SUMMARY We instrumented six dog hearts in vivo to

Our results suggest that the response to graded dobutamine infusion in the post-cardiopulmonary bypass period differs from that previously reported. After cardiopulmonary bypass, the dominant mechanism by which dobutamine improves left ventricular performance is by increasing heart rate. Dobutamine affects blood pressure minimally.

A simple expression is derived to describe the partial pressure at 50% hemoglobin saturation with oxygen (P50) that maximizes venous oxygen tension (PO2) for a given arterial PO2 and oxygen consumption. That "optimal P50" also maximizes arteriovenous saturation differences for given arterial and venous PO2 values. The optimal P50 can be expressed as the square root of the product of arterial and venous PO2 values. Alternatively, it can be expressed as a simple function of the arterial PO2 and the arteriovenous saturation difference. Nomograms summarize the relationships between the variables, and published observations that suggest an observational basis for our theoretical analysis are reviewed. We conclude that for normoxia or moderate hypoxia a high P50 is advantageous, whereas for more severe hypoxia or increased metabolic demands, a low P50 is advantageous.

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SUMMARY Because of anatomic differences between the right and left heart, we hypothesized that the reduction in left ventricular stroke volume during acute cardiac tamponade is the passive result of decreased pulmonary venous return due primarily to right heart compression. To test this hypothesis, two types of experiments were performed in dogs. Group 1: in fresh, potassium-arrested hearts, rightand left-sided cardiac chambers were filled to previously determined physiological end-diastolic pressures. When saline then was introduced into the pericardial space, right heart volumes were reduced more rapidly than left heart volumes in all cases. These data indicate that the right heart is compressed more easily than the left heart by pericardial fluid when initial filling pressures are in the physiological range. BOTH left and right ventricular volumes are decreased during cardiac tamponade (Merlich, 1951;Isaacs et al., 1954;Sharp et al., 1960;Craig et al., 1968;DeCristofaro and Liu, 1969;D'Cruz et al., 1975;Pegram et al., 1975;Schiller and Botvinick, 1977; Settle et al., 1977). Since left ventricular systolic function is not severely compromised in most cases of tamponade (Isaacs et al., 1954;Sharp et al., 1960;DeCristofaro and Liu, 1969), reduced left ventricular end-diastolic volume is primarily responsible for the systemic hypotension and low cardiac output characteristics of this condition. It is uncertain, however, whether decreased left ventricular volume results primarily from direct left ventricular compression by pericardial fluid, or from underfilling due to impaired pulmonary venous return.The importance of underfilling due to right heart compression in the pathophysiology of tamponade is suggested by anatomic differences between rightand left-sided cardiac structures. The right ventricle is thinner walled than the left ventricle and may therefore be more easily compressed by pericardial fluid under pressure. Based on this consideration, we hypothesized that acute cardiac tamponade results in a selective decrease in right heart volume because initially the right heart is more easily compressed, and that reduced left ventricular filling is largely a passive event resulting from decreased pulmonary venous return. To test this hypothesis we designed two types of experiments. First, we sought to determine directly the relative susceptibility of the right-and left-sided cardiac chambers to compression by pericardial fluid under pressure. Second, we predicted that there would be a phase lag between the effects of abrupt tamponade on right and left ventricular stroke volumes, and we sought to verify this experimentally. Methods Group 1Nine mongrel dogs were studied using pentobarbital anesthesia (25 mg/kg, iv) and positive pressure ventilation. A midline sternotomy was performed and the heart within the intact pericardium was exposed. BD vinyl tubing #6209 with multiple sideholes was placed through a small (approximately 1 cm) incision in the anterior pericardium and the tip located away from the free wall of...