Pulmonary Blood Flow (Cardiac Output) and the Effective Lung Volume Determined from a Short Breath Hold Using the Differential Fick Method

Abstract

Objectives.This work attempts to show how pulmonary blood flow (Qp), cardiac output (CO_Fick) and the lung volume of effective gas exchange (ELV ) can be determined from breath-by-breath measurements of the tidal exhaled CO₂ elimination V (litre/min) and the end tidal CO₂ concentration P (%) using the differential Fick method. The measurements are made during steady state ventilation and when the CO₂ balance in the lungs changes subsequent to a perturbation of the gas exchange conditions. Methods.A short breath hold is used to implement such a perturbation. V and P were measured in patients on mechanical ventilation. When the end tidal CO₂ values were stable, the end inspiratory pause of a single breath was prolonged 3 seconds as compared to the normal ventilation pattern. From the changes induced in P and V, Qp, CO_Fick and ELV are obtained. Results:Cardiac output values were measured in 18 patients after CABG. CO_Fick was found to be in good agreement with the thermodilution cardiac output reference CO_th. (n= 51 and (CO_th–CO_Fick) with Mean = −0.17 litre/minute and SD = 0.62 litre/minute). Conclusions.With a single breath perturbation, the differential Fick method can yield cardiopulmonary information using 2–3 breaths only and with a minimum of interference with the patient. Complete data analysis results in multiple determinations of the Qp and ELV values which improve the attainable precision. Our investigation points to the possibility to determine Qp, CO_Fick and ELV also during spontaneous breathing, by using the natural tidal variations of V and P.