CARDIAC OUTPUT AND TOTAL PERIPHERAL RESISTANCE MEASUREMENTS IN EXPERIMENTAL DOGS

Abstract

Though perfectly sound in principle, the procedure for determining cardiac output as proposed by G. N. Stewart was found to invite so many technical inaccuracies that the values could not be regularly relied upon. Much effort was directed with the aim of overcoming these weaknesses. The technical procedures were improved by proper adjustment of the conc. and rate of infusion of the NaCl soln. so that the salt appeared in sufficient conc. in the femoral blood stream to be easily detected without at the same time provoking any abnormality of cardiac action and a consequent artificial change in cardiac output; technical and instrumental improvements for detecting the arrival of the uniform blood-NaCl mixture in a femoral artery; advancements in the technique of withdrawing a sample of the uniform mixture for electro-analysis; and improvement in the accuracy of electro-titration of the control sample to match the blood-NaCl sample. As a result of the above expts., the method has been improved to the extent that constantly reproducible measurements of cardiac output were obtained in 145 tests on 42 dogs. Furthermore, statistical analyses revealed that the standard deviation from the mean of these measurements was smaller than any which have been reported for dogs by other investigators. Comparisons of values obtained almost simultaneously by this modified Stewart method and by the more generally accepted Fick method were as comparable as could be expected in both normal and hypotensive dogs. In 20 dogs, in which arterial blood pressure remained very stable, the mean circulatory index, expressed in liters/sq.m./min. was 2.77 and the avg. of the individual deviations was only 0.05. In the remaining 22 dogs, in which arterial pressures fluctuated somewhat (18 mm. Hg.; S.D. [plus or minus] 13) the circulatory index was of the same order of magnitude (2.94) but the avg. of the individual variations was greater (0.19). There appears therefore to be a definite relation between the constancy of consecutive measurements to be expected and the stability of arterial blood pressure. Although it appears from the results of this and other investigations that the control circulatory index of anesthetized mongrel laboratory dogs generally ranges from 2.5-3.3, such values cannot be considered as sufficiently restricted to justify their adoption as controls for any given expt. On any occasion where it is anticipated that the proposed exptl. procedure will modify the cardiac output, it is absolutely essential that several consistent control measurements be obtained. The modified Stewart method excels in its ability to provide quickly such repetitive cardiac output values for consideration by the investigator. An analysis of these and previously reported data indicates that equivalence in values for the circulatory index in man and dog, or for that matter, among any of the animal species has not been adequately demonstrated. The total peripheral resistance of the intact dog under control conditions, concerning which only very meagre information has been published, ranged in these dogs from 3100 to 9400 A.U. In 105 of 145 measurements, however, it fell within narrower limits (3600-5400 A.U.). In certain chronic expts. in which control T.P.R. values cannot be obtained prior to the institution of exptl. procedures (say states of hypotension or hypertension), the above range for control detns. offers some guide as to the extent to which T.P.R. changes may be concerned. Since little correlation was observed between the total peripheral resistance and body surface area, the advantage of introducing surface area into the equation for computing T.P.R. seems dubious.