A new method for non‐invasive determination of effective pulmonary blood flow and cardiac output in spontaneously breathing subjects

The differential Fick method is well established for measuring effective pulmonary blood flow (EPBF) and cardiac output (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 a dead space of precisely known volume and recording the resulting rise in the end-tidal CO₂ value allowed the determination of EPBF. The shunted blood flow fraction was estimated from the arterial oxygen saturation to obtain cardiac output (FickCO). Two measurements were made on each subject 15 min 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 RefCO using a Bland-Altman analysis, we obtained a mean bias of 0.03 L/min, limits of agreement (LoA) of +1.43 to -1.37 (95% CI) L/min and a percentage error (PE) of 0.25. For the mean of two FickCO observations, we obtained a mean bias of -0.04 L/min, LoA +0.94 to -1.01 (95% CI) and PE of 0.17. 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.