Continuous cardiac output monitoring after cardiopulmonary bypass:: a comparison with bolus thermodilution measurement

The interchangeability of continuous measurement of cardiac output (CO) with the traditional bolus method in patients after cardiopulmonary bypass (CPB) is uncertain. Prospective observational clinical study. A 20-bed surgical ICU at a university hospital. Fourteen deeply sedated, ventilated, post-cardiac surgery patients, all equipped with a pulmonary artery catheter. Six hours after the end of the CPB, 56 simultaneous bolus and continuous measurements were compared by a linear regression analysis and Bland-Altman analysis. Bolus CO was estimated by averaging triplicate injections of 10ml room-temperature NaCl 0.9%, delivered randomly during the respiratory cycle. A stringent maximum difference of 0.55 min(-1) (about 10% of the mean bolus measured) was considered as a clinically acceptable agreement between the two types of measurements. To be interchangeable the limits of agreement (+/- 2 SD of the mean difference between the two methods) should not exceed the chosen acceptable difference. Continuous was correlated with bolus CO, with a correlation coefficient of r(2)=0.68. (p < 0.01). The Bland-Altman analysis demonstrated an objective mean bias of 0.33 +/- 0.6min(-1) (confidence interval of -0.87 +/- 1.58) with 34% of measured values falling outside of the clinically acceptable limits. Our results suggest that, in the first 6h after CPB, continuous and bolus CO determinations are not interchangeable; one third of the values obtained by continuous CO fell outside the strict limits of clinically useful precision.