A STUDY OF MINUTE TO MINUTE CHANGES OF ARTERIO-VENOUS OXYGEN `CONTENT DIFFERENCE, OXYGEN UPTAKE AND CARDIAC OUTPUT AND RATE OF ACHIEVEMENT OF A STEADY STATE DURING EXERCISE IN RHEUMATIC HEART DISEASE

Abstract

A technique was developed whereby the behavior of the mixed venous and arterial blood saturation can be followed accurately during rapidly changing states. It was thus possible to determine the minute to minute changes of cardiac output in patients with rheumatic heart disease during 5 minutes exercise and subsequent recovery. Errors in cardiac output determination during early exercise due to the use of mean (in time) A-V differences and the difficulty of measuring true blood O2 uptake were shown to be of such a small order in these studies that they were of no importance. Particular study was made of the rate of achievement of a steady state after the commencement of exercise. In 10 of 16 patients a steady state of O2 uptake, A-V difference and cardiac output was achieved after 2-3 minutes. A number of exceedingly disabled patients who were unable to raise their cardiac output on exercise and had very abnormally increased A-V difference reached a steady hemodynamic state in a few minutes. The constancy of the ventilation, respiratory quotient, pulse rate and O2 uptake were all found to be unreliable criteria of steadiness of cardiac output. Mixed venous blood saturation fell to exceedingly low levels in a number of patients who were, however, able to continue to exercise without undue distress for several minutes. There was a remarkably prompt recovery of the mixed venous blood saturation to resting levels after the exercise stopped. There appeared to be no correlation between the degree of dyspnoea and the ventilation either in absolute terms or expressed as a percentage of the maximum breathing capacity. Similarly no important relation was found between the degree of dyspnoea and the level of mixed venous blood saturation, the behavior of the cardiac output, the pulmonary artery pressures, or the resting transpulmonary pressures (P.C.P.) Brief mention is made of evidence suggesting that there is considerable reduction of blood flow to the upper body, especially to the arms, when these disabled patients exercise.