MEASURING FUNCTIONAL RESIDUAL CAPACITY IN NORMAL AND OLEIC ACID-INJURED LUNGS

Functional residual capacity (FRC) is an important oxygen reserve that is often depleted in acute respiratory failure. Recent interest in th e mechanisms of liquid ventilation and limited experience in measuring FRC in paralyzed, mechanically ventilated, normal and lung-injured an imal models have mandated development of accurate laboratory technique s. Eight sheep, from 17 to 27 kg, were anesthetized and instrumented t o provide a tracheostomy, a pulmonary artery catheter, and carotid art erial Line. They were randomized to two groups, one of which received 0.07 ml/kg of intravenous oleic acid to induce lung injury, Gas ventil ation of both groups was identical except for respiratory rate, which was adjusted to normalize PaCO2. FRC was measured in duplicate by both helium dilution (HD) and body plethysmography (BP). When measurements were completed, the animals were euthanized and their endotracheal tu bes clamped at end expiration. The lungs were then removed and their w ater displacement (WD) FRC values were measured. FRC was the differenc e between WD and tissue weight assuming 1 ml = 1 g. Pearson's correlat ion coefficient (R(2)) was calculated. During in vitro measurement of test lungs, HD had an R(2) value of 0.99 and BP had an R(2) value of 0 .98. When compared to WD, in vivo measurement of FRC by HD had an R(2) value of 0.94 while the value for BP was 0.97. In conclusion, both HD and BP are accurate methods of determining FRC in an uninjured and in jured lung model when compared to postmortem WD. Documenting changes i n FRC will aid in elucidating the mechanisms of alternative ventilator y techiques. (C) 1996 Academic Press, Inc.