BREATHING Breathing: It is taking air in (inspiration) and out
of your lungs (expiration). It can be consciously controlled
(voluntary action) Breathing involves two stages ventilation and
gas exchange. Ventilation is the movement of air in and out of
lungs and gas exchange is the absorption of oxygen from the lungs
and release of carbon dioxide.
Respiration is a process where the body breaks down the oxygen,
so that the cells in the body can use it . Therefore ,Breathing is
a physical process and respiration is a chemical process
NORMAL MECHANICS OF SPONTANEOUS VENTILATION AND RESPIRATION
Spontaneous breathing or spontaneous ventilation is simply the
movement of air into and out of the lungs.The main purpose of
ventilation is to bring in fresh air, for gas exchange into the
lungs and to allow the exhalation of air that contains CO2.
RESPIRATION It is defined as movement of gas molecules across a
membrane. EXTERNAL RESPIRATION is movement of O2 from the lungs
into bloodstream and of CO2 from bloodstream into alveoli. INTERNAL
RESPIRATION is movement of CO2 from the cells into the blood and
movement of O2 from the blood into cells.
Normal inspiration is accomplished by the expansion of thorax
or chest cavity. It occurs when the muscles of inspiration
contract. During contraction , the diaphragm descends and enlarges
the vertical size of thoracic cavity. The external intercostal
muscles contract and raise the ribs slightly, increasing the
circumference of thorax. The activities of these muscles represent
the work required to inspire.
Normal exhalation is passive and does not require any work.
During normal exhalation, the muscles relax, the diaphragm moves
upward to its resting position, and the ribs return to their normal
position. The volume of thoracic cavity decreases, and air is
forced out of alveoli. INSPIRATION EXPIRATION
BASIC PHYSIOLOGY - Negative pressure circuit - Gradient between
mouth and pleural space is the driving pressure - need to overcome
resistance - maintain alveolus open overcome elastic recoil forces
- Balance between elastic recoil of chest wall and the
lung=FRC
BASIC PHYSIOLOGY
VENTILATION Ventilation is the process by which Oxygen and CO2
are transported to and from the lungs. Pulmonary ventilation
Alveolar ventilation
GAS FLOW AND PRESSURE GRADIENTS DURING VENTILATION Basic
concept of Air flow is that, for air to flow through a tube or
airway, pressure at one end must be higher than the pressure at the
other end. Air always flows from the high pressure point to the low
pressure point (pressure gradient). The conductive airway begins at
the mouth & nose, and ends at the small airways near the
alveoli. Therefore, gas flows into the lungs, when the pressure in
the alveoli is lower than the pressure at the mouth and nose.
Conversely, gas flows out of lungs, when the pressure in the
alveoli is greater than the pressure at the mouth and nose. When
the pressure at the mouth and alveoli are same, as occurs at the
end of inspiration or the end of expiration, then no gas flow
occurs as there is no pressure gradient.
DEFINITION OF PRESSURES AND GRADIENTS IN THE LUNGS Airway
opening pressure (Paw)/ Mouth pressure(PM) is often called airway
pressure (Paw). Unless pressure is applied to mouth or nose, Paw is
Zero (atmospheric). Body surface pressure (Pbs) is the pressure at
body surface . This is equal to Zero unless the person is using a
pressurized chamber or a negative pressure.
Ppl = Intrapleural pressure; pressure in the intrapleural space
; generally negative because the lungs are naturally smaller than
the chest wall; the negative pressure helps to keep the airways
open and helps the lungs from collapsing. Palv = Intra-alveolar
pressure; pressure within the alveoli; positive on expiration,
negative on inspiration, and zero (same as atmospheric) when there
no air movement.
Four basic pressure gradients are used to describe normal
ventilation: 1.Trans-Airway pressure (PTA): It is the pressure
gradient between the air opening and the alveolus 2. Trans-Thoracic
pressure(Pw or Pt): It is the pressure difference between the
alveolar space(lung) and the Body surface. Pw = PA - Pbs (Pw
represents the pressure needed to expand or contract the lungs and
the chest wall at the same time.)
3. 3.Trans-Pulmonary pressure (PL or PTP )/Trans-Alveolar
pressure It is the pressure difference between the alveolus and the
pleural space. PL = PA - Ppl 4. Trans - respiratiory pressure (PTR
): It is the pressure gradient between airway opening and the body
surface PTR = Paw - Pbs
D During normal spontaneous inspiration, as the volume of
thoracic space increases, the intrapleural pressure becomes more
negative in relation to atmospheric pressure . This negative
intrapleural pressure goes from -5cm H2O at end expiration to -10cm
H2O at end inspiration. The negative intrapleural pressure is
transmited to the alveolar space.
