Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9e

Chapter 29B: Mechanical Ventilation Skyler Lentz; Patricia Ruth Atchinson; Matthew A. Roginski

MANAGING VENTILATORS SETTINGS

The primary function of mechanical ventilation is to provide respiratory support while treating theunderlying process that caused respiratory failure. Safe oxygenation and ventilation can be accomplished ineither a volume- or pressure-targeted mode. The chosen mode is o�en based on provider preference and

experience. Patient outcomes are unlikely to be changed by the mode of ventilation.1,2

VOLUME TARGETED

In a volume-targeted mode (e.g., volume control; Figure 29B-1), the ventilator provides an inspiratory flowover time to target a set volume. To avoid ventilator-induced injury, multiple trials have shown that limitingthe tidal volume improves patient outcomes compared to using the larger volumes more common in the past

(o�en ≥10 mL/kg to start).3-6 One approach is to start volume-targeted ventilation at 6 mL/kg ideal bodyweight for patients with or at risk for acute respiratory distress syndrome and 6 to 8 mL/kg ideal body weightfor other patients. The definition of acute respiratory distress syndrome is provided in Table 29B-1. Those atrisk for acute respiratory distress syndrome have pneumonia, sepsis, trauma, pancreatitis, or any shock

state.7 To choose ventilation settings, calculate ideal body weight using the patient’s height, an importantcaveat in obese patients in whom harm is risked if tidal volumes choice is based on actual body weight.

FIGURE 29B-1.

Normal ventilator waveforms for volume control with a square inspiratory waveform flow. Yellow is thepressure waveform, green is the flow waveform, and blue is the volume waveform. This figure is an exampleof volume control with a square inspiratory waveform where the tidal volume of 600 mL is delivered over 1second. PEEP = positive end-expiratory pressure.

Abbreviations: FIO2 = fraction of inspired oxygen; PaO2 = partial pressure of arterial oxygen; PEEP = positive end-

expiratory pressure.

Source: Data from The ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA

2012;307(23):2526-2533.

TABLE 29B-1

Acute Respiratory Distress Syndrome Definition (Berlin Definition)8

Onset within 1 week of a known clinical insult or new or worsening respiratory symptoms

Bilateral opacities on chest imaging not fully explained by lobar/lung collapse or nodules

Respiratory failure not fully explained by cardiac failure or volume overload

PaO2/FIO2 ratio ≤300 mm Hg with PEEP ≥5 cm H2O

Mild PaO2/FIO2 200–300 mm Hg

Moderate PaO2/FIO2 100–200 mm Hg

Severe PaO2/FIO2 ≤100 mm Hg

In volume control mode, the airway pressures depend on the tidal volume plus the size and compliance ofthe respiratory system. Peak pressures are the highest pressures seen by the respiratory system and aremeasured every breath by the ventilator. They are a combination of airway resistance and alveolar andthoracic pressures. Peak pressures o�en reflect proximal airway resistance either in the endotracheal tube(as with mucous plugging or tube obstruction) or conducting airways (as with bronchospasm), but do notrepresent the pressure applied to the alveoli (Figure 29B-2). To approximate alveolar pressure, measure aplateau pressure with an end inspiratory pause in a passively breathing or paralyzed patient. The end

inspiratory pause means flow in the respiratory circuit is zero and the pressure measured at the ventilator is

the same at the alveolus9 (Figure 29B-3). Target a plateau pressure of <30 cm H2O.3,4

FIGURE 29B-2.

Pressure waveform of a patient with high airway resistance from obstructed airways (e.g., chronic obstructivepulmonary disease or asthma). The high peak pressure and low plateau pressure indicate the problem is inthe proximal airways. It also illustrates that the high peak pressure (69 cm H2O) is not being transmitted to

the alveoli.

FIGURE 29B-3.

Pressure waveform of a patient with low lung compliance (i.e., acute respiratory distress syndrome). Theplateau is 35 cm H2O in this example. A high plateau pressure in relation to the peak pressure indicates that

there is reduced pulmonary compliance and not proximal airway resistance.

