Effect of selective and nonselective beta-adrenoceptor blockade on thermoregulation during prolonged exercise in heat

Effects of Medications on Heat Loss Capacity in Chronic Disease Patients: Health Implications Amidst Global Warming

Pharmacological agents used to treat or manage diseases can modify the level of heat strain experienced by chronically ill and elderly patients via different mechanistic pathways. Human thermoregulation is a crucial homeostatic process that maintains body temperature within a narrow range during heat stress through dry (i.e., increasing skin blood flow) and evaporative (i.e., sweating) heat loss, as well as active inhibition of thermogenesis, which is crucial to avoid overheating. Medications can independently and synergistically interact with aging and chronic disease to alter homeostatic responses to rising body temperature during heat stress. This review focuses on the physiologic changes, with specific emphasis on thermolytic processes, associated with medication use during heat stress. The review begins by providing readers with a background of the global chronic disease burden. Human thermoregulation and aging effects are then summarized to give an understanding of the unique physiologic changes faced by older adults. The effects of common chronic diseases on temperature regulation are outlined in the main sections. Physiologic impacts of common medications used to treat these diseases are reviewed in detail, with emphasis on the mechanisms by which these medications alter thermolysis during heat stress. The review concludes by providing perspectives on the need to understand the effects of medication use in hot environments, as well as a summary table of all clinical considerations and research needs of the medications included in this review. SIGNIFICANCE STATEMENT: Long-term medications modulate thermoregulatory function, resulting in excess physiological strain and predisposing patients to adverse health outcomes during prolonged exposures to extreme heat during rest and physical work (e.g., exercise). Understanding the medication-specific mechanisms of altered thermoregulation has importance in both clinical and research settings, paving the way for work toward refining current medication prescription recommendations and formulating mitigation strategies for adverse drug effects in the heat in chronically ill patients.

Altered core and skin temperature responses to endurance exercise in heart failure patients and healthy controls

BACKGROUND

Exercise training represents a central aspect of rehabilitation of heart failure patients. Previous work on passive heating suggests impaired thermoregulatory responses in heart failure patients. However, no previous study directly examined thermoregulatory responses to an exercise bout, that is, active heating, as typically applied in rehabilitation settings in heart failure.

DESIGN

Cross-sectional observational study to compare changes in core body temperature (Tcore) and skin temperature (Tskin) during exercise between heart failure patients and controls.

METHODS

Fourteen heart failure subjects (65 ± 7 years, 13:1 male:female) and 14 healthy controls (61 ± 5 years, 12:2 male:female) were included. Tcore (telemetric temperature pill) and Tskin (skin thermistors) were measured continuously during a 45-min cycle exercise at comparable relative exercise intensity.

RESULTS

Tcore increased to a similar extent in both groups (controls 1.1 ± 0.4℃, heart failure patients 0.9 ± 0.3℃, 'time*group': p = 0.15). Tskin decreased during the initial phase of exercise in both groups, followed by an increase in Tskin in controls (1.2 ± 1.0℃), whilst Tskin remained low in HF patients (-0.3 ± 1.4℃) ('time*group': p < 0.001). Furthermore, we found that a given change in Tcore was associated with a smaller increase in Tskin in heart failure patients compared with controls. When comparing heart failure patients and controls who performed exercise at similar absolute workload, between-group differences disappeared (p-values > 0.05).

CONCLUSION

Heart failure patients and controls show comparable exercise-induced increase in Tcore, whilst heart failure patients demonstrate altered Tskin responses to exercise and attenuated elevation in Tskin per increase in Tcore. These impaired thermoregulatory responses to exercise are, at least partly, explained by the lower absolute workload and lower physical fitness level in heart failure patients.

Both beta1- and beta2-adrenoceptors contribute to feedforward coronary resistance vessel dilation during exercise

During exercise, beta-feedforward coronary vasodilation has been shown to contribute to the matching of myocardial oxygen supply with the demand of the myocardium. Since both beta(1)- and beta(2)-adrenoceptors are present in the coronary microvasculature, we investigated the relative contribution of these subtypes to beta-feedforward coronary vasodilation during exercise as well as to infusion of the beta(1)-agonist norepinephrine and the beta(1)- and beta(2)-agonist isoproterenol. Chronically instrumented swine were studied at rest and during graded treadmill exercise (1-5 km/h) under control conditions and after beta(1)-blockade with metoprolol (0.5 mg/kg iv) and beta(1)/beta(2)-blockade with propranolol (0.5 mg/kg iv). The selectivity and degree of beta-blockade of metoprolol and propranolol were confirmed using isoproterenol infusion (0.05-0.4 microg. kg(-1).min(-1)) under resting conditions. Isoproterenol-induced coronary vasodilation was mediated through the beta(2)-adrenoceptor, whereas norepinephrine-induced coronary vasodilation was principally mediated through the beta(1)-adrenoceptor. Exercise resulted in a significant increase in left ventricular norepinephrine release and epinephrine uptake. beta(1)-Adrenoceptor blockade with metoprolol had very little effect under resting conditions. However, during exercise, metoprolol attenuated the increase in myocardial oxygen supply in excess of the reduction in myocardial oxygen demand, as evidenced by a progressive decrease in coronary venous Po(2). Consequently, metoprolol caused a clockwise rotation of the relationship between myocardial oxygen consumption and coronary venous Po(2). Additional beta(2)-adrenoceptor blockade with propranolol further inhibited myocardial oxygen supply during exercise, resulting in a further clockwise rotation of the relationship between myocardial oxygen consumption and coronary venous Po(2). In conclusion, both beta(1)- and beta(2)-adrenoceptors contribute to the beta-feedforward coronary resistance vessel dilation during exercise.