VENTILATION PARAMETERS A. Lung Volumes 1. Basic volumes: a.
Tidal Volume (VT, TV): volume of gas exchanged each breath; can
change as ventilation pattern changes .(500 ml) b. Inspiratory
Reserve Volume (IRV): maximum volume that can be inspired, starting
from the end inspiratory position (potential volume increase at the
end of inspiration).(3000ml) c. Expiratory Reserve Volume (ERV):
maximum volume that can be expired, starting from the end
expiratory position (potential volume decrease at the end of
expiration)(1200ml) d. Residual Volume (RV): volume remaining in
the lungs and airways following a maximum expiratory effort (1300
ml)
2. Capacities:combined volumes a. Vital Capacity (VC): maximum
volume of gas that can be exchanged in a single breath VC = TV +
IRV + ERV (4700 ml) b. Total Lung Capacity (TLC): maximum volume of
gas that the lungs(and airways) can contain TLC = VC + RV = TV +
IRV + ERV + RV (6000 ml) c. Functional Residual Capacity (FRC):
volume of gas remaining in the lungs (and airways) at the end
expiratory position FRC = RV + ERV (2500 ml) d. Inspiratory
capacity (IC): maximum volume of gas that can be inspired from the
end expiratory position . IC = TV + IRV (3500 ml)
3. Measurement of volumes: Spirometery
Ventilation 1. Frequency /Respiration rate (f or RR): breaths
per unit time. At rest: 12/min 2. Ventilation rate: total volume
inspired or expired per unit time ; sometimes called Minute Volume
(MV) when measured per minute; to avoid ambiguity, usually measured
as volume expired, VE MV or VE = f TV , At rest= 12/min 0.5L = 6
L/min
A a. Peak velocity (e.g. peak expired flow rate) normal value
400-600 liters/minute b. Timed vital capacity: volume of gas that
can be expired from the lungs with maximum effort in a given time .
1) Usually expressed as a fraction of the total volume expired in a
maximum effort, the Forced Vital Capacity (FVC) 2) Normal value of
FEV1 / FVC 80%
DEFINITION Mechanical ventilation is a positive or negative
pressure artificial breathing device that can maintain ventilation
and oxygen delivery for prolonged periods. (It is indicated when
the patient is unable to maintain safe levels of oxygen or CO2 by
spontaneous breathing even with the assistance of other oxygen
delivery devices)
HISTORY OF MECHANICAL VENTILATION The Roman physician Galen may
have been the first to describe mechanical ventilation. In 1908
George Poe demonstrated his mechanical respirator by asphyxiating
dogs and seemingly bringing them back to life.
ORIGINS OF MECHANICAL VENTILATION Negative-pressure ventilators
(iron lungs) Non-invasive ventilation first used in Boston
Childrens Hospital in 1928 Used extensively during polio outbreaks
in 1940s 1950s Positive-pressure ventilators Invasive ventilation
first used at Mass achusetts General Hospital in 1955 Now the
modern standard of mechanical ventilation Iron lung polio ward at
Rancho Los Amigos Hospital in 1953.
MECHANICAL VENTILATION Ventilator delivers gas to lungs using
positive pressure at certain rate. The amount of gas delivered can
be limited by time, pressure , volume. The duration can be cycled
by time , pressure and flow.
CONTROL VARIABLE
1.Pressure controller: The ventilator maintains the same
pressure waveform, at the mouth regardless of changes in lung
characteristics. 2. Flow controller: Ventilator volume delivery and
volume waveform remain constant and are not affected by changes in
lung characteristics. Flow is measured 3. Volume controller:
Ventilator volume delivery and volume waveform remain constant and
are not affected by changes in lung characteristics. Volume is
measured 4.Time controller: Pressure, volume, and flow curves can
change as lung characteristics change. Time remains constant.
PHASES OF VENTILATORY CYCLES: 1. INITIATION OF INSPIRATION
(triggering) 2. INSPIRATORY PHASE 3. CHANGE OVER FROM INSPIRATION
TO EXPIRATION (cycling) 4. EXPIRATORY PHASE CYCLING
1.INITATION OF INSPIRATION TRIGGERING This is how inspiration
is initiated in association with patients breath. It can be by
changes in time, flow or pressure TIME TRIGGERING : The rate of
breathing is controlled by the ventilator. The breath is controlled
or mandatory. The patient cannot obtain air from the machine.
