Common misconceptions and mistakes
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Believing that patients with acute respiratory distress syndrome (ARDS) require advanced modes of mechanical ventilation (eg, airway pressure release ventilation [APRV])
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Changing the ventilator mode from volume control to pressure control because of high peak airway pressures, without urgently working up the change in pulmonary mechanics
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Attempting lung-protective ventilation while allowing patients to have tidal volumes (TVs) > 6 mg/kg by ignoring “double triggering” of the ventilator
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Missing breath stacking and severe intrinsic positive end-expiratory pressure (PEEP)
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Missing the tell-tale warning signs of endotracheal tube (ET) tube occlusion with biofilm—namely, increasing peak inspiratory pressures; unchanged plateau pressures and reports of intermittent difficulty passing the suction catheter
Invasive mechanical ventilation
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The goal of invasive mechanical ventilation is always the same (in every patient):
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to provide adequate respiratory support without causing ventilator-associated lung injury
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This requires a lung-protective ventilation strategy where low lung volumes (6–8 mg/kg) are prioritized
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Low lung volumes have been proven to improve survival in individuals with ARDS (Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome, ARDSNET, N Engl J Med 2000; 342:1301–1308, May 4, 2000)
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Mechanical ventilation is capable of causing life-threatening injury by causing either (or both):
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Noncardiogenic pulmonary edema/ARDS (by alveolar overdistension and trauma)
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Pneumothorax (PTX)
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Most likely to occur when:
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Lung volumes are high (> 8 mg/kg)
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Peak airway pressures are high (> 40 cm H 2 O)
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Plateau pressures (P plat ) are high (> 30 cm H 2 O)
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Adequate as opposed to full support; prioritizes low lung volumes over normal pH and p co 2
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In patients with abnormal lung mechanics, this often requires permitting a respiratory acidosis (a.k.a. permissive hypercapnia) to maintain a safe TV of ~ 6 mg/kg, ideal body weight (IBW) ( Table 19.1 )
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Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet
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Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet
Table 19.1
Safe Tidal Volumes Based on Height (Ideal Body Weight)
Height (ft)
Inches
Centimeters
Female Tidal Volume (mL) 6 mg/kg
Female Tidal Volume (mL) 8 mg/kg
Male Tidal Volume (mL) 6 mg/kg
Male Tidal Volume (mL) 8 mg/kg
5′
60
152
270
360
300
400
5′3
63
160
310
420
340
455
5′6
66
168
350
480
380
510
5′9
69
175
400
530
420
560
6′
72
182
430
580
460
610
6′6
78
198
520
690
550
730
6′10
82
208
575
770
600
800
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Permissive hypercapnia requires tolerating hemodynamically asymptomatic pH decreases (often in the low 7.1–7.2 range)
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Prioritizing low lung volumes means that increasing the minute ventilation is done by increasing the rate, not the TV
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Maximum respiratory rate is determined by exhalation time and varies widely based on the pulmonary mechanics (ie, 15 breaths/min for obstructive disease vs 35 breaths/min for restrictive disease)
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Practically speaking, maximum respiratory rate is determined by examining the expiratory flow waveform and ensuring that flow returns to zero before the next breath is delivered ( Fig. 19.1A )
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The rate is then slowly increased to the point just before breath stacking (delivery of the next breath before complete exhalation of the last one) ( Fig. 19.1B )
Fig. 19.1
(A) Representative tracing of a flow vs time ventilator waveform, showing three mechanical breaths delivered under three hypothetical situations; normal pulmonary physiology, obstructive physiology, and restrictive physiology, highlighting the difference in expiratory flow rate and thus expiratory time. Obstructive physiology is evidenced by a decreased expiratory flow rate and a long exhalation time, where restrictive physiology is evidenced by a supranormal expiratory flow rate, and a shortened exhalation time. (B) Flow vs time tracing of a patient with obstructive physiology demonstrating breath stacking, occurring after an increase in respiratory rate. This is evidenced by the delivery of a breath before the complete exhalation of the last one. (C) Pressure vs time and a flow vs time tracing illustrating “double triggering,” a phenomenon where the patient triggers a mechanical breath before exhalation of the last one, leading to the delivery of nearly twice the set tidal volume (TV). This is evident by comparing the set TV in the lower left of the figure with the actual exhaled TV in the upper right portion of the figure. Double triggering must be avoided during lung-protective ventilation.