PRESSURE TARGETED

In a pressure-targeted mode (e.g., pressure control), the ventilator will raise the airway pressures aprescribed amount over the positive end-expiratory pressure (PEEP) for a set amount of time (inspiratorytime). The tidal volume depends on the resistance and compliance of the respiratory system. In pressure-control ventilation, follow tidal volumes and minute ventilation to ensure adequate ventilation. Start with apressure control of 10 cm H2O above PEEP and adjust the pressure up or down to target tidal volumes of 6 to

8 mL/kg ideal body weight. In a spontaneously breathing patient who is not intubated for severe hypoxemiaor obstructive lung disease, consider transitioning to pressure support a�er a volume-targeted modebecause the former may be more comfortable for the patient.

SYNCHRONOUS INTERMITTENT MANDATORY VENTILATION

Synchronous intermittent mandatory ventilation allows the patient to spontaneously breath betweenmandatory volume or pressure-targeted breaths. In this mode, the provider prescribes the mandatory breathtype (volume or pressure targeted) and the amount of pressure support for spontaneous breaths. The setmandatory respiratory rate ensures a minimal minute ventilation. In heavily sedated or paralyzed patientsnot spontaneously breathing, synchronous intermittent mandatory ventilation is no di�erent than pressure-or volume-targeted control modes (i.e., volume control–assist control). Similarly, if the mandatory rate is setclose to zero, almost all breaths will be spontaneous and appear more similar to pressure support. The typeof breath delivered to spontaneously breathing patients depends on the timing of the triggered breath. If thebreath is triggered near a mandatory breath time, the patient will receive a controlled breath. If the breath istriggered outside of the set timing of the mandatory breath, ventilator support will depend on the set

pressure support.10 When using synchronous intermittent mandatory ventilation in critically ill patients, besure to guarantee safe tidal volumes during spontaneous breaths.

RESPIRATORY RATE

The ideal respiratory rate depends on many factors. Most adult patients with normal respiratory physiologyhave adequate ventilation at a respiratory rate of 10 to 20 breaths/min. In sedated or paralyzed patients, theprescribed machine rate will be the actual respiratory rate. In patients who are triggering their own breaths,patients may determine their respiratory rate above the set rate. When deciding on a rate, anticipatedemands; for example, in severe metabolic acidosis (e.g., diabetic ketoacidosis), set a high respiratory rate tomaintain an adequate minute ventilation.

POSITIVE END-EXPIRATORY PRESSURE

Oxygenation can improve with mechanical ventilation by increasing the fraction of inspired oxygen (FIO2) or

increasing the mean airway pressure with PEEP.9 PEEP improves oxygenation by recruiting additional lungunits, increasing the surface area for gas exchange, and redistributing lung water. Set initial PEEP at 5 cm H2O

in most patients. Obese patients or those with a tense abdomen require a higher PEEP; for these patients,

start PEEP at 8 to 10 cm H2O.4 When you encounter a patient who is di�icult to oxygenate despite using these

initial PEEP and FIO2 settings, titrate using a table (Table 29B-2) to incrementally increase your PEEP until the

desired oxygenation is achieved. Improvements through increased PEEP are not immediate, so useincremental changes in PEEP of 2 cm H2O every 10 to 20 minutes rather than rapidly increasing or decreasing

because there is potential for unanticipated hemodynamic, intrathoracic, or intrapulmonary changes.

Abbreviation: FIO2 = fraction of inspired oxygen.

Source: Adapted from http://www.ardsnet.org/files/ventilator_protocol_2008-07.pdf (ARDSNet: NIH NHLBI ARDS

Clinical Network Mechanical Ventilation Protocol Summary). Accessed January 22, 2019.

TABLE 29B-2

Positive End-Expiratory Pressure (PEEP) Tables

Lower PEEP/higher FIO2

FIO2 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7

PEEP 5 5 8 8 10 10 10 12

FIO2 0.7 0.8 0.9 0.9 0.9 1.0    

PEEP 14 14 14 16 18 18–24    

Higher PEEP/lower FIO2

FIO2 0.3 0.3 0.3 0.3 0.3 0.4 0.4 0.5

PEEP 5 8 10 12 14 14 16 16

FIO2 0.5 0.5–0.8 0.8 0.9 1.0 1.0    

PEEP 18 20 22 22 22 24    

HYPEROXIA

Hyperoxia (too much supplemental oxygen) increases patient mortality in a dose-dependent relationship.11-

13 Many recommend titrating the FIO2 to target an oxygen saturation of no greater than 96% as soon as the

patient recovers from the apneic induction period (Table 29B-3).

