INTERFACING BETWEEN SPONTANEOUS BREATHING AND MECHANICAL VENTILATION AFFECTS VENTILATION-PERFUSION DISTRIBUTIONS IN EXPERIMENTAL BRONCHOCONSTRICTION

The effect of interfacing between spontaneous and mechanical ventilati on on ventilation-perfusion (VA/Q) distributions was determined during pressure-support ventilation (PSV) and in the presence and absence of spontaneous breathing during biphasic positive airway pressure (BIPAP ) in 10 pigs with methacholine-induced bronchoconstriction. Whereas BI PAP without spontaneous breathing provides full and PSV breath-to-brea th synchronized ventilatory support, BIPAP allows unrestricted spontan eous breathing throughout the mechanical cycle. Compared with BIPAP wi th and without spontaneous breathing, PSV effected an increase in vent ilatory rate (p < 0.05) and a higher minute ventilation (VE) (p < 0.05 ). Spontaneous breathing during BIPAP accounted for 15 +/- 1% of the V E and increased cardiac output (CO) from 4.5 +/- 0.2 to 5.3 +/- 0.2 L/ min (p < 0.05), PaO2 from 55 +/- 3 to 80 +/- 4 mm Hg (p < 0.05), and o xygen delivery (DO2) from 442 +/- 39 to 630 +/- 43 ml/min (p < 0.05). PSV did not increase CO, PaO2, and DO2. Spontaneous breathing did not affect oxygen consumption. During BIPAP spontaneous breathing accounte d for a 15 +/- 2% decrease (p < 0.05) in blood flow to shunt units and a 16 +/- 2% increase (p < 0.05) in the perfusion of normal VA/Q units . Perfusion of shunt and normal VA/Q units was similar during PSV and BIPAP without spontaneous breathing. Dead space ventilation decreased with spontaneous breathing during BIPAP by 12% compared with PSV (p < 0.05). Dispersion of ventilation distribution was lowest during BIPAP. Uncoupling of spontaneous and mechanical ventilation during BIPAP imp roved gas exchange by allowing better VA/Q matching during experimenta l bronchoconstriction. Apparently, the spontaneous respiratory effort during PSV is not sufficient to counteract the VA/Q maldistribution ca used by airway occlusion and positive pressure lung inflation.