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In 40 lightly anesthetized dogs, 5 to 30 days after surgical preparation, flow was measured simultaneously in the venae cavae, pulmonary artery, pulmonary vein, and aorta with ultrasonic flowmeters. Intrapleural and pericardial pressures were measured via silastic cannulas. Pulmonary vein diameter was monitored by miniature mutual inductance coils. In the resting animal with sinus arrhythmia, inspiration increased heart rate and flow in the vena cava, and to a lesser extent, in the pulmonary vein. Left ventricular stroke volume (LVSV) varied directly with the right ventricular stroke volume (RVSV) in dogs with slow heart rates. Cardiac tamponade invariably caused tachycardia and a marked decrease in cardiac output, arterial pressure, pulse pressure, and stroke volume; venous pressure and diameter increased. Pericardial pressure, although markedly elevated, fell with inspiration paralleling the fall in intrapleural pressure. Flow in the pulmonary vein rose or remained constant with inspiration. Pulmonary vein diameter frequently increased with inspiration during tamponade, but only after the pulmonary artery diameter increased with the inspiratory surge. LVSV did not decline sharply with inspiration, and actually increased within 2 beats of the increase in RVSV. The sum of LVSV plus RVSV increased markedly with inspiration, contradicting the concept of fixed intrapericardial volume. Almost all of the changes of pulsus paradoxus reflect the normal respiratory effects on the RVSV, delayed by transit through the pulmonary bed and exaggerated by the small LVSV in a vasoconstricted state.

Effect of Respiration on Venous Return and Stroke Volume in Cardiac Tamponade