The intimate relationship between the mechanism of the circulatory system and the total metabolism has been frequently pointed out in publications from the Nutrition Laboratory. The heart rate is, with the same individual, a remarkably significant index of the total metabolism. When it is considered that the total carbon-dioxide production is directly proportional to muscular activity and heat production, and furthermore that the blood must carry away the carbon dioxide and supply fresh oxygen to the tissues, in proportion to the need therefor, it is not surprising that the work of the heart bears a general relationship to the total metabolism. If the systolic discharge from the heart were uniform under all conditions, one could predicate that the pulse-rate would be proportional to the total metabolism. Such a proportionality of relationship, however, is by no means established or to be inferred from experimental evidence thus far obtained.

While a reasonably close correlation between the pulse-rate and the total metabolism of a given individual appears to be substantiated by a large number of experiments, this does not apply in any sense to a comparison of the pulse-rate and total metabolism of different individuals. For example, when a subject has a pulse-rate of 60 at one time and a pulse-rate of 80 at another time, one can be sure that the metabolism will be measurably higher with the higher pulse-rate, but it is by no means certain that subject A with a pulse-rate of 60, even with an equivalent weight and height, will have a metabolism lower than subject B of the same height and weight with a pulse-rate of 80. Indeed, the absence of correlation between pulse-rate and total metabolism with different individuals has been frequently noted and commented on in this Laboratory. On the other hand, a recent biometric treatment1 of the basal metabolism data of the Nutrition Laboratory has indicated the existence of a slight but apparently significant correlation between these two variables, slightly higher gaseous exchange being associated with higher pulse-rate, even with men and women in complete muscular repose and in the post-absorptive state.

In this research on low diet it was imperatively necessary to obtain every possible index of metabolism or physiological activity under the varying conditions of normal and restricted diet. Hence we required careful records of the pulse-rate, since it is one of the most important physiological indices. Throughout the entire series of observations pulse-rates were very frequently obtained. Realizing that those found during complete muscular repose and with the post-absorptive condition would be of greatest significance, we invariably secured records of the pulse-rate during the respiration experiments with Squad A which were made each morning at Springfield. Records were also obtained at the conclusion of the night experiments with the large respiration chamber in Boston. For Squad A we have but a relatively few observations of pulse-rate with normal diet for comparison with the records after the diet restriction began. Pulse-rates for Squad B with muscular repose and post-absorptive condition were secured only at the end of the Boston night experiments. As indicated in the section on technique, the pulse-rate was taken chiefly by count at the wrist by an experienced observer, although a number of pulse-rates were recorded photographically with body electrodes and the string galvanometer.

1 Harris and Benedict, Carnegie Inst. Wash. Pub. No. 279, 1919, p. 79.

In view of the rather important relationship between pulse-rate and the body position and general physical activity, we have subdivided our pulse measurements into several sections according to the experimental conditions and particularly the body position of the subject: (1) lying in the post-absorptive condition; (2) standard electrocardiograms; (3) lying in the middle of the forenoon or middle of the afternoon prior to bicycle riding; (4) sitting with pulse counted either by the subject himself or by an observer at various times throughout the day, invariably with food in the stomach; (5) standing during respiration experiments, or on the treadmill; (6) reclining after short periods of muscular exertion; (7) in the periods of transition from standing to walking, and walking to standing; (8) during the actual process of walking on the treadmill; and (9) lying after a few minutes of brisk bicycle riding, during a special study of the influence of muscular work upon the return of the pulse-rate to normal.