PATIENT TRIGGERING When pressure is the trigger , a decrease in the
pressure within the inspiratory circuit is sensesd and inspiration
begins. The sensitivity setting reflects the amount of pressure
drop baseline pressure that the patient must develop in the
ventilator circuit , on inspiration , to initate the flow of
gas.
FLOW TRIGGERING: The ventilator delivers a constant background
flow (flow by). Any change caused by patient effort is sensed by
the flow sensor. A breath is delivered to the patient. This
requires less work of breathing when compared to pressure
triggering. 2. INSPIRATORY PHASE (Inspiration is timed from the
beginning of inspiratory flow to the beginning of expiratory flow)
A limit variable is the maximum value that a variable(pressure,
volume, flow, or time) can attain. This limits the variable during
inspiration but does not end the inspiratory phase.
3.CHANGE OVER FROM INSPIRATION TO EXPIRATION (cycling) Of the
four variables the ventilator can control to cycle out of
inspiration ( i.e.pressure, time, volume, or flow), only one can
operate at a given time. VOLUME-CYCLED VENTILATION The inspiratory
phase of a volume-cycled breath is terminated when the set volume
has been delivered. In most cases the volume remains constant even
when lung characteristics change. However, the pressures required
to deliver the volume and gas flow vary, as compliance and
resistance change.
TIME CYCLED VENTILATION In this , the inspiration ends and
expiration begins after a pre-determined time interval is reached.
Cycling may be controlled by a simple timing mechanism or by
setting the rate and adjusting the I:E ratio, or percentage of
inspiratory time With time-cycled pressure ventilation, both volume
and flow vary. Ex- IPPB FLOW-CYCLED VENTILATION With flow-cycled
ventilation, the ventilator cycles into the expiratory phase once
the flow has decreased to a predetermined value during inspiration.
Volume,pressure,and time vary according to changes in lung
characteristics. Flow cycling is the most common cycling mechanism
in the pressure-support mode.Ex-PSV
PRESSURE-CYCLED VENTTIILLATTIION When a preset pressure
threshold (limit) is reached at the mouth or upper airway, a
ventilator set to pressure cycle ends inspiration. The exhalation
valve opens, and expiratory flow begins. The volume delivered to
the patient depends on the flow delivered, the duration of
inspiration, lung characteristics, and the set pressure.
EXPIRATION The variable controlled during the expiratory time
on the ventilator is known as the baseline variable. In all
commonly used ventilator , pressure is the variable controlled
during expiration. Exhalation occurs passively because of the
elastic recoil of the lung, but patient passively exhales to a
controlled baseline pressure . The end expiratory pressure when in
equilibrium with atmospheric pressure ,is zero. A baseline pressure
above atmospheric pressure is known as Positive end- expiratory
pressure (PEEP)
INDICATIONS Acute lung injury (including ARDS, trauma) Apnea
with respiratory arrest, including cases from intoxication Chronic
obstructive pulmonary diseas(COPD) Acute respiratory acidosis with
partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH <
7.25, which may be due to paralysis of the diaphragm due to
Guillain-Barr syndrome, Myasthenia Gravis, spinal cord injury, or
the effect of anaesthetic and muscle relaxant drugs Increased work
of breathing as evidenced by significant tachypnea, and other
physical signs of respiratory distress
Hypotension including sepsis, shock, congestive heart failure
Neurological diseases such as Muscular Dystrophy and Amyotrophic
Lateral Sclerosis. Inefficiency of thoracic cage in generating
pressure gradient necessary for ventilation (chest injury, thoracic
malformation) Cardiac insufficiency (elimination WOB, reduce oxygen
consumption) Ventilatory failure or oxygenation failure due to 1.
Increased airway resistance 2. Changes in lung compliance 3.
Hypoventilation 4. V/Q mismatch 5. Intrapulmonary shunting 6.
diffusion defect
Disorders of Pulmonary Gas Exchange 1. Acute respiratory
failure 2. Chronic respiratory failure 3. Hypoxemia( not responding
to supplemental oxygen and fluid resuscitation) 4. Acute
hypercapnia ( with worsening acidosis) 5. Pulmonary disease
resulting in diffusion abnormality 6. Pulmonary diseases resulting
in ventilation-perfusion mismatch
UNDERLYING PHYSIOLOGICAL PRINCIPLES GUIDING MECHANICAL
VENTILATION o Control of CO2 elimination o Improved impaired
oxygenation o Assist respiratory muscles FACTORS AFFECTING
VENTILATION 1.Compilance 2. Resistance 3. Time constants for lung
elasticity 4. Work of breathing
LUNG COMPLIANCE It is the change in volume per unit change in
pressure Types: Static compliance= Exhaled tidal volume Plateau
pressure-PEEP Dynamic compliance = Exhaled tidal volume Peak
inspiratory pressure-PEEP
STATIC COMPLIANCE- is measured when there is no air flow.
