Hemodynamic response to exercise after beta-adrenergic and parasympathetic blockade

The effect of β-adrenoceptor blockade on body temperature and plasma renin activity in heat-exposed man

1 The effect of propranolol (5 mg i.v.) on rectal and skin temperatures, heart rate, blood pressure, plasma renin activity (PRA) and plasma renin substrate concentration (PRS) was investigated in twelve men exercising in the heat. The effect of practolol (10 mg i.v.) on PRA was investigated in five men. 2 Body temperatures were insignificantly affected by propranolol, while heart rate elevation in response to exercise in the heat was 21% lower than in the same subjects receiving saline. Diastolic blood pressure during exercise was elevated by propranolol. 3 The normal increase in PRA seen in heat exposure was suppressed by propranolol to levels seen when the same exercise was carried out at 25°C. Practolol did not affect the renin response to heat exposure. 4 PRS was not altered significantly by exercise or heat. 5 The results indicate that the increase in PRA seen in the heat is largely a result of increased sympathetic activity.

Effects of atropine on left ventricular volumes and ejection and filling rates at rest and during exercise

1. The influence of the parasympathetic nervous system on left ventricular function including ejection and filling rates was studied by radionuclide cardiography in eight healthy young men at rest and during upright exercise. 2. After parasympathetic blockade induced by atropine, the mean heart rate (HR) at upright rest increased from 67 to 114 beats min-1, cardiac output (CO) from 4.05 to 5.17 1 min-1 (both P less than 0.001), and the diastolic blood pressure by 13 mm Hg (P less than 0.01). 3. Stroke volume (SV), left ventricular end diastolic and end systolic volume all decreased significantly after atropine. The relative ejection time increased from 0.33 to 0.51 of the cardiac cycle length (P less than 0.001), and the appropriate ejection and filling rates increased by 13% (P less than 0.05) and 147% (P less than 0.001), respectively. Haemodynamic changes in the supine position were virtually the same. 4. During exercise atropine increased HR from 115 to 146 beats min-1 (P less than 0.001) and CO by 12% (P less than 0.05), whereas SV decreased by 12% (P less than 0.05) and the systolic blood pressure by 16 mm Hg (P less than 0.001). Changes in ejection and filling rate of the left ventricle were of the same nature as those found at rest. 5. Thus apart from its HR limiting properties, secondary effects of parasympathetic nervous tone are dilatation of the left ventricle and enhancement of ejection, effects that are counteracted by atropine.

The effect of cardiac denervation and beta-blockade on control of cardiac output in exercising dogs

Normal and cardiac denervated dogs, with an electromagnetic aortic flowprobe implanted at least 14 days before the experiments, ran at different speeds on a 25% graded treadmill. The experiments were carried out before and after blockade of betareceptors in the heart by PO administration of 125 mg X kg-1 practolol per day. Changes in stroke volume, heart rate, and cardiac output were measured. After beta-adrenergic blockade, only two of the seven dogs with denervated hearts were prepared to run at a limited number of speeds. Time constants of the cardiac output changes at the onset of exercise were significantly different (P less than 0.001) for the normal (11.5 +/- 0.7 s, mean +/- SEM) and the denervated dogs (29.5 +/- 1.1 s), but in normal dogs did not change with practolol (11.8 +/- 0.8 s). The steady state relationship between cardiac output (CO) and work per unit time performed on the treadmill (P) was for normal dogs: CO = 156 + 1.55P, for normal dogs after practolol treatment: CO = 156 + 0.43P (slope significantly different, P less than 0.05), and for dogs with denervated hearts: CO = 121 + 2.06P (not significantly different from normal dogs). It was concluded that changes in the venous or arterial system alone are not sufficient to increase cardiac output appreciably during exercise. The magnitude of the cardiac output increase depends more on the presence of intact beta-receptors than on the presence of intact cardiac nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of thoracic epidural analgesia on the circulation at rest and during physical exercise in man

The effects of thoracic epidural analgesia (TEA) on the circulation at rest, during, and after physical exercise were investigated in 10 volunteers. At rest, TEA did not affect oxygen consumption (VO2) or cardiac output (Q), but heart rate (HR) was, increased by 7 beats/min, and stroke volume decreased correspondingly by 13 ml. Systolic arterial blood pressure was slightly reduced, but no other changes in systemic or pulmonary circulatory parameters were produced. TEA did not change VO2 during exercise, a 2.11/min reduction of Q being accompanied by a 4.2% increase of oxygen utilization coefficient. HR showed a relative reduction of about 10%. At moderate work loads during TEA, systemic arterial blood pressures were significantly lower than during control exercise. The resulting rate pressure product (RPP) was markedly reduced, while total peripheral resistance (TPR) remained unchanged. During recovery after the termination of exercise during TEA, VO2, Q, HR, RPP and TPR fell more rapidly towards values obtained at rest. From a clinical point of view the results support the safeness of TEA and also support its use in surgical patients with heart disease.

Effects of cold, eating, efferent nerve stimulation and angiotensin on heart rate in sheep before and after autonomic blockade

1. Effects of the natural stimuli of cold exposure and feeding on heart rate were tested in conscious sheep in which blockade of the cardiac efferent nerves was achieved by administration of propranolol and atropine. Effects of direct nerve stimulation, isoproterenol and angiotensin on heart rate before and after autonomic blockade were studied in acute preparations anaesthetized with pentobarbitone sodium.2. Propranolol reduced the extent of cardioacceleration induced both by exposure to cold and by eating. Heart rate in propranolol treated animals exposed to cold did not exceed about 120 beats/min. During eating, heart rate in propranolol treated animals showed no upper limit and increased on occasions to over 170 beats/min.3. Heart rate in conscious sheep following administration of atropine and propranolol was about 110 beats/min; in anaesthetized sheep following vagal section and propranolol it was usually about 120 beats/min. This was taken as intrinsic heart rate.4. Angiotensin administered to normal anaesthetized sheep usually reduced heart rate. After vagal section and propranolol, angiotensin injection usually, and angiotensin infusion invariably, increased heart rate.5. It was concluded that there exists in sheep a mechanism which during eating elevates heart rate by a means other than through the parasympathetic and sympatho-adrenal systems. It is suggested that this mechanism involves a direct chronotropic effect of angiotensin secreted in association with eating.

Exercise haemodynamics of coarctation of the aorta. Acute effects of propranolol

Eight patients with aortic coarctation were studied at rest and during supine bicycle exercise before and after intravenous propranolol 0.17 mg./kg. The mean pressure gradient across the aortic narrowing increased from 16 to 45 mm. Hg during exercise, and was reduced to 11 and 31 mm. Hg after propranolol. Total peripheral resistance was increased by propranolol both at rest and during exercise, and the diminished pressor response was due to a 20 per cent decrease in exercise cardiac output. Brachial artery systolic pressure was reduced more than central aortic systolic pressure in 2 of the 3 patients in whom both pressures were monitored. In one patient descending aortic pressure fell during exercise; in the remainder, slight increases were noted.