Journal of Applied Physiology, Vol 54, Issue 5 1427-1433, Copyright © 1983 by American Physiological Society
Cardiac output measurement using N2O uptake: a method to subtract cardiogenic oscillations
W. T. Dawson and R. L. Johnson Jr
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.
