Study design and population
The prospective, randomized, controlled, single-blind trial Catastim 2 was carried out at the Division of Cardiothoracic and Vascular Anesthesia and Intensive Care Medicine of the General Hospital affiliated to the Medical University of Vienna from May 2011 to July 2012 (clinicaltrials.gov identifier: NCT02391103). Patients were blinded to the NMES/sham stimulation. Nonblinded assessors performed the ultrasound scans; however, the evaluators measuring MLT were blinded to patients’ group assignment. The assessors of muscle strength and the secondary outcomes were not blinded to patients’ group assignment. Ethical approval was obtained from the ethics committee of the Medical University of Vienna in 2010 (number 1072/2010). Patients were included if they underwent cardiothoracic surgery and were anticipated to stay in the ICU for at least 48 hours. Patients were excluded if they were younger than 18 years, had a body mass index of more than 40 kg/m2, had metal implants or skin lesions in the stimulation area, had neuromuscular diseases, had an implanted ventricular assist device or intra-aortic balloon pump. After submission of a protocol amendment in June 2011, a twofold study setting was defined in order to recruit patients not only before, but also after cardiothoracic surgery. In cohort A, patients were recruited before surgery. In cohort B, patients were recruited after surgery. Recruitment of patients after surgery primarily allowed inclusion of patients after urgent nonelective surgeries such as heart transplantations, who could not be seen preoperatively. Written informed consent was obtained from patients in cohort A. Patients in cohort B were included in the trial while sedated to be later informed of their participation. The inclusion of sedated patients in cohort B without providing written informed consent was approved by the ethics committee of the Medical University of Vienna.
Randomization
For all patients, randomization was performed on postoperative day 1. Patients in cohort A and B were separately randomized using a web-based randomizer for clinical trials [26]. In the previous Catastim 1 trial taking place in the same ICU [24], the NMES and control groups had slightly different Simplified Acute Physiology Score II (SAPS II) scores. Hence, in the Catastim 2 trial, randomization was stratified by the SAPS II score on the first postoperative day to ensure balance of the NMES and control groups with respect to disease severity. The four strata were deduced from the quartiles of SAPS II scores in a retrospective analysis of 2708 patients of the same ICU.
Intervention
In the intervention group, all parts of the quadriceps muscle (rectus femoris muscle, vastus intermedius muscle, vastus lateralis muscle, vastus medialis muscle) of both thighs were electrically stimulated. The muscles were stimulated twice a day (2 × 30 minutes of NMES with an interval of at least 30 minutes between both sessions [24]) 7 days a week during the entire ICU stay but no longer than 14 days, starting on postoperative day 1. Highest tolerable intensity just below the pain threshold was applied. A visible and palpable contraction was the goal to achieve. The Compex 3 Professional (CefarCompex Medical AB, Freiburg, Germany) stimulator delivered biphasic rectangular pulses of 0.4 ms pulse duration at 66 Hz [19]. The duty cycle was 3.5 s on and 4.5 s off, ramping up and down were both set at 0.5 s [24]. Dual Snap (5 × 10 cm and 5 × 5 cm) electrodes (CefarCompex Medical AB) were used. In the control group, the electrodes were applied, connected to the stimulator, but no electricity was delivered. Patients were blinded to the intervention: All patients were told they would get NMES, whether or not electricity was delivered.
