Some aspects of determination of the cardiac output by the rebreathing method

Abstract

It is shown that the rebreathing method can be used to study the cardiac output even when the ventilation does not correlate with the pulmonary blood flow. pCO₂ was measured in mixed venous blood by equilibrating it with pCO₂ in the alveolar air by the rebreathing method. A gas mixture with a CO₂ concentration close to that in venous blood was produced by the subject himself during rebreathing into a bag with a capacity of 2–3 liters, filled with oxygen. Irregularity of distribution of ventilation relative to blood flow was judged from the shape of the CO₂ concentration versus time curve. If signs of irregularity are present, it is impossible to make p_etCO₂ equal to p_ACO₂ and for that reason pCO₂ of arterialized blood was determined. By means of this correction it is possible to determine the cardiac output of patients with cardiac and pulmonary diseases. The possibility of using standard nomograms for calculating the CO₂ concentration in the arterial and venous blood in the presence of appreciable disturbances of the acid-base balance is discussed.