PARTIAL CO2 REBREATHING FICK TECHNIQUE FOR NONINVASIVE MEASUREMENT OF CARDIAC OUTPUT

Abstract

Introduction: The noninvasive partial CO2 rebreathing technique uses a differential form of the Fick Equation tocalculate cardiac output (CO). The ratio of the change in end-tidal CO2 and CO2 excretion, in response to a brief period of rebreathing, gives a non-invasive estimate of the CO. [1,2] The method assumes that the difference between end-tidal and arterial CO2 content does not change during rebreathing. We have developed a partial CO2 rebreathing algorithm based on a two-compartment lung model, which minimizes error in estimation of CO when this assumption is not true. This model compensates for the changing end-tidal to arterial CO2 gradient during partial rebreathing. Methods: After IRB approval and patient consent the system was tested in seven patients (6 Male, 1 Female, Age 47-78 Years) undergoing cardiac surgery. End-tidal CO2 was measured using a mainstream CO2 analyzer and a disposable pneumotachograph (COSMO[registered sign], Novametrix Medical Systems, Wallingford, CT). CO2 production was calculated as the integral of the flow and CO2 signals over the entire breath. Actuation of a pneumatic valve under computer control resulted in breathing circuit changes, which increased airway dead space by 120 ml thereby causing partial rebreathing of exhaled gas. The valve was actuated for 50 seconds once every 3 minutes. Changes in CO2 excretion and etCO2 were used to calculate the partial CO2 rebreathing CO using the differential Fick equation. An average of three consecutive thermodilution cardiac output (TDco) measurements made during end expiration were compared with corresponding partial CO2 rebreathing cardiac output measurements. Measurements made during the first 45 minutes after bypass were discarded since thermal noise can make the TDco measurement unreliable [3]. Results: A total of 44 comparisons were made with CO ranging from 2.5 to 9.4 L/min. Regression analysis gives an r = 0.90 with a slope of 0.71 and an offset of 1.49 L/min. Bland-Altman analysis resulted in a bias of 0.07 L/min with a precision (1SD) of +/- 0.85 L/min. Discussion: The results suggest that partial CO2 rebreathing technique may be a clinically acceptable method for measurement of CO. Observed differences between the two techniques may have resulted from intrapulmonary shunts. The partial CO2 rebreathing system is completely automatic and can provide frequent, nearly continuous measurements of CO. (Figure 1)Figure 1