The Effect of Cardioacceleration by Methylscopolamine Nitrate on the Circulation at Rest and During Exercise in Supine Position, with Special Reference to the Stroke Volume1

Cardiovascular responses to hot skin at rest and during exercise

Integrative cardiovascular responses to heat stress during endurance exercise depend on various variables, such as thermal stress and exercise intensity. This review addresses how increases in skin temperature alter and challenge the integrative cardiovascular system during upright submaximal endurance exercise, especially when skin is hot (i.e. >38°C). Current evidence suggests that exercise intensity plays a significant role in cardiovascular responses to hot skin during exercise. At rest and during mild intensity exercise, hot skin increases skin blood flow and abolishes cutaneous venous tone, which causes blood pooling in the skin while having little impact on stroke volume and thus cardiac output is increased with an increase in heart rate. When the heart rate is at relatively low levels, small increases in heart rate, skin blood flow, and cutaneous venous volume do not compromise stroke volume, so cardiac output can increase to fulfill the demands for maintaining blood pressure, heat dissipation, and the exercising muscle. On the contrary, during more intense exercise, hot skin does not abolish exercise-induced cutaneous venoconstriction possibly due to high sympathetic nerve activities; thus, it does not cause blood pooling in the skin. However, hot skin reduces stroke volume, which is associated with a decrease in ventricular filling time caused by an increase in heart rate. When the heart rate is high during moderate or intense exercise, even a slight reduction in ventricular filling time lowers stroke volume. Cardiac output is therefore not elevated when skin is hot during moderate intensity exercise.

Cardiovascular responses to exercise when increasing skin temperature with narrowing of the core-to-skin temperature gradient

The decline in stroke volume (SV) during exercise in the heat has been attributed to either an increase in cutaneous blood flow (CBF) that reduces venous return or an increase in heart rate (HR) that reduces cardiac filling time. However, the evidence supporting each mechanism arises under experimental conditions with different skin temperatures (T_sk; e.g., ≥38°C vs. ≤36°C, respectively). We systematically studied cardiovascular responses to progressively increased T_sk (32°C-39°C) with narrowing of the core-to-skin gradient during moderate intensity exercise. Eight men cycled at 63 ± 1% peak oxygen consumption for 20-30 min. T_sk was manipulated by having subjects wear a water-perfused suit that covered most of the body and maintained T_sk that was significantly different between trials and averaged 32.4 ± 0.2, 35.5 ± 0.1, 37.5 ± 0.1, and 39.5 ± 0.1°C, respectively. The graded heating of T_sk ultimately produced a graded elevation of esophageal temperature (T_es) at the end of exercise. Incrementally increasing T_sk resulted in a graded increase in HR and a graded decrease in SV. CBF reached a similar average plateau value in all trials when T_es was above ~38°C, independent of T_sk. T_sk had no apparent effect on forearm venous volume (FVV). In conclusion, the CBF and FVV responses suggest no further pooling of blood in the skin when T_sk is increased from 32.4°C to 39.5°C. The decrease in SV during moderate intensity exercise when heating the skin to high levels appears related to an increase in HR and not an increase in CBF. NEW & NOTEWORTHY This study systematically investigated the effect of increasing skin temperature (T_sk) to high levels on cardiovascular responses during moderate intensity exercise. We conclude that the declines in stroke volume were related to the increases in heart rate but not the changes in cutaneous blood flow (CBF) and forearm venous volume (FVV) during moderate intensity exercise when T_sk increased from ~32°C to ~39°C. High T_sk (≥38°C) did not further elevate CBF and FVV compared with lower T_sk during moderate intensity exercise.

A historical perspective on peripheral reflex cardiovascular control from animals to man

Although drug treatment of human hypertension has greatly improved, there is renewed interest in non-drug methods of blood pressure reduction. Animal experiments have now shown that arterial baroreflexes do control long-term blood pressure levels, particularly by nervously mediated renal excretion of sodium and water. This Paton Lecture provides a review of the historical development of knowledge of peripheral circulatory control in order to supplement prior Paton Lectures concerned with cerebral cortical and other areas of influence. I also discuss how improved understanding of nervous control of the circulation has led to current methods of non-drug blood pressure control in man by implanted carotid baroreceptor pacemakers or by renal denervation. Finally, the role of other therapy, particularly listening to music, is reviewed.

