Comparison of cardiac output (CO) measurement before and after cardiopulmonary bypass: bolus thermodilution (TDco) and noninvasive partial CO₂ rebreathing (NICO)

Introduction

NICO is a noninvasive cardiopulmonary monitor that provides an alternative to invasive TDco for measurement of CO. NICO uses a differential form of the Fick equation (change in CO₂ excretion and end-tidal CO₂, in response to a brief period of partial rebreathing) to provide noninvasive estimates of the CO [1]. The objective of this study was to compare the degree of agreement in measurements of CO using NICO and TDco, at predetermined intervals in patients undergoing coronary artery bypass graft surgery (CABG).

Methods

Matched sets of CO measurements from a standard PA Catheter and NICO (Novametrix Medical Systems Inc, Wallingford, CT, USA, software version 4.2) were collected in 10 patients (age 35-62 years, weight 32-68 kg) undergoing elective CABG. Measurements were made at predetermined intervals during the surgery (during pre-cardiopulmonary bypass and post-cardiopulmonary bypass; chest wall open and closed). The NICO circuit was attached between the endotracheal tube and the breathing circuit of the patient. An average of three consecutive bolus (10 ml) TDco measurements made during end-expiration was compared with corresponding NICO measurements. Bland-Altman analysis was performed to compare the degree of agreement between the two methods.

Results

Bias, precision (1 standard deviation), and limits of agreement between the NICO and TDco during pre-CPB vs post CPB are shown in the Table.

Table

Conclusion

The overall agreement between NICO and TDco was -0.03 (bias) and ± 0.80 (precision) l/min. There was no statistical difference in the CO measured by the two techniques neither during pre-CPB nor during post-CPB (P < 0.01) with the chest open or closed. The degree of agreement between NICO and TDco is within the recommended value [2] for NICO to be a clinically acceptable method for CO measurement during CABG.