Clinical review: biphasic positive airway pressure and airway pressure release ventilation

Abstract

This review focuses on mechanical ventilation strategies that allow unsupported spontaneous breathing activity in any phase of the ventilatory cycle. By allowing patients with the acute respiratory distress syndrome to breathe spontaneously, one can expect improvements in gas exchange and systemic blood flow, based on findings from both experimental and clinical trials. In addition, by increasing end-expiratory lung volume, as occurs when using biphasic positive airway pressure or airway pressure release ventilation, recruitment of collapsed or consolidated lung is likely to occur, especially in juxtadiaphragmatic lung legions. Traditional approaches to mechanical ventilatory support of patients with acute respiratory distress syndrome require adaptation of the patient to the mechanical ventilator using heavy sedation and even muscle relaxation. Recent investigations have questioned the utility of sedation, muscle paralysis and mechanical control of ventilation. Furthermore, evidence exists that lowering sedation levels will decrease the duration of mechanical ventilatory support, length of stay in the intensive care unit, and overall costs of hospitalization. Based on currently available data, we suggest considering the use of techniques of mechanical ventilatory support that maintain, rather than suppress, spontaneous ventilatory effort, especially in patients with severe pulmonary dysfunction.

Figure 1

Computed tomography of a lung region above the diaphragm in a pig with oleic acid induced lung injury during airway pressure release ventilation/biphasic positive airway pressure (a) with and (b) without spontaneous breathing while maintaining airway pressure limits equal.

Figure 2

Electro-impedance tomography used to estimate regional ventilation in patients with acute respiratory distress syndrome during continuous positive airway pressure (CPAP) and airway pressure release ventilation (APRV)/biphasic positive airway pressure (BiPAP) with and without spontaneous breathing. Spontaneous breathing with CPAP is associated with better ventilation in the dependent well perfused lung regions. Spontaneous breathing with APRV/BiPAP is associated with better ventilation in the dependent well perfused lung regions and the anterior lung areas. When spontaneous breathing during APRV/BiPAP is abolished, mechanical ventilation is directed entirely to the less well perfused, nondependent anterior lung areas. PCV, pressure-controlled ventilation.

Figure 3

Spontaneous breathing during airway pressure release ventilation (APRV)/biphasic positive airway pressure (BiPAP) accounted for a decrease in blood flow to shunt units (ventilation/perfusion [V_A/Q] < 0.005) and an increase in perfusion of normal V_A/Q units (0.1 < V_A/Q < 10), without creating low V_A/Q areas (0.05 < V_A/Q < 0.1). Pressure support ventilation had no effect on pulmonary blood flow distribution when compared with controlled mechanical ventilation (APRV/BiPAP without spontaneous breathing).

Figure 4

Oxygen consumption plotted against oxygen delivery during airway pressure release ventilation (APRV)/biphasic positive airway pressure (BiPAP) with and without spontaneous breathing and during inspiratory assistance with pressure support ventilation (PSV). Oxygen consumption was determined by indirect calorimetry. Bars indicate standard deviation.

Figure 5

Urine volume (Uvol), effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) during airway pressure release ventilation (APRV) with and without spontaneous breathing (SB). During APRV without SB, airway pressure was adjusted to produce equal minute ventilation (V_E; normocapnia) while APRV without spontaneous breathing was administered with equal airway pressure limits (P_aw; permissive hypercapnia). Vertical bars indicate standard deviation.