Dr Lee Wai Chuen, Intensive Care Unit
Queen Elizabeth Hospital (December 2001 Critical Care Medicine Exit Assessment Exercise)
Introduction Thermodilution technique has been used for many years but recently being under critical review. Carbon dioxide rebreathing technique has been used as a non-invasive technique for cardiac output measurement. However, its accuracy and usefulness in intensive care unit is not being verified. We have tried the NICO system, one of the non-invasive cardiac output measurement, in our intensive care unit. Thermodilution technique was used for comparison. Design retrospective, comparison study.
Method Simultaneous cardiac output measurement by NICO system and thermodilution were compared by Pearson correlation coefficient and Bland-Altman method to determine the bias, precision and agreement between the two methods. The difference in the measurement of the two methods was further tested with paired-t test. Stepwise regression was used to determine the contributing factors of the difference in cardiac output measurement.
Result 8 patients with 59 paired measurements were obtained for comparison. The Pearson correlation coefficient was 0.64, which was statistically significant (p < 0.001). Bland-Altman analysis resulted in a bias of -0.38 L/min and the precision was 1.3. The difference in cardiac output was statistically significant (P = 0.29). The percentage difference in cardiac output in relation to the average of cardiac output showed a mean percentage difference of – 6.9 ± 24.4 % (range – 61.02 to 45.5 %). Factors contributing to the difference included (1) the PaCO2 before rebreathing, (2) the PEEP level and (3) the tidal volume.
Conclusion In our study, the agreement between thermodilution and CO2 rebreathing technique by NICO system was moderate. There was a statistically significant difference in the cardiac output measurement obtained from the thermodilution and NICO system. Further studies are needed to verify its usefulness in intensive care setting.