Measurement accuracy of cardiac output in humans: indicator-dilution technique versus geometric analysis by ultrafast computed tomography

Royal Brompton National Heart and Lung Hospital, London, England, United Kingdom.

OBJECTIVES. The aim of this study was to evaluate measurement accuracy of cardiac output in humans by comparing the indicator-dilution technique with geometric analysis by ultrafast computed tomography. BACKGROUND. Ultrafast computed tomography can be used to measure cardiac output by two methods. First, by scanning to obtain end-systolic and end-diastolic short-axis images of the ventricular cavities at sequential tomographic levels, the stroke volume and therefore the cardiac output can be calculated. Second, indicator-dilution theory (the Stewart-Hamilton equation) can be applied to measurements of the concentration of radiographic contrast in the blood pool after a bolus injection. The latter method has not been validated in humans. METHODS. The accuracy of the geometric method itself was first established by comparing left and right ventricular stroke volumes in 29 patients without valvular regurgitation or an intracardiac shunt, whose left and right ventricular stroke volumes should have been identical (group A). In a subset of 17 patients, the geometric method was compared with the indicator-dilution method (group B). RESULTS. Geometric analysis showed that the mean difference between left and right ventricular stroke volume was 1.8 +/- 7.3 ml, with a percentage SD of the differences of 9.3% (r = 0.9). Comparison wih indicator dilution-calculated cardiac output showed a mean difference of 0.079 +/- 1.22 liters/min, with a percent SD of the differences of 23.7% (r = 0.6). There was no improvement in this comparison with individual calibration of the scanner for each patient. CONCLUSIONS. The disparity found between data obtained with the geometric and indicator-dilution methods may be a result of the hemodynamic effects of contrast medium or it may suggest the possibility that some assumptions of indicator-dilution theory are not valid.

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