Evaluating neuromuscular electrical stimulation for preventing and managing intensive care unit-acquired weakness: Current evidence and future directions

Abstract

Intensive care unit-acquired weakness (ICU-AW) is a prevalent issue in critical care, leading to significant muscle atrophy and functional impairment. Aiming to address this, Neuromuscular Electrical Stimulation (NMES) has been explored as a therapy. This systematic review assesses NMES's safety and effectiveness in enhancing functional capacity and mobility in pre- and post-cardiac surgery patients. NMES was generally safe and feasible, with intervention sessions varying in frequency and duration. Improvements in muscle strength and 6-minute walking test distances were observed, particularly in preoperative settings, but postoperative benefits were inconsistent. NMES showed promise in preventing muscle loss and improving strength, although its impact on overall functional capacity remained uncertain. Challenges such as short ICU stays and body composition affecting NMES efficacy were noted. NMES also holds potential for other conditions like cerebral palsy and stroke. Further research is needed to optimize NMES protocols and better understand its full benefits in preventing ICU-AW and improving patient outcomes.

Key Words: Neuromuscular electrical stimulation; Intensive care unit-acquired weakness; Cardiac surgery; Muscle atrophy; Functional capacity

Core Tip: Muscle weakness, termed intensive care unit-acquired weakness (ICU-AW), commonly affects limb and respiratory muscles, causing severe atrophy and functional impairment. Neuromuscular Electrical Stimulation (NMES) is a promising therapy that induces muscle contractions without patient effort. While NMES is safe and feasible, its effectiveness in improving post-surgery functional capacity is limited. It shows potential in preventing neuromyopathy and enhancing muscle strength, especially when used preoperatively. NMES may also benefit conditions beyond cardiac surgery, such as cerebral palsy and stroke. Further research is needed to fully understand and optimize NMES for preventing ICU-AW and improving outcomes in critical care settings.

TO THE EDITOR

As emphasized by Kourek et al[1], to tackle the issues of diminished functional capacity and muscle function in patients recovering from cardiac surgery, and to counteract complications such as intensive care unit-acquired weakness (ICU-AW) and polyneuromyopathy, neuromuscular electrical stimulation (NMES) is advised. NMES has proven to be both safe and practical, even in high-risk patients, with a low incidence of severe complications or adverse effects. Implementing NMES as a prehabilitation strategy before cardiac surgery and continuing its use until discharge can lead to notable improvements in muscle strength, exercise tolerance, and overall mobility. This method also holds potential for reducing the length of intensive care unit (ICU) stays and minimizing complications. Nevertheless, while NMES shows considerable promise, further research with large-scale randomized controlled trials is needed to fully understand its benefits and refine its application.

WHY OPT FOR NEUROMUSCULAR ELECTRICAL STIMULATION?

Muscle weakness is a common issue in the ICU, often arising as a secondary disorder during treatment for life-threatening conditions. This condition, termed ICU-AW, primarily affects limb and respiratory muscles, with reduced muscle tone and reflexes. It can result from neurogenic or myogenic disturbances, or a combination of both known as critical illness neuromyopathy[2]. Severe muscle atrophy may occur, contributing to functional impairment. In some studies, it was shown that ICU-AW caused more than 10% loss of muscle mass, just in the first week of stay in the ICU[3]. ICU-AW stands out as the predominant neuromuscular issue encountered in patients facing critical illness in hospitals and it has been estimated to affect approximately 43% of ICU patients[4]. As this is such a common occurrence, a few therapies have emerged as possible treatments, with NMES being one of the most promising[1].

The study outlines that ICU-AW is exacerbated by prolonged ICU stays and lack of mobilization, impacting patient quality of life and mortality rates. Therapeutic strategies focus on preventing muscle atrophy and degeneration, with early mobilization showing promise but facing challenges due to patient limitations. As such, NMES has emerged as a viable alternative, promoting muscle contraction without patient effort, showing benefits in preventing neuromyopathy progression, shortening ICU stays, and improving muscle strength and functional capacity. Their systematic review aims to evaluate NMES's safety and effectiveness in improving functional capacity and mobility in pre- and post-cardiac surgery patients, with potential implications for improving patient outcomes and quality of life.

