Fully sedated patients, being treated in the intensive care unit (ICU), experience substantial skeletal muscle loss. Consequently, survival rate is reduced and full recovery after awakening is compromised. Neuromuscular electrical stimulation (NMES) represents an effective method to stimulate muscle protein synthesis and alleviate muscle disuse atrophy in healthy subjects. We investigated the efficacy of twice-daily NMES to alleviate muscle loss in six fully sedated ICU patients admitted for acute critical illness [n=3 males, n=3 females; age 63±6 y; APACHE II (Acute Physiology and Chronic Health Evaluation II) disease-severity-score: 29±2]. One leg was subjected to twice-daily NMES of the quadriceps muscle for a period of 7±1 day whereas the other leg acted as a non-stimulated control (CON). Directly before the first and on the morning after the final NMES session, quadriceps muscle biopsies were collected from both legs to assess muscle fibre-type-specific cross-sectional area (CSA). Furthermore, phosphorylation status of the key proteins involved in the regulation of muscle protein synthesis was assessed and mRNA expression of selected genes was measured. In the CON leg, type 1 and type 2 muscle–fibre-CSA decreased by 16±9% and 24±7% respectively (P<0.05). No muscle atrophy was observed in the stimulated leg. NMES increased mammalian target of rapamycin (mTOR) phosphorylation by 19±5% when compared with baseline (P<0.05), with no changes in the CON leg. Furthermore, mRNA expression of key genes involved in muscle protein breakdown either declined [forkhead box protein O1 (FOXO1); P<0.05] or remained unchanged [muscle atrophy F-box (MAFBx) and muscle RING-finger protein-1 (MuRF1)], with no differences between the legs. In conclusion, NMES represents an effective and feasible interventional strategy to prevent skeletal muscle atrophy in critically ill comatose patients.

CLINICAL PERSPECTIVES

  • Fully sedated patients experience substantial skeletal muscle loss that reduces survival rate and compromises full recovery. We investigated the efficacy of twice-daily NMES to attenuate skeletal muscle loss in the fully sedated ICU patients admitted for acute critical illness.

  • The non-stimulated leg showed substantial type 1 and type 2 muscle fibre atrophy (a 16±9% and 24±7% decline in muscle fibre CSA respectively; P ≤ 0.05). In contrast, no atrophy was observed in the muscle fibres collected from the stimulated leg. Both mRNA and protein expression of key proteins involved in muscle protein metabolism were assessed to understand the molecular mechanisms involved.

  • In conclusion, NMES represents an effective and feasible interventional strategy to prevent skeletal muscle atrophy in critically ill comatose patients.

INTRODUCTION

Critically ill patients suffer from extensive muscle wasting, which occurs rapidly at the onset of an intensive care unit (ICU) stay [13]. Aside from an increased risk of mortality [4,5], consequences to this muscle loss include muscle weakness, prolonged mechanical ventilation, fatigue, decreases in muscle strength, impaired glucose homoeostasis and delayed recovery and rehabilitation [69]. Muscle atrophy in ICU patients exceeds that seen in normal hospitalized or bed-ridden persons [10,11]. Moreover, the ICU patients, who are mechanically ventilated and deeply sedated, are thought to be even more susceptible to muscle wasting and subsequent negative health consequences due to a complete lack of muscle contraction. Despite this, no data are currently available concerning muscle fibre atrophy in this specific ICU-patient subpopulation.

Early ambulation has been proven a successful rehabilitation strategy in non-sedated ICU patients in terms of improving functional outcomes and overall prognosis [12]. However, in fully sedated patients, early ambulation is not feasible and, as such, alternative strategies should be defined to alleviate muscle wasting. Neuromuscular electrical stimulation (NMES) is an effective means to invoke involuntary muscle contractions. Previously, NMES has been shown to attenuate the loss of muscle mass and strength experienced by non-sedated ICU patients [13] and healthy individuals subjected to limb immobilization [14]. However, the potential for NMES to rescue muscle mass in fully sedated comatose ICU patients has not been investigated. In the present study, we tested our hypothesis that daily NMES attenuates skeletal muscle fibre atrophy in fully sedated comatose ICU patients. Fully sedated ICU patients, expected to be sedated for a minimum of 3 days, were included in the present study. NMES was performed twice-daily on the quadriceps of one leg, whereas the other leg served as a sham-treated control (CON). Prior to and immediately after the intervention, plasma samples were taken to assess any systemic changes in amino acid availability during the experiment and muscle biopsies were taken from both legs to assess muscle fibre atrophy and myocellular characteristics. Additionally, real-time-PCR and Western blotting were performed on collected muscle tissue samples to assess the potential affect of NMES on basal mRNA and protein expression levels of the key genes involved in the regulation of muscle mass maintenance.

