The Science Journal of the American Association for Respiratory Care

2009 OPEN FORUM Abstracts

EVALUATION OF A NEW PARTIAL REBREATHING CARDIAC OUTPUT MONITOR

Lara Brewer, Joseph Orr, Matthias Goerges; Anesthesiology, University of Utah Health Sciences Center, Salt Lake City, UT

Background: The partial CO2 rebreathing method makes use of measured changes in end-tidal CO2 (PetCO2) and CO2 excretion (VCO2) to noninvasively calculate cardiac output (CO). The ratio of the change in these two signals, which occurs due to a period of partial rebreathing, is used as the input to the differential Fick equation. We tested a newer version of the partial rebreathing system which compensates for the non steady-state transfer of CO2 between the blood and the FRC, thereby improving measurement accuracy. This algorithm has been implemented in a new monitor (NM3, Philips, Wallingford, CT). The new monitor also incorporates improvements to the measurement of airway flow and pressure, making the measurements more reliable. Method: We evaluated the performance of a new monitor in five 30-40 kg pigs. Each animal was intubated and anesthetized using 1 MAC of isoflurane. The partial rebreathing sensor (flow, CO2 and rebreathing volume) was placed in the breathing circuit between the endotracheal tube and the wye adapter. Rebreathing CO measurements were taken automatically and saved to a notebook computer once every three minutes. A pulmonary artery catheter was placed (Edwards Lifesciences, Irvine, CA) and reference CO measurements were taken as the average of three to five bolus thermodilution measurements (10 ml iced 5% dextrose solution) which had been randomized with respect to respiration. Comparison between the two CO measurements was made at baseline conditions, during infusions of either dobutamine or norepinephrine, and immediately following discontinuation of the drug infusions. Arterial blood gas measurements were entered into the NM3 monitor for shunt estimation. Noninvasive CO measurements were compared to corresponding average bolus thermodilution measurements. Results: The average noninvasive measurement was 6.1 L/min and the average thermodilution measurement was 5.4 L/min (range of 1.9 to 10.6 L/min). The bias was 0.72 L/min. The standard deviation of the difference was 0.74 L/min. Figure 1 shows a scatter plot comparing the two methods (R2 = 0.95). Conclusions: The partial CO2 rebreathing method as implemented in the new NM3 monitor measures cardiac output accurately as compared to bolus thermodilution measurements. The updated algorithm removes the need for steady state conditions during both rebreathing and non-rebreathing states and decreases the likelihood of error caused by venous blood recirculation during rebreathing. Sponsored Research - Philips Medical

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