A new method for noninvasive determination of effective pulmonary blood flow (EPBF) and cardiac output (CO) in spontaneously breathing subjects.

The differential Fick method is well established for measuring EPBF and CO but until now it has only been used for patients on mechanical ventilation. Here we present and evaluate a new approach adapted to spontaneous breathing situations. Ten healthy subjects with diverse anthropometric and respiratory parameters were studied in the sitting position. Rebreathing through dead space of precisely known volume and recording the resulting rise in the end-tidal CO2 value allows the determination of EPBF. The shunted blood flow fraction is estimated from the arterial oxygen saturation to obtain cardiac output (FickCO). Two measurements were made on each subject 15 minutes apart. Reference values for cardiac output (RefCO), were calculated as the product of stroke volume and heart rate where the stroke volume was measured with established echocardiography techniques. Heart rate and arterial oxygen saturation were measured with an ordinary pulse oximeter Comparing FickCO to Ref CO using the Bland-Altman analysis we obtain bias = 0.03 l/min and SD = 0.71 l/min resulting in a percentage error PE = 0.25 The differential Fick method can be adapted to spontaneously breathing situations with good absolute accuracy using simple equipment. Short data collection times make it possible to use the mean of repeated observations and thereby get adequate precision. The new method could therefore be of value both in the pre-operative and the post-operative setting.