MATERIALS AND METHODS

Patients

All patients admitted to the ICU of Jessa Hospital, Hasselt, Belgium between March 2012 and July 2013 were assessed for eligibility for the present study (Supplementary Figure S1). First, patients admitted to the ICU were screened by the nursing staff and were excluded if one or more of the following exclusion criteria were met: <18 or >80 years of age, not expected to undergo complete sedation, suffering from spinal cord injury, recent arterial surgery on the legs, local wounds that prohibit the application of NMES, chronic use of corticosteroids, intake of certain anti-thrombotic drugs or the presence of an implantable cardioverter-defibrillator (ICD) and/or pacemaker. Secondly, the expected sedation time was estimated by the responsible physician and patients were excluded if this was <3 days. All patients who were excluded based on an expected short sedation time were re-evaluated after 24 h and included if the revised expected sedation time was >3 days. Participants were accepted into the study after written informed consent was obtained from their legal representatives. The study was approved by the Medical Ethical Committee of the Jessa Hospital in accordance with the Declaration of Helsinki. Clinical trial registration was NCT01521637.

Study design

An overview of the experimental protocol is depicted in Supplementary Figure S2. Patients were included in the study directly after informed consent was obtained from their legal representatives, which was generally given within 2.5 days after admission to the ICU (depicted in column ‘Time to inclusion’ in Table 1). After this, patient's legs were randomly assigned as either the CON or the stimulated (NMES) leg, counter-balanced for left and right legs. Randomization was performed by an independent investigator and treatment allocation was performed by using sequentially labelled envelopes, which were opened after the inclusion of subjects. Baseline measurements were then taken, which consisted of assessment of leg circumference (measured at different locations on the upper leg), obtaining an arterial blood sample and obtaining a muscle biopsy from both legs. After the pre-measurements, NMES was performed twice-daily on one leg (NMES), whereas the other leg served as a CON. Post-measurements were performed on the final day of sedation, with a minimum study duration of 3 days and a maximum of 10 days. The study duration for each patient is depicted in Table 1. Post-measurements were performed prior to the subjects being awake. Standard medical care was not altered and passive mobilization was performed on both legs according to the standard care procedures.

Table 1
Patients with critical illness

Fully sedated patients were subjected to one-legged NMES for a period of 3–10 days, whereas the other leg served as non-stimulated CON. Pre-intervention muscle measurements were performed immediately after obtaining informed consent. Post-intervention measurements were performed prior to awakening after a minimum of 3 and a maximum of 10 days. CABG, coronary artery bypass graft; D, death; HES, hydroxyethyl starch; S, survival; M, male; F, female.

