schemesinclude average VT (for adaptive targeting using pressurecontrol); work rate of breathing and minute ventilation (foroptimal targeting); and combined end-tidal PCO2

, volume,and frequency values describing a zone of comfort (forintelligent targeting, eg, SmartCare/PS [Drager Evita In-

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finity V500] or IntelliVent-ASV [S1 ventilator, HamiltonMedical, Reno, Nevada]).

The targeting scheme (or combination of targetingschemes) is what distinguishes one ventilatory pattern fromanother. There are currently 7 basic targeting schemes thatcomprise the wide variety seen in different modes of ven-tilation. (1) Set-point is a targeting scheme for which theoperator sets all of the parameters of the pressure wave-form (pressure control modes) or volume and flow wave-forms (volume control modes). (2) Dual is a targetingscheme that allows the ventilator to switch between vol-ume control and pressure control during a single inspira-tion. (3) Bio-variable is a targeting scheme that allows theventilator to automatically set the inspiratory pressure (orVT) randomly to mimic the variability observed duringnormal breathing. (4) Servo is a targeting scheme for whichthe output of the ventilator (eg, inspiratory pressure) au-tomatically follows a varying input (eg, inspiratory effort).(5) Adaptive is a targeting scheme that allows the venti-lator to automatically set one target (eg, pressure within abreath) to achieve another target (eg, average VT overseveral breaths). (6) Optimal is a targeting scheme thatautomatically adjusts the targets of the ventilatory patternto either minimize or maximize some overall performancecharacteristic (eg, work rate of breathing). (7) Intelligent isa targeting scheme that automatically adjusts the targets ofthe ventilatory pattern using artificial intelligence programssuch as fuzzy logic, rule-based expert systems, and artifi-cial neural networks.

(10) A mode of ventilation is classified according to itscontrol variable, breath sequence, and targeting scheme(s).The preceding 9 maxims create a theoretical foundationfor the taxonomy of mechanical ventilation. Taxonomy isthe science of classification. A full explanation of howtaxonomies are created, as it applies to mechanical venti-lation, has been published previously.15 In short, the firststep is to create a standardized set of definitions. We haverefined such a vocabulary over the last 20 years (see thesupplementary materials at http://www.rcjournal.com). Se-lected terms in the vocabulary are used to create a hierar-chical classification system (essentially an outline) thatforms the structure of the taxonomy.

The taxonomy has 4 hierarchical levels (analogous toorder, class, genus, and species used in biological taxon-omies): (1) control variable (pressure or volume, for theprimary breath), (2) breath sequence (for CMV, IMV, orCSV), (3) primary breath targeting scheme (for CMV,IMV, or CSV) , and (4) secondary breath targeting scheme(for IMV). The primary breath is either the only breath thatoccurs (mandatory breaths in CMV and spontaneous breathsin CSV) or the mandatory breath in IMV. We consider itprimary because if the patient becomes apneic, it is theonly thing keeping the patient alive.

The targeting schemes can be represented by single low-ercase letters: set-point � s, dual � d, servo � r, bio-variable � b, adaptive � a, optimal � o, and intelligent �i. For example, on the Covidien PB840 ventilator, there isa mode called A/C volume control (volume assist control).This mode is classified as VC-CMV with set-point target-ing, represented by VC-CMVs.

Minor differences in a species of modes (such as uniqueoperational algorithms) can be accommodated by adding afifth level we could call variety (as is done in biology). Asan example, there are 3 varieties of PC-CSV using servotargeting. One makes inspiratory pressure proportional tothe square of inspiratory flow (automatic tube compensa-tion), one makes it proportional to the electrical signalfrom the diaphragm (neurally adjusted ventilatory assist),and one makes it proportional to the patient-generated vol-ume and flow (proportional assist ventilation). The firstcan support only the resistive load of breathing, whereasthe other two can support both the elastic and resistiveloads.

Application of the Taxonomy

Translating a name of a mode into a mode classificationusing the taxonomy is a simple 3-step procedure. In step 1,the primary breath control variable is identified. Simplyput, if inspiratory pressure is set or if pressure is propor-tional to inspiratory effort, then the control variable ispressure. In contrast, if the VT and inspiratory flow are set,then the control variable is volume. Figure 6 shows thedecision rubric with a few refinements to accommodatedual targeting. In step 2, the breath sequence is identified.Figure 7 shows the decision rubric. In step 3, the targetingschemes for the primary and, if applicable, secondarybreaths are identified (Table 2).

