O2 Content of ‘Mixed’ Venous Blood in Man During Various Phases of the Respiratory and Cardiac Cycles in Relation to Possible Errors in Measurement of Cardiac Output by Conventional Application of the Fick Method

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Abstract
Two methods were described for the investigation of phasic variations in the blood O2 saturation. The first consists of a synchronizer circuit controlling an electromagnetic valve which can be activated by the R of the ecg or by the respirations, and blood samples can be withdrawn from the heart or great vessels during any desired phase of the cardiac or respiratory cycles. The second method consists of the rapid withdrawal (about 18 ml/minute) of blood through a cardiac catheter and cuvette oximeter for continuous analysis of O2 saturation. These methods were applied to a study of the O2 content of the "mixed" venous blood in man in relation to possible errors in the conventional application of the Fick principle. No phasic variations in O2 saturation with the cardiac cycle were shown, but small cyclic changes at respiratory frequency were detected. These fluctuations were abolished by breath holding and were increased by deep breathing and by exercise in most subjects. The mean of the recorded fluctuations in O2 saturation in the pulmonary artery was 0.3% with the subjects at rest, and 1.1% during moderate exercise. The magnitude of these fluctuations is probably underestimated owing to the attenuation caused by the slow dynamic response of the catheter-cuvette oximeter recording system. Preliminary studies of the dynamic response characteristics of the recording system indicate that the variations recorded at respiratory frequency may be in the range of 10-20% of the actual fluctuations in oxygen saturation occurring in the pulmonary artery. The largest possible error that such fluctuations might cause in the calculation of the cardiac output of resting subjects by the conventional application of the Fick method was estimated to be usually small enough (4%) so that for practical purposes it could be disregarded. However, especially during conditions of exercise and until the true magnitude of these fluctuations can be estimated, the possibility of significant systematic errors in Fick determinations of cardiac output cannot be disregarded.

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