Diagnosis Age Sex APACHE II at admission Time to inclusion (day) Days in study Survival Medication 
Urgent CABG 78 32 3.5 3.5 Acetylcysteine, acetylsalicylic acid, alprazolam, amiodarone, amoxicillin, bisoprolol, ciprofloxacine, furosemide, haloperidol, hydrocortisone, insulin, ipratropium bromide/fenoterol hydrobromide, isosorbide mononitrate, midazolam, milrinone, molsidomine, morphine, nadroparin, norepinephrine pantoprazole, piracetam, ramipril, spironolacton 
Pneumonia 74 26 2.5 7.5 Acetylsalicylic acid, ceftazidime, ciproflocacine, erythromycin, methylprednisolone, nadroparin, pantoprazole, ranitidine 
Herpetic encephalitis 39 25 Acetylcysteine, acyclovir, amoxicillin, diazepam, furosemide, HES, levetiracetam, midazolam, nadroparin, norepinephrine, paracetamol, phenytoin, propofol, thiopental, valproate 
Cerebral haemorrhage 46 29 7.5 Aacidexam, acetylcysteine, bumetanide, ciprofloxacine, clindamycin, fluconazole, furosemide, insulin, ipratropium bromide/fenoterol hydrobromide, lactulose, meropenem, methylprednisolone, nadroparin, norepinephrine, nystatin, pantoprazole, paracetamol, piritramide, propofol, spironolacton, valproate 
Cerebral haemorrhage 78 29 4.5 7.5 Aacidexam, acetylcysteine, amoxicillin, ciprofloxacine, dobutamine hydrochloride, furosemide, insulin, lactulose, metoclopramide, midazolam, nebivolol, nimodipine, norepinephrine, pantoprazole, paracetamol, piritramide, pravastatin, propofol, spironolactone, timolol 
Cerebral haemorrhage 65 35 2.5 Aacidexam, amlodipine, amoxicillin, bisoprolol, cefuroxime, ciprofloxacine, dobutamine hydrochloride, furosemide, hydrocortisone, insulin, metoclopramide, midazolam, norepinephrine, pantoprazole, paracetamol, piritramide, propofol 
Diagnosis Age Sex APACHE II at admission Time to inclusion (day) Days in study Survival Medication 
Urgent CABG 78 32 3.5 3.5 Acetylcysteine, acetylsalicylic acid, alprazolam, amiodarone, amoxicillin, bisoprolol, ciprofloxacine, furosemide, haloperidol, hydrocortisone, insulin, ipratropium bromide/fenoterol hydrobromide, isosorbide mononitrate, midazolam, milrinone, molsidomine, morphine, nadroparin, norepinephrine pantoprazole, piracetam, ramipril, spironolacton 
Pneumonia 74 26 2.5 7.5 Acetylsalicylic acid, ceftazidime, ciproflocacine, erythromycin, methylprednisolone, nadroparin, pantoprazole, ranitidine 
Herpetic encephalitis 39 25 Acetylcysteine, acyclovir, amoxicillin, diazepam, furosemide, HES, levetiracetam, midazolam, nadroparin, norepinephrine, paracetamol, phenytoin, propofol, thiopental, valproate 
Cerebral haemorrhage 46 29 7.5 Aacidexam, acetylcysteine, bumetanide, ciprofloxacine, clindamycin, fluconazole, furosemide, insulin, ipratropium bromide/fenoterol hydrobromide, lactulose, meropenem, methylprednisolone, nadroparin, norepinephrine, nystatin, pantoprazole, paracetamol, piritramide, propofol, spironolacton, valproate 
Cerebral haemorrhage 78 29 4.5 7.5 Aacidexam, acetylcysteine, amoxicillin, ciprofloxacine, dobutamine hydrochloride, furosemide, insulin, lactulose, metoclopramide, midazolam, nebivolol, nimodipine, norepinephrine, pantoprazole, paracetamol, piritramide, pravastatin, propofol, spironolactone, timolol 
Cerebral haemorrhage 65 35 2.5 Aacidexam, amlodipine, amoxicillin, bisoprolol, cefuroxime, ciprofloxacine, dobutamine hydrochloride, furosemide, hydrocortisone, insulin, metoclopramide, midazolam, norepinephrine, pantoprazole, paracetamol, piritramide, propofol 

Data collection

At baseline, data on demographic and clinical characteristics of the patients were obtained, including information necessary to determine the severity of illness. These data were scored according to the Acute Physiology and Chronic Health Evaluation II (APACHE II) system with higher values indicating more severe illness and more therapeutic interventions respectively [15].

Arterial blood samples were collected from the catheter already placed in the arteria radialis. Blood (10 ml) was collected into EDTA-containing tubes and immediately centrifuged at 1000 g for 10 min at 4°C. Aliquots of plasma were directly snap-frozen in liquid nitrogen and stored at −80°C until further analysis. Processing and storage of the samples was done by UBiLim (Universitaire Biobank Limburg). Plasma amino acid concentrations were measured using ultra-performance liquid chromatography tandem mass spectrometry, as described previously [16], and results are displayed in Supplementary Table S2.