Examples

To demonstrate these steps, we will classify some of themost commonly used modes in ICUs, starting with A/Cvolume control (Covidien PB840 ventilator). For this mode,both inspiratory volume and flow are pre-set, so the con-trol variable is volume (see Fig. 6). Every breath is vol-ume-cycled, which is a form of ventilator cycling. Anybreath for which inspiration is ventilator-cycled is classi-fied as a mandatory breath. Hence, the breath sequence isCMV (see Fig. 7). Finally, the operator sets all of theparameters of the volume and flow waveforms, so thetargeting scheme is set-point (see Table 2). Thus, the modeis classified as VC-CMV with set-point targeting (VC-CMVs).

Another common mode is SIMV volume control plus(Covidien PB840 ventilator). For this mode, the operatorsets the VT, but not inspiratory flow. Because setting vol-

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ume alone (like setting flow alone) is a necessary but notsufficient criterion for volume control, the control variableis pressure (see Fig. 6). Spontaneous breaths are allowedbetween mandatory breaths, so the breath sequence is IMV(see Fig. 7). The ventilator adjusts inspiratory pressure formandatory breaths to achieve an average pre-set VT, so thetargeting scheme for the mandatory breaths is adaptive(see Table 2). For spontaneous breaths, inspiratory pres-sure is set by the operator (eg, pressure support), so the

targeting scheme for these breaths is set-point. The modetag is PC-IMVa,s.

A very common mode for spontaneous breathing trials(or for assistance of spontaneous breaths in IMV modes) ispressure support or pressure support ventilation (note thatalthough ubiquitous, pressure support is a name, not aclassification). For this mode, the operator sets an inspira-tory pressure, so the control variable is pressure. All breathsare patient-triggered and patient-cycled (note that flow cy-

Fig. 6. Rubric for determining the control variable of a mode. Paw � airway pressure, SIMV � synchronized intermittent mandatoryventilation; VT � tidal volume; P � pressure; E � elastance; V � volume; R � resistance; V � inspiratory flow. Courtesy Mandu Press.

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Fig. 7. Rubric for determining the breath sequence of a mode. Courtesy Mandu Press.

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cling is a form of patient cycling, as discussed above), sothe breath sequence is CSV. Because the ventilator doesnot automatically adjust any of the parameters of the breath,the targeting scheme is set-point, and the tag is PC-CSVs.

If carefully applied, the taxonomy has the power toclarify and unmask hidden complexity in a mode that hasa cryptic name. Take, for example, the mode calledCMV � AutoFlow on the Drager Evita XL ventilator. Al-though CMV on this ventilator is a mode equivalent tovolume assist control (described above), adding the Auto-Flow feature changes it to a completely different mode.For CMV � AutoFlow, the operator sets a target VT, butnot inspiratory flow. Indeed, inspiratory flow is highlyvariable because the ventilator automatically sets the in-spiratory pressure within a breath. Thus, according to theequation of motion, the control variable is pressure. Everyinspiration is time-cycled and hence mandatory, and thebreath sequence is CMV. The ventilator adjusts the in-spiratory pressure between breaths to achieve an average

VT equal to the pre-set value using an adaptive targetingscheme. Thus, the mode is classified as PC-CMV withadaptive targeting (PC-CMVa).

On the other hand, the taxonomy can unmask the com-plexity in an apparently simple mode. The mode calledvolume control (Servo-i, Maquet, Wayne, New Jersey)allows setting the VT and inspiratory time. Setting bothvolume and inspiratory time is equivalent to setting meaninspiratory flow (flow � volume/time); hence, the controlvariable is volume. Every breath is normally time-cycledand hence mandatory, so our initial thought is that thebreath sequence is CMV. The tricky part is the targetingscheme. The operator’s manual states that “. . . if a pres-sure drop of 3 cm H2O is detected during inspiration, theventilator (switches) to Pressure Support with a resultingincrease in inspiratory flow.” This indicates dual targetingas described above (see Table 2). Noting that the breathmay switch to pressure support alerts us that the breathsequence is not what it first seemed to be. A breath may be

Table 2. Targeting Schemes

Name(Abbreviation) Description Advantage Disadvantage Example Mode Name Ventilator

(Manufacturer)

Set-point (s) The operator sets allparameters of the pressurewaveform (pressure controlmodes) or volume and flowwaveforms (volume controlmodes).

