Theoretical validation of the thermal dilution method for cardiac output determination

Method of assessing the reproducibility of blood flow measurement: factors influencing the performance of thermodilution cardiac output computers

Measurements of blood flow by three different makes of thermodilution cardiac output computer in an artificial circulation were analysed by linear regression against absolute flow measured by timed blood volume collection. For each computer the horizontal distance between the 95% confidence limits for a single prediction was calculated at a standard flow rate of 5 litres per minute. This measurement represents the range of flow rates that could give rise to an identical measurement and provides a summary of the reproducibility of the computer's results and its ability to detect a change of flow rate. This measurement was used to evaluate the effect on each computer's performance of pulsatile or continuous flow, injectate volume, and injectate temperature. With continuous flow the optimum results were 1.8, 0.85, and 0.85 litres per minute and with pulsatile flow they were 1.3, 1.05, and 1.65 litres per minute. There was generally a deterioration in performance when pulsatile flow was evaluated. Under the conditions of the experiment optimum performance in both flow modes was obtained with 5 ml of ice cold injectate, but these findings cannot necessarily be extrapolated to the clinical situation. With pulsatile flow the overall range of blood flows that could give rise to identical measurements were for each computer 2.0, 1.5, and 3.1 litres per minute, corresponding to 40, 30, and 62% changes of the standard flow rate of 5 litres per minute.

The thermodilution technique in on-line computation of the left ventricular volumes on anaesthetized dogs

The thermal dilution technique and its suitability for automatic computation of left ventricular volumes were evaluated on 28 anaesthetized dogs. The end-diastolic volume (EDV) was computed from ventricular washout curves in two ways, first utilizing the ratio between the indicator amount and its peak concentration (first ejection method); and secondly the ratio between stroke volume and ejection fraction with the latter obtained by subtracting the residual fraction from 1.0. The residual fraction was determined as the mean of ratios between successive steps of four ventricular ejections on the downslope of the curve (downslope method). The linear correlation coefficient between the EDV determinations (n = 366) was 0.926, and slope y = 1.14 X + 0.5 ml with 4.6 ml as standard error of estimate. The first ejection method gave on average 15% higher EDV values than the downslope method. When heart rate was increased by pacing from 88 to 240/min, both methods showed a reduction of the EDV, but the downslope method to a greater degree. When afterload was increased but heart rate decreased by phenylephrine, the first ejection method but not the downslope method indicated an increase in EDV. Otherwise, when pronounced tachy- or bradycardia were avoided, ventricular volumes yielded by the two methods were comparable. Indicator injection did not substantially affect ventricular pressure dynamics. The first ejection method seems to be a simple means for repeated measurements of EDV even in the case of diseased hearts.