In addition, during the pre- and post-measurements, a muscle biopsy sample was collected from each leg. After an injection of local anaesthesia, percutaneous needle biopsy samples were collected from musculus vastus lateralis, approximately 15 cm above the patella using the Bergström technique [17].

Neuromuscular electrical stimulation

NMES sessions were performed both in the morning (11:00 h) and in the afternoon (16:30 h). Four self-adhesive electrodes (2 mm thick, 50 × 50 mm) were placed on the distal part at the muscle belly of the musculus rectus femoris and the musculus vastus lateralis and at the inguinal area of both muscles. The electrodes were connected to an Enraf-Nonius TensMed S84 stimulation device (Enraf-Nonius), discharging biphasic symmetric rectangular-wave pulses. The position of the electrodes was re-marked daily with a semi-permanent marker to maintain the same location of stimulation for each session. The NMES protocol was composed of a warm-up phase (5 min, 5 Hz, 250 μs), a stimulation period [30 min, 100 Hz, 400 μs, 5 s on (0.75 s rise, 3.5 s contraction, 0.75 s fall) and 10 s off] and a cooling-down phase (5 min, 5 Hz, 250 μs). The intensity of the stimulation was set to a level at which full contractions of musculus quadriceps femoris were both visible and palpable. The intensity was raised approximately every 3 min when a full muscle contraction was no longer achieved with the current intensity. This protocol was based on our previous work showing increased rates of muscle protein synthesis after a single bout of NMES [18] and applied on the immobilized leg of healthy young adults [14]. During the NMES sessions, four electrodes and compatible cables were also applied to the CON leg to standardize all procedures (representing a sham treatment).

Dietary intake

When patients were haemodynamically stable, enteral feeding was started according to routine guidelines of the ICU at Jessa Hospital, as early as possible. Patients were fed Nutrison Multi Fibre (containing 420 kJ, 16 en% protein, 49 en% carbohydrates and 35 en% fat per 100 ml). Generally, patients were fed maximally 80 ml/h with short intervals during which nutritional supply was paused. Gastric emptying was determined by the nursing staff and food administration was altered accordingly. Nutritional support was not modulated and was applied according to the standard medical care in this ICU.

Muscle analyses

Muscle samples were freed from any visible non-muscle tissue and separated into different sections. The first part (~30 mg) was imbedded in Tissue-Tek (Sakura Finetek), frozen on liquid nitrogen cooled isopentane and used to determine muscle-fibre-type-specific cross-sectional area (CSA) and satellite cell content, as done previously [19]. The second part (~15 mg) was snap-frozen in liquid nitrogen and used for real time-PCR analysis to determine the mRNA expression of selected genes, as described before [14,20], and compared with the mRNA expression of n=6 healthy, age- and gender-matched CONs. The third part (~40 mg) was snap-frozen in liquid nitrogen for Western Blot analysis to determine the total content and phosphorylation status of several key proteins of interest, as described previously [18]. All muscle analyses were performed by an investigator blinded to treatment. A detailed overview of the muscle analyses is presented in the Supplementary material.

Statistics

Based on data from previous studies in the healthy subjects in our laboratory [14,21], we calculated that eight patients would be required to detect an 8% difference in muscle fibre CSA between CON and NMES over 7 days (using an α level of 0.05 and a β level of 0.10). All data presented are expressed as means±S.E.M. Baseline differences between legs were compared with a paired-samples Student's t test. Pre- and post-intervention data were analysed using repeated measures ANOVA with time (pre compared with post) and treatment (CON compared with NMES) as factors. Fibre type (type 1 compared with type 2) was added as a third within-subjects factor when analysing all muscle fibre characteristics. In case of significant interaction (time × treatment), paired-samples Student's t tests were performed to determine time-effects within the CON and the NMES legs separately. Alternatively, when a time × treatment effect was observed for muscle fibre characteristics, a two-way ANOVA was performed for the CON and the NMES leg separately, with time and treatment as factors. For the mRNA analyses, differences between patients and healthy CONs were tested by means of an independent samples Student's t test between the mean value of the CON and the NMES leg in the patients and the values observed in the healthy CONs. Statistical analyses were performed using the SPSS version 20.0 software package (SPSS), with P<0.05 as the value for statistical significance.

