Thermodilution cardiac output

The thermodilution method for measuring cardiac output, after injection of the thermal indicator through the central venous port and registration of the temperature change by the thermistor at the distal port of the pulmonary artery catheter, has been extensively validated by comparison with the gold standard, i.e. the cardiac output calculated, using the Fick principle, from measurements of the oxygen uptake (indirect calorimetry, analysis of inspiratory and expiratory breath) and the arterial and mixed venous oxygen contents. In critically ill patients, at low cardiac outputs and with small thermal indicator volumes, the thermodilution method may overestimate the true cardiac output (R§nn§L§Lal 1993).

There is continuing controversy regarding the optimal phase during which the thermal indicator should be injected for proper measurement of cardiac output during spontaneous and mechanical ventilation. It is generally recommended that the average result for three consecutive thermal indicator injections at either end-expiration or end-inspiration should be taken for a proper cardiac output estimation. However, during mechanical ventilation and relatively large swings in intrathoracic pressure, transmitted to the heart and great vessels, there is a large variation in right-sided thermodilution cardiac output following changes in right ventricular loading during the respiratory cycle. It is difficult to predict the phase and amplitude of these changes in an individual patient because many factors may influence the relation between the cyclic variation in ventilation-induced intrathoracic pressure and thermodilution cardiac output. These factors include the intravascular volume status, the cardiac output itself, and the level of intraalveolar pressure swings which are determined by respiratory rate, pattern, and volume (compliance). Therefore, for clinical purposes, it is recommended that the thermodilution cardiac output in ventilated patients should be determined as the mean of at least four thermodilution injections (with valid curves) made at random in the ventilatory cycle, irrespective of their variation.

By altering the steady-state baseline blood temperature, on which the change in injectate temperature is superimposed and cardiac output is calculated from the area under the curve of the thermal signal, rapid infusion of relatively cold fluids changes the area under the thermodilution curve and thus leads to either under- or overestimation of cardiac output, depending on the infusion rate. Thermal noise can be reduced by warming infusion fluids to blood temperature.

Another controversial point is the widely recommended use of ice-cold injectates for enhancing signal-to-noise ratio and improving reproducibility ( R§DD.§.L§ta.l 1993)

However, many authors have shown that room temperature injectates may be as good as cold injectates, provided that the patient is not severely hypothermic or hyperdynamic. The use of room temperature injectates also avoids the need to warm the injectate before injection and slowing of the heart rate and associated stroke volume changes during cardiac passage of the cold solution. The first error can be avoided by measuring the temperature of the injectate at the injection site. Furthermore, tricuspid regurgitation, which is a relatively frequent phenomenon in critically ill mechanically ventilated patients, may result in systematic and accidental measurement errors of cardiac output compared with measurements using other methods. A systematic underestimation usually results. Finally, the accuracy of thermodilution cardiac output measurements depends on the volume of thermal indicator injectate used. Since reproducibility is improved by using 10 ml rather than 5 ml, the former is widely advocated. The injectate port must be downstream from the introducer sheath of the pulmonary artery catheter, without concomitant infusions via the side-arm, to avoid errors. Otherwise, the side-arm may substitute for the central venous port, if the latter becomes non-functional. Continuous cardiac output

Thermodilution techniques have been developed for (semi)continuous monitoring of cardiac output ( HaJ.Ie.riefna/ 1995). This technology utilizes a thermal filament in the catheter at the level of the right ventricle. The generated heat (pulse) is detected downstream by a thermistor close to the tip of the catheter. This method yielded results equivalent to those obtained by intermittent thermodilution, even though the respiratory variations in venous return tend to level out with the continuous technique. However, the response time is relatively long. A Doppler pulmonary artery catheter has been developed for intermittent and continuous measurement of blood flow. In some experiments, Doppler blood flow correlated better with electromagnetic blood flow than with thermodilution blood flow.

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