Validation of continuous thermodilution cardiac output in critically ill patients with analysis of systematic errors

D L Medin, D T Brown, R Wesley, R E Cunnion, F P Ognibene
Journal of Critical Care 1998, 13 (4): 184-9

PURPOSE: Bolus thermodilution cardiac output (BCO) measurements are affected by variations in injectate volume, rate, and temperature. These variations are eliminated when CO is measured by a continuous automated thermal technique (CCO). Further, CCO eliminates the need for fluid boluses, reduces contamination risk, requires no operator, and provides a continuous CO trend. We prospectively evaluated CCO versus BCO in a population of critically ill adults with low, normal, and high CO states. We sought to discern any systematic effects of temperature fluctuations or signal-to-noise-ratios (SNR) on disparities between BCO and CCO measurements and also sought to assess the relative cost effectiveness of the CCO system.

MATERIALS AND METHODS: Pulmonary artery catheterizations were performed in a convenience sample of 20 patients over 6 months. BCO data were obtained using a standardized protocol. Three bolus injections of 5% dextrose were given when each CO was within 10% of the median before averaging; otherwise five boluses were given, with the high and low values eliminated before averaging. Injectates were administered randomly through the respiratory cycle and at 1-minute intervals. CCO measurements were recorded from a Vigilance monitor pre and post BCO measurements, yielding an average CCO value. Also recorded were pre- and post-core temperatures and SNR during the first CCO measurement. Cost data included estimates of operator time for BCO determinations as well as costs of Intellicath (Baxter-Edwards, Irvine, CA) pulmonary artery catheters, Vigilance (Baxter-Edwards, Irvine, CA) monitors, conventional catheters, and injectates.

RESULTS: Of the 20 patients, 15 were mechanically ventilated. A total of 306 paired CO values were obtained for analysis. CCO ranged from 2.5 to 14.4 L/min and BCO from 2.4 to 13.3 L/min. Absolute differences between CCO and BCO measurements increased with increasing CO, but percentage differences did not. Of the paired values, 77% were within 1 L/min of one another. Temperature instability and SNR independently had weak correlations with CCO/BCO disparities. The Vigilance system had a slightly higher net cost than conventional BCO, although no economical value was assigned to the clinical usefulness of continuous, as opposed to intermittent, CO monitoring.

CONCLUSIONS: Continuous CO is a reliable and cost-effective alternative to bolus thermodilution CO for critically ill patients in low, normal, and high CO states.


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