Abbreviations: FIO2 = fraction of inspired oxygen; PEEP = positive end-expiratory pressure.

TABLE 29B-3

Initial Ventilator Settings

Mode Volume control Pressure control

Control variable Tidal volume 6–8 mL/kg ideal body weight 10 cm H2O

Measure-dependent variable Peak and plateau pressures Tidal volume

Respiratory rate 10–20 breaths/min

PEEP 5–8 cm H2O

FIO2 Titrate to saturation of <96%

SEDATION OF THE INTUBATED PATIENT

Sedation decisions during mechanical ventilation center on a fundamental question: Does the ventilatorneed to be adjusted to accommodate the patient, or does the patient need to be sedated to accommodatethe ventilator? Next, decide how much sedation is needed; not all patients require the same depth. Patientsintubated for severe hypoxic/hypoxemic or hypercapnic respiratory failure o�en need to be more deeplysedated to accommodate lung protective ventilation. Patients who are intubated for airway protection do noto�en require the same level of sedation as others being mechanically ventilated. Finally, although o�en notan ED issue, ending sedation or using sedation holidays allows for more prompt return to spontaneousventilation.

Target adequate analgesia before targeting sedation because pain can be a major source of agitation anddyssynchrony (i.e., discoordinated breathing—an injury mechanism along with volume-induced lung injury).Use a standardized scale to assess pain and agitation, such as the Richmond Agitation-Sedation Scale. In theED, target a Richmond Agitation-Sedation Scale score of –2 (awakens and makes eye contact to voice) to 0

(awake, alert, and calm).14,15 If possible, avoid continuous benzodiazepine infusions as they have been

associated with higher mortality, increased delirium, and increased sedation duration.15,16

OBSTRUCTIVE LUNG DISEASE

Abbreviation: PEEP = positive end-expiratory pressure.

Source: PEEP Table Adapted from ARDSNet NIH NHLBI ARDS Clinical Network Mechanical Ventilation Protocol

Summary. http://www.ardsnet.org/files/ventilator_protocol_2008-07.pdf

Acutely decompensated asthma and chronic obstructive lung disease patients are di�icult to manage on theventilator because the primary pathology is not improved by intubation (Table 29B-4). The problemsencountered are increased airway resistance (with resultant high peak pressures; Figure 29B-2), pulmonary

hyperinflation, and increased dead space causing hypercapnia.10 The obstructive nature of these diseases

leads to an e�ort-independent prolonged exhalation time.17,18 Arterial hypercapnia tempts the treatingphysician to increase the respiratory rate to exhale more carbon dioxide; however, increasing the respiratoryrate is counterproductive because it shortens the exhalation time, leading to an additional breath prior to the

lungs completely emptying. This latter event is called dynamic hyperinflation.10,19 As dynamic hyperinflationworsens, lung volumes increase, and the risk of barotrauma, pneumothorax, and hypotension also

increases.20 Incomplete emptying can be detected by monitoring the expiratory flow limb on the ventilator(Figure 29B-4) in a passively breathing patient. Measure the intrathoracic pressure from dynamichyperinflation during an end-expiratory hold (Figure 29B-4). The di�erence between the total PEEP (fromend-expiratory hold) and set PEEP is called auto-PEEP or intrinsic PEEP, signifying trapped air.

TABLE 29B-4

Ventilator Management for Obstructive Lung Disease

Slow

respiratory

rate

Start with a rate of 10–14 breaths/min.

A slow respiratory rate is the best way to increase emptying time compared with a decrease in

inspiratory time.

Tolerate

hypercapnia

Tolerate a pH of ≥7.20.

A respiratory acidosis with a pH >7.20 is safe assuming there is not concomitant pulmonary

hypertension.19,21 If a respiratory acidosis is causing systemic e�ects, increase the tidal

volume >8 mL/kg while keeping a low respiratory rate to maximize alveolar ventilation.

Check for

gas trapping

Check for auto-PEEP.

Ensure the expiratory flow limb of the flow curve reaches zero before the next breath (in a

passively breathing patient). Perform an end-expiratory hold to calculate auto-PEEP. If

hypotension occurs, disconnect the patient from the ventilator for 15–20 seconds and

decrease the respiratory rate a�er reconnection.

FIGURE 29B-4.

