Airway pressure release ventilation: An alternative mode of mechanical ventilation in acute respiratory distress syndrome
ABSTRACTAcute respiratory distress syndrome (ARDS) results in collapse of alveoli and therefore poor oxygenation. In this article, we review airway pressure release ventilation (APRV), a mode of mechanical ventilation that may be useful when, owing to ARDS, areas of the lungs are collapsed and need to be reinflated (“recruited”), avoiding cyclic alveolar collapse and reopening.
KEY POINTS
- The advantages and disadvantages of APRV are related to its two components: high mean airway pressure and spontaneous ventilation.
- Several studies show APRV to have physiologic benefits and to improve some measures of clinical outcome, such as oxygenation, use of sedation, hemodynamics, and respiratory mechanics.
- No study has reported that fewer patients die if they receive APRV compared with conventional protective ventilation.
- APRV is a promising mode, and further research is needed to strengthen support for its more widespread use.
HUMAN STUDIES OF APRV
Garner et al (1988)30 studied 14 patients after operative coronary revascularization, giving them volume control mode (12 mL/kg) and then, when they were hemodynamically stable, APRV. While APRV and volume control supported ventilation and arterial oxygenation equally in all cases, peak airway pressure was greater with volume control.
Räsänen et al (1991)31 designed a prospective, multicenter, crossover trial in which 50 patients with ALI were ventilated with conventional ventilation and subsequently with APRV. Patients in both groups were adequately ventilated and oxygenated. However, as described in the aforementioned study,24 the peak airway pressure was lower in the APRV group.
Davis et al (1993)32 studied 15 patients with ARDS requiring ventilatory support who received intermittent mandatory ventilation plus PEEP and then were placed on APRV. Peak airway pressure was lower, but mean airway pressure was higher with APRV. There were no statistically significant differences in gas exchange or hemodynamic variables.
Putensen et al,33 in a study designed on the basis of prior publications,15 randomized 30 patients with multiple trauma to either APRV with spontaneous breathing (n = 15) or pressure-control ventilation (n = 15) for 72 hours. Weaning was performed with APRV in both groups. APRV was associated with increases in lung compliance and oxygenation and reduction of shunting. Interestingly, the use of APRV was associated with shorter duration of ventilatory support (15 vs 21 days), shorter length of intensive care unit stay (23 vs 30 days), and shorter duration of sedation and use of vasopressors.
An important confounder in this trial was that all patients on pressure-control ventilation were initially paralyzed, favoring the APRV group.
Varpula and colleagues34 performed a prospective randomized intervention study to determine whether the response of oxygenation to the prone position differed between APRV vs pressure-controlled synchronized intermittent mandatory ventilation with pressure support. Forty-five patients with ALI were randomized within 72 hours of initiation of mechanical ventilation to receive one of these two modes; 33 ultimately received the assigned treatment. All patients were positioned on their stomachs for 6 hours once or twice a day. The response in terms of oxygenation to the first pronation was similar in both groups, whereas there was a significant improvement after the second pronation in the APRV group. The authors concluded that prone positioning and allowance of spontaneous breathing during APRV had advantageous effects on gas exchange.
In 2004, the same investigators35 randomized 58 patients with ALI after stabilization to either APRV or pressure-controlled synchronized intermittent mandatory ventilation. There were no significant differences in the clinically important outcomes such as ventilator-free days, sedation days, need of hemodialysis, or intensive care unit-free days.
Dart et al,36 in a retrospective study of 46 trauma patients who were ventilated with APRV for 72 hours, found an improvement in the Pao2/Fio2 ratio and a decrement in peak airway pressure after APRV was started.
Table 2 summarizes the randomized clinical trials of APRV.33–35,37
CONCERNS ABOUT APRV
Overstretching. One of the major concerns when applying APRV is overstretching the lung parenchyma.26,38 It is important to recognize that, when choosing a P high setting, this variable is not the only determinant of the tidal volume. Spontaneous breathing causes the pleural pressure to become less positive. As a result, there is an increase in the transpulmonary pressure (pressure in alveoli minus pressure in the pleura). This augmentation of transpulmonary pressure will result in a higher tidal volume and the risk of overdistention and volume-induced lung injury.
Atelectrauma. As mentioned earlier, damage may occur when airways open and close with each tidal cycle. This is particularly worrisome when the end-expiratory pressure is below the lower inflection point, as some diseased alveolar units may collapse. In APRV, the airway pressure is released to zero. Even though the intentional auto-PEEP might maintain a certain end-expiratory pressure, this parameter is truly uncontrolled.39
If the patient cannot breath spontaneously. Another consideration is that many of the benefits of APRV are based on the spontaneous breathing component. Unfortunately, patients who need heavy sedation or neuromuscular paralysis with lack of spontaneous breathing efforts may lose the physiologic advantages of this mode.
Despite these limitations, APRV presents many attractive benefits as an alternative mode of mechanical ventilation in patients who do not respond to conventional modes.
Table 3 summarizes the advantages and disadvantages of each component of APRV.
