VENOUS DELAY, A MAJOR SOURCE OF ERROR IN ISOTOPIC CARDIAC-OUTPUT DETERMINATION

The high correlation reported for cardiac output determinations by 99mTc-HSA has not held for other unselected series. To investigate possible causes, cardiac output was determined simultaneously by two indicators: 99mTc-HSA and indocyanine green both injected by rapid flush technique, the 99mTc via antecubital vein and the dye (DD) via superior vena cava. Precordial counting rates were obtained by scintillation camera and dilution curves were derived by computer program from selected areas in right (RV) and left ventricles (LV). Thirty consecutive studies showed significant (p < 0.001) correlation (r = 0.81) between 99mTc and DD output but a determination index of only 66 per cent. Varying degrees of persistence of counting rate were noted in the subclavian region in the absence of demonstrable venous obstruction. In seven studies (Group I) there was no isotope hang-up in the subclavian region; in 17 (Group II), counts in the subclavian region persisted only through RV visualization, and in six studies (Group III) counts persisted even when radioactivity was at its peak concentration over LV. Correlation coefficients (99mTc output with DD output) varied inversely with delay; r was 0.90 for Group I and Group II, and 0.64 for Group III (determination indices 81 per cent, 81 per cent, and 41 per cent, respectively). Results suggest that unexpected delay in arrival of radioactive bolus into the heart results in erratic distortion of cardiac output values. Unless monitored, delay often remains unsuspected; in a consecutive series of 63 99mTc studies severe delay occurred in 16 per cent. The reason for the delay is not clear. It did not seem related to the speed of circulation, to the presence of venous obstruction, or to anatomical variations. It could not be related to the speed of injection or to the physical characteristics of the injectate. © 1979.