Simplicity Changing patient conditionsmay make settingsinappropriate.

Volume control CMV Evita Infinity V500(Dräger)

Dual (d) The ventilator canautomatically switchbetween volume controland pressure control duringa single inspiration.

It can adjust to changingpatient conditions andensure either a pre-setVT or peak inspiratorypressure, whichever isdeemed most important.

It may be complicated toset correctly and mayneed constantreadjustment if notautomatically controlledby the ventilator.

Volume control Servo-i (Maquet)

Servo (r) The output of the ventilator(pressure/volume/flow)automatically follows avarying input.

Support by the ventilator isproportional toinspiratory effort.

It requires estimates ofartificial airway and/orrespiratory systemmechanical properties.

Proportional assistventilation plus

PB840 (Covidien)

Adaptive (a) The ventilator automaticallysets target(s) betweenbreaths in response tovarying patient conditions.

It can maintain stable VTdelivery with pressurecontrol for changinglung mechanics orpatient inspiratory effort.

Automatic adjustment maybe inappropriate ifalgorithm assumptionsare violated or if they donot match physiology.

Pressure-regulatedvolume control

Servo-i

Bio-variable (b) The ventilator automaticallyadjusts the inspiratorypressure or VT randomly.

It simulates the variabilityobserved during normalbreathing and mayimprove oxygenation ormechanics.

Manually set range ofvariability may beinappropriate to achievegoals.

Variable pressuresupport

Evita Infinity V500

Optimal (o) The ventilator automaticallyadjusts the targets of theventilatory pattern to eitherminimize or maximizesome overall performancecharacteristic (eg, work rateof breathing).

It can adjust to changinglung mechanics orpatient inspiratory effort.

Automatic adjustment maybe inappropriate ifalgorithm assumptionsare violated or if they donot match physiology.

ASV G5 (HamiltonMedical)

Intelligent (i) This is a targeting schemethat uses artificialintelligence programs suchas fuzzy logic, rule-basedexpert systems, andartificial neural networks.

It can adjust to changinglung mechanics orpatient inspiratory effort.

Automatic adjustment maybe inappropriate ifalgorithm assumptionsare violated or if they donot match physiology.

SmartCare/PS Evita Infinity V500IntelliVent-ASV S1 (Hamilton

Medical)

CMV � continuous mandatory ventilationASV � adaptive support ventilationVT � tidal volume

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patient-triggered with a patient inspiratory effort, and ifthe effort is large enough and long enough, inspiration isflow-cycled, not time-cycled. Flow cycling (at a certainpercent of peak inspiratory pressure) is, as we describedabove, a form of patient cycling because the time constantof the patient’s respiratory system determines when thecycle threshold is met for passive inhalation (hence, in-spiratory time is determined by the patient). Alternatively,the patient may make an expiratory effort that cycles in-spiration off. Either way, a patient-triggered and patient-cycled breath is a spontaneous breath. Thus, spontaneousbreaths may occur between mandatory breaths, and thebreath sequence is actually IMV. Finally, the tag for thismode is VC-IMVd,d. Note that with dual targeting modes,we need to identify which control variable is in effect atthe start of inspiration, and in this case, it is volume. Incontrast, the mode called pressure A/C with machine vol-ume (CareFusion Avea) is also dual targeting but starts outin pressure control and may switch to volume control. Thisconvention is used because if the criterion that causes theswitch between control variables is never met during abreath, the original control variable remains in effectthroughout the inspiratory time.