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In patients with severely abnormal lung mechanics, lung-protective ventilation typically requires deep sedation and paralysis to avoid dyssynchrony, which can:
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Prevent effective ventilation (via high pressures generated by patient struggling/misplaced effort)
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Cause breath stacking (previously described)
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Cause double triggering, where acidosis and air hunger cause the patient to trigger a breath immediately after the last breath terminates (before complete exhalation), leading to the delivery of nearly twice the set TV ( Fig. 19.1C )
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Choosing a mode of ventilation (volume controlled vs Pressure controlled)
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All individuals requiring mechanical ventilation can be appropriately managed with either volume-controlled (VC) or pressure-controlled (PC) ventilation
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There is no disease state that requires exotic modes of ventilation
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Volume controlled
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TV is set and fixed
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Airway pressures vary based on airway resistance and lung compliance
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Pressure controlled
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Peak airway pressure is set and fixed
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TV varies based on airway resistance and lung compliance
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Volume-controlled ventilation is the preferred mode of ventilation for individuals with normal to moderately abnormal pulmonary mechanics, because:
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It prioritizes control of tidal volume, which is the hallmark of a lung-protective ventilatory strategy
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It is the most commonly used mode, making it the safest
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Staff familiarity (specifically with high airway pressure alarms) makes this the easiest mode to troubleshoot
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It is more comfortable than pressure control, requiring less sedation
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Volume-controlled ventilation is only problematic when patients have severely abnormal pulmonary mechanics (ie, extremely increased airway resistance or low compliance)
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In these situations it can take time to find the minimally acceptable TV (based on airway pressure alarming), delaying adequate support and placing the individual at risk for barotrauma
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Pressure-controlled ventilation is the preferred mode of ventilation for individuals with severely abnormal pulmonary mechanics
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Pressure control can protect patients with severe obstruction or poor compliance from barotrauma, while rapidly establishing the minimally effective/safe TV
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However, this protection from barotrauma puts them at risk for underventilation , as TV is sacrificed to avoid high airway pressures
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PIPs are fixed; therefore TV drops when mechanics worsen
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In pressure control mode the low exhaled TV alarm (used to detect circuit disconnect in VC mode) becomes the most important alarm, signaling a change in mechanics (akin to the peak airway alarm in VC mode)
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Ensure that it is set high enough, reflecting the lowest TV you would be comfortable with (eg, 350 mL in a patient with a TV target of 450 mL)
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The low minute ventilation alarm is also important in pressure control, screening for underventilation
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Volume Control : Initial Ventilator Settings and Adjustments for Patients with Normal to Moderately Abnormal Mechanics ( Fig. 19.2 )
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Normal lung mechanics:
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TV: 8 mL/kg IBW
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Rate: 15 breaths/min
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Fi o 2 : start with 50% Fi o 2
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PEEP: 5 cm H 2 O
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Mild to moderate increase in airway resistance:
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TV: 8 mL/kg IBW
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Rate: 8–15 breaths/min
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Fi o 2 : start with ≥ 100% Fi o 2
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PEEP: 5 cm H 2 O
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Mild to moderate decrease in lung compliance:
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TV: 6 ml/kg IBW
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Rate: 20–25 breaths/min
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Fi o 2 : start with 100% Fi o 2
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PEEP: 5–10 cm H 2 O
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Adjusting the ventilator in volume control mode based on safe airway pressures and adequate arterial blood gas (ABG) values:
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Obtain the PIP, the P plat and ABG values
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If the PIP is > 35 cm H 2 O or the P plat is > 30 cm H 2 O, decrease the TV
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If the pH is < 7.35, increase the rate (to the maximum tolerated before breath stacking), then consider increasing the TV, watching airway pressures closely
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If the pH is > 7.44, decrease the TV to 6 mg/kg before decreasing the rate
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If the Pa o 2 is < 60 mm Hg, increase the Fi o 2 to 100% and increase the PEEP by 2–5 cm H 2 O
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If the Pa o 2 is > 80 mm Hg, decrease the Fi o 2 to ≤ 60% before decreasing PEEP by 2–5 cm H 2 O
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Things to remember about volume control:
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Patient always gets full TV with every breath; PIP and P plat vary
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Breath stacking (where the next breath is triggered before complete exhalation) is a major problem when ventilating patients with obstructive lung disease in acute respiratory failure because of both:
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Prolonged exhalation and a rapid intrinsic respiratory rate (increased drive from acidosis)
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Flow vs time waveform reveals that the next breath is delivered before flow returns to zero (ie, before exhalation is complete)
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This can lead to dynamic hyperinflation, high intrinsic PEEP, and ultimately an increase in intrathoracic pressure high enough to impair venous return
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This causes shock and failure to ventilate
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Physical examination reveals tachycardia, hypotension, and failure to ventilate secondary to “high pressure” limits, with no air movement despite connection to the ventilator
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Temporarily disconnecting the patient from the ventilator, allowing complete exhalation, provides instantaneous resolution to the acute ventilatory failure and hemodynamic instability
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Rate must be decreased and/or the patient must be sedated (± paralyzed) to prevent rapid recurrence
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