Reflects the elastic properties of the lung and the chest wall.
DYNAMIC COMPLIANCE -is measured when air flow is present. Reflects
the airway resistance (non elastic resistance) and elastic
properties of lung and chest wall Low lung compliance increases the
work of breathing. High compliance exhalation is often incomplete
due to lack of elastic recoil by the lungs.
AIRWAY RESISTANCE I It is defined as airflow obstruction in the
airways. Normal airway resistance is between 0.6 and 2.4 cm
H2o/l/sec at a flow rate of 30 l/min. Airway resistance varies
directly with the length & inversely with diameter of ET
Calculated by Raw= pressure change/flow Increase in airway
resistance is equal to increase in work of breathing.
TIME CONSTANTS I It is product of compilance and resistance.
The time constant is the time required, in seconds , to inflate a
lung region to 60% of its filling, if the filling pressure was to
remain constants. Areas of the lung that have either increased
resistance or decreased compilance will have a longer time
constants.
WORK OF BREATHING The total work of breathing (WOB) is the sum
of physiologic work plus the work imposed by the breathing appratus
. The work that the respiratory muscles must perform to expand the
lung is that which will overcome elastic and non elastic forces :
compilance & resistance respectively. When compilance
decreases/ resistance increases a greater force is required to move
volume in the lung . That is WOB increases.
VENTILATOR CONTROLS/PARAMETERS: 1. Fraction of Inspired Oxygen
(FiO2): Amount of oxygen delivered to the patient. Adjusted to
maintain O2 sat of > 90%. Concern with oxygen toxicity with FiO2
> 60% required for 12-24 hours. 2. Respiratory Rate: Number of
breaths/min. ventilator is to deliver 3. Tidal Volume: Amount of
air delivered with each ventilator breath, usually set at 6-8
ml/kg. 4. Sigh: Ventilator breath with greater volume than preset
tidal volume, used to prevent atelectasis,however not always
used.
5. Pressure limit: Limits highest pressure allowed by
ventilator. 6. Positive End Expiratory Pressure (PEEP): Pressure
maintained in lungs at end of expiration used to improve
oxygenation by opening collapsed alveoli, improving
ventilation/perfusion, increasing oxygenation; can be used to
reduce FiO2. 7. Adjuncts to Mechanical Ventilation PEEP, CPAP, PSV
8. Alarms ventilator alarms must never be ignored or
disarmed!!!!
9. Peak Inspiratory Pressure: Peak pressure registered on the
airway pressure gauge during normal ventilation; PIP value used to
set high and low pressure alarms; increased PIP may indicate
decreased lung compliance or increased lung resistance. 10. Minute
Volume or Minute Ventilation (Ve): Respiratory rate times the tidal
volume. RR x vt = Ve Normal minute volume for adults is 5-10 liters
11. Ventilatory Mode CMV, IMV, SIMV, A/C, PCV
Power Electrical failure alarms are a must for all ventilators
12. Frequency Alarms if RR goes above or below set levels 13.
Volume Volumes go above or below preset levels (i.e. VT/ minute
volume) 14. Pressure Change in inspiratory or peak airway pressure
above or below preset limits
Mechanical ventilation BREATHS TYPES DESCRIPTION MACHINE
-CYCLED MANDATORY A breath that is triggered , limited & BREATH
cycled by ventilator . Ventilator performs all of the work of
breathing throught the the phases of ventilation ASSISTED A breath
that is triggered by the patient , BREATH then limited & cycled
by the ventilator
PATIENT CYCLED SUPPORTED A breath that is triggered by the
patient, BREATH limited by the ventilator and cycled by patient. A
spontaneous breath with an inspiratory pressure greater than
baseline. SPONTANEOUS A breath that is triggered , limited and
BREATH cycled by the patient . The patient performs all of the work
of ventilation
FULL VERSES PARTIAL VENTILATOR SUPPORT Ventilatory support can
be classified according to two general approches: 1. FULL
VENTILATORY SUPPORT (FVS) It constitutes mechanical ventilation in
which the ventilator performs all of the WOB without any
contribution from the patient. The ventilator alone provides the
minute volume of gases required to satisfy the patients respiratory
needs. 2. PARTIAL VENTILATORY SUPPORT(PVS) PVS occurs when both the
ventilation and the patient contribute toward the WOB and meeting
the minute volume of gases required to satisfy respiratory needs.
