Interval Training and Hormones in Chronic Heart Failure

According to European Society of Cardiology the prevalence of Heart Failure (HF) is between 2 and 3% in general population and increases with age, so the prevalence in 70- to 80-years-old people is between 10 and 20%. HF is the cause of 5% of acute hospital admissions, is present in 10% of patients in hospital beds, and accounts for high national expenditure on health, mostly due to the cost of hospital admissions. Although some patients can live for many years and the great improvement of medical therapy during last decades, overall 50% of patients are dead at 4 years.

Despite different hypothesis that explain the underlying physiopathology of heart failure have been proposed over time, no single paradigm for heart failure was established definitively. One logical explanation of the inability to define the syndrome of heart failure in precise mechanistic model is that the clinical syndrome of heart failure almost certainly represents the summation of multiple anatomic, functional, and biological alterations that interact together in a complex way. Thus, it is not surprising that investigators have used a variety of complex model in an attempt to describe the syndrome of heart failure. Nowadays, the most accepted hypothesis explaining HF physiopathology and its progression is the "neurohormonal model". According to this paradigm, heart failure progresses as a result of the overexpression of biologically active molecules that exert toxic effects on the heart and circulation. A variety of molecules including norepinephrine, angiotensin II, endothelin, aldosterone, and tumor necrosis factor have been implicated as some of the factors that contribute to disease progression in the failing heart.

Despite the effectiveness of the neurohormonal model to explain disease progression and the many insights that it provided for the development of new therapies, there is increasing clinical evidence that suggests that our current models fail to completely explain disease progression. Thus, neurohormonal models may be necessary but not sufficient to explain all aspects of disease progression in the failing heart.

Because the prognosis of HF patients is still unsatisfactory despite optimal therapies, other mechanisms that contribute to HF progression need to be elucidated. Mounting evidence suggest that in heart failure there is a metabolic imbalance characterized by a predominance of the catabolic status over anabolic drive. The most impressive example is seen in end-stage HF known as "cardiac cachexia" characterized by strong weight loss, particularly lean mass and rapid deterioration of clinical conditions, attributed to a prevalence of catabolic pathways. If the hormonal imbalance is an epiphenomenon or an important pathophysiological mechanism in the HF progression is still matter of debate.

In particular deficit of each anabolic axis (adrenal, gonadal and somatotropic axes) is an independent marker of poor prognosis in HF patients and the coexistence of more than one deficiency identifies a subgroup of patients with a higher mortality.

The most involved hormonal axes include growth hormone (GH), its tissue effector insulin-like growth factor-1 (IGF-1), thyroid hormone, and anabolic steroids. Taken together, these alterations could be recognized as a multiple hormonal and metabolic deficiency syndrome (MHD) in HF patients. MHD has a significant impact on cardiac performance and HF progression.

The most involved hormonal axes include growth hormone (GH), its tissue effector insulin-like growth factor-1 (IGF-1), thyroid hormone, and anabolic steroids. Taken together, these alterations could be recognized as a multiple hormonal and metabolic deficiency syndrome (MHD) in HF patients. MHD has a significant impact on cardiac performance and HF progression.

Moreover, a pattern of Insulin-resistance (IR) is quite common in diabetic as well as non-diabetic CHF patients. IR has been found in about 30% of non-diabetic CHF patients and was related to underlying disease severity. Few studies have considered reduction of IR as a new therapeutic target.

In brief, it could be argued that CHF patients showed an anabolic/catabolic imbalance due to multiple neurohormonal axis disequilibrium. Anabolic hormonal deficiency is usually described in men with chronic heart failure (CHF) contributing to the anabolic/catabolic imbalance ultimately resulting in skeletal muscle waist and cardiac cachexia. Counteracting the anabolic deficit seems to play beneficial clinical effects in CHF patients. In fact, the increase of serum levels of testosterone and growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis obtained by hexogen administration, improves key symptoms of CHF such as exercise intolerance and muscle fatigue and positively impact quality of life. Besides the hexogen administration, an increase of the levels of anabolic hormones can be obtained through physical exercise. In healthy subjects, testosterone may increase remarkably as an acute response to both endurance and heavy resistance exercise. Similarly, GH concentrations generally increase in response to both strength and endurance exercise thereby stimulating IGF-1 production. Although this hormonal modulation could be one of the mechanisms by which exercise training exerts its beneficial effects on CHF patients, there are few data on endogenous exercise-induced increase of anabolic hormones in such patients.

Aside from the nature of the training activity, the effects of training may vary with different dose parameters, specifically program length, session duration and frequency and workload or intensity. In the most severely impaired patients, with initial exercise intolerance, sessions may initially be limited to 3-5 minutes duration with 3 or 4 sessions completed during the course of the day; however, recent work has suggested that if total exercise energy expenditure is standardized then intermittent exercise training programs may elicit superior benefits to heart failure patients compared to continuous exercise training sessions.

High intensity, repeated intermittent work periods separated by recovery periods have been shown to be efficacious in heart failure patients, and interval stress has been shown to be as effective as continuous workloads in older, healthy and post coronary artery bypass surgery populations. In a systematic review of 81 heart failure ExT studies only two of these reported peak VO2 changes of 10% and 20% respectively compared with 16.5% overall change in continuous exercise training and similar improvements with strength training. The underlying theory is that higher intensity, intermittent stress is more likely to promote peripheral adaptations and produce concurrent improvements in functional capacity. Recent work has shown that reductions of brain natriuretic peptide, a marker of myocardial stretch, may be greater in high intensity (90% peak VO2), rather than moderate intensity (70% peak VO2) exercise training in patients with severe left ventricular dysfunction. A recent meta-analysis showed that intermittent exercise elicits superior improvements in peak VO2 and VE/VCO2 slope compared to continuous exercise training in heart failure patients.

Few studies evaluated the hormonal response to interval training in CHF. However, the relatively small sample size, the lack of control group or the relatively short time exercise intervention limits the conclusions.

The present study aims at investigating whether 24-week high intensity interval training might exert beneficial effects by modulating neurohormonal axis in CHF patients. Furthermore, the effect of detraining on neurohormonal axis in CHF patients will be evaluated.