respirator. A midline sternotomy was performed and the aorta and its branch vessels were isolated and cleaned of adventitia and fat. The azygos vein was then ligated and the pericardium opened. Stab incisions were made in the lateral right atrial wall and right atrial appendage, through which retrograde cannulation of the venae cavae using /-inch tubing was achieved. This technique has been described.5 These cannulas drained by gravity to a 5-liter reservoir from which a 3/4-inch tube passed through a mechanical roller pump, and the return tube from the pump was sutured to the right atrial free wall and passed through the tricuspid valve into the pulmonary artery. Heparin (5000 U) was given in a bolus and the reservoir primed with 1 liter of Ringer's lactate. Ligatures around the venae cavae and pulmonary artery were tightened as right-heart bypass was instituted. The roller pump, which had been calibrated using a stopwatch and graduated cylinder, could then be set to achieve and maintain left cardiac output within strict limits. When venous return did not keep up with forward flow, a maximum of 1 liter of Ringer's lactate was added to the reservoir. Cardiac output using the roller pump could be varied between 0.5 and 6 1/min flow and Doppler flow measurement made over flows varying in increments of 0.5 1/min. Calibrated electromagnetic flow probes (Gould-Statham SP2204) were periodically also placed 2 cm distal to the aortic valve to verify roller pump accuracy, and were used in each of the dogs over two to three cardiac outputs, but not for every roller pump setting.Ultrasound and Doppler Method A prototype two-dimensional sector scanner with range-gated Doppler capability (Electronics Medicine/ Honeywell) was used for both imaging and Doppler flow studies. The instrument has a 3.5-MHz mechanical transducer that is oscillated through an angle of 60-75°. A

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