Interfacing between spontaneous breathing and mechanical ventilation affects ventilation-perfusion distributions in experimental bronchoconstriction.

The effect of interfacing between spontaneous and mechanical ventilation 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 BIPAP without spontaneous breathing provides full and PSV breath-to-breath synchronized ventilatory support, BIPAP allows unrestricted spontaneous breathing throughout the mechanical cycle. Compared with BIPAP with and without spontaneous breathing, PSV effected an increase in ventilatory rate (p < 0.05) and a higher minute ventilation (VE) (p < 0.05). Spontaneous breathing during BIPAP accounted for 15 +/- 1% of the VE 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 oxygen 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 accounted 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 improved gas exchange by allowing better VA/Q matching during experimental bronchoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)