Effect of beta-blockade on exercise core temperature in coronary artery disease patients.

The effect of pharmacologic blockade of beta-adrenoceptors on the relationship between rectal (Tre) and pulmonary artery (Tpa) temperatures was studied in six coronary artery disease patients during 30 min of exercise. Exercise was performed at a set work rate (120 W) on a cycle ergometer before and 2 h after the ingestion of 80 mg propranolol. The heart rate on completion of exercise was reduced (P less than 0.001) from 140 +/- 5 to 108 +/- 3 beats.min-1 by propranolol demonstrating a considerable degree of beta-adrenoceptor blockade. At rest, neither Tre nor Tpa were modified by propranolol. Likewise, propranolol failed to modify the Tre response to exercise. However, propranolol induced an alteration of the normal relationship between Tre and Tpa during exercise. In particular, although Tre was essentially unchanged, propranolol accentuated both the initial fall (by 0.28 degrees C, P less than 0.001) and the subsequent rise (by 0.46 degrees C, P less than 0.01) in Tpa during exercise. The present data therefore demonstrate a considerable limitation to the use of Tre when assessing thermal homeostasis during acute beta-adrenoceptor blockade. Furthermore, although research with more chronic therapy is warranted, our study suggests an accentuated risk of hyperthermia and, by implication, its adverse physiologic consequences during prolonged exercise performed by coronary artery disease patients treated with propranolol.

Effect of dual beta-blockade and calcium antagonism on endurance performance.

The effect of oral clinically used doses of propranolol, atenolol, nifedipine, propranolol + nifedipine, and atenolol + nifedipine on endurance performance and ventilatory responses to graded treadmill testing was studied in 12 healthy physically active men. Maximal exercise duration was reduced by propranolol (8.5%, P less than 0.001) and its combination with nifedipine (11.1%, P less than 0.001), and to a lesser degree by atenolol (3.2%, 0.05 less than P less than 0.1), nifedipine (2.1%, P less than 0.05), and atenolol + nifedipine (3.9%, P less than 0.01). Exercise duration and heart rate (HR) and ventilatory responses to maximal exercise were equivalent with a beta-blocker and its combination with nifedipine. At submaximal exercise, beta-blockade reduced the HR and oxygen uptake, nifedipine accentuated the HR but did not alter ventilation, and all drugs modified the relative oxygen uptake corresponding to 85% of the maximal HR. Physiologic responses to submaximal exercise during combination therapy were similar to those during beta-blockade alone. This study concludes that, in physically active men, nifedipine induces a small impairment of maximal performance, but does not accentuate the reduction in effort tolerance resulting from beta-blockade. Furthermore, HR and ventilatory responses to exercise during combined beta-blockade and calcium antagonism can be predicted from those during beta-blockade alone.

Effect of beta-adrenoceptor blockade and calcium antagonism, alone and in combination, on thermoregulation during prolonged exercise.

The effect of clinically used doses of propranolol, atenolol, nifedipine, propranolol plus nifedipine, and atenolol plus nifedipine on thermoregulatory responses of 11 healthy men was studied during 2-h block-stepping in heat. Drug intervention did not alter ventilation during exercise. In contrast, propranolol and atenolol produced equivalent reductions in exercise tachycardia, implying a similar level of beta 1-adrenoceptor blockade. The heart rate response to exercise was unaffected by nifedipine and during dual beta-adrenoceptor blockade and calcium antagonism was equivalent to that with beta-adrenoceptor blockade alone. While rectal temperature rises were not modified by drug ingestion, propranolol and, to a lesser degree, atenolol and combination therapy, but not nifedipine alone, attenuated skin temperature rises. Moreover, although atenolol, nifedipine, and their combination did not alter sweating, propranolol and its combination with nifedipine enhanced sweating during the 1st and 2nd h of exercise. This study concludes that nifedipine does not modify thermoregulation during exercise and allows for greater confidence of its use during cardiac rehabilitation. Furthermore, the present data confirm that propranolol does enhance sweating during exercise and demonstrate that this effect is not mediated simply by an earlier onset of rapid sweating nor abolished by concomitant calcium antagonism.

Comparison of continuous and intermittent multistage maximal exercise testing during beta-adrenoceptor blockade in physically active men.

Cardiorespiratory responses of 11 healthy males were studied, with placebo and propranolol, during a continuous and an intermittent multistage maximal treadmill test. With placebo, equivalent maximal heart rates were attained for the disparate test modes, and the intermittent protocol yielded a slightly higher (2%, P less than 0.01) maximal O2 consumption. In contrast, during beta-adrenoceptor blockade, higher maximal heart rates (5.1%, P less than 0.01) and O2 consumptions (4.4%, P less than 0.02) were reached with intermittent compared with continuous testing. Values were, however, markedly lower (P less than 0.001) for both protocols than with placebo. These results demonstrate that the precise degree of attenuation of maximal heart rate and O2 consumption observed in physically active persons receiving propranolol is partly dependent upon the actual test protocol utilized. Furthermore, the present data suggest an advantage for intermittent testing when accurate evaluation of the maximal cardiorespiratory capacity is desired in such individuals.