RESULTS

Patients

Between March 2012 and July 2013, nine patients were included in the present study. Two patients awoke after <3 study days and one patient died. Therefore, the presented results represent data collected from six patients. Clinical characteristics of the included patients are listed in Table 1. Energy intake per day averaged 5.31±0.56 MJ, with a mean protein intake of 0.56±0.06 g·(kg of body weight)−1·day−1.

Neuromuscular electrical stimulation

Within 5 min of the start of the actual 30 min stimulation period, a full muscle contraction was achieved. The intensity of the NMES intervention for subjects averaged 29.9 mA during the first session and was progressively increased to 32.3 mA in the final session.

Muscle fibre characteristics

Figure 1 illustrates the Δ-change in muscle fibre CSA in both the NMES and the CON legs throughout the study. Table 2 details skeletal-muscle-fibre-type-specific characteristics at baseline and following 7±1 day of full sedation in both legs. In the CON leg, a significant decline of 16±9% and 24±7% was observed in type 1 and 2 muscle fibre CSA respectively (time effect; P<0.05). In contrast, the NMES leg showed no atrophy in either type 1 or 2 muscle fibres (time × treatment interaction effect; P<0.05). Muscle fibre type distribution showed an overall significant time × treatment interaction-effect (Table 2; P<0.05), with a shift from type 1 towards type 2 fibres in the CON leg and a shift towards more type 1 fibres in the NMES leg. At baseline, satellite cell content was greater in type 1 compared with type 2 muscle fibres (expressed per muscle fibre, per mm2 and as a percentage of total myonuclei). No differences in muscle-fibre-type-specific myonuclear content, myonuclear domain size or satellite cell content were observed between the legs or over time.

Changes in muscle fibre CSA in the CON and stimulated (NMES) leg of sedated patients, after 7±1 days of twice-daily NMES

Figure 1
Changes in muscle fibre CSA in the CON and stimulated (NMES) leg of sedated patients, after 7±1 days of twice-daily NMES

A significant interaction effect (P<0.05) was observed and a time- effect in the CON leg (P<0.05). *Significantly different change from zero (P<0.05).

Figure 1
Changes in muscle fibre CSA in the CON and stimulated (NMES) leg of sedated patients, after 7±1 days of twice-daily NMES

A significant interaction effect (P<0.05) was observed and a time- effect in the CON leg (P<0.05). *Significantly different change from zero (P<0.05).

Table 2
Muscle fibre characteristics

Data represent means±S.E.M. Abbreviations: SC, satellite cell; SCs/myonuclei (%), the number of SCs as a percentage of the total number of myonuclei (i.e. number of myonuclei + number of SCs). *Significantly different from pre-intervention value (P<0.05). #Significantly different from type 1 fibre value (P<0.05). †Significant treatment × time × fibre type interaction effect (P<0.05).