Finally, some modes are composed of compound tar-geting schemes. For example, some ventilators offer tubecompensation, a feature that increases inspiratory pressurein proportion to flow to support the resistive load of breath-ing through an artificial airway. This is a form of servotargeting. On the Drager Evita XL ventilator, tube com-pensation can be added to CMV � AutoFlow to obtain amode classified as PC-CMVar, where ar represents thecompound targeting scheme composed of adaptive plusservo (note the absence of a comma between a and r be-cause we are referring only to the primary breaths, and nosecondary breaths exist). A mode classified as PC-IMVwith set-point control for both primary (mandatory) andsecondary (spontaneous) breaths would have the tag PC-IMVs,s (note the comma indicating set-point for primarybreaths and set-point for secondary breaths). If tube com-pensation is used for the spontaneous breaths (eg, Covi-dien PB840 ventilator), the tag would change to PC-IMVs,r. If it is added to both mandatory and spontaneousbreaths (eg, Drager Evita XL ventilator), the tag wouldchange to PC-IMVsr,sr. Another example is IntelliVent-ASV (Hamilton Medical S1 ventilator), which uses opti-mal targeting to minimize the work rate and intelligenttargeting to establish lung-protective limits and adjust PEEPand FIO2

. The tag for this mode is PC-CMVoi,oi.The utility of this taxonomy becomes evident when com-

paring modes on different ventilators (eg, for making apurchase decision), as shown in a recent issue of HealthDevices.6 We have extended this type of comparison toinclude several common ICU ventilators (Table 3). Table3 is sorted by manufacturer and model, control variables,

breath sequences, and targeting schemes (simple to com-plex). Table 4 shows the most commonly used modes forthe ventilators described in Table 3, sorted by classifica-tion (tag). Table 4 illustrates how modes that are essen-tially the same or very similar are given very differentnames. We have constructed a table of all of the modes on30 different ventilators from 11 different manufacturers(not shown). The table lists 290 unique names of modesrepresenting 45 different classifications (tags). Dealing withthis level of complexity is not unlike the challenge facingclinicians when applying clinical diagnostic reasoning. Theelements of the mode taxonomy can thus be seen as anal-ogous to the discriminating and defining features of a setof diagnostic hypotheses,16 as shown in Figure 8.

Discussion

The Problem of Growing Complexity

We mentioned in the introduction how modes of me-chanical ventilation have evolved to a high level of com-plexity. If we agree that the goal is to be able to appro-priately use all modes of ventilation (even if we restrictthis goal to a single type of ventilator that might be avail-able), then this implies the ability to compare and contrasttheir advantages and disadvantages. The urgency for deal-ing with the complexity of mechanical ventilation is ulti-mately based on the growing concern for patient safety.The ECRI “has repeatedly stressed the need for users tounderstand the operation and features of ventilators . . . Thefact that ventilators are such an established technology byno means guarantees that these issues are clearly under-stood . . . We continue to receive reports of hospital staffmisusing ventilators because they’re unaware of the de-vice’s particular operational considerations.”17

To deal with this complexity, researchers are designingeven more complex systems that attempt to better servethe 3 goals of mechanical ventilation (ie, safety, comfort,and liberation).18 For example, Tehrani19 has recently de-scribed a system designed to automatically control thesupport level in proportional assist ventilation to guaranteedelivery of a patient’s required ventilation (serving thegoal of safety). This system can also be used to controlthe proportional assist ventilation support level based onthe patient’s work of breathing (serving the goal of com-fort). Blanch et al20 have developed the Better Care sys-tem, which reliably detects ineffective respiratory effortsduring expiration (ie, inability of a patient to trigger breaths)with accuracy similar to that of expert intensivists and theEAdi signal (serving the comfort goal). According to Kilicand Kilic,21 conventional weaning predictors ignore im-portant dimensions of weaning outcome. They describe afuzzy logic system that provides an approach that canhandle multi-attribute decision making as a tool to over-

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Table 3. Classification of Modes on Common ICU Ventilators

Mode Name Tag

Covidien PB840A/C volume control VC-CMVs*SIMV volume control with pressure support VC-IMVs,sSIMV volume control with tube compensation VC-IMVs,rA/C pressure control PC-CMVsA/C volume control plus PC-CMVaSIMV pressure control with pressure support PC-IMVs,sSIMV pressure control with tube compensation PC-IMVs,rBi-level with pressure support PC-IMVs,sBi-level with tube compensation PC-IMVs,rSIMV volume control plus with pressure support PC-IMVa,sSIMV volume control plus with tube compensation PC-IMVa,rSpontaneous pressure support PC-CSVsSpontaneous tube compensation PC-CSVrSpontaneous proportional assist PC-CSVrSpontaneous volume support PC-CSVa

