Special instrumentation problems encountered in physiological research concerning the heart and circulation in man.

T HE FUNCTION OF THE HEART is to maintain an adequate flow of blood through the lungs and to the body. A quantitative study of this function therefore requires physical measurements of such variables as pressure, volume, velocity of flow, volume of flow, and others. For the most part, measurement of physiologic variables concerned in the heart and circulation of man requires that these determinations be carried out on intact unanesthetized human beings. Therefore, direct measurements of many of the variables must commonly be made through small needles or long, narrowbore, flexible tubes. Since variables such as blood pressure have both static and dynamic components, their high-fidelity recording under such circumstances requires close attention to the frequency and damping characteristics of the instruments used. Adequate instrumentation must be capable of faithful reproduction of both the static component and all dynamic components of a magnitude to be of practical importance. The highest frequencies of the dynamic components of practically important magnitudes in a complex wave form, such as an arterial pressure pulse or the action potential complex of the heart muscle (electrocardiogram), are not accurately known. It is generally considered, however, that instruments with a uniform dynamic sensitivity to the tenth harmonic of the fundamental frequency of such complex wave forms are suitable for high-fidelity recording of the wave concerned. By this criterion, since the heart rate of human beings seldom exceeds 240 beats per minute, an instrument with a uniform sensitivity from 0 to 40 cycles per second should be adequate for the recording of arterial blood pressure and most other physiologic variables associated with the cardiovascular system. Recent direct evidence (22) indicates that manometer systems with a uniform dynamic response to 10 cycles per second will record peripheral arterial pressure in man without significant amplitude distortion. It is of interest also that the sensitivity 'Abstract of paper presented at Gordon Research Conference on Instrumentation, August 4, 1950. of the majority of clinically acceptable electrocardiographs is diminished by more than 20 per cent at a frequency of 40 cycles per second. In the functioning cardiovascular system, the various factors of pressure, flow, velocity, rate, and so forth are all mutually interrelated and continuously varying. Therefore, accurate studies of the over-all function of the circulatory system require continuous recording of multiple variables. The desirability of recording multiple physiologic variables in studies of cardiovascular function is well illustrated by the results of studies concerning the nature of the blackout and unconsciousness sometimes experienced by pilots as a result of exposure to positive acceleration or centrifugal force (12, 16, 26). Sudden exposure of a fighter pilot to a commonly experienced positive acceleration of five times the force of gravity (5g) will, because of the effective increase in the weight of the blood, reduce arterial pressure at head level to zero. Studies of the physiologic effects of this type of stress in human beings are carried out under controlled laboratory conditions by means of human centrifuges (26), perhaps the largest type of physiologic instrument designed for the study of the circulation (1) (Fig. 1). Physiologic recordings taken during exposure of a normal subject to positive acceleration on the human centrifuge are shown in Fig. 2. The left panel shows the changes produced by an exposure to 4.6g when the subject was unprotected. The increase in weight of the blood associated with this acceleration reduced arterial blood pressure temporarily to zero at head level, and a temporary loss of vision resulted. The reflex compensatory increases in blood pressure at heart level induced by the lowered blood pressure in the head region (carotid sinus) are evident during the latter portion of this exposure. The center panel illustrates the increase in arterial pressure produced by inflation of a pneumatic antiblackout suit. The panel on the right was recorded from the same subject during exposure to 5.5g while protected by the antiblackout suit. Protecticn is afforded by the increase in blood pressure at heart level produced by inflation of the suit.