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) over dot (T)) using non-invasive measurements. Changes in CO2 elimination (Delta (V) over dot CO2) and partial pressure of end-tidal CO2 (Delta PETCO2) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow ((Q) over dot (PCBF)). A non-invasive estimate of anatomic and intrapulmonary shunt fraction ((Q) over dot (S)/(Q) over dot (T)), based on oxygen saturation from pulse oximetry (SpO(2)) and inspired oxygen concentration (FIO2), is added to compute total cardiac output [(Q) over dot (T)=(Q) over dot (PCBF)/(1-(Q) over dot (S)/(Q) over dot (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.