The adaptive responses of the cardiovascular system to regular physical activity appear to include a reduction in sympathetic (SNS) activity and an increase in parasympathetic (PNS) activity during rest and at different absolute intensities of exercise. In a cross-sectional design, trained individuals who exercised at least 5 days/week for 45 min/day or more were compared with age- and gender-matched untrained controls. There was a relative bradycardia in the trained groups at rest and at the same absolute intensity of exercise in both young and middle-aged subjects. There were no differences in indicators of PNS and SNS activity at rest in young subjects. There was a reduced SNS activity and increased PNS activity in middle-aged trained subjects versus their age-matched controls. Spectral analysis was capable of showing changes in autonomic control of heart rate at rest and across intensities of exercise. These data showed significant shifts to increased PNS and reduced SNS indicators at rest in trained versus untrained middle-aged subjects and in females in both age groups versus males.

Although the use of ultrasound as a therapeutic agent in the field of physical medicine continues to grow, there are still a number of questions concerning its physiologic effects which remain unanswered. With regard to the thermal response, it is generally accepted that practically the entire amount of energy applied to the tissues through this medium is absorbed and converted into heat. As a result, there is a rise in tissue temperature, most marked in the bones and nerves in which selective heating appears to occur (1-3). The effect of ultrasound on blood flow has been investigated by a number of workers. Bickford and Duff (4) reported that in man, exposure of the forearm to 2 watts per square centimeter produced minor increases in local circulation in one-half the cases and no change in the others, while only with dosages of 3-334 watts was a significant augmentation consistently noted. Paul and Imig (5) performed a similar study and obtained essentially the same results. In the case of dogs, Paul and his associates (5, 6) found 1 watt per square centimeter to be an effective quantity of ultrasonic irradiation for the production of a vascular response. In the present study it was decided to

The advent of the use of hypothermia as an adjunct to surgery has revived interest in the effect of alterations of local temperature on tissue metabolism. In the early part of this century, several investigators (1, 2) performed in v4tro studies and found that increasing the temperature of muscle slices raised their oxygen consumption, while an opposite effect was produced by decreasing the temperature. The Q10 of this relationship was generally found to be slightly above the value of two (2). However, one limitation of the work was that the absolute oxygen consumption was closely dependent upon the composition of the bath in which the tissue was suspended.In more recent years a number of workers (3-5) have noted an increase in local circulation with exposure of a limb to high external temperatures and a decrease with cooling of the tissues. From such evidence it has been concluded that the alterations in blood flow produced by local heat and cold are due both to the direct action of these agents on the blood vessels and to changes in the metabolic needs of the tissues being supplied. However, it is apparent that data on the circulation alone are not sufficient for the determination of tissue oxygen utilization, and that for this purpose, the simultaneous measurement of the quantity of oxygen removed from the blood as it passes through the structures under study is likewise essential.Studies on the effect of hypothermia on the whole organism have also contributed to the elucidation of the problem. In the rat (6) and in the dog (7,8) rise in oxygen consumption, corresponding to a stress reaction developed in the. body, and then a fall as cooling is continued. If shivering, which is elicited in the first phase, is prevented by the use of barbiturates, the increase in oxygen consumption is no longer observed (7). In poikilothermic animals hypothermia produces only a reduction in oxygen consumption (9).The purpose of this paper is to present the effects of local heating and cooling on the resting oxygen uptake of the tissues of the forearm. METHODThe investigation was performed on 17 healthy subjects between the ages of 19 and 51 years. Thirteen were males and four were females. In all, 27 experiments were done, their duration averaging six hours. In this period no food was served to the subject other than one or two small chocolate bars.Three different procedures were carried out during each experiment: 1) blood flow recordings on a six inch segment of forearm enclosed in a venous occlusion plethysmograph, according to the technique previously described3 (10) ( Figure 1); 2) determinations of oxygen content of venous blood from deep or superficial veins of the forearm; and 3) studies of muscle, subcutaneous tissue, skin, and rectal temperatures, using thermocouples. The temperature of the water in the plethysmograph (bath temperature) was first maintained at 450 C. and then lowered to 320 and finally to 17°,4 and at each level all of the above procedures were done, together with blood pressure and pulse rate readi...

In 17 experiments, performed on the forearm of normal subjects, the effect of 2½, 5, and 10 min of arterial occlusion was studied. Blood flow was obtained with the venous occlusion plethysmograph, and oxygen uptake was calculated using the Fick principle. Arterial occlusion resulted in the production of an oxygen debt which was subsequently repaid. With progressively longer periods of anoxia there was a proportionate increase in the magnitude of the debt. Similar conclusions could not be drawn from blood flow studies alone, since the vascular change represented only one means of repayment of the oxygen debt during reactive hyperemia, the other being a greater extraction of oxygen from each unit of blood early in the postocclusion period. The constant overswing on either side of the control base line, observed in the records of oxygen uptake, suggested the absence of delicately balanced and efficient checks on the mechanisms responsible for repayment of the oxygen debt incurred in the period of tissue anoxia. Submitted on March 27, 1961

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