Background: Improved matching between ventilation and perfusion (VA/Q) has been proposed to be a major advantage of partial ventilatory support compared with controlled mechanical ventilation. This study was designed to determine whether a difference in gas exchange exists between partial ventilatory support techniques that allow unsupported spontaneous breathing to occur during any phase of the mechanical ventilatory cycle and those that provide mechanical support for each spontaneous inspiratory effort.
Methods: Ten anesthetized dogs with oleic acid-induced lung injury received, in random order, pressure-support ventilation (PSV) and airway pressure-release ventilation (APRV) with and without spontaneous breathing using equivalent airway pressure limits. Gas exchange was assessed by conventional blood gas analysis and by estimating the VA/Q distributions using the multiple inert-gas elimination technique.
Results: During APRV, spontaneous breathing accounted for 10 +/- 1% of the total expiratory minute ventilation. Breath-to-breath ventilatory support with PSV resulted in the highest total expiratory minute ventilation (P < 0.05). During spontaneous breathing with APRV, cardiac output increased from 3.9 +/- 0.3 to 4.6 +/- 0.41.min-1 (P < 0.05), arterial oxygen tension from 75 +/- 3 to 107 +/- 8 mmHg (P < 0.05), and oxygen delivery from 567 +/- 47 to 719 +/- 73 ml.kg.min-1 (P < 0.05). PSV did not increase cardiac output, arterial oxygen tension, and oxygen delivery. Spontaneous breathing did not increase oxygen consumption. During APRV spontaneous breathing accounted for a 13 +/- 2% decrease (P < 0.05) in blood flow to shunt units (VA/Q < 0.005) and a 14 +/- 2% increase (P < 0.05) in the perfusion of normal VA/Q units (0.1 < VA/Q < 10). Pulmonary blood flow distribution to shunt and normal VA/Q units was similar during PSV and APRV without spontaneous breathing. Dead space (VA/Q > 100) ventilation decreased by 6% during APRV with spontaneous breathing compared with PSV (P < 0.05).
Conclusions: Spontaneous breathing superimposed on mechanical ventilation contributes to improved VA/Q matching and increased systemic blood flow. Apparently, the spontaneous contribution to a mechanically assisted breath during PSV is not sufficient to counteract the VA/Q maldistribution of positive pressure lung insufflation during acute lung injury.