Noninvasive determination of effective (nonshunted) pulmonary blood flow in normal and injured lungs

: PEEP is utilized in acute respiratory failure to decrease intrapulmonary shunting and improve oxygenation. Despite these beneficial effects, PEEP may adversely affect cardiac output, thus reducing oxygen delivery. To monitor some of the cardiopulmonary effects of PEEP, we utilized a noninvasive rebreathing technique to measure effective (nonshunted) pulmonary blood flow (Qepr) and compared the results to those measured by thermodilution (Qepi) in normal and oleic acid-injured canine lungs. Qepr was highly correlated with Qepi (r = .92, r2 = .85, p less than .001) despite large variations in PEEP before lung injury (0 to 15 cm H2O) and after lung injury (0 to 20 cm H2O). This close correlation was found even with wide ranges in cardiac output (1.01 to 6.45 L/min) and intrapulmonary shunt fractions (0.03 to 0.67). This technique may prove valuable as a noninvasive method by which to monitor and adjust PEEP therapy in patients with acute lung injury.