POSTURAL CHANGES IN CARDIAC OUTPUT AND RESPIRATION IN MAN

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1. The acetylene method for the determination of cardiac output is considered in its practical details:(a) Modifications are recommended in the technique of sampling which reduce the possibility of vitiation of the results by the recirculation of acetylene‐containing blood.(b) The use of the usual modified Haldane apparatus for the gas analyses is open to serious error due to solution of acetylene in the CO2 absorbent. This analytical error can be eliminated by the use of a Van Slyke manometric apparatus.2. Studies in normal subjects of the postural changes in the circulation reveal an invariable increase of the arterio‐venous oxygen difference in the erect as compared with the reclining position: this is usually associated with a definite drop in cardiac output in the erect position, although this change may be masked by the increased rate of oxygen consumption usually found in the standing position.3. When a subject at rest on a tilting table is suddenly changed from the erect to the flat position, there occurs at once an increase in cardiac output as shown by an increase in the rate of oxygen uptake during the first 30 seconds or so after the change over. The increase in cardiac output calculated in this manner agrees closely with that calculated by the acetylene method.4. The pulmonary ventilation per 100 c.c. oxygen consumed is greater in the erect than in the reclining position. This hyperventilation can be correlated with the fall in absolute cardiac output which occurs in the erect position.This work was carried out with the aid of a grant for expenses from the Medical Research Council.