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.
WEANING FROM APRV
Weaning from APRV is done carefully to avoid derecruitment. Some authors recommend lowering P high by 2 to 3 cm H2O at a time and lengthening T high by increments of 0.5 to 2.0 seconds.13,17
Once P high is about 16 cm H2O, T high is at 12 to 15 seconds, and spontaneous respiration accounts for most or all of the minute volume, the mode can be changed to continuous positive airway pressure (CPAP) and titrated downwards. Usually, when CPAP is at 5 to 10 cm H2O, the patient is extubated, provided that mental status or concerns about airway protection or secretions are not contraindications.
PHYSIOLOGIC EFFECTS OF APRV WITH SPONTANEOUS BREATHING
Effects on the respiratory system
During spontaneous breathing, the greatest displacement of the diaphragm is in dependent regions. These regions are the best ventilated.18 Compared with spontaneously breathing patients, mechanically ventilated patients have a smaller inspiratory displacement of the dependent part of the lung.19
A study using computed tomography demonstrated that the reduction of lung volume observed in patients with acute lung injury (ALI) predominantly affects the lower lobes (dependent areas).20 Causative mechanisms could be an increase in lung weight related to ALI and a passive collapse of the lower lobes associated with an upward shift of the diaphragm.
In a preliminary study, the topographic distribution of lung collapse was different in spontaneously breathing ARDS patients than in patients who were paralyzed. In particular, lung densities were not concentrated in the dependent regions in the former group.21
Oxygenation is better with APRV with spontaneous breathing than with mechanical ventilation alone. This effect is at least partly attributable to recruitment of collapsed lung tissue and increased aeration of the dependent areas of the lung.22
Putensen et al15 compared ventilation-perfusion distribution in 24 patients with ARDS who were randomized to APRV with spontaneous breathing (more than 10% of the total minute ventilation), APRV without spontaneous breathing, or pressure-support ventilation. Spontaneous breathing during APRV improved ventilation-perfusion matching and increased systemic blood flow.
Neumann et al23 recently compared the effect of APRV with spontaneous breathing vs APRV without spontaneous breathing in terms of ventilation perfusion in an animal model of lung injury. APRV with spontaneous breathing increased ventilation in juxta-diaphragmatic regions, predominantly in dependent areas. Spontaneous breathing had a significant effect on the spatial distribution of ventilation and pulmonary perfusion.
Based on these studies, we generally use APRV with no pressure support. This strategy permits recruitment and expansion of dependent lung areas.
Effects on the cardiovascular system and hemodynamics
Räsänen et al,24 in an animal model, compared cardiovascular performance during APRV, spontaneous breathing, and continuous positive pressure ventilation. No significant differences in cardiovascular function were detected between APRV and spontaneous breathing. In contrast, continuous positive pressure ventilation decreased blood pressure, stroke volume, cardiac output, and oxygen delivery.
Falkenhain et al,25 in a subsequent case report, found that a change in mode from intermittent mandatory ventilation with PEEP to APRV resulted in improvement in the cardiac output of a patient requiring mechanical ventilation.
The lack of deleterious effect of APRV on cardiovascular function is probably a result of its spontaneous breathing component. The reduction in mean intrathoracic pressure during spontaneous breathing (compared to paralysis) improves venous return and biventricular filling, boosting cardiac output and oxygen delivery.26
Hering et al27 compared APRV with spontaneous breathing (at least 30% of the total minute ventilation) vs APRV with no spontaneous breathing in 12 patients with ALI. This study showed higher renal blood flow, glomerular filtration, and osmolar clearance in the APRV-with-spontaneous-breathing group.
The same investigators evaluated the effects of spontaneous breathing with APRV on intestinal blood flow in an animal model of lung injury.28 Spontaneous breathing with APRV improved arterial oxygenation, the systemic hemodynamic profile, and regional perfusion to the stomach and small bowel compared with full ventilatory support.
ANIMAL STUDIES OF APRV
Stock et al,11 in their original description of APRV in 1987, reported experimental results in dogs. In that study, 10 dogs with and without ARDS were randomized to APRV with a custom-built device vs volume-control mode with a Harvard pump ventilator plus PEEP. APRV delivered adequate alveolar ventilation, had lower peak airway pressures, and promoted better arterial oxygenation (at the same tidal volume and mean airway pressure) compared with volume control.
Martin et al (1991)29 studied seven neonatal lambs with ALI with four ventilatory modes: pressure-support ventilation, APRV, volume control, and spontaneous breathing. APRV maintained oxygenation while augmenting alveolar ventilation compared with pressure-support ventilation. APRV also provided ventilation at a lower peak pressure in contrast to volume control. The authors concluded that APRV was an effective mode to maintain oxygenation and assist alveolar ventilation with minimal cardiovascular impact in their animal model of ALI.