  CON NMES 
Parameter Fibre type Pre Post Pre Post 
Muscle fibre CSA (μm24560±261 3879±484* 4414±441 4512±550 
 3412±530 2647±512* 3168±607 3246±590 
% Fibre 53±8† 45±5† 42±6† 46±6† 
 47±8† 55±5† 58±6† 54±6† 
% Fibre area 59±9 56±8 51±8 55±7 
 41±9 44±8 49±8 45±7 
Nuclei/fibre 2.4±0.1 2.5±0.2 2.3±0.2 2.6±0.2 
 2.1±0.2# 2.1±0.2# 1.9±0.2# 2.3±0.3# 
Myonuclear domain (μm21853±69 1574±183 1931±110 1760±142 
 1573±162# 1255±203# 1618±153# 1452±220# 
Number of SCs per fibre 0.083±0.014 0.085±0.012 0.075±0.006 0.092±0.006 
 0.061±0.015# 0.049±0.013# 0.048±0.008# 0.055±0.009# 
Number of SCs/mm2 18.8±2.9 22.7±2.7 17.6±1.7 22.1±4.1 
 18.6±2.9# 17.8±2.0# 17.0±3.9# 19.8±4.6# 
SCs/myonuclei (%) 3.4±0.5 3.5±0.4 3.4±0.4 3.7±0.4 
 2.7±0.4# 2.3±0.5# 2.5±0.3# 2.5±0.3# 
  CON NMES 
Parameter Fibre type Pre Post Pre Post 
Muscle fibre CSA (μm24560±261 3879±484* 4414±441 4512±550 
 3412±530 2647±512* 3168±607 3246±590 
% Fibre 53±8† 45±5† 42±6† 46±6† 
 47±8† 55±5† 58±6† 54±6† 
% Fibre area 59±9 56±8 51±8 55±7 
 41±9 44±8 49±8 45±7 
Nuclei/fibre 2.4±0.1 2.5±0.2 2.3±0.2 2.6±0.2 
 2.1±0.2# 2.1±0.2# 1.9±0.2# 2.3±0.3# 
Myonuclear domain (μm21853±69 1574±183 1931±110 1760±142 
 1573±162# 1255±203# 1618±153# 1452±220# 
Number of SCs per fibre 0.083±0.014 0.085±0.012 0.075±0.006 0.092±0.006 
 0.061±0.015# 0.049±0.013# 0.048±0.008# 0.055±0.009# 
Number of SCs/mm2 18.8±2.9 22.7±2.7 17.6±1.7 22.1±4.1 
 18.6±2.9# 17.8±2.0# 17.0±3.9# 19.8±4.6# 
SCs/myonuclei (%) 3.4±0.5 3.5±0.4 3.4±0.4 3.7±0.4 
 2.7±0.4# 2.3±0.5# 2.5±0.3# 2.5±0.3# 

mRNA expression

Figure 2 displays the relative muscle mRNA expression of the key genes involved in the regulation of muscle protein synthesis and breakdown in the CON and the NMES leg before and after the intervention, as well as for a group of healthy, age- and gender-matched CONs. At baseline, mRNA expression did not differ between the NMES and the CON legs. However, MAFBx (muscle atrophy F-box), MuRF1 (muscle RING-finger protein-1), FOXO1 (forkhead box protein O1), mTOR (mammalian target of rapamycin) and p70S6K (p70S6 kinase) were all more highly expressed in the patients compared with the healthy CONs (P<0.01). There was a significant time effect (P<0.05) such that FOXO1 and p70S6K expression decreased during the period of sedation, with no differences between the legs. Expression levels for all other genes did not reveal any interaction or time effects. The mRNA expression of additional genes involved in the regulation of myogenesis, oxidative metabolism, mechano-sensing and cellular amino acid transport are presented in Supplementary Figure S3.

Skeletal muscle mRNA expression of genes of interest

Figure 2
Skeletal muscle mRNA expression of genes of interest

*Significantly different from patients at baseline (P<0.05). #Significantly different from pre-value (P<0.05).

Figure 2
Skeletal muscle mRNA expression of genes of interest

*Significantly different from patients at baseline (P<0.05). #Significantly different from pre-value (P<0.05).

Signalling proteins

The skeletal muscle content and phosphorylation status of key proteins involved in the regulation of muscle protein synthesis are displayed in Figure 3. Neither total protein content nor phosphorylation status of Akt was affected by time or intervention (both P>0.05). Although muscle mTOR content was unaffected by time or treatment, a significant time × treatment interaction effect (P<0.05) was found for the phosphorylation status of mTOR. mTOR phosphorylation increased by as much as 19±5% in the NMES leg (P<0.05), with no changes in the CON leg (P>0.05). Muscle p70S6K total protein content decreased following the intervention in both legs (time effect, P<0.05), without changes in phosphorylation status (P>0.05).

Skeletal muscle protein expression of Akt, mTOR and p70S6K in the CON and stimulated (NMES) leg, before (white bars) and after (black bars) 7±1 days of twice-daily NMES

Figure 3
Skeletal muscle protein expression of Akt, mTOR and p70S6K in the CON and stimulated (NMES) leg, before (white bars) and after (black bars) 7±1 days of twice-daily NMES

Left graphs: total protein expression, right graphs: phosphorylated/total expression. *Significantly different from pre-intervention values (P<0.05).

