Am. J. Respir. Crit. Care Med., Vol 151, No. 4, Apr 1995, 993-999.
Interfacing between spontaneous breathing and mechanical ventilation affects ventilation-perfusion distributions in experimental bronchoconstriction
C Putensen, J Rasanen and FA Lopez
Department of Anesthesiology, University of South Florida, College of Medicine, Tampa.
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)