Outcomes
Muscle layer thickness (MLT)
For all patients, MLT of the quadriceps muscle of both thighs using two-dimensional B-mode ultrasound (Vivid i, General Electric Healthcare, Little Chalfont, UK) was measured on postoperative day 1, every other day until ICU discharge and at hospital discharge. Patients in cohort A had an additional assessment of MLT before surgery. In cohort A, the preoperative assessment took place on the preoperative day in most patients (n = 23). Due to postponement of surgery, the preoperative assessment was between 2 and 8 days before surgery in six patients. In previous studies [7, 27, 28] the cross-sectional area of the rectus femoris muscle was measured. However, the rectus femoris muscle only constitutes approximately 10 % of the total quadriceps cross-sectional area [29]. Even though there is no data about differing wasting rates of the quadriceps muscle components in critically ill patients, hypertrophy may occur at different rates in the quadriceps muscle components in healthy young males during training [30]. Thus we measured not a cross-sectional area of a single muscle, but the muscle layer thickness of all parts of the quadriceps muscle. During the ICU stay, edema may enlarge the cross-sectional area of the muscle. Furthermore the ultrasound transducer is only 3.8 cm wide. Thus it is impossible to measure all parts of the quadriceps muscle at only one measuring point by ultrasonography. In order to account for all parts of the quadriceps muscle in the most reliable way, we used three measuring points per leg in two orthogonal planes to increase precision. Twelve measurements per patient per observation day were averaged to account for all parts of the quadriceps muscle. Three measuring points per leg were marked with a permanent marker pen for scanning in the transverse and sagittal plane to make sure that the exact same measuring point was measured across time [31]: (1) lateral point: in the middle of the distance between the superior border of the greater trochanter and the lateral knee joint space; (2) ventral point: on a line from the anterior superior iliac spine to the superior border of the patellar base, on the same level as the lateral point; (3) medial point: on a line from the pubis to the medial knee joint space, on the same level as the lateral point. Frequency was set at 6 MHz. A pillow was placed under the patient’s knees to ensure muscular relaxation. A gel pad (Geliperm 260 × 120 × 3.3 mm, Geistlich Pharma, Wolhusen, Switzerland) placed on the patient’s thigh and additional ultrasound gel was used to apply the least amount of pressure necessary while scanning until the muscles were neatly displayed. Prior to trial start, inter-examiner variation in MLT scanning between the five trained assessors was assessed on 24 measuring points. The intra-class correlation coefficient (ICC) for the five assessors was 0.9 (95 % CI, 0.82 to 0.95) (P < 0.001). The five nonblinded assessors effectuated the ultrasound scans and muscle strength assessment and then applied the electrodes for NMES or sham stimulation. During the recruitment phase, none of the nonblinded assessors had access to any actual MLT values. Thus it appears unlikely that any observer bias was introduced by the nonblinded assessors who effectuated the ultrasound scans. After the end of data collection, MLT was evaluated with the software EchoPAC (General Electric Healthcare). The two evaluators were blinded to whether the patient belonged to the NMES or control group. Scans were evaluated per patient from the first to the last taken ultrasound scan to ensure consistency since muscular form is very individual. In that way, the level of the fascia lata could be discerned consistently in each patient. In a pilot testing we observed that the intramuscular fascia cannot be precisely delimitated from the muscular parts in patients during early ICU stay (Fig. 1 and Additional file 1). Thus we opted to measure the entire muscle layer thickness from the fascia lata (or deep fascia) to the femur, because these two landmarks can be precisely delineated. Beginning in the exact center of the scan at the level of the lowest layer of the fascia lata, MLT was measured down to the femur drawing the shortest possible line. At the lateral measuring point in the transverse and sagittal plane, MLT of vastus lateralis and vastus intermedius muscles was measured from the lowest layer of the fascia lata to the femur (Fig. 1 and Additional file 1). Similarly, MLT of rectus femoris and vastus intermedius muscles was measured in both planes at the ventral measuring point and MLT of vastus medialis and vastus intermedius muscles at the medial measuring point. MLT measurement in both the sagittal and transverse plane of one measuring point is shown in Additional file 1. MLT measured in the transverse plane highly correlated with the corresponding MLT measured in the sagittal plane (r = 0.97, P < 0.001). We averaged the tranverse and sagittal measurements for each measuring point. This accounts for interscan variation per measuring point. Since preliminary results indicated that the mean of all six measuring points in both planes is representative of all 12 measurements, mean MLT of all 12 measurements was calculated for each observation day.
Muscle strength
For all patients, muscle strength was measured on postoperative day 1 or as soon as they were awake, every day until ICU discharge and at hospital discharge. The patient was considered awake when he could follow orders, which is necessary for evaluation of active muscle contractions. Patients in cohort A had an additional assessment of muscle strength before surgery. Based on the diagnostic criteria of ICUAW, which is established when mean MRC score of all testable muscle groups is below 4 [5], muscle strength was evaluated in all muscle groups of the upper and lower extremities using the MRC scale [32]. Mean MRC score of all muscle groups was calculated [5]. In addition quadriceps mean MRC score was calculated as mean MRC score of knee extension and hip flexion of both legs to evaluate the function of the electrically stimulated quadriceps muscles. According to the MRC scale [33], mean MRC score ranges from a minimum of 0 to a maximum of 5 points. Bilateral grip strength was measured with a Jamar hand dynamometer (Jamar, Duluth, MN, USA). Patients were asked to perform the hand dynamometer test twice for each side and the highest value was noted for each side.