Circulatory effects in healthy young men of atrial pacing at rest and during isometric handgrip

1. The influence of a fixed heart rate and cardiac output on the cardiovascular response to isometric handgrip at one third of maximal voluntary contraction has been studied by means of atrial pacing. 2. At rest, atrial pacing with a mean heart rate of 109 beats/min increased cardiac output and forearm blood flow while total systemic and forearm vascular resistance decreased. 3. During handgrip, total systemic resistance increased both with and without pacing. A slow lowering of forearm vascular resistance was noted in the former situation, no change in the latter. 4. It is concluded that atrial pacing per se increases cardiac output in healthy, young volunteers. Handgrip elicits a vasoconstriction on other vascular beds than the resting forearm.

The cardiovascular effects of neuroleptanaesthesia

Oxygen uptake, cardiac output, stroke volume and arterial and central blood pressures were measured before and after induction of neuroleptanaesthesia in 27 subjects. Nine were elderly patients operated on for obliterative arteriosclerotic disease, and the other 18--nine elderly and nine younger patients--underwent operation for varicose veins. Cardiac output, stroke volume and systolic arterial blood pressure decreased significantly with a corresponding decrease in oxygen uptake. The changes were most pronounced in the patients with arteriosclerotic disease. The arterio-venous oxygen difference was unchanged in the arteriosclerotics and decreased in the other two groups. The central pressures remained unchanged in all groups. It is concluded that the cardiovascular changes induced by neuroleptanaesthesia are due to a decrease in oxygen uptake and not to myocardial depression.

Effects of autonomic blockade on the baroreflex in man at rest and during exercise

The reflex bradycardia produced by a transient phenylephrine-induced rise of arterial pressure was investigated in man during rest and supine exercise, before and after autonomic blockade of the heart. Reflex bradycardia diminished proportionally to the tachycardia of exercise. Propranolol slowed the heart at rest and during exercise, but increased the reflex response only at rest, having no effect during exercise. Atropine, or atropine with propranolol, blocked the reflex during rest and exercise. The tachycardia following hypotension induced by amyl nitrite was similarly affected by the two drugs. Tachycardia induced by standing up and by isoprenaline also diminished the reflex bradycardia. It is concluded that reflex heart rate changes following sudden changes of arterial pressure are predominantly parasympathetic, and diminish during exercise in parallel with the decrease of parasympathetic tone. The reflex response is determined partly by the interaction of parasympathetic and sympathetic impulses at the sinoatrial node, shown by the effects of peripheral sympathetic stimulation and blockade at rest. During exercise central depression of the reflex may also occur.

Hemodynamic response to exercise after beta-adrenergic and parasympathetic blockade

Cardiac responses to supine bicycle exercise were studied in six normal subjects after the intravenous administration of 5 mg of propranolol and again after the additive effect of 0.03 mg/kg of atropine. Mean resting heart rate was decreased 15 beats/min by propranolol and then increased by 38 beats/min after atropine. Mean exercise heart rate was 157 for control exercise, 119 after propranolol and 143 after propranolol + atropine. The resting cardiac index was reduced 21% by propranolol and restored to control values after atropine. The mean control exercise cardiac index was 7.2 l/min per m2; this was reduced to 5.8 after propranolol and increased to 6.2 after atropine. The mean stroke index was not altered by control exercise, increased 3 ml/beat per m2 with exercise after propranolol and increased 6 ml/beat per m2 with exercise after propranolol + atropine. Resting pulmonary artery, aortic and left-ventricular end-diastolic pressures were not altered by propranolol or by propranolol + atropine. The mean control exercise left-ventricular end-diastolic pressure was 9 mm Hg, increased to 19 mm Hg after propranolol and fell to 12 mm Hg after propranolol + atropine. The mean exercise aortic systolic pressure was 10 mm Hg below the value for control exercise after propranolol and 8 mm Hg below control after propranolol + atropine. Atropine lessens the inhibition of the cardiac response to exercise with beta adrenergic inhibition, possibly by increasing the heart rate.