The advantages of PVS include allowing the patient to respond to
increase in CO2 by increasing VE and promiting use of the
respiratory muscles , thereby preventing disuse atrophy.
CLASSIFICATION OF VENTILATOR
POSITIVE PRESSURE VENTILATORS Volume-cycled terminate
inspiration after delivering a preset volume of gas delivered
regardless of required pressure to do so volume remains the same
unless high peak pressures reached Pressure-cycled terminate
inspiration when a preset pressure is reached varying degrees of
resistance will interfere with gas flow best used with drug
overdose patients not good for post- operative or severe
respiratory infections
NEGATIVE PRESSURE VENTILATORS They exert a negative pressure on
the external chest wall. This causes decreasing the intrathoracic
pressure during inspiration which allows air to flow into the
lungs, filling its volume. Physiologically this type of assisted
ventilation is similar to spontaneous breathing. USES 1. It is used
mainly in chronic respiratory failure associated with neuromuscular
conditions such as poliomyelitis, muscular dystrophy, amyotrophic
lateral sclerosis and myasthenia gravis. 2. Not used for serious
patients 3. Simple to use 4. Do not require intubation 5. Adaptable
for home use EXAMPLES Iron lung, body wrap and chest cuirass
COMPLICATIONS WITH NEGATIVE PRESSURE VENTILATION Limited access
for patient care. Inability to properly monitor pulmonary
mechanics. Patient discomfort.
IRON LUNG Encloses patients body except for the head and neck
in a tank and the air in it is evacuated to produce a negative
pressure around the chest. This negative pressure surrounding the
chest & underlying alveoli results in chest wall and alveolar
expansion. The tidal volume delivered to the patient is directly
related to the negative presssure gradient.
IRON LUNG CIRCA 1950s
MODERN(IZED) IRON LUNG
CHEST CUIRASS It is a form of negative pressure ventilation
that was intended to alleviate the problems of patient acess &
TANK SHOCK associated with iron lungs. It covers only the patients
chest and leaves the arms and lower body exposed. To overcome the
problem of air leakage, individually designed cuirass minimise air
leaks, & they have been used successfully to ventilate patients
with chest wall diseases such as scoliosis
CHEST CUIRASS
POSITIVE PRESSURE VENTILATORS Positive pressure ventilators
inflate the lungs by exerting positive pressure on the airway,
forcing the alveoli to expand during inspiration. Exhalation is
passive. Endotracheal intubation or tracheotomy is necessary in
most cases. There are three types of positive pressure ventilators,
which are classified by the method of ending the inspiratory phase
of respiration: 1. Pressure cycled Ventilators 2. Time Cycled
ventilators 3. Volume Cycled Ventilators 4. Non-invasive positive
pressure ventilator
NONINVASIVE POSITIVE -PRESSURE VENTILATION Positive pressure
ventilation can be given via face mask that covers the nose and the
mouth, nasal masks or other nasal devices. Ventilation can be
delivered by volume ventilator, pressure controlled ventilator,
continuous positive pressure device or bi- level positive pressure
ventilator. The most comfortable mode for the patient is pressure
controlled ventilation with pressure support. This eases the work
of breathing and enhances the gas Exchange.
Indications for NIPPV 1. Acute or chronic respiratory failure
2. Acute pulmonary edema 3. COPD 4. Chronic congestive heart
failure with a sleep rated breathing disorder 5. Obstructed sleep
apnea Contraindications 1. Hemodyanamically unstable 2.Respiratory
arrest 3. Inability to protect airway 4. Excessive secrections 5.
Unco-operative patients 6. Patients with facial odema , trauma,
burns.
