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438 PART 4: Pulmonary Disorders

70 a) b) c) d) e) f)
P ET CO 2 mm Hg 35

0
20
Paw cm H 2 O 10
0
2
Volume L 0

−2
PAV PSV VACV
FIGURE 50-3. Comparison of proportional assist ventilation (PAV), pressure support ventilation (PSV), and volume assist control (VACV) during stimulation of the respiratory drive using
inhaled CO administration. Plotted are CO concentrations (upper panel), airway pressure (middle panel), and volume (bottom panel). Each mode is depicted before and after CO administration.
2
2
2
Note in the right examples, VACV delivers no additional volume during increased demand and thus the airway pressure graphic is pulled downward by patient effort. In the middle examples,
PSV provides additional volume with a constant airway pressure during increased demand. In the left examples, PAV provides both additional volume and additional pressure during increased
demand.
control the car’s ultimate direction just as the patient ultimately Like PAV, NAVA depends exclusively on patient effort for timing,
must control the magnitude of the breath and the timing of the breath- intensity, and duration of the breath. Thus, like PAV, clinicians must set
ing pattern. appropriate alarms and backup positive pressure ventilation, especially
With PAV, the greater the patient effort, the greater the delivered pres- for patients with unreliable respiratory drives. Also like PAV, clinicians
sure, flow, and volume. This is in contrast with volume assist where flow must set PEEP and Fi O 2 .
and volume are not affected by effort and where, in fact, applied pressure Small clinical studies have demonstrated improved trigger and cycle
may be “pulled down” by effort. PAV also contrasts with pressure assist/ synchrony with NAVA compared to conventional assisted modes. 89-93
support where flow and volume are affected by effort but pressure is not However, consensus on what level of support to begin with and how
(Fig. 50-3). it should be subsequently manipulated does not exist. Like PAV, some
Because PAV requires sensors in the ventilator circuitry to measure argue to start at a high level and wean as tolerated while others point
patient effort, it is susceptible to the same sensor performance and out that maintaining a constant level of support coupled with regular
intrinsic PEEP issues that affect breath triggering in other assisted SBTs makes the most sense. Also like PAV, data demonstrating improved
modes. Also like conventional assisted modes, the clinician must outcomes (eg, duration of mechanical ventilation, sedation needs) are
24
. Finally, breath termination (cycling) is much like lacking. Another concern with NAVA is the expense associated with the
set PEEP and Fi O 2
pressure support and is determined by a clinician adjustable percentage EMG sensor.
of maximal inspiratory flow.
PAV has been shown in multiple studies to perform as designed. 86-88 CONCLUSIONS
These studies have also shown that safety mechanisms to prevent exces-
sive pressures (“runaway”) are effective. These studies also emphasize As noted at the beginning of this chapter, the overarching goal of posi-
the importance of having appropriate alarms and backup positive pres- tive pressure mechanical ventilation is to provide adequate gas exchange
sure modes since PAV provides minimal support with small efforts and support while not causing harm. Clinicians face important challenges
no support if effort ceases. Thus PAV must be used with caution in every day in providing mechanical ventilatory support. Two of the most
patients with unreliable respiratory drives (eg, neurological disorders, important of these challenges are balancing adequate gas exchange with
fluctuating sedation/opioid use). the risk of VILI in acute respiratory failure; and ensuring patient com-
Clinical studies have compared PAV to other forms of assisted venti- fort during interactive support in the recovery period. Over the last two
lation and it has been found to be useful in terms of muscle unloading decades a number of novel approaches have been introduced that may
and patient comfort. 85-88 However, consensus on what level of support help clinicians address these challenges. While all of these approaches
to begin with and how it should be subsequently manipulated does not have conceptual appeal, most still await good clinical outcome data to
exist. Some argue to start at a high level and wean as tolerated while justify their widespread use.
others point out that maintaining a constant level coupled to regular
SBTs makes the most sense. Whether PAV improves meaningful clinical
outcomes (eg, sedation needs, shorter needs for mechanical ventilation)
remains to be determined. KEY REFERENCES
Neurally Adjusted Ventilatory Assistance: Neurally adjusted ventilatory • Clavieras N, Wysocki M, Coisel Y, et al. Prospective random-
assistance (NAVA) utilizes a diaphragmatic EMG signal to trigger, ized crossover study of a new closed-loop control system versus
govern flow, and cycle ventilatory assistance. 89,90 The EMG sensor is an pressure support during weaning from mechanical ventilation.
array of electrodes mounted on an esophageal catheter that is positioned Anesthesiology. 2013;119(3):631-641.
in the esophagus at the level of the diaphragm. Ventilator breath trigger- • Dongelmans DA, Paulus F, Veelo DP, Binnekade JM, Vroom MB,
ing is thus virtually simultaneous with the onset of phrenic nerve excita- Schultz MJ. Adaptive support ventilation may deliver unwanted
tion of the inspiratory muscles and breath cycling is tightly linked to the respiratory rate-tidal volume combinations in patients with
cessation of inspiratory muscle contraction. Flow delivery is driven by acute lung injury ventilated according to an open lung strategy.
the intensity of the EMG signal (Electrical Activity of the Diaphragm or Anesthesiol. 2011;114:1138-1143.
EADi) and the clinician sets an mL/mV gain factor.

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