Cardiovascular diseases, especially coronary heart disease, have become the main cause of morbidity and mortality in India [1]. In 2017, 17.8 million deaths due to cardiovascular diseases were reported globally [2]. Worldwide, coronary artery bypass graft (CABG) surgery is performed daily in patients with coronary artery disease [3]. Valve repair or replacement is the treatment of choice for patients with valve diseases [4].

Despite advances in surgical procedures, the risk of postoperative complications still exists, as surgeries are now being performed on high-risk patients [5]. It is well known that cardiac surgery causes postoperative pulmonary dysfunction [6], which results in increased effort for breathing, hypoventilation, ineffective cough, and hypoxemia, which leads to postoperative pulmonary complications (PPCs), such as atelectasis, bronchospasm, pneumonia, respiratory failure, acute respiratory distress syndrome, prolonged mechanical ventilation, diaphragmatic dysfunction, and pleural disorders [7,8].

Respiratory muscle dysfunction is a consequence of numerous factors. The grafts harvested for CABG, specifically the internal mammary artery graft, reduces blood supply to the intercostal muscles, which leads to respiratory muscle weakness [9,10]. The median sternotomy incision results in pain and altered chest wall mechanics. Sedation and pain together can result in shallow breathing and hypoventilation. All these reduce pulmonary function [9,11].

In addition, the use of cardiopulmonary bypass, topical cooling for myocardial protection (phrenic nerve injury) [6], use of chest drains, especially intercostal drains, further reduces pulmonary function [9]. Even after 1 year, the pulmonary function remains reduced and is not restored to preoperative levels, although the reduction is lesser than that after 4–5 months; compared with the 6%–13% reduction in pulmonary function at 4–5 months, there is a reduction of 4%–5% at 1 year [12]. Pulmonary dysfunction is significant in the early postoperative period, and is the primary cause of PPCs, morbidity, and mortality. However, the clinical significance of decreased pulmonary function at 1 year postoperatively is not known [12,13].

Inspiratory muscle training (IMT) targets the respiratory muscles by increasing the load to inspiration, thereby increasing strength and endurance. Inspiratory muscles respond in a manner similar to that of the other skeletal muscles. IMT has been found to increase strength in healthy individuals, mechanically ventilated patients, and patients with chronic heart failure [14]. Many interventions, such as early mobilization, diaphragmatic breathing exercises, and the active cycle of breathing technique, are traditionally practiced postoperatively in the intensive care unit to prevent the risk of postsurgical pulmonary complications. In addition, devices such as positive expiratory pressure devices, IMT devices, and incentive spirometers are used to reduce the incidence of PPCs after cardiac surgeries [7,15]. Prerehabilitation also plays a vital role as it assists the individual to withstand predicted cardiorespiratory and musculoskeletal challenges postcardiac surgery [14]. Mobilization of the extremities; IMT; and IMT using a device that includes isocapnic/normocapnic hyperpnea training, inspiratory resistive flow training, inspiratory threshold pressure training, and incentive spirometry is part of prehabilitation [14]. IMT can also be used in phase II of cardiac rehabilitation along with aerobic training or resistance exercises in patients with chronic heart failure and chronic obstructive pulmonary disease, as evidence has shown improvements in respiratory muscle strength (RMS) in this population [3].

The purpose of this systematic review was to determine the effects of IMT on RMS, pulmonary function, and functional capacity in individuals undergoing cardiac surgery.

The present systematic review and meta-analysis was conducted following the Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) guidelines. This systematic review protocol was registered in PROSPERO on July 7, 2020 (Registration No.CRD42020179130) and the Center for Open Science (OSF) registry (https://doi.org/10.17605/OSF.IO/U6VNM).

The computerized database search retrieved 184 articles, of which 16 were selected for full-text review, and five were included in the final review.

Articles were excluded based on study methodology, languages other than English, outcome measures other than those to be analyzed in the review, and articles with inadequate data on outcome measures. Data from 293 participants were analyzed. The intervention and control groups included 144 and 149 patients admitted for cardiac surgery, respectively. All participants were older than 18 years. The characteristics of the participants are presented in Table 1. Participants were recruited from hospitals or outpatient clinics.