Pressure support ventilation (PSV) Adaptive support ventilation
(ASV) Proportional assist ventilation (PAV) Volume assured pressure
support (VAPS) Pressure regulated volume control (PRVC) Volume
ventilation plus (VV+) Pressure control ventilation (PCV) Airway
pressure release ventilation Inverse ratio ventilation (IRV)
Automatic tube compensation (ATC)
Spontaneous ventilation A ir w a y p r e s s u r e Is not an
actual mode on the ventilator since the rate and tidal volume are
determined by the patient It provides inspiratory flow to the
patient in a timely manner Used with adjunctive modes like
PEEP
Positive End Expiratory Pressure (PEEP) PEEP is positive
pressure that is applied by the ventilator at the end of
expiration. This mode does not deliver breaths, but is used as an
adjunct to CV, A/C, and SIMV to improve oxygenation by opening
collapsed alveoli at the end of expiration.
ADVANTAGES Improves oxygenation by increasing FRC Decreases
physiological shunting Improved oxygenation will allow the Fio2 to
be lowered Increased lung compliance DISADVANTAGES Increased
incidence of pulmonary brotrauma Potential decrease in venous
return Increased work of breathing Increased intracranial pressure
Complications from the increased pressure can include decreased
cardiac output, pneumothorax, and increased intracranial
pressure.
BIPAP This offers independent control of inspiratory and
expiratory pressures while providing pressure support ventilation.
Can be used as a Cpap device by setting IPAP and EPAP at the same
level ADVANTAGES It is provided via a nasal or oral mask, nasal
pillow, or mouthpiece with a tight seal with a portable ventilator.
INDICATION It is most often used for patients who require
ventilator assistance at night, such as patients with severe COPD
or sleep apnea.
CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP) It is simply a
spontaneous breath mode, with the baseline pressure elevated above
zero. Advantages Improves oxygenation by increasing FRC Decreases
physiological shunting Improved oxygenation will allow the Fio2 to
be lowered Increased lung compliance Disadvantages Increased
incidence of pulmonary brotrauma Potential decrease in venous
return Increased work of breathing Increased intracranial
pressure
Air leaks Pressure lesion on the skin Irritation of eyes
Gastric distention Facial pain INDICATION Where FRC is increased
(ARDS, Pneumonia, lung collapse) Improved V/Q mismatch In post
operative patients & In ARDS premature infants (to treat
HYPOXIA) If patient breath spontaneously helps to maintain airway
patency CONTRAINDICATION -Surgical emphysema -Bullae -Undrained
pneumothorax - Excessive secrections
CONTROLLED MANDATORY VENTILATION (CMV) Patient has no control
over ventilation Breaths are delivered at a rate and volume that
are deterimned by adjusting ventilator , regardless of patients
attempts to breath(i.e. controls both the tidal volume and
respiratory rate of the patient). Should only be used with a
combination of sedatives, respiratory depressants and neuromuscular
blockers. INDICATION Patients fighting or bucking the ventilator
,means the patient is severely distressed and vigrously struggling
to breathe. Teatnus or seizure activites Complete rest for the
patient for 24 hrs
Crushed chest injury patients (in whom paradoxical chest wall
movement produced due to spontaneous inspiratory efforts) Where
complete control is mandatory (i.e. undergoing surgery) Patient who
are unable to breath at all (GBS, Anaesthetic patient)
ADVANTAGES Rests muscles of respiration DISADVANTAGES Heavy
sedation is required More haemodynamic depression Risk of intrinsic
PEEP is significant Patient does not like to be controlled
(uncomfortable) COMPLICATIONS Disconnection or ventilator fails to
operate is a primary hazard- in a sedated or apneic patient is the
potential for apnea and hypoxia.
Assist/Control Mode Control Mode Pt receives a set number of
breaths and cannot breathe between ventilator breaths Similar to
Pressure Control Assist Mode Pt initiates all breaths, but
ventilator cycles in at initiation to give a preset tidal volume Pt
controls rate but always receives a full machine breath
Assist/Control Mode Assist mode unless pts respiratory rate falls
below preset value Ventilator then switches to control mode Rapidly
breathing pts can overventilate and induce severe respiratory
alkalosis and hyperinflation (auto-PEEP) Ventilator delivers a
fixed volume
ADVANTAGES Small WOB Guarantee minute ventilation allows
control over RR DISADVANTAGES In a trachypneic patient > lead to
over ventilaton and severe respiratory alkalosis>>
Hyperinflation . INDICATION Heavy sedation Paralysis
INTERMITENT MANDATORY VENTILATION (IMV) Allows patient to
breathe spontaneously through ventilator circuitry. In between the
mandated breaths, the patient is free to breath at his desired
respiratory rate. ADVANTAGE 1. Between the mandatory breaths the
patient is free to choose his own respiratory rate, tidal volume
and flow rate. 2. The mandatory breath is delivered in synchrony
with patient effort, making for comfortable breathing. 3. The
patients respiratory muscles are active and so disuse atrophy is
less common. 4.Facilitates weaning
DISADVANTAGES 1. Hypoventilation is possible if the mandatory
breath rate is not set high enough. 2. Work of breathing may be
high, if trigger-sensitivity and flow rate are inappropriate to
patients needs. 3. Excessive work of breathing may occur during the
spontaneous breaths unless an adequate level of pressure support is
added. INDICATION Normal respiratory drive but respiratory muscles
unable to perform all WOB In maintaining normal PaCO2 Weaning from
mechanical ventilation
SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION(SIMV) The
ventilator attempts to synchronize the set number mandatory breaths
with the patients respiratory efforts The ventilator waits for a
patient effort during a sensitive peroid before every breath.