Figure 3
Skeletal muscle protein expression of Akt, mTOR and p70S6K in the CON and stimulated (NMES) leg, before (white bars) and after (black bars) 7±1 days of twice-daily NMES

Left graphs: total protein expression, right graphs: phosphorylated/total expression. *Significantly different from pre-intervention values (P<0.05).

DISCUSSION

In the present study, we demonstrate for the first time that fully sedated patients experience substantial type 1 and type 2 muscle fibre atrophy during a ~7 day stay in the ICU. Daily application of NMES effectively prevents skeletal muscle fibre atrophy, offering an effective and feasible interventional strategy to alleviate muscle wasting in comatose ICU patients.

General admission to the ICU has been shown to cause substantial muscle wasting [22] with a decline in type 1 and type 2 muscle fibre CSA of 3% and 4% per day respectively [2]. In keeping with this, we show a 2.8% and 4.4% decline in the muscle fibre size in type 1 and 2 muscle fibres respectively, in the fully sedated patients (i.e. no possibility of voluntary muscle contraction) during an average 7 days in the ICU (Figure 1). By way of comparison, muscle atrophy brought about by disuse only in healthy humans (i.e. limb immobilization) leads to a 0.5% and 0.9% per day decline in type 1 and 2 muscle fibre CSA respectively [21]. This implies that the mechanisms responsible for muscle wasting in the ICU are not simply attributed to disuse. One possible contributing factor could be inadequate nutritional status. Sufficient dietary protein is considered a key factor in the maintenance of muscle mass [2325] and previous research has shown that sufficient protein intake is associated with reduced mortality rates in critically ill patients [26,27]. In the current study, the patients received 0.56±0.06 g of protein·(kg of body weight)−1·day−1, which is below the current guidelines of 1.3–2.0 g of protein·(kg of body weight)−1·day−1 recommended during critical illness [28,29] and has probably contributed to the extensive level of muscle wasting. In support, plasma amino acid concentrations in our patients declined throughout the sedated state (Supplementary Table S2). In agreement, previous work has reported declines in circulating amino acid concentrations during critical illness [30]. Such a decline in circulating amino acid concentrations probably reduces amino acid uptake in muscle [31] and, as such, could modulate the efficacy of NMES as a means to stimulate muscle protein synthesis rates.

From a mechanistic viewpoint, disuse atrophy has been primarily attributed to declines in muscle protein synthesis rates [20,3234]. However, it has been suggested that in various conditions associated with rapid muscle wasting, a multitude of other factors (e.g. increased inflammation, higher metabolic stress responses etc.) may stimulate muscle proteolysis, driving much of the muscle loss [35]. In line with this, we see evidence of the severely metabolically compromised condition of our patients as demonstrated by numerous clinical chemistry indictors obtained throughout the study [e.g. high white blood cell counts and C-reactive protein (CRP) concentrations; Supplementary Table S1]. In keeping with this, molecular markers that have been used as a proxy for changes in the muscle protein breakdown rate were elevated upon admission to the ICU when compared with a group of healthy subjects (i.e. MAFBx, MuRF1 and FOXO1; Figure 2). The subsequent decline in the expression levels of these genes suggest a decline in the muscle protein turnover during hospital stay but expression levels remained elevated when compared with the healthy CONs. This is not unexpected given the metabolic stress response upon ICU admission [36]. In contrast with previous work investigating the effect of NMES on an immobilized leg [14], we observed no significant differences in the expression levels of various genes between the stimulated and unstimulated leg in this comatose ICU setting. The absence of such differences may be attributed to various factors, but underline our understanding that changes in the expression and phosphorylation levels of various genes being used as a proxy for changes in muscle protein breakdown and synthesis do not necessarily represent changes in muscle protein breakdown and synthesis rates and do not necessarily translate to a net increase or decrease in the muscle mass [37]. Taken together, the present data highlight the need for immediate and effective intervention at the onset of ICU admission to stimulate muscle protein synthesis and inhibit proteolysis, thereby preventing or attenuating extensive muscle wasting. An interesting observation in the stimulated leg was that NMES reversed the decline in phosphorylation status of mTOR (Figure 3D), which seems to be in line with previous work showing that NMES increases muscle protein synthesis rates [18].

