Although cardiac surgery is an effective option and a safe procedure that increase quality of life and survival in patients with heart failure, it is still a complex procedure with many possible complications.1^-3 Muscle proteolysis has been known to accelerate within 48 h after cardiovascular surgery,2 when patients are under significant mobilization restrictions, and use of mechanical ventilation, vasoactive drugs, sedatives, and analgesics, and the presence of catheters, thoracic, and mediastinal drains. In this context, the rehabilitation of patients after cardiac surgery becomes a challenge in the intensive care unit (ICU).1^-3

Early mobilization and physical exercise have been considered as fundamental components in the rehabilitation of patients in the postoperative period of cardiac surgery.4^,5 The functional status is known as being prognostic in patients submitted to cardiac surgery at hospital discharge.6 In this context, neuromuscular electrical stimulation (NMES) seems to be a promising option to intensify the rehabilitation of patients in the immediate postoperative period of cardiac surgery.7

Previous studies have already shown the positive effects of NMES on the exercise capacity of patients with cardiovascular disorders,8^,9 as well as its safety regarding the hemodynamic and respiratory responses to its use.10^,11 However, studies evaluating the use of NMES immediately after cardiac surgery are lacking, wherein patients are more restricted to the bed and often require vasoactive drugs.

The aim of this study was to evaluate the hemodynamic and respiratory responses of patients to NMES session performed in the immediate postoperative period of cardiac surgery.

The study included 30 patients submitted to cardiac surgery, with 15 patients in the experimental group and 15 in the control group, respectively (Figure 1). In the experimental group, the use of NMES occurred in the first 23.13 ± 5.24 h and ,in the control group, 22.20 ± 5.46 h after cardiac surgery.

Figure 1

No complications were observed during our protocol, and none of the patients were excluded. The sample characteristics are shown in Table 1.

No change was found in the hemodynamic and respiratory variables in the patients submitted to neuromuscular electrostimulation, as well as in the control group patients. In addition, all hemodynamic and respiratory parameters remained within normal limits (Table 2).

The main finding of this study was that an NMES session did not result in any changes in HR, SBP, DBP, MBP, RR, and SpO₂ in patients in the immediate postoperative period of cardiac surgery.

In the past, many professionals working with cardiovascular rehabilitation hesitated to prescribe NMES to patients with heart disease, contraindicating electrotherapy due to the risk of cardiac arrhythmia.3 Additional concerns that could contribute to the non-indication of NMES would be the concern that repeated sustained muscle contractions would elevate total peripheral resistance, resulting in acute elevations in blood pressure and hemodynamic overload, increasing the risk of cardiovascular complications in critically-ill patients.12

Despite these facts, NMES has been proposed as a promising adjuvant therapy to increase the physical capacity of patients involved in cardiovascular rehabilitation programs, such as patients hospitalized for heart failure,13 and in patients in the postoperative period of cardiac surgery.14

Some authors have already investigated hemodynamic responses to the use of NMES in healthy subjects,15 exercise plus NMES in patients with heart failure,10 and in patients under critical care.11 However, only one study investigated the safety of NMES immediately after cardiac surgery.3 In this study, no patient showed changes in blood pressure and HR that exceeded the safety criteria defined by the study. The mean variation was a maximum of 2.1 mmHg for SBP and 1.7 bpm for HR.3

In the present study, the majority of the patients used inotropic agents, such as dopamine, dobutamine, noradrenaline, and/or needed vasopressor support to maintain their hemodynamic stability, which would lead to extra concerns regarding the cardiovascular system during NMES application. However, no statistical or clinical changes were observed (4.53 bpm for HR; 2.93 mmHg for SBP, 3.27 mmHg for DBP and 1.73 mmHg for MBP). No cardiac arrhythmia was reported either.

A previous study found that a session of NMES in critically-ill patients caused an increase in SBP and HR of 6 mmHg and 5 bpm, respectively, although the authors stated that this result was not clinically significant.16 Another author also found small changes in HR, of approximately 1 bpm, and in SBP and DBP, of approximately 1 mmHg, with no statistical significance when NMES was applied on the femoral quadriceps of critically-ill patients.17 These borderline increases in BP and HR after the use of NMES in critically-ill subjects are in agreement with the results presented by this study, wherein one can observe similar variations of these variables, including in the control group, with no statistical and clinical difference between the groups.

Moreover, no reports of muscle pain, discomfort, or fatigue that could lead to interruption of NMES therapy, or even a dyspnea complaint, confirmed by the maintenance of the respiratory variables of RR and SpO₂, were observed. Our findings are in agreement with those of another study that found no significant changes in RR and SpO₂ with the use of NMES in critically-ill patients.17

Our study had no dropouts. This is in accordance to previous studies that reported low dropout rates of 1.5% in patients in the postoperative period3 and 11% for patients with heart failure.18 These data support the use of NMES, because it was well tolerated by the patients during the acute phase, when they are submitted to invasive procedures and subject to pain.2^,19^,20

The parameters used in the NMES studies are divergent, ranging from 250 ms-400 ms for pulse duration, 1.75 Hz-100 Hz for frequency, and 2-12 s to 4-24 s for the clicks.21 Some authors report that high frequencies ≥ 50 Hz improved muscle strength,22 whereas others state that the intensity of NMES response varies based on the patient’s interaction.16

Differently from the present study, another study has shown that NMES induced energy expenditure and cardiovascular response similar to other types of exercise in other patient profiles, with higher HR increases, but using low-frequency NMES techniques.23 This same study even suggested that this technique using low frequency would lead to higher physiological responses than other classically used protocols with high and moderate frequencies, similar to that used in the present study. In this aspect, another factor that must be taken into account is the stimulated muscle mass, because larger muscular masses could lead to higher physiological responses.23

However, regardless of the variety of parameters used in the studies, a systematic review of NMES efficacy in critically-ill patients indicates that this is a relatively safe method for use in this type of patient,21 which is in agreement with our data.

Thus, we proposed that NMES be used as post-surgical therapy considering the possible benefits related to the use of this resource, as a shorter period of exercise restriction, smaller strength decline and faster recovery of muscle strength, consequently resulting in higher tolerance by patients to recover their ambulation capacity, functional levels and performance of activities of daily life.

In the present study, NMES did not promote changes in hemodynamic and respiratory responses in the immediate postoperative period of patients submitted to cardiac surgery.