INDICATION: -In weaning - Initially after full ventilatory support
to partial ventilatory support - Heavy sedation & paralysis
CONTRAINDICATION- Respiratory muscles fatigue
ADVANTAGES Prevention of respiratory muscles atrophy Decreased
requirement of sedation Lower mean airway pressure DISADVANTAGES
Respiratory muscles fatigue Increased risk of CO2 rentation
Increased WOB
MANDATORY MINUTE VENTILATION (MMV) It is a mode where the
patient breathe spontaneously, yet a constant minute ventilation
(VE) is guaranteed. If the patients spontaneous ventilation does
not match the target VE , the ventilator provides whatever part of
the VE the patient does not achieve. INDICATION To prevent
hypercapnia To prevent hypoventilation & respiratory acidosis
Apneic patient
ADVANTAGES Better patient ventilator interaction Less
hemodynamic effects DISADVANTAGES Higher work of breathing than
CMV, AC Risk of lung injury due to high peak airway pressures
PRESSURE SUPPORT VENTILATION The pressure support ventilation
is patient-triggered, flow cycled, pressure supported mode where
each inspiratory effort of the patient is augmented by the
ventilator at a preset level of inspiratory pressure. Pressure
support may be used independently as a ventilator mode or used in
conjunction with CPAP or SIMV. Advantages 1. Maximizing patient
control of respiration, thereby enhancing patient comfort on the
ventilator. 2. Increase the patients spontaneous tidal volume 3.
Decrease the patients spontaneous respiratory rate 4. Decrease work
of breathing 5. Providing alternative mode of weaning from
mechanical ventilation
Disadvantages 1. PSV is not used as a sole ventilator support
during acute respiratory failure because of the risk of
hypoventilation. 2. Not suitable for the management of patient with
central apnea. 3. Developed of atelactasis due to smaller tidal
volume in patients with brief inspiratory times and high
respiratory impedance. 4.Requires spontaneous respiratory effort
5.Delivered volumes affected by changes incompliance Indication
Patient who dont have sufficient capacity (i.e. SIMV mode) To
faciliatate weaning Contraindication If patient needs mandatory
breaths
PRESSURE CONTROL VENTILATION(PCV) Pressure - controlled breaths
are time triggered , pressure limited, time cycled Advantages Can
minimize the peak inspiratory pressure while still maintaining
adequate PaO2 & PaCO2. Decreased mean airway pressure Control
frequency Disadvantages Requires sedation or paralysis Ventilation
does not change in response to clinical changing needs Indication-
Sedation
ADAPTIVE SUPPORT VENTILATION(ASV) A mode of ventilation that
changes the number of mandatory breaths and pressure support level
according to the patients breathing pattern Indication Designed to
reduce episodes of central apnea in CHF: Improvement in sleep
quality, decreased daytime sleepiness Can be used for patients who
are at risk for central apnea like those with Brain damage.
PROPORTIONAL ASSIST VENTILATION (PAV) PAV , there is no target
flow, volume, or pressure during mechanical ventilation Advantages
The pressure used to provide the pressure support is variable and
is in proportion to the patients pulmonary Characterstics and
demand. Has the ability to track changes in breathing effort over
time. Disadvantages Where the elastance / airflow resistance shows
sudden improvement , the pressure PAV may be too high . This may
lead to overdistension , increased air trapping , and
barotrauma.
All clinical situations characterized by high ventilatory
output uncoupled with ventilatory requirements (i.e. respiratory
alkalosis) may be potentially worsened by PAV Indication ARDS
Hypercapnic respiratory failure in COPD Adaptability of ventilator
to changing patients ventilatory demands Increases sleep efficiency
Non- invasive use of PAV in COPD &Kyphoscoliotic
patients:delivered through nasal mask; improves dyspnea score.
