A comparative evaluation of thermodilution and partial CO2 rebreathing techniques for cardiac output assessment in critically ill patients during assisted ventilation.

OBJECTIVE: To evaluate the reliability and clinical value of partial noninvasive CO2 (NICO2) rebreathing technique for measuring cardiac output compared with standard thermodilution in a group of intensive care nonpostoperative patients. DESIGN AND
SETTING: Clinical investigation in a university hospital ICU. PATIENTS: Twelve mechanically ventilated patients with high (n=6) and low (n=6) pulmonary shunt fractions. MEASUREMENTS AND
RESULTS: Thirty-six paired measurements of cardiac output were carried out with NICO2 and thermodilution in patients ventilated in pressure-support mode and sedated with a sufentanil continuous infusion to obtain a Ramsay score value of 2. The mean cardiac output was: thermodilution 7.27+/-2.42 l/min; NICO2 6.10+/-1.66 l/min; r2 was 0.62 and bias -1.2 l/min+/-1.5. Mean values of cardiac output were similar in the low shunt group (Qs/Qt < 20), with r2=0.90 and a bias of 0.01 l/min+/-0.4; conversely, in the high pulmonary shunt group (Qs/Q > 35%) the mean was 9.32+/-1.23 l/min with thermodilution and a mean NICO2CO value was 6.97+/-1.53 l/min, with r2 of 0.38 and a bias of -2.3 l+/-1.2 min.
CONCLUSIONS: The partial CO2 rebreathing technique is reliable in measuring cardiac output in nonpostoperative critically ill patients affected by diseases causing low levels of pulmonary shunt, but underestimates it in patients with shunt higher than 35%.