Cardiac output measurement using N2O uptake: a method to subtract cardiogenic oscillations

Plethysmographic (box) methods using N2O uptake to measure pulmonary capillary blood flow, right ventricular stroke volume, and pulmonary arterial flow pulse conduction time have been complicated by the presence of so-called "cardiogenic oscillations" in the box flow records on air and N2O breathing. The exact cause of these oscillations is unknown but believed to be secondary to instantaneous changes in net blood flow into and out of the thorax during the cardiac cycle. We examined these oscillations and pulmonary capillary blood flow in normal human subjects in a flow plethysmograph using a pressure compensation circuit to extend the frequency response. The cardiogenic oscillations in flow recorded at the mouth (with glottis open) are much greater in amplitude and 90 degrees out of phase from those recorded from the plethysmograph. We derived a theoretical analysis of this and designed an analogue electrical circuit to enable us to eliminate these oscillations by electrical subtraction from the box flow record generated by N2O uptake in the pulmonary capillary bed. Measurements of pulmonary capillary blood flow and ventricular stroke volume using our method correlate with measurements made with acetylene-rebreathing blood flow measurements performed sequentially at the same sitting.