Pressure waveform demonstrating auto–positive end-expiratory pressure (PEEP) with an end-expiratoryhold. The total PEEP is 16 cm H2O, and the set PEEP is 5 cm H2O, which means the auto-PEEP is 11 cm H2O. In

the green flow waveform, inspiration is positive (above the baseline flow of zero) and expiration is negative(below the baseline flow of zero). The expiratory flow limb not returning to zero (baseline) means there iscontinued expiration when the next breath is delivered. This leads to dynamic hyperinflation and airtrapping.

SEVERE HYPOXIA/HYPOXEMIA, ACUTE RESPIRATORY DISTRESSSYNDROME, AND TRAUMA

Determining the etiology of hypoxia (depressed alveolar oxygen) or hypoxemia (depressed arterial oxygen)starts with determining if there is unilateral or bilateral lung disease; this strategy guides potential therapiesor interventions. In patients with bilateral lung disease at risk for acute respiratory distress syndrome, uselung-protective ventilator settings discussed earlier. Although it is o�en di�icult to completely exclude acardiogenic source of bilateral infiltrates and all patients at risk will not develop acute respiratory distress

syndrome, this approach of limiting tidal volume can provide benefit.6

The mainstays of suspected acute respiratory distress syndrome treatment are low tidal volume ventilation,adequate PEEP, and treatment of the underlying condition (Figure 29B-5). If the plateau pressure is above 30cm H2O, the tidal volume should be incrementally decreased by 1 mL/kg to as low as 4 mL/kg. These

protective lower tidal volumes may lead to hypercapnia and acidemia, although a pH of >7.20 is usually welltolerated. Most patients with restrictive lung physiology tolerate a respiratory rate of 30 to 35 breaths/min

without air trapping.5 As noted earlier, avoid hyperoxia; do not routinely seek a saturation of 98% to 100% in

most patients.11,12 In acute respiratory distress syndrome, many seek an oxygen saturation of 88% to 95% or

partial pressure of arterial oxygen of 55 to 80 mm Hg.3

FIGURE 29B-5.

Pressure waveform of a patient with low lung compliance (i.e., acute respiratory distress syndrome).Decreasing the tidal volume is one method to decrease the plateau pressure. In the top waveform, the tidal

volume is 8 mL/kg ideal body weight and the plateau is 35 cm H2O. In the bottom waveform, the tidal volume

is 6 mL/kg ideal body weight and the plateau is 30 cm H2O.

Setting appropriate PEEP aids by improving oxygenation through recruiting additional lung, increasing thesurface area for gas exchange, and preventing atelectrauma. Titrate PEEP based on the PEEP/FIO2 tables in

Table 29B-2.3

In those with refractory hypoxemia and criteria for severe ARDS, other options include neuromuscularblockade, prone positioning, pulmonary vasodilators, or transfer to an extracorporeal membrane

oxygenation–capable center.22-25

Trauma patients are at risk for either lung injury by direct trauma to the chest or secondary lung injury fromsystemic inflammation. Use the same principles noted earlier, targeting normoxia and maintaining plateau

pressure <30 cm H2O and adequate PEEP.26 In traumatic brain injury, the current goal is normoxia and

normocapnia with a goal partial pressure of carbon dioxide of 35 to 45 mm Hg while reserving

hyperventilation for a temporizing therapy in those with an increased intracranial pressure.27

VENTILATOR CARE BUNDLE

Implement a ventilator care bundle in every intubated patient as soon as possible. This includes routinemeasurement of patient height to determine appropriate tidal volume for ventilation, elevating the head ofbed to at least 30 degrees, decompressing the GI tract, and oral care with a chlorhexidine solution every 2

hours (Figure 29B-6).4,28 Although many recommend stress ulcer prophylaxis with a proton pump inhibitor ora H2 blocker, the overall e�ect on mortality and morbidity, including ventilator-associated pneumonia, gut

infection, and bleeding, is uncertain.29

FIGURE 29B-6.

1. 

2. 

3. 

Reassess intubated patients a�er initial neuromuscular blockade has worn o� to assess adequacy ofsedation, decrease the FIO2 quickly to avoid hyperoxia, check an arterial blood gas, and adjust ventilatory

strategy to meet the patient’s needs. Start a venous thromboembolism prophylaxis approach, and think earlyabout sedation holidays and spontaneous breathing trials in patients expected to be housed in the ED for≥24 hours.

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