Analysis of errors in measurement of cardiac output by simultaneous dye and thermal dilution in cardiothoracic surgical patients

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Abstract
The purpose of this study was a detailed analysis of the random and systematic errors encountered in the measurement of cardiac output by dye and thermal dilution under actual clinical conditions in cardiothoracic surgical patients. Two hundred simultaneous dye and thermal cardiac output measurements were performed in 20 patients by injection of a 10 cm3 bolus of cold indocyanine green dye with on-line analysis employing two portable analogue computers. Agreement between the dye and thermal methods was significantly better in the intensive care unit (r values of 0.88 to 0.94) when compared with the operating room (0.68 to 0.84), a difference thought to reflect the better haemodynamic stability of patients in the intensive care unit. A statistical method was devised allowing separation of random and calibration errors for both the dye and thermal methods. The random error (reproducibility) of a single measurement of cardiac output by either the dye or thermal method, expressed as a standard deviation, approximated ± 7%. The calibration error of a single measurement was estimated to have a standard deviation approximating ± 15% for the dye method, but of less than 5% for the thermal method. A significant difference between simultaneous dye and thermal cardiac outputs was found only for measurements done in the operating room following the induction of anaesthesia. With this exception, it is concluded that both the dye and thermal methods are clinically reliable despite the changing postoperative state of cardiothoracic patients. The ease of performance of thermal dilution measurement recommends its preferential use with no loss of accuracy.

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