The Doppler method for measuring cardiac output in conscious rabbits: validation studies, uses, and limitations.

The velocity of blood in the rabbit aorta is very fast, approaching the limits of some pulsed directional Doppler flow meters. Therefore, we thoroughly evaluated a 20- and a 10-MHz device for measuring cardiac output in rabbits. Flow probes were implanted around the ascending aorta and catheters were implanted into the left atrium (for microsphere injection), femoral artery, and vein. About 2 weeks later, cardiac output was determined with the Doppler method and simultaneously with tracer microspheres. Cardiac output was manipulated with isoproterenol, dihydralazine, guanfacine, or alinidine, intravenously. With the 20-MHz device, only normal and decreased cardiac output could be measured accurately, even with a 60 degrees implantation angle of the crystals. The 10-MHz device yielded accurate measurements also at very high flow. Surprisingly, at high aortic flow rates, both the 10- and the 20-MHz devices were unable to measure correctly diastolic flow, which is close to zero. It was necessary to adjust the position of the late diastolic Doppler signal manually to the electrical zero line. With this precaution, the 10-MHz device yielded an excellent correlation between mean Doppler signal and cardiac output. Cardiac output can be measured in absolute flow units if the flow-probe can be calibrated in vivo with an independent, accurate method about 2 wk after implantation. The baroreflex was not affected by the implanted flow probes. Within these limits, Doppler flow meters are good tools to assess drug effects on cardiac output in conscious rabbits.