Isoproterenol sensitivity in heat tolerant and relatively heat intolerant men.

Recent research has demonstrated that pharmacologic blockade of beta-adrenoceptors predisposes to hyperthermia during prolonged exercise. To investigate the hypothesis that beta-adrenoceptor sensitivity to catecholamines may be an important determinant of exertional heat tolerance, we performed a cross-sectional study comparing the heart rate responses to graded doses of isoproterenol in 6 heat tolerant and 6 relatively heat intolerant men. We observed no significant difference (p greater than 0.1) between the heat tolerant (0.9 +/- 0.68 microgram) and heat intolerant (1.19 +/- 0.61 microgram) subjects in the dose of isoproterenol that produced a 25 beat.min-1 increment in heart rate. Although the possibility of a relationship between beta-adrenoceptor sensitivity and the ability to tolerate exercise in heat cannot be entirely excluded on the basis of these data, our study clearly demonstrates the lack of a correlation between cardiac pacemaker sensitivity to isoproterenol and exertional heat tolerance.

Effect of acute calcium slow-channel antagonism on the cardiorespiratory response to graded exercise testing.

The effect of acute calcium slow-channel antagonism on the cardiorespiratory responses of 12 healthy males was studied during multistage maximal exercise testing. Oral diltiazem (60 mg), nifedipine (10 mg), verapamil (80 mg), and placebo were administered in a randomized double-blind crossover fashion 1.5 h before an intermittent multistage cycle ergometer exercise test. At peak effort, heart rate, pulmonary ventilation, CO2 production, and respiratory exchange ratio were not significantly modified by drug ingestion. In contrast, maximal O2 consumption (2.6% reduction, P less than 0.02) and performance time (6.4% reduction, P less than 0.02) were reduced by nifedipine. During submaximal exercise, the chronotropic response was accentuated by nifedipine but not by diltiazem or verapamil. Furthermore, nifedipine altered the relationship of percentage of maximal O2 consumption to percentage of maximal heart rate, and both the relative and absolute O2 consumption corresponding to 70% and 85% of the maximal heart rate were significantly reduced after nifedipine ingestion. The results indicated that in persons without symptomatic coronary artery disease exercise performed at a given percentage of the maximal heart rate will yield a lower than expected relative and absolute intensity in the presence of acute nifedipine-induced calcium slow-channel antagonism.

The South African Defence Force physical training programme. Part I. Effect of 1 year's military training on endurance fitness.

The effect of the South African Defence Force's (SADF) military training on the endurance fitness of recruits was investigated. Pre-military training maximal O2 consumption (53,14 ml/kg/min) did not change significantly during the study. In contrast, lactate turnpoint (ml O2/kg/min) and treadmill performance time were moderately enhanced (7,5% and 8% respectively; P less than 0,05) by the initial 10-week basic training programme. Further analysis of the results showed this endurance-training effect to be limited to recruits of average and, in particular, below-average fitness. The salutary effect of basic training on fitness levels was, however, transient, the values on completion of 1 year's military training being unaltered from those before conscription. This preliminary assessment of the SADF physical training programme suggests a need for the introduction of changes during and, in particular, after basic training.

The South African Defence Force physical training programme. Part II. Effect of 1 year's military training on muscular strength, power, power-endurance, speed and flexibility.

The effect of the South African Defence Force (SADF) military training on recruits' muscular strength, power, power-endurance, speed and flexibility was investigated. In the entire study group, a significant enhancement by basic training was observed for the isokinetic muscular strength of the right elbow extensors alone (17% increase; P less than 0,05). Stratification of training responses on the basis of initial fitness levels revealed significant improvements with basic training for all measures of muscular strength, power, power-endurance and speed, but not flexibility, in the below-average fitness recruit. For all variables the training effect was transient and values documented on completion of 1 year's military training differed insignificantly from those recorded before conscription. In contrast, with the exception of elbow extensor muscular strength, fitness levels of the average and above-average recruits were not increased by basic training. While further research to assess physical requirements of various military work and emergency situations is needed to evaluate the desirability of this observed selective enhancement of recruit fitness levels during basic training, it is evident that greater emphasis should be placed on flexibility training, and physical conditioning after basic training.

Body cooling as a method for reducing hyperthermia. An evaluation of techniques.

