Published in:
Open Access 01-04-2002 | Research
Comparison of bedside measurement of cardiac output with the thermodilution method and the Fick method in mechanically ventilated patients
Published in: Critical Care | Issue 2/2002
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Introduction
Bedside cardiac output determination is a common preoccupation in the critically ill. All available methods have drawbacks. We wished to re-examine the agreement between cardiac output determined using the thermodilution method (QT THERM) and cardiac output determined using the metabolic (Fick) method (QT FICK) in patients with extremely severe states, all the more so in the context of changing practices in the management of patients. Indeed, the interchangeability of the methods is a clinically relevant question; for instance, in view of the debate about the risk–benefit balance of right heart catheterization.
Patients and methods
Eighteen mechanically ventilated passive patients with a right heart catheter in place were studied (six women, 12 men; age, 39–84 years; simplified acute physiology scoreII, 39–111). QT THERM was obtained using a standard procedure. QT FICK was measured from oxygen consumption, carbon dioxide production, and arterial and mixed venous oxygen contents. Forty-nine steady-state pairs of measurements were performed. The data were normalized for repeated measurements, and were tested for correlation and agreement.
Results
The QT FICK value was 5.2 ± 2.0 l/min whereas that of QT THERM was 5.8 ± 1.9 l/min (R = 0.840, P < 0.0001; mean difference, -0.7 l/min; lower limit of agreement, -2.8 l/min; upper limit of agreement, 1.5 l/min). The agreement was excellent between the two techniques at QT THERM values <5 l/min but became too loose for clinical interchangeability above this value. Tricuspid regurgitation did not influence the results.
Discussion and conclusions
No gold standard is established to measure cardiac output in critically ill patients. The thermodilution method has known limitations that can lead to inaccuracies. The metabolic method also has potential pitfalls in this context, particularly if there is increased oxygen consumption within the lungs. The concordance between the two methods for low cardiac output values suggests that they can both be relied upon for clinical decision making in this context. Conversely, a high cardiac output value is more difficult to rely on in absolute terms.