After their literature review, the study ended up looking at 10 randomized control trials with a total of 703 patients, most of them being male patients, who had undergone surgeries ranging from valve replacements to heart transplants. In these patients, NMES was consistently applied to the intervention group across all studies, with variations in intensity and session duration noted. In three studies, electrodes were placed on the control group without electricity delivery, while in seven studies, the control group received usual care post-surgery. Most studies involved at least five NMES sessions and session frequency ranged from 2 to 5 times weekly, with durations spanning from 30 to 90 minutes. Functional capacity was evaluated in nine out of ten studies on NMES. However, only two recent studies demonstrated improvement in functional capacity. One study also observed a significant increase in distance measured on the 6-Minute Walking Test (6MWT) within the NMES group as compared to the control group when it came to prehabilitation before cardiac surgery. Additionally, some studies reported significant improvements in muscle strength with NMES, while others did not find significant differences compared to controls.

Overall, NMES appears safe and feasible for pre- and post-cardiac surgery patients, potentially benefiting muscle strength to prevent ICU-AW However, its impact on functional capacity post-surgery seems limited, particularly compared to its effectiveness as prehabilitation before surgery. Muscle atrophy, a key component of ICU-AW, is influenced by various factors including age and inactivity-induced oxidative stress. NMES offers a safe alternative exercise method, potentially reducing ICU stays and improving muscle strength, particularly in the limbs, though its impact on overall functional capacity remains uncertain.

The effectiveness of NMES may be hindered by the short duration of ICU stays and limited sessions. Preoperative NMES as a form of prehabilitation shows promise in enhancing functional capacity and muscle strength, yet data on this approach are still scarce. Mechanistically, NMES activates muscle fibers directly, potentially inducing muscle growth and modulating catabolic processes. By potentially preventing ICU-AW and polyneuromyopathy, NMES could lead to better outcomes, shorter ICU stays, fewer complications, and improved exercise tolerance and mobility for cardiac surgery patients. Additionally, NMES may offer further benefits such as enhancing quality of life and improving hemodynamic and respiratory responses.

In addition to those mentioned in the paper, another limiting factor is that the review was only done on those with a body mass index (BMI) ranging from 19.3 to 29.1 kg/m². Obese patients, or patients with a BMI > 30.0 kg/m², face challenges in utilizing NMES due to their body composition[5]. The presence of excess subcutaneous fat can hinder NMES effectiveness by increasing the distance between the stimulating electrode and the axon terminals[5]. Additionally, body fat's poor conductivity means higher current intensities are needed to produce muscle contractions in obese individuals[5]. Furthermore, this heightened intensity may inadvertently activate nociceptors along with muscle fibers, which may lead to poorer outcomes[5]. Obese individuals, especially women, have also showed a lower tolerance to motor stimulation compared to non-obese individuals so this is another limiting factor to consider[6]. Furthermore, studies have shown that 4 months post-discharge from the intensive care unit, the 6MWT provided a moderately accurate prediction of long-term physical functional status after the ICU[7]. Therefore, to truly see the validity of NMES, we would need to compare the postoperative 6MWT distance with the participant’s baseline 6MWT distance.

CONCLUSION

Be that as it may, NMES has a broad range of applications. There is mounting evidence that NMES can have a beneficial effect by increasing muscle strength, range of motion, and muscle function in patients with diseases such as cerebral palsy[8] or after surgeries such as total knee arthroplasties[9]. NMES has also been shown to be an excellent treatment choice in conditions such as oropharyngeal dysphagia or even stroke[10,11]. Studies have shown that NMES can have peripheral effects of boosting muscle contractility and resilience to fatigue, as well as increasing muscle mass and reducing swelling, effectively reversing muscle atrophy due to inactivity[11,12]. Certain NMES techniques may influence the central nervous system's regulation of movement, fostering motor relearning by facilitating synchronized activity between nerve endings[11]. Moreover, studies have shown that NMES-induced isometric contractions can lead to extensive brain activation patterns, similar to those observed during voluntary movements[13]. Despite discomfort being a significant constraint on NMES efficacy[13], these findings and others like it suggest the potential of NMES for developing customized stimulation protocols to target specific or impaired cerebral brain regions in future research endeavors. However, further research, particularly larger randomized controlled trials, is needed to fully understand NMES's benefits, optimize its application, and explore its potential in preventing ICU-AW after cardiac surgery.