Dräger Evita XLCMV VC-CMVsCMV with pressure-limited ventilation VC-CMVdSIMV VC-IMVs,sSIMV with automatic tube compensation VC-IMVs,srSIMV with pressure-limited ventilation VC-IMVd,sSIMV with pressure-limited ventilation and automatic tube compensation VC-IMVd,srMandatory minute volume ventilation VC-IMVa,sMandatory minute volume ventilation with automatic tube compensation VC-IMVa,srMandatory minute volume with pressure-limited ventilation VC-IMVda,sMandatory minute volume with pressure-limited ventilation and automatic tube compensation VC-IMVda,srPressure control ventilation plus assisted PC-CMVsCMV with AutoFlow PC-CMVaCMV with AutoFlow and tube compensation PC-CMVarPressure control ventilation plus/pressure support PC-IMVs,sAPRV PC-IMVs,sMandatory minute volume with AutoFlow PC-IMVa,sSIMV with AutoFlow PC-IMVa,sMandatory minute volume with AutoFlow and tube compensation PC-IMVar,srSIMV with AutoFlow and tube compensation PC-IMVar,srPressure control ventilation plus/pressure support and tube compensation PC-IMVsr,srAPRV with tube compensation PC-IMVsr,srCPAP/pressure support PC-CSVsSmartCare PC-CSViCPAP/pressure support with tube compensation PC-CSVsr

Hamilton Medical G5Synchronized controlled mandatory ventilation VC-CMVsSIMV VC-IMVs,sSIMV with tube-resistance compensation CV-IMVs,srPressure control CMV PC-CMVsAdaptive pressure ventilation CMV PC-CMVaAdaptive pressure ventilation CMV with tube-resistance compensation PC-CMVarPressure control CMV with tube-resistance compensation PC-CMVsrPressure SIMV PC-IMVs,sNIV-spontaneous timed PC-IMVs,sNasal CPAP-pressure support PC-IMVs,sAPRV PC-IMVs,sDuoPAP PC-IMVs,s

(continued)

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come the weaknesses of currently used weaning predictors(serving the goal of liberation). Wysocki et al22 provide avery good overview of technical and engineering consid-erations regarding closed-loop controlled ventilation, aswell as tangible clinical evidence that such systems makemechanical ventilation safer and more efficient.

The Problem of Identifying Unique Modes

Before we can compare modes, we must have a list ofavailable modes. To generate such a list, we cannot justcopy the names of modes found in ventilator operators’manuals for 2 reasons. First, there is no consistency amongmanufacturers regarding how modes are named: somenames are the same but the modes are different, and some

names are different but the modes are the same. Second,complex ICU ventilators offer features that, when acti-vated, result in modes that are not named by the manu-facturer and, in many cases, not even recognized as dif-ferent modes. Such ambiguity makes learning about howventilators work very difficult.

Thus, before we can generate a list of modes to com-pare, we first have to find some way to deal with thisconfusion. One way is to distinguish between a mode name,which is determined by the manufacturer, and a mode tagor general classification. However, before we can do that,we must have a practical taxonomy. And before a taxon-omy can be constructed, there must be a standardized glos-sary (or controlled vocabulary, as it is called by taxono-mists) of relevant terms.15,23 The need for a standardized

Table 3. Continued

Mode Name Tag

Adaptive pressure ventilation SIMV PC-IMVa,sAdaptive pressure ventilation SIMV with tube-resistance compensation PC-IMVar,srASV PC-IMVoi,oiIntelliVent-ASV PC-IMVoi,oiASV with tube-resistance compensation PC-IMVoir,oirIntelliVent-ASV with tube-resistance compensation PC-IMVoir,oirPressure SIMV with tube-resistance compensation PC-IMVsr,srAPRV with tube-resistance compensation PC-IMVsr,srSpontaneous with tube-resistance compensation PC-CSVrSpontaneous PC-CSVsNIV PC-CSVs

