EFFECT OF INTERFACING BETWEEN SPONTANEOUS BREATHING AND MECHANICAL CYCLES ON THE VENTILATION-PERFUSION DISTRIBUTION IN CANINE LUNG INJURY

Background: Improved matching between ventilation and perfusion (V-A/Q ) has been proposed to be a major advantage of partial ventilatory sup port compared with controlled mechanical ventilation. This study was d esigned to determine whether a difference in gas exchange exists betwe en partial ventilatory support techniques that allow unsupported spont aneous breathing to occur during any phase of the mechanical ventilato ry cycle and those that provide mechanical support for each spontaneou s inspiratory effort. Methods: Ten anesthetized dogs with oleic acid-i nduced lung injury received, in random order, pressure-support ventila tion (PSV) and airway pressure-release ventilation (APRV) with and wit hout spontaneous breathing using equivalent airway pressure limits. Ga s exchange was assessed by conventional blood gas analysis and by esti mating the V-A/Q distributions using the multiple inert-gas eliminatio n technique. Results: During APRV, spontaneous breathing accounted for 10 +/- 1% of the total expiratory minute ventilation. Breath-to-breat h ventilatory support with PSV resulted in the highest total expirator y minute ventilation (P < 0.05). During spontaneous breathing with APR V, cardiac output increased from 3.9 +/- 0.3 to 4.6 +/- 0.4 1.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 t ension, and oxygen delivery. Spontaneous breathing did not increase ox ygen consumption. During APRV spontaneous breathing accounted for a 13 +/- 2% decrease (P < 0.05) in blood flow to shunt units (V-A/Q < 0.00 5) and a 14 +/- 2% increase (P < 0.05) in the perfusion of normal V-A/ Q units (0.1 < V-A/Q < 10). Pulmonary blood flow distribution to shunt and normal V-A/Q units was similar during PSV and APRV without sponta neous breathing. Dead space (V-A/Q > 100) ventilation decreased by 6% during APRV with spontaneous breathing compared with PSV (P < 0.05). C onclusions: Spontaneous breathing superimposed on mechanical ventilati on contributes to improved V-A/Q matching and increased systemic blood flow. Apparently, the spontaneous contribution to a mechanically assi sted breath during PSV is not sufficient to counteract the V-A/Q maldi stribution of positive pressure lung insufflation during acute lung in jury.