The most important objective in the treatment of heatstroke and related conditions is to reduce the body core temperature to safe levels. Subjects performed at an external work rate of 54 W in hot, humid conditions (dry-bulb 40 degrees C, wet-bulb 35 degrees C), and a recent innovation to achieve body cooling, i.e. strategically placing instant cold packs (ICPs) (Coldpak; Medac) over the large vessels of the neck, axillae and groin, was evaluated under controlled laboratory conditions. We found that this procedure was not significantly more effective (P greater than 0.5) than passive body cooling in bringing about a 2 degrees C reduction in rectal temperature. Moreover, covering the whole body with ICPs plus induced evaporative cooling produced higher cooling rates than those achieved by the strategic placement of ICPs (0.0340 degrees C/min and 0.0344 degrees C/min respectively) (P less than 0.01).

Physiologic and biochemical changes during a triathlon competition.

The triathlon has come to be recognized as a distinct endurance event and although a substantial body of research information exists with regard to the individual components (canoeing, cycling, running, etc.), the physiologic demands imposed by combining the respective activities into one continuous event have not yet been assessed. Twenty-three male athletes (age = 33 +/- 5.2 yr; mass = 71.79 +/- 7.42 kg; height = 176.0 +/- 7.21 cm; means +/- SD) participated in the study on the basis of informed consent. Venous blood samples, taken immediately prior to and on completion of the Iron Man Triathlon held in Johannesburg during 1983, were assayed for parameters of energy metabolism, fluid and electrolyte balance, and stress (tissue enzymes in serum). On the basis of post-race blood glucose (6.17 mmol. l-1), free fatty acids (1943 mumol. l-1), and blood lactate (3.45 mmol. l-1) changes, it was concluded that the triathlon is primarily, in an overall sense, an aerobic event. With the exception of magnesium and iron, serum electrolyte changes agreed with literature findings. However, body mass reductions (average 3.23 kg, i.e., 4.5%) indicated that the extent of dehydration is more severe in the triathlon than in comparable endurance events. Post-race enzyme levels (LD, CK, and AST) were significantly elevated, and LD negatively correlated with total performance time (P less than 0.01). The general conclusion is that the physiologic demand of the triathlon exceeds that of other comparable endurance events.

Effect of beta1 selective adrenoceptor blockade on physiological response to exercise.

The effect of the beta1 selective adrenoceptor blocker, atenolol, on the physiological response to exercise was studied in 12 healthy young men. Oral atenolol (100 mg) and placebo were administered in a randomised double blind crossover fashion an hour and a half before an intermittent multistage cycle ergometer exercise test. At maximal effort oxygen consumption, pulmonary ventilation, carbon dioxide output, and respiratory exchange ratio were not modified by atenolol. In contrast, maximal heart rate and performance time were significantly reduced after atenolol. Nevertheless, because the relation of percentage of maximal oxygen consumption to percentage of maximal heart rate was not changed by atenolol both the absolute and relative oxygen consumption corresponding to 70% and 85% of the maximal heart rate remained unaltered. These data suggest that recommendations of exercise intensity may be determined on the basis of a calculated percentage of the predetermined maximal heart rate in persons without symptomatic coronary heart disease receiving beta1 selective adrenoceptor blockers.

Effect of beta-adrenoceptor blockade on thermoregulation during prolonged exercise.

The effect of clinically used equipotent doses of nonselective (beta 1/beta 2; propranolol) and selective (beta 1; atenolol) beta-adrenoceptor blockers on thermoregulation was studied during prolonged exercise in the heat. Oral propranolol (160 mg/day), atenolol (100 mg/day) or matching placebo were taken for 6 days each by 11 healthy young adult caucasian males. Subjects participated in 2 h of block-stepping at a work rate of 54 W in an environmental chamber with a temperature of 33.2 +/- 0.3 degree C dry bulb and 31.7 /+- 0.3 degree C wet bulb, 2 h after ingestion of the final dose of each drug. Both active agents produced similar marked (P less than 0.001) increases in subjective perception of effort, the mechanism of which was not immediately evident from changes in serum electrolytes, blood glucose, blood lactate, or ventilatory parameters. Propranolol did, however, cause a greater rise in serum K+ than placebo (P less than 0.02) and atenolol (P = NS) after exercise. Although rectal and mean skin temperatures were insignificantly altered by beta-adrenoceptor blockade, an increased total sweat production was noted with propranolol (P less than 0.01 vs. placebo) and to a lesser degree atenolol (P = NS vs. placebo) therapy. Analysis of the time course of sweat production showed the propranolol-mediated enhancement of sweating to ensue largely during the initial hour of block-stepping and to be transient in nature. The scientific and clinical implications of this observation will be dependent upon the precise underlying mechanism, a factor not identified by the present study.