Thermal Dilution Curves in the Intact Animal

The acceptance of thermal dilution curves for the estimation of cardiac output has boon discouraged by unconvincing comparisons with the Fick method and by the possibility of the loss of "indicator " to the tissues of the central circulation. The present studies were undertaken to examine the technical and physiologic factors affecting thermal curves, with particular reference to tissue heat exchanges, and to provide adequate data for estimating the usefulness of the thermal curves for simultaneous measurements of right and left ventricular outputs. PRESENT methods for obtaining indica-tor-dilution curves in intact animals are beset by many problems which make difficult or uncertain the repeated, accurate quantifi-cation of the cardiac output from such curves-Indicators such as Evans blue dye or radio-iodinated albumin have serious dosage limita-tions in human subjects, whereas agents such as K42 or saline may be lost to some extent in the lungs. Adequate detection of the time-concentration curves of these indicators in arterial blood requires the cumbersome tecli-nic associated with drawing blood steadily and quickly through a cuvette using tubing of suitable maximal dimensions.1 When dyes are used, the calibration of colorimetric cuvettes presents additional problems involving the linearity of photocell or phototube responses with increasing background dye concentra-tions, and the peculiar optical properties of flowing blood. The calibration difficulty is augmented when indirect detectors are used, and, with baseline instability, is a major dis-advantage in the use of a conductivity cell.-Most of these problems were avoided in th