Treating hypertension in active patients: which agents work best with exercise?

Hypertension treatment typically involves drug and nondrug therapies. For patients already exercising and those trying to start, physicians need to be aware of the intricacies of antihypertensive medications, including side effect profiles and whether they are banned by some sports organizations. Although diuretics and beta-blockers have been used in the past by most physicians, in active patients we typically opt for newer agents such as ACE inhibitors, angiotensin-II-receptor blockers, dihydropyridine calcium channel blockers, and alpha-1 blockers. These drugs typically have fewer side effects and impediments to athletic performance. Other drugs may be used in active patients who have other medical problems or when cost is a concern.

Management of Hypertension

In brief Current recommendations for brief managing mild to very severe high blood pressure need to be adapted for the special concerns of physically active patients. First-line treatment involves dietary changes, smoking cessation, and aerobic exercise. The next step is to add drug therapy, and diuretics and beta-blockers are the initial drugs of choice because of their proven long-term efficacy. Diuretics may, however, produce hypokalemia or dehydration, and beta- blockers may cause hyperkalemia, reduce exercise capacity, or increase perceived exertion. Other antihypertensive agents may be preferable in specific situations.

Effects of sympathomimetic and sympatholytic drugs on exercise performance

This article has presented information on the importance of the sympathetic nervous system in the response to exercise. The authors have reviewed the very limited information on the effects of sympathomimetic and sympatholytic drugs on exercise performance in the horse. Most of these drugs are specifically prohibited under the rules of racing, and they have significant side effects that either decrease performance or make their use dangerous to both the horse and horse-man. Additionally, all of these drugs or their metabolites are readily detected by current drug testing protocols. Further information is needed to expand our knowledge of how each of these substances affects the health of the equine athlete.

Hypertension

In brief When exercise alone does not NMM control hypertension, a medical ISMS regimen can be used in conjunction with exercise or other nonpharmacologic interventions. Diuretics, beta blockers, calcium antagonists, and angiotensin-converting enzyme (ACE) inhibitors are widely used to treat hypertension. ACE inhibitors and calcium antagonists are good choices for exercising patients because they do not limit exercise performance and have few side effects. The author gives guidelines for changing the regimen from exercise alone to exercise and medication, and for stepping down from medical therapy.

Beta-blockers and central nervous system side effects

Beta-adrenergic blocking drugs are a widely used, well tolerated and effective treatment for a variety of cardiovascular and noncardiovascular disorders. Over the years, beta-blockers have been associated with an incidence, albeit low, of CNS side effects. The question of interest, however, is whether the incidence is the same for all members of the class or whether other properties, such as hydrophilicity, have a bearing on the incidence of this type of side effect? This article addresses this question. In pharmacokinetic terms the lipophilic beta-blockers have been shown, both in animals and man, to readily cross the blood-brain barrier in contrast to hydrophilic beta-blockers. This is thought to have possible clinical relevance with respect to the relative incidence of CNS side-effects. To clarify the situation every published clinical paper, in which the beta-blockers propranolol (highly lipophilic, nonselective, no intrinsic sympathomimetic activity (ISA)), pindolol (moderately lipophilic, nonselective, moderate ISA), metoprolol (moderately lipophilic, beta 1-selective, no ISA) and atenolol (hydrophilic beta 1-selective, no ISA) were compared, was assessed for information pertaining to CNS side effects. This comprehensive review of the literature has shown, with few exceptions, that the incidence of CNS side effects such as sleep disturbances, dreaming, nightmares and hallucinations following clinically accepted doses of the four beta-blockers under scrutiny is generally low and that effects on short-term memory are minimal or absent. However, within this group of four drugs the incidence of these side effects is lowest with hydrophilic atenolol and generally highest with pindolol and propranolol. Metoprolol occupies an intermediate position. This order is in agreement with the pharmacokinetic observation that the more hydrophilic the molecule, the less is found in the brain tissue of both animals and man, although in the case of pindolol other factors may be important. The clinical relevance of studies involving psychometric testing is not clear.