Maquet Servo-iVolume control VC-IMVd,dSIMV (volume control) VC-IMVd,dAutomode (volume control to volume support) VC-IMVd,aPressure control PC-CMVsPressure-regulated volume control PC-CMVaSIMV (pressure control) PC-IMVs,sBiVent PC-IMVs,sAutomode (pressure control to pressure support) PC-IMVs,sSIMV pressure-regulated volume control PC-IMVa,sAutomode (pressure-regulated volume control to volume support) PC-IMVa,aSpontaneous/CPAP PC-CSVsPressure support PC-CSVsNeurally adjusted ventilatory assist PC-CSVrVolume support PC-CSVa

* Targeting schemes are represented by single lowercase letters: s � set-point, r� servo, a � adaptive, d � dual, i � intelligent, and o � optimal. Combinations include: sr � set-point with servo,da � dual with adaptive, as � adaptive with set-point, ar � adaptive with servo, oi � optimal with intelligent, and ois � optimal with intelligent and servo.A/C � assist controlSIMV � synchronized intermittent mandatory ventilationCMV � continuous mandatory ventilationsCSV � continuous spontaneous ventilationIMV � intermittent mandatory ventilationNIV � noninvasive ventilationAPRV � airway pressure release ventilationDuoPAP � dual positive airway pressureASV � adaptive support ventilationVC � volume controlPC � pressure control

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vocabulary is the reason we included one in the supple-mentary materials (http://www.rcjournal.com). This vocab-ulary has been carefully developed by the authors over thelast 20 years with the specific purpose of establishing ba-sic concepts that are logically consistent across all appli-cations.

The Problem of Teaching Mechanical Ventilation

We believe that there is a growing and under-recog-nized problem regarding training in the art and science ofmechanical ventilation that frequently leads to operationalerrors. The reason is that technology is expanding fasterthan our educational resources. Not even a 4-year respi-ratory care program can afford the time to ensure the above-mentioned mechanical ventilation competencies for RRTs.The challenge for physicians may be even greater becausethey generally rely on their residency (rather than medicalschool) to learn mechanical ventilation. But according toat least one study, “. . . internal medicine residents are notgaining important evidence-based knowledge needed to

provide effective care for patients who require mechanicalventilation.”24

So how do instruction programs deal with the chal-lenge? We recently conducted an informal survey of mem-bers of the AARC specialty section on education. Weasked program directors to send us their outlines for teach-ing mechanical ventilation. On the basis of what we found,we contend there are 3 categories of organizational struc-ture: (1) simple lists of skills needed to operate specificventilators, (2) lists of topics ranging from indications forventilation to weaning (and everything in between) withno apparent order, and (3) topic organization identical toor closely following the table of contents in textbooks onmechanical ventilation. Having written ventilator-relatedcontent in such textbooks, we can safely say that suchmaterial was never designed to be used as the basis for aclass syllabus. In most of our previous writings focusingon modes of mechanical ventilation,7,14,25,26 the emphasishas been on descriptions of modes rather than the back-ground technical knowledge required to understand them.That knowledge was assumed on the part of the reader

Table 4. Most Common Modes in Table 3 Sorted by Tag to Show Which Mode Names Have Equivalent Mode Classifications

Tag Covidien PB840 Dräger Evita XL Hamilton G5 Maquet Servo-i

VC-CMVs* A/C volume control CMV Synchronized controlledmandatory ventilation

NA†

VC-IMVs,s SIMV volume control withpressure support

SIMV SIMV NA‡

PC-CMVs A/C pressure control Pressure control ventilationplus assisted

Pressure control CMV Pressure control

PC-CMVa A/C volume control plus CMV with AutoFlow Adaptive pressure ventilationCMV

Pressure-regulated volumecontrol

PC-IMVs,s SIMV-pressure control withpressure support

Pressure control ventilationplus/pressure support

Pressure SIMV SIMV (pressure control)

Bi-level with pressuresupport

APRV NIV-spontaneous timed BiVentNasal CPAP-pressure support Automode (pressure control to

pressure support)APRVDuoPAP

PC-IMVa,s SIMV volume control pluswith pressure support

Mandatory minute volumewith AutoFlow

Adaptive pressure ventilationSIMV

SIMV pressure-regulatedvolume control

SIMV with AutoFlowPC-CSVs Spontaneous pressure support CPAP/pressure support Spontaneous Spontaneous/CPAPPC-CSVa Spontaneous volume support NA NA Volume support