VOLUME ASSURED PESSURE SUPPORT ( VAPS) A mode of ventilation
that assures a stable tidal volume by incorporating inspiratory PSV
with conventional volume- assistd cycles (VAV) ADVANTAGE o VAPS
incorporates pressure support ventilation with conventional volume-
assisted cycles to provide stable tidal volume in patient with
irregular breathing patterns DISADVANTAGE o VAPS may prolong the
inspiratory time. o Patients with airflow obstruction should be
monitored closely in order to prevent air trapping.
Pressure Regulated Volume control ( PRVC) It provides volume
support with the lowest pressure possible by changing the flow and
inspiratory time Advantage Decelerating inspiratory flow pattern
Pressure automatically adjusted for changes in compliance and
resistance within a set range Tidal volume guaranteed Prevents
hypoventilation Disadvantage Pressure delivered is dependent on
tidal volume achieved on last breath Intermittent patient effort
variable tidal Volumes Asynchrony with variable patient effort
VOLUME VENTILATION PLUS ( VV+) An option that combines volume
control plus and volume support Volume control Plus (VC+) It is
used to deliver mandatory breaths during AC and SIMV modes of
ventilation VC+ is intented to provide a higher level of synchrony
than standarad volume control ventilation In VC + , the clinician
sets the target tidal volume inspiratory time. Volume Support ( VS)
It is intended to provide a control tidal volume and increased
patient comfort Indicated in weaning from anesthesia
AIRWAY PRESSURE RELEASE VENTILATION( APRV) APRV- A mode of
ventilation in which the spontaneous breaths are at an elevated
basline(i.e.CPAP).This elevated baseline is periodically released
to facilitate expiration. ADVANTAGE Preservation of spontaneous
breathing and comfort with most spontaneous breathing occurring at
high CPAP WOB Barotrauma Circulatory compromise Better V/Q
matching
Disadvantage of APRV Volumes change with alteration in lung
compliance and resistance Limited access to technology capable of
delivering APRV An adequately designed and powered study to
demonstrate reduction in mortality or ventilator days compared with
optimal lung protective conventional ventilation May be less
comfortable than the PSV and SIMV modes , and synchonization with
mechanical breaths may also be a problem Indication In patient with
ARDS ( decreased lung compilance)
Inverse Ratio Ventilation (IRV) Advantage IRV Improves
Oxygenation by- 1)Decrease intrapulmonary shunting 2)Increasing V/Q
matching 3) Decrease dead space ventilation Disadvantages
Exacerbation of hemodynamic instability Barotrauma Requires deep
sedation and paralysis Changes in lung compliance result in changes
in delivered
INDICATION 1.I:E ratio is greater than 1, in which inspiration
is longer than expiration 2. Used in patients with acute severe
hypoxemic respiratory failure. 3. Used with heavily sedated
patients 4. Used in ARDS and acute lung injury
AUTOMATIC TUBE COMPENSATION(ATC) A mode of ventilation that
offsets and compensates for he air - flow resistance imposed by the
arificial airway. It allows the patient to have a breathing pattern
as if breathing Spontaneously without an artificial airway. With
ATC, the pressure delivered by the ventilator to compensate for the
airflow resistance is acitve during inspiration and expiration. It
is dependent on the airflow characteristics and the flow demand of
the patient.
WEANING
Unnecessary delays in this discontinuation process can increase
the complication rate the ventilation(pneumonia , airway trauma) as
well as cost. Prematuration discontinuation carries its own set of
problem, including difficulty in reestabilishing airtificial
airways and compromised gas exchange.
ESSENTIAL TO BEGIN WEANING Patient parameters Awake, alter
& co-operative Haemodianamically stable RR> 30/min. No
effect of sedation / neuromuscular blockade Minimal secrection
Nutritional status good Ventilator parameters Spontaneous TV >
5-8 ml/Kg VC > 10- 15 ml/kg PEEP requriment 30 ml/mm of H2O MV
< 10 L
Oxygenation crieteria PaCO2 < 50 mm of Hg with normal PH
PaO2 > 60 @ FiO2 0.4 / less SaO2 > 90% @ FiO2 0.4 / less PaO2
/ FiO2 > 200
CONVENTIONAL MODES NEWER MODES MODES OF WEANING
References: 1. Clinical application of mechanical ventilation
by David W. Chang 2. Management of the mechanically ventilated
patient by Lynelle N. B. Pierce 3. Internet refrences