To our knowledge, this is the first systematic review and meta-analysis to examine the effect of IMT in patients undergoing cardiac surgeries. The results of the meta-analysis suggest that there is a remarkable improvement in RMS, functional capacity, and lung function. However, the included studies showed heterogeneity in variables such as training period (preoperative IMT or postoperative IMT) and duration of sessions. Only one study used IMT in the preoperative phase. Dos Santos et al. [3] reported on IMT in the second phase of cardiac rehabilitation, whereas in three studies, it was provided during the first phase of cardiac rehabilitation. The first phase of cardiac rehabilitation refers to rehabilitation during hospitalization. The second phase includes rehabilitation up to 4 months after discharge from the hospital and consists of monitored outpatient physical activity as recommended by the physician. Subsequently, the third phase of cardiac rehabilitation involves a permanent, unmonitored exercise program [19]. We conducted a meta-analysis of all included studies, which indicated an improvement in RMS, functional capacity as measured by the 6-minute walk distance, and lung function.

RMS was evaluated as the primary outcome in this systematic review and meta-analysis. RMS is reduced in all individuals undergoing cardiac surgery. Previous studies have shown that neural conditioning caused by repeated exposure to the same task improves RMS by improving neuromuscular recruitment patterns [8]. According to Zanini et al. [18], RMS improves by the 30th postoperative day and that there is no significant improvement in the IMT group compared to other groups. Moreover, reversal of RMS has been reported due to the additional effect of expiratory positive airway pressure and not IMT alone. However, Cordeiro et al. [16] noted better RMS at the time of discharge in a population who underwent IMT post-surgery. This difference could be due to the higher resistance used by Cordeiro et al. [16] or the difference in the population. Meta-analysis reported the highest heterogeneity, I²=69%.

De Menezes et al. [20] reported that reduced RMS was associated with decreased functional capacity and an increased period of recovery of lung function, as it is directly related to peripheral muscle strength. Therefore, improvement in RMS can improve FVC and FEV₁ and thereby the 6-minute walking distance. We included two studies to analyze the effects of IMT on FVC and FEV₁. A meta-analysis of both studies showed an improvement in FVC, irrespective of whether IMT was administered preoperatively or postoperatively. Nevertheless, the study by Zanini et al. [18], which assessed postoperative intervention, reported better improvement than the study by Chen et al. [8]. The study by Shenkman et al. [21] reported that individuals with CABG had a 70% reduction in FVC immediately postoperatively and a reduction of 35% at 3 weeks after surgery.

PPCs usually prolong the postoperative length of hospital stay. Previous studies by Hulzebos et al. [22] reported that preoperative IMT reduced the incidence of PPCs. In addition, a systematic review and meta-analysis conducted by Gomes et al. [23] showed similar findings. In our review, only the study by Chen et al. [8] reported the reduction of PPCs after preoperative IMT, but other studies have not focused on this aspect.

Four studies with no heterogeneity (I²=0%) were used for meta-analysis of the length of hospital stay. The meta-analysis indicated that there was a significant improvement in the intervention group compared to the control group. A reduction in the length of stay can be associated with a reduction in PPCs in the intervention group. Improved inspiratory muscle strength improves the efficacy of cough, and thereby, clearance of secretions and reduction in PPCs.

This systematic review and meta-analysis has a few limitations. Only five articles, with intervention administered either preoperatively or postoperatively, were eligible for the systematic review. We were unable to compare studies with preoperative IMT with those with postoperative IMT, as very few studies with independent preoperative IMT were available. The sample size was small, and one study included data from only males in the interpretation of results [16]. In addition, we included studies in which intervention was provided at different time intervals. Furthermore, differences in exercise intensities and duration of sessions may have also contributed to the heterogeneity, which was unavoidable as there are very few studies on this subject. Hence, further studies are required to determine whether IMT helps improve RMS, functional capacity, and pulmonary function.

In conclusion, the authors recommend the use of IMT, either before or after cardiac surgery, as it successfully improves pulmonary function, inspiratory muscle strength, functional capacity, and duration of hospitalization.