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

Objective. Evaluation in animals of a non-invasive and continuous cardiac o utput monitoring system based on partial carbon-dioxide (CO2) rebreathing i ndirect Fick technique. Methods. We have developed a non-invasive cardiac o utput (NICO) monitoring system, based on the partial rebreathing method. Th e partial rebreathing technique employs a differential form of the Fick equ ation for calculating cardiac output ((Q) over dot (T)) using non-invasive measurements. Changes in CO2 elimination (Delta (V) over dot CO2) and parti al pressure of end-tidal CO2 (Delta PETCO2) in response to a brief period o f 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 satura tion from pulse oximetry (SpO(2)) and inspired oxygen concentration (FIO2), is added to compute total cardiac output [(Q) over dot (T)=(Q) over dot (P CBF)/(1-(Q) over dot (S)/(Q) over dot (T))]. The performance of the NICO wa s compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and ha lothane, and by clamping of the inferior vena cava. Two hundred and forty-s ix (n = 246) paired measurements of TDco and NICO over a range of cardiac o utputs (TDco range = 0.60-8.87 l/min) were compared using Bland-Altman anal ysis and weighted correlation coefficient. Results. The Bland-Altman techni que 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 parti al CO2 rebreathing technique for measurement of cardiac output is non-invas ive, automated, and based on the well accepted Fick principle. The limits o f agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The re sults of this canine study show that NICO performed as well, and in some ca ses better, than other currently available non-invasive cardiac output tech niques over a wide range of cardiac outputs.