Functional outcomes
For all patients, the Functional Independence Measure (FIM) score [34], the Timed Up and Go test [35] and the 12-Item Short Form (SF-12) health survey [36] were evaluated at hospital discharge. The average mobility level proposed by Brown [37] was assessed at ICU and hospital discharges. Patients in cohort A had an additional assessment of all functional outcomes before surgery.
Patient satisfaction
At ICU discharge, patients were asked if the NMES/sham stimulation was comfortable or uncomfortable.
Statistical analysis
In the previous Catastim 1 trial [24], MRC muscle strength was evaluated for hip flexion: mean changes in MRC score from the first postoperative day until ICU discharge were −0.80 points [standard deviation (SD), 0.70 points] in the NMES group (n = 20) versus −1.75 points (SD, 1.52 points) in the control group out of a maximum MRC score of 5 points (n = 20) (P = 0.015). Based on these results, sample size calculation with Stata yielded 25 patients per group at a two-sided significance level of α = 0.05 and 80 % power. Assuming a dropout rate of 20 %, 60 patients needed to be randomized.
All data was analyzed according to the intention-to-treat principle with no imputation for any missing data. Normality was assessed with Kolmogorov-Smirnov and Shapiro-Wilk statistics. If quantitative variables were normally distributed, they were expressed as mean ± SD or 95 % confidence interval (CI). If they were not normally distributed, median (range) was indicated.
In previous muscle ultrasound studies, linear regression analyses were performed [7, 27, 28]. Similarly, we presented results of actual raw data (Additional file 2 and Table 7) and results based on linear regression modeling (Tables 2, 3, 4, 5, 6 and Additional file 3). Linear mixed models were used to assess the treatment effects while accounting for the repeated measurements per patient. Thus, the random effect ‘patient’ was defined in each model. Linear mixed models were calculated for mean MLT, mean MRC score of all muscle groups, quadriceps mean MRC score and grip strength. Fixed effects of interest were postoperative day or study day, NMES/control group, interaction between NMES/control group and postoperative day, and daily fluid balance in the ICU. Fixed effects were either considered as quantitative or categorical variables: for example, the postoperative day or the daily fluid balance at the ICU were quantitative variables. The study day (preoperative day, first postoperative day, ICU discharge, hospital discharge) or the group (NMES group, control group) were categorical variables. By definition the results presented as linear mixed models are derived statistics by linear regression modeling (‘proc mixed’ in SAS software version 9.4; SAS Institute Inc., Cary, NC, USA).
Because of bandages to protect arterial catheters, patients were not always able to perform every movement in all 12 muscle groups. If there were missing values for muscle groups, mean MRC score was calculated for the muscle groups that could be assessed. Similarly, due to armboards because of radial arterial catheter, patients were not always able to perform the hand dynamometer test on both sides. Thus, either the right or left grip strength was chosen for statistical analysis per patient according to the lowest number of missing values over all study days on each side. If the number of missing values was equal for right and left grip strength for a patient, either the right or left grip strength was chosen according to the highest mean grip strength over all study days on each side.
In addition, intraoperative fluid balance, changes over time in MLT or mean MRC score were compared between both groups with independent-groups t tests or Mann-Whitney U tests as appropriate. Change in MLT was correlated with the cumulative fluid balance in the first three postoperative days with Pearson product-moment correlation.
The secondary functional outcomes were evaluated in a sensitivity analysis for patients seen both on preoperative day and at hospital discharge (n = 12) in order to evaluate whether preoperative functional levels could be regained at hospital discharge. Changes over time in secondary functional outcomes were analyzed with Wilcoxon signed-rank tests. Changes in functional outcomes were compared between both groups with Mann-Whitney U tests. The patient satisfaction related to the intervention was evaluated in 42 patients with Pearson’s chi-squared test or Fisher’s exact test as appropriate.
Significance level was set at 0.05. All P values were two-tailed. For statistical analysis, SPSS version 22 (IBM Corp., Armonk, NY, USA) and SAS version 9.4 were used. For figure construction, GraphPad Prism version 6.0 (GraphPad Software, Inc., La Jolla, CA, USA) was used.