Continuous cardiac output measurement with a Doppler-equipped pulmonary artery catheter.

Background: We developed a Doppler-equipped pulmonary artery catheter that provides continuous measurement of the true main pulmonary blood flow velocity independent of the angle of incidence formed by the pulmonary artery catheter and the main pulmonary artery blood flow. This device uses 2 orthogonally positioned Doppler transducers that allow trigonometric correction for differences in the angle of blood flow between each transducer. We tested the accuracy of the Doppler-equipped pulmonary artery catheter by comparing its cardiac output measurements with those done by conventional techniques in animals.

Methods: The Doppler-equipped pulmonary artery catheter was evaluated in dogs. A pair of ultrasound Doppler transducers positioned at a fixed angle (90°) was mounted on the distal part of the thermodilution pulmonary artery catheter. The Doppler shifts (Δf1, Δf2) were detected by the 2 transducers sampling at 2 closely spaced points in the main pulmonary artery. The values of Δf1 and Δf2 were used to compute 2 velocity measurements. The true flow velocity of the main pulmonary artery was calculated with the following equation: V(pulm) = {(V(transducer1))(2) + (V(transducer2))(2)}(1/2) (V(pulm) = true main pulmonary artery velocity; V(transducer1) and V(transducer2) = velocity detected by transducers 1 and 2, respectively). The flow velocities were calculated by using a phase differential technique. Cardiac output was calculated as V(pulm) multiplied by a coefficient value. The coefficient value was calculated by dividing cardiac output, derived from conventional techniques, by V(pulm) at the beginning of each experiment. After thoracotomy, an electromagnetic flowprobe was placed around the main pulmonary artery in dogs. Cardiac output was simultaneously measured by the Doppler-equipped pulmonary artery catheter (CO-Doppler), and the electromagnetic flowmeter (CO-EMF) or the thermodilution technique (CO-Thermo). Cardiac output was manipulated by dobutamine and propranolol.

Results: CO-Doppler was highly correlated with CO-EMF (y = 1.16 × -0.26, r(2) = 0.99, P < 0.001) and CO-Thermo (y = 1.24 × -0.90, r(2) = 0.85, n = 48, P < 0.001). The bias between CO-EMF and CO-Doppler was -0.02 L/min; 95% limits of agreement were -0.32 to 0.28 L/min. The percentage error was 16%. The bias between CO-Thermo and CO-Doppler was 0.18 L/min; 95% limits of agreement were -0.62 to 0.98 L/min.

Conclusions: The newly developed Doppler-equipped pulmonary artery catheter with 2 orthogonally positioned Doppler transducers allowed accurate and continuous measurements of cardiac output independent of the angle of incidence formed by the pulmonary artery catheter and the main pulmonary artery blood flow.