Isotope external counting method for cardiac output analyzed in glass model circulation

¹ Duke University Medical Center, Durham, North Carolina

Certain aspects regarding degree of accuracy and some of the theoretical sources of error of the isotope-external counting method for determination of cardiac output have been further evaluated by the use of a glass model central circulation. By comparing calculated flow with actually timed flow, and by observing the constant relationship between dilution curve area and flow, it was found that single as well as multichambered systems could be used for this method with valid and reproducible results. Variations in chamber size, rate of flow, isotope dosage and instrument settings were used to evaluate some of the possible sources of error. Variable mixing was the most important factor responsible for discrepancies. Errors in extrapolation of the primary dilution curve to zero and instrumental lag were considered to be of less importance and can most often be avoided. A change in the recording time constant altered the curve shape, and therefore the calculated mean transit time, but not curve area and calculated flow.