Daily application of NMES has been shown to prevent muscle atrophy in healthy subjects during a week of leg immobilization [14]. Moreover, clinical trials have demonstrated beneficial effects of NMES on muscle function in various bed-rested populations, including patients suffering from chronic obstructive pulmonary disease (COPD) [38,39] and sepsis [40,41]. The current study demonstrates, for the first time, that NMES is capable of preventing muscle wasting in fully sedated patients during 7 days in the ICU (with a +7±12% change in mixed muscle fibre CSA in the stimulated leg compared with a −21±8% decline in mixed muscle fibre CSA in the CON leg; Figure 1). The prevention of muscle atrophy in these individuals can have profound clinical implications. For instance, maintaining muscle mass during critical illness has been shown to reduce mortality rates [4,5]. Additionally, since muscle mass is vital for functional capacity [42], metabolic homoeostasis [9] and immune function [43], maintaining muscle mass during an ICU stay is essential to allow proper recovery during rehabilitation. As such, preventing muscle wasting is imperative for promoting quality of life after a hospital discharge and reducing the likelihood of re-hospitalization. NMES in fully sedated patients can be easily applied by nursing staff, is relatively cheap and does not seem to cause any adverse effects on vital parameters during or after the sessions [44]. Some difficulties applying NMES in the ICU patients have been reported previously and are probably due to increased skin/soft tissue impedance and/or oedema [13]. Despite experiencing similar problems in the present study, all NMES sessions could be successfully performed without any adverse effects. Taken together, our data demonstrate that NMES is practical and feasible as a counter-measure for muscle wasting in clinically compromised ICU patients. Future studies should address whether these findings would translate into longer-term benefits such as increased survival rates, reduced hospitalization length of stay and/or improved rehabilitation outcomes.

Conclusions

NMES represents an effective and feasible interventional strategy to prevent skeletal muscle wasting in critically ill comatose patients. NMES may be applied effectively to offset negative consequences of muscle wasting and, as such, may increase survival and improve subsequent rehabilitation in these patients.

Abbreviations

     
  • APACHE II

    Acute Physiology and Chronic Health Evaluation II

  •  
  • CON

    control

  •  
  • CSA

    cross-sectional area

  •  
  • FOXO1

    forkhead box protein O1

  •  
  • ICU

    intensive care unit

  •  
  • MAFBx

    muscle atrophy F-box

  •  
  • mTOR

    mammalian target of rapamycin

  •  
  • MuRF1

    muscle RING-finger protein-1

  •  
  • NMES

    neuromuscular electrical stimulation

  •  
  • p70S6K

    p70S6 kinase

AUTHOR CONTRIBUTION

Study concept and design: Marlou Dirks, Dominique Hansen, Aimé van Assche, Paul Dendale and Luc van Loon. Acquisition of data: Marlou Dirks and Dominique Hansen. Analysis and interpretation of the data: Marlou Dirks, Dominique Hansen, Paul Dendale and Luc van Loon. Drafting of the manuscript: Marlou Dirks. Critical revision of the manuscript for important intellectual content: Dominique Hansen, Aimé van Assche, Paul Dendale and Luc van Loon. Study supervision: Aimé van Assche, Paul Dendale and Luc van Loon. Marlou Dirks had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data. None of the authors disclose any conflicts of interest.

We gratefully acknowledge the enthusiasm and assistance of the physicians and nursing staff of the ICU in Jessa Hospital, with special thanks to Dr P. Vranckx for his practical assistance. We would also like to thank Marika Leenders and Lex B. Verdijk for their practical support and Benjamin T. Wall for his assistance in drafting the manuscript. Furthermore, we are thankful for the assistance of Dr E. Bijnens, Department of Radiology at Jessa Hospital and for the support and assistance provided by Biobank UbiLim at Jessa Hospital for processing and storage of the samples.

FUNDING

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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Supplementary data