Comparison of bedside measurement of cardiac output with the thermodilution method and the Fick method in mechanically ventilated patients

Abstract

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 (QTTHERM) and cardiac output determined using the metabolic (Fick) method (QTFICK) 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). QTTHERM was obtained using a standard procedure. QTFICK 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 QTFICK value was 5.2 +/- 2.0 l/min whereas that of QTTHERM 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 QTTHERM 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.

Figure 1

Comparison of cardiac output determined using the thermodilution method (QTTHERM) and cardiac output determined using the metabolic (Fick) method (QTFICK) according to (a) the Bland and Altman graphic method [28], and (b) the Passing and Bablok regression method [29]. Determined using the whole set of data after removal of one data point identified as an outlier (48 pairs obtained in the 18 patients), irrespective of the cardiac output value and of the presence of a tricuspid regurgitation. CI, confidence interval; SD, standard deviation.

Figure 2

Passing and Bablok regression of cardiac output determined using the metabolic (Fick) method (QTFICK) against cardiac output determined using the thermodilution method (QTTHERM) [29] restricted to (a) QTTHERM values <5 l/min and (b) QTTHERM values >5 l/min (after removal of one outlier). CI, confidence interval.

Figure 3

Comparison of cardiac output determined using the metabolic (Fick) method (QTFICK) and cardiac output determined using the thermodilution method (QTTHERM) according to (a) the Bland and Altman graphic method [28], and (b) the Passing and Bablok regression method [29]. Restricted to the patients in whom cardiac echography ruled out tricuspid regurgitation (14 patients, 40 pairs of measurements, after removal of one outlier). CI, confidence interval; SD, standard deviation.