Relationship between CO and transit times for dye and thermal indicators in central circulation

We investigated two factors that may influence the estimation of lung water by the thermal-dye double-indicator-dilution method: 1) changes in cardiac output (CO), and 2) thermal equilibration with cardiac tissue. In theory, the difference between mean transit times of thermal and dye indicators (delta MTT) is proportional to the extravascular volume of distribution of the thermal indicator (VODev) and inversely related to CO. The delta MTT also includes a time element DT due to the difference in response times of the measuring instruments such that delta MTT = VODev/CO + DT. In nine anesthetized dogs we recorded 286 aortic thermal and dye curves following left atrial (LA) and right atrial (RA) injections as CO was increased from 2.35 to 6.65 ml X s-1 X kg-1 by isoproterenol infusion, and a regression of delta MTT on CO-1 was performed. DT was measured in vitro for comparison with the y-intercept. In six of nine dogs the slope of the regression for LA injections was not different from zero, indicating that there is no measurable volume of distribution for thermal indicator in cardiac tissue. For RA injections the relationship between delta MTT and CO-1 was linear in all experiments, with an average correlation coefficient of 0.97 +/- 0.01 (SE), indicating that the VODev was constant over a threefold increase in CO. Although the in vitro measurement of DT agreed closely with the average of the y-intercepts of the regressions, small between-subject differences in DT can lead to apparent flow-related changes in extravascular thermal volume computed in the conventional fashion using the in vitro estimate of DT.