Am. J. Respir. Crit. Care Med., Vol 150, No. 1, Jul 1994, 101-108.
Ventilation-perfusion distributions during mechanical ventilation with superimposed spontaneous breathing in canine lung injury
C Putensen, J Rasanen and FA Lopez
Department of Anesthesiology, University of South Florida, College of Medicine, Tampa.
Biphasic positive airway pressure (BIPAP) allows unrestricted spontaneous
breathing throughout mechanical ventilation. Effects of spontaneous
breathing during BIPAP on pulmonary gas exchange were studied on a
randomized basis in 12 dogs with oleic acid-induced lung injury using the
multiple inert gas elimination technique. Spontaneous breathing during
BIPAP, accounting for 10% of minute ventilation (VE), increased PaO2 from
61 +/- 2 to 78 +/- 3 mm Hg (mean +/- SE) (p < 0.01), cardiac output from
4.2 +/- 0.3 to 4.6 +/- 0.3 L/min (p < 0.05), and oxygen delivery from
537 +/- 51 to 716 +/- 58 ml/kg/min (p < 0.05), whereas oxygen
consumption and total VE remained unchanged. Improved pulmonary gas
exchange caused by better ventilation/perfusion (VA/Q) matching was
indicated by a 17 +/- 3% decrease (p < 0.01) in blood flow to shunt
units (VA/Q < 0.005), a 15 +/- 3% increase (p < 0.05) in perfusion of
normal VA/Q units (0.1 < VA/Q < 10), and a 6 +/- 3% reduction in
ventilation of dead space (VA/Q > 100) areas (p < 0.05). Spontaneous
breaths superimposed on mechanical ventilation may convert shunt VA/Q units
to normal by increased ventilation of poorly or nonventilated units and/or
increase blood flow to previously minimal or nonperfused areas.
