Functional Electrical Stimulation of Lower Limb Muscles as an Alternative Mode of Exercise Training in Chronic Heart Failure: Practical Considerations and Proposed Algorithm

Conventional exercise programmes constitute a cornerstone in the management of patients with chronic heart failure (CHF).1, 2 Physical exercise has beneficial effects on endothelial dysfunction, neurohormonal activation, oxidative stress, inflammatory activation and depressive symptoms.2-6 As a result, according to a meta-analysis (ExTraMATCH),7 exercise training leads to a relative risk reduction of 23% for death or hospitalization. In addition, the randomized controlled trial HF-ACTION showed a reduction in cardiovascular mortality and hospitalization rate after adjustment for key prognostic factors.8 However, the compliance of CHF patients with training programmes is not satisfactory, even in the context of clinical trials.8 This poor adherence is related to limitations resulting from advanced disease state or coexistent comorbid conditions, along with the lack of proper motivation.9 In addition to conventional physical exercise, alternative modes of training have been studied in CHF patients.10, 11 In this context, neuromuscular or functional electrical stimulation (FES) of leg muscles offers an alternative mode of training and represents an attractive option for CHF patients who are unable, non-adherent or unwilling to exercise. Self-adhesive surface electrodes are positioned on the thighs (5 cm below the inguinal fold and 3 cm above the upper patella border) and calf muscles (2 cm below the knee joint and just over the proximal end of the Achilles tendon). The patient lies in supine position with their thighs slightly bent in a 120° angle to the torso that enables muscles to produce their maximal force. Electrical stimulation is performed through the surface electrodes. Skeletal muscular contractions are produced by the percutaneous stimulation of peripheral nerves, aiming to maintain muscle mass and strength and prevent muscle atrophy. The stimulator delivers a biphasic current of low frequency (10–50 Hz), with gradually increasing stimulation amplitude of 40–80 mA, maximized to pain threshold. A FES rehabilitation programme can be performed either at the hospital or at home, using a dual-channel battery-powered stimulator. A series of small clinical studies in CHF patients comparing FES with conventional exercise or sham stimulation showed that FES had beneficial effects on muscular strength, exercise capacity, quality of life, emotional stress and biomarkers of neurohormonal or immuno-inflammatory activation.12-17 Moreover, FES was safe and well tolerated, whilst the patient is sedentary and relaxed in the environment of his/her choice. Therefore, FES may be considered as a substitute for traditional rehabilitation programmes in CHF patients that are unable to perform conventional exercise, either owing to advanced heart failure with extremely poor cardio-respiratory fitness or to comorbidities, such as coexisting neuromuscular or skeletal muscular abnormalities. In those cases, FES may serve as an alternative rehabilitation programme. Otherwise, FES can defer muscle atrophy until the patient's limitations subside, thus acting as a ‘bridge’ to conventional exercise programmes. The initial participation in a FES protocol aims at improving muscle strength and endurance as well as exercise capacity up to a point where transition to conventional aerobic exercise may be feasible, with or without FES as an adjunct regimen. In fact, the incremental value of neuromuscular electrical stimulation combined with conventional exercise may be an interesting target for future research. It has been shown that FES enhanced maximal aerobic capacity not only in sedentary adults but also in a physically active healthy population, supporting the concept of bearing incremental value.18 For patients with advanced or end-stage CHF in particular, a recent position statement of an expert panel on quality of life clearly stressed the importance of non-pharmacological therapies, including exercise training.9 In this particular CHF population, who are unable to perform conventional exercise, FES may serve as an alternative mode of training. As previously emphasized, FES is an easily applicable, well-tolerated and safe modality in a wide range of HF patients, including elderly ones19 and those with severe symptoms New York Heart Association (NYHA) class III or IV.17 In addition, it has been shown that FES might exert a greater benefit on clinical and neurohormonal parameters in NYHA III–IV patients compared with NYHA II patients.17 However, evidence on FES in advanced or end-stage CHF is lacking. It is evident that patients able to participate in conventional rehabilitation programmes should be urged to adhere to a tailored exercise regimen. Patients' compliance and uptitration of exercise in time are of great importance in order to ensure major benefits. Evaluation of the response to exercise programmes is also of key importance. The lack of improvement in exercise capacity after a conventional training programme seems to have a strong prognostic value for adverse cardiac events, independently of classical predictive factors.20 In those ‘non-responders’ to classical exercise regimens, FES may constitute a valuable alternative. Finally, one last group worth a mention are patients who, despite being able to exercise, lack the motivation to participate in training programmes or show an extremely low compliance with them. In this context, FES has two potential advantages: the amelioration of depressive feelings that often underlie patients' lack of interest or incentive, and the enhancement of compliance with training. Taking advantage of both, this unmotivated group of patients may be offered this alternative mode of training in order, on one hand, to experience the aforementioned benefits and, on the other, to be potentially motivated to undertake conventional exercise at a later stage, with or without FES. The duration of FES training seems to correlate with the magnitude of induced adaptations, with superior results elicited by increasing hours of training. However, gradually increasing FES frequency, initially targeting muscle endurance and uptitrated to target muscular strength as well, should also be tested. In addition to escalating FES protocols, future research could further focus on identifying transition thresholds from FES to conventional exercise training, facilitating complementation or replacement of FES by traditional exercise. Recently, investigators studied FES in hospitalized heart failure patients and reported an improvement in 6-min walking distance.21 If the benefit of FES during hospitalization for heart failure is confirmed by further studies, neuromuscular stimulation may potentially be part of the therapies prescribed to patients during discharge. An algorithm for the integration of FES into the care of CHF patients is outlined in Figure 1. Patients, who are able to exercise physically, should be encouraged to follow conventional training programmes. Those patients should be monitored for compliance and response. Among them, those who fail to comply with the prescribed physical training regimen and those who show a poor response, as defined by the lack of improvement in exercise capacity, quality of life and potentially other parameters (i.e. emotional stress, endothelial function) should be considered for enrolment in a FES programme. In those patients, FES may serve either as an adjunct or a ‘bridge’ to conventional training. However, patients who are unable to exercise physically because of comorbidities or advanced or end-stage CHF should also be considered for enrolment in a FES program. In those patients, FES may serve either as an alternative mode of training when no lack of improvement in clinical status that would allow physical exercise is expected or as an adjunct or a ‘bridge’ to conventional training when improvement is expected. In conclusion, FES constitutes an alternative form of exercise for patients unable or unwilling to engage in conventional exercise programmes. It may effectively substitute, promote or complement physical training and increase adherence to rehabilitation protocols. Despite its benefits for exercise capacity and quality of life, this modality remains currently underutilized. The algorithm proposed herein (Figure 1) may allow the integration of FES into the care of CHF patients provided that some missing pieces of supporting evidence, particularly in end-stage CHF patients, will become available and that the algorithm will be validated by future research. Furthermore, studies with larger sample sizes, longer duration of follow-up and ‘harder’ clinical endpoints are required to establish the value of FES in CHF. Conflict of interest: none declared.