Continuous measurement of pulmonary blood flow using a retractable pulsed Doppler probe

The feasibility of measuring pulmonary blood flow (PAQ) continuously using a removable, extraluminal 20 MHz pulsed Doppler probe, which has been used successfully to measure aortic blood flow, was assessed in seven anesthetized mongrel dogs. Simultaneous recordings were made from the Doppler probe (range-gated 5-6 mm from the anterior wall of the main pulmonary artery) and an electromagnetic flow probe (encircling the aorta) over cardiac outputs (CO) ranging from 0.2 to 5.5 L/min. Assuming a flat velocity profile and a fixed cross-sectional area, PAQ was initially calculated as the product of area and mean velocity. Regression analyses (PAQ = a + b X CO) indicated good intraanimal linear correlations in six animals (r greater than or equal to 0.84) and no correlation in one animal (r = 0.003); however, PAQ was consistently higher than CO and interanimal variability was marked, as suggested by large deviations in mean intercept and slope values (a = 1.67 +/- 1.09 L/min and b = 0.70 +/- 0.33). Results improved (r greater than or equal to 0.79 in all animals, a = 0.47 +/- 0.52 L/min, and b = 0.77 +/- 0.21) when the method to estimate PAQ was altered to assume that the starting cross-sectional area was the area that would make baseline PAQ and CO agree, and that the area during each subsequent CO level changed as a function of pulmonary artery pressure and an estimate of pulmonary artery compliance. Results of this study imply that it will be more difficult to use this Doppler probe to monitor CO from the pulmonary artery than it was from the aorta due to the elliptical, more compliant pulmonary vessel walls and the irregular pulmonary artery velocity profile.