The cardiac output is measured with a CO2 rebreathing method suggested by Defares (1956). The mixed vmous CO2 pressure is determined graphically from the breath by breath‐determined CO2 content in a rubber bag during rebreathing from the bag. Arterial CO2 pressure is assumed to be the same as in end‐tidal air. CO2 eliminated per time unit is measured by collecting the expired air in a Douglas bag and the cardiac output is determined with the Fick equation. For the CO2 analysis a rapid infrared CO2 metcr is used. The obtained values at rest are of the same size as those obtaincd with the Crollman acetylene method. The values from the work cxperiments arc of the same magnitude as those obainted with dye‐diliution or direct Fick methods. Stroke volume shows increasr from rrst to work about 60–80 per cent.

The solubility of acetylene in lung tissue is measured by a rebreathing technique. Enough acetylene is dissolved in the lung tissue to simulate an extra gaseous lung volume between 0.274–0.48 liters. This should give an error of 7–12 % in cardiac output determination with the acetylene rebreathing method.