* Targeting schemes are represented by single lowercase letters: s � set-point, and a � adaptive.† Volume control continuous mandatory ventilation (VC-CMV) is not available because the mode called volume control allows some breaths to be patient-triggered and patient-cycled; hence, theyare spontaneous, making the breath sequence intermittent mandatory ventilation (IMV) rather than CMV.‡ VC-IMV is available only with dual targeting, called SIMV (volume control) with the tag VC-IMVd,d.PC � pressure controlCSV � continuous spontaneous ventilationA/C � assist controlSIMV � synchronized intermittent mandatory ventilationNA � not availableAPRV � airway pressure release ventilationDuoPAP � dual positive airway pressureNIV � noninvasive ventilation

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(and instructor). This seems to be a universal theme amongauthors of both articles and book chapters on how venti-lators work, but as the technology grows more complex,the gap between assumed and actual knowledge becomeswider. We trust that this article helps to narrow that gap.

The Problem of Implementation

The first problem regarding implementation of any tax-onomy (and its underlying standardized vocabulary) is, ofcourse, reaching a tipping point in acceptance by stake-holders. We believe that such acceptance is achievableonly if the tools for implementing the taxonomy are sim-ple, practical, and efficient in permitting both the identi-fication and comparison of modes. The acceptance of thistaxonomy by the ECRI is a step in the right direction. Wehope that the detailed definitions, descriptions, and algo-rithms provided here will address the deficiencies in theavailable textbook references. Indeed, such tools are ame-nable to dissemination using mobile computing technol-ogy such as tablet computers and smartphones.

The second problem is ongoing maintenance of the tax-onomy. Terms and concepts necessarily change as tech-nology evolves. Ideally, a professional organization shouldtake responsibility for this function. The International Or-

ganization for Standardization and the Integrating theHealthcare Enterprise Rosetta Terminology Mapping proj-ect are working toward such a goal. Alternatively, theAARC might be an appropriate venue for maintaining thetaxonomy as they do for clinical practice guidelines.

Conclusions

The rapid increase in the number and complexity ofmechanical ventilators, and specifically the modes theyoffer, has outpaced development of tools for describingthem. A key problem has been the lack of a practicalclassification system or taxonomy. Partial solutions to thatproblem have been offered in our previous publications. Inthis paper, we developed a full taxonomy and standardizedvocabulary for modes of mechanical ventilation. Weshowed how the taxonomy is based on 10 fundamentalconstructs, or maxims, describing ventilator technology.Finally, we showed how to use the taxonomy to classifymodes of ventilation found on common ICU ventilators.Identifying and classifying modes are necessary steps be-fore being able to compare their relative merits and ulti-mately to choose the most appropriate mode to serve thegoals of care for a particular patient.18 The tools offeredin this paper (including the standardized vocabulary and

Fig. 8. Venn diagram illustrating how the mode taxonomy can be viewed in terms of discriminating features and defining features.VC � volume control; PC � pressure control; CMV � continuous mandatory ventilation; IMV � intermittent mandatory ventilation;CSV � continuous spontaneous ventilation; PETCO2

� end-tidal partial pressure of carbon dioxide; a � adaptive targeting; s� set-pointtargeting. Courtesy Mandu Press.

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2 summary handouts in the supplementary materials athttp://www.rcjournal.com) serve as a means for achievingthe mechanical ventilation competencies of the respiratorytherapist in 2015 and beyond.4 Indeed, they serve the needsof all stakeholders, including clinicians (to understand treat-ment options), researchers (to evaluate treatment options),educators (to prepare the next generation of ventilator ex-perts), administrators (to make purchase decisions), and,perhaps most important of all, manufacturers (to explainthe technical capabilities of their products and serve theneeds of the other stakeholders).

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RESPIRATORY CARE Paper in Press. Published on August 12, 2014 as DOI: 10.4187/respcare.03057

Copyright (C) 2014 Daedalus Enterprises ePub ahead of print papers have been peer-reviewed, accepted for publication, copy edited and proofread. However, this version may differ from the final published version in the online and print editions of RESPIRATORY CARE