Potential Therapeutic Effects of Warming on Patients with Heart Failure

This article refers to ‘Acute lower-limb heating improves exercise performance in individuals with heart failure with reduced ejection fraction’ by F.G. O'Connor et al., published in this issue on pages xxx. Doctors used to recommend bedrest and avoiding strenuous exertion to patients with heart failure1 but randomized trials subsequently showed that training programmes improved exercise capacity and wellbeing without a detrimental effect on cardiac function or prognosis.2 Exercise training is now recommended for chronic heart failure by national and international guidelines. The duration and intensity of exercise may influence its benefits, which may be increased by a preliminary warm-up, either passive (hot showers or saunas) or active (light exercise), which is common practice amongst athletes.3 Prospective cohort studies suggest that individuals who take more frequent saunas have a lower risk of cardiovascular death.4 Might warming interventions help patients with heart failure? (Figure 1) In this issue of the Journal, O'Connor and colleagues report the results of a randomized, crossover trial investigating the effects of lower-limb heating on exercise capacity in 22 patients with heart failure.5 Participants first did an endurance shuttle walk test to determine how long they could walk at a constant, personally tailored, sub-maximal speed. Patients were then randomized to 45 min of immersion of the lower leg in a water bath maintained at either 42°C (hot) or 30°C (control). After hot water immersion, resting femoral arterial blood flow (by Doppler ultrasound) and calf muscle oxygenation (by near infra-red spectroscopy) were substantially higher and subsequent walking distance was much greater (696 m vs. 544 m). Despite the large difference in exercise distance there were no differences in heart rate, perceived exertion or in end-exercise calf muscle oxygenation, implying that exercise reserve had increased. There are limitations to the study, which the authors acknowledge. The trial was small. Although the study is labelled ‘double-blind’, the patients were aware of the water temperature. However, the difference in walking distance was large. In health, increasing skin temperature to >40°C using water-perfused suits increases cardiac output by 29–125%.6, 7 Hydrostatic pressure alone increases venous return, by compressing the peripheral and splanchnic venous system, and cardiac output by about a third with further increases at higher water temperatures due to vasodilatation of skin arterioles, reducing vascular resistance, blood pressure and afterload.8 Shah et al.9 reported that warm water immersion up to the neck in 17 patients with mild heart failure increased stroke volume immediately by 26% and cardiac output by 30% and reduced systemic vascular resistance by 33%. Haemodynamic changes persisted throughout 15 min of warm water immersion. Intermittent passive heat exposure, such as sauna bathing, seems to offer similar physiological benefits to moderate-intensity aerobic exercise. Michalsen and colleagues did a randomized crossover trial comparing a 6-week hydrotherapy programme to a control period in 15 patients with mild chronic heart failure. Hydrotherapy consisted of alternating warm (maximum 40°C for ≤15 min) or cold (<18°C for ≤5 min) baths or wet sheets at least thrice daily. Hydrotherapy was well tolerated, improving quality of life and heart failure symptom scores.10 Ohori and colleagues investigated the effects of Waon therapy (repeated sauna therapy; five times a week for 3 weeks) in an observational study of 41 patients with chronic heart failure.11 Compared to baseline, Waon therapy increased the 6-min walk distance by 12%, reduced plasma concentrations of norepinephrine by 25% and B-type natriuretic peptide by 24%. SAUNA-HFpEF is currently enrolling patients with heart failure and preserved left ventricular ejection fraction to investigate the effects of saunas on cardiac function, exercise capacity and wellbeing.12 How might pre-heating the calves lead to such a large increase in exercise capacity in the current trial? The authors cite ‘vasodilatation in the cutaneous vasculature’ as ‘the primary mechanism’, but that seems implausible. Immersion in hot water increased resting femoral blood flow and calf muscle oxygenation. Better calf muscle oxygenation prior to exercise might increase energy stores and reduce muscle lactate.3 The endurance shuttle walk test is meant to be conducted at a constant speed, but perhaps patients walked faster at the onset of exercise. Perhaps heating has a systemic effect, causing venous and arteriolar relaxation thereby reducing the pre- and afterload on the failing heart. However, maintenance of core body temperature within the physiological range is essential. The skin controls heat loss in several ways: convection, conduction, radiation and evaporation (sweating). In health, skin temperature (measured by infrared thermography) gradually increases during exercise due to cutaneous vasodilatation which allows the radiation of heat generated by exercising muscles. Patients with heart failure have disordered thermoregulation,13, 14 with a decline in skin temperature during exercise, suggesting tonic vasoconstriction of skin arterioles.14 Failure to dissipate heat during exercise could lead to a potentially dangerous increase in core temperature. There is growing interest in the possible beneficial effects of warming before exercise but, so far, the studies have been small and short-term. O'Connor's findings are intriguing, but whether any acute increase in exercise performance translates into longer-term gains awaits longer and larger trials. Most research has been done in countries with a temperate climate. Will warming also help patients from hotter climes? Moreover, there are growing concerns about the effect of prolonged exposure to extreme environmental temperatures, especially amongst older patients who often have low thermal awareness and precarious renal function.15, 16 Conflict of interest: none reported.