Partial CO2 rebreathing indirect Fick technique for non-invasive measurement of cardiac output

Objective.Evaluation in animals of a non-invasive and continuous cardiac output monitoring system based on partial carbon-dioxide (CO₂) rebreathing indirect Fick technique. Methods.We have developed a non-invasive cardiac output (NICO) monitoring system, based on the partial rebreathing method. The partial rebreathing technique employs a differential form of the Fick equation for calculating cardiac output (Q_T) using non-invasive measurements. Changes in CO₂ elimination (ΔVCO₂) and partial pressure of end-tidal CO₂ (Δ PETCO₂) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow (Q_PCBF). A non-invasive estimate of anatomic and intrapulmonary shunt fraction (Q_S/Q_T), based on oxygen saturation from pulse oximetry (SpO₂) and inspired oxygen concentration (FIO₂), is added to compute total cardiac output [Q_T=Q_PCBF/(1−Q_S/Q_T)]. The performance of the NICO was compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and halothane, and by clamping of the inferior vena cava. Two hundred and forty-six (n = 246) paired measurements of TDco and NICO over a range of cardiac outputs (TDco range = 0.60–8.87 l/min) were compared using Bland-Altman analysis and weighted correlation coefficient. Results.The Bland–Altman technique yielded a NICO precision of ± 0.70 l/min (13.8%) with a mean bias of −0.07 l/min (−1.4%) compared to TDco. The weighted correlation coefficient between TDco and NICO values was: r= 0.93 (n= 246). Conclusion.The partial CO₂ rebreathing technique for measurement of cardiac output is non-invasive, automated, and based on the well accepted Fick principle. The limits of agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The results of this canine study show that NICO performed as well, and in some cases better, than other currently available non-invasive cardiac output techniques over a wide range of cardiac outputs. This revised version was published online in July 2006 with corrections to the Cover Date.