Inspiratory Muscle Strength as a Determinant of Functional Capacity Early After Coronary Artery Bypass Graft Surgery

Article Outline

• Abstract
• Methods
  • Patient Population
  • Design and Intervention
  • Pulmonary Function and Respiratory Muscle Testing
  • Measures of Functional Capacity
  • Statistical Analysis
• Results
  • Patients
  • Pulmonary Function and Respiratory Muscle Testing
  • Measures of Functional Capacity
• Discussion
  • Clinical Implications
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Stein R, Maia CP, Silveira AD, Chiappa GR, Myers J, Ribeiro JP. Inspiratory muscle strength as a determinant of functional capacity early after coronary artery bypass graft surgery.

Objective

To evaluate the effects of a 6-day postoperative in-hospital cardiopulmonary rehabilitation program on inspiratory muscle strength and its potential association with improved functional capacity after coronary artery bypass graft (CABG) surgery.

Design

Prospective, randomized controlled trial.

Setting

Tertiary public hospital in Brazil.

Participants

Men (N=20) after CABG were randomized to cardiopulmonary rehabilitation (n=10; age, 64±8y) or to usual care (n=10; age, 63±7y).

Interventions

Ten subjects underwent a 6-day postoperative in-hospital program, which included the use of expiratory positive airway pressure mask and bronchial hygiene techniques, coupled with progressive distance walking and calisthenics as well as cardiopulmonary training. Ten controls were followed by their own physicians and received routine nursing assistance but were not exposed to any specific respiratory or motor physical intervention.

Main Outcome Measures

Maximal inspiratory and expiratory pressure were measured by a pressure transducer, and the highest pressure obtained in 6 measurements was used for analysis (before surgery, and 7 and 30d after surgery). The six-minute walk test (6MWT) was performed 7 days after surgery, and maximal cardiopulmonary exercise testing was performed 30 days after CABG.

Results

After randomization, clinical and functional characteristics were similar in the 2 groups. Rehabilitation resulted in maintenance of maximal inspiratory pressure (PImax) measured at 7 and 30 days postoperatively, respectively (from 68±19% at baseline to 58±22% and to 61±22% predicted), while it was significantly reduced in the control group. 6MWT distance was longer 7 days after CABG in rehabilitation subjects (416±78m) than controls (323±67m). Peak oxygen uptake (Vo₂peak) at day 30 was also higher (28%) in the rehabilitation group and was correlated with PImax (r=.90).

Conclusions

A 6-day rehabilitation program attenuated the postoperative reduction in respiratory muscle strength and also improved the recovery of functional capacity after CABG. The correlation between PImax and Vo₂peak during the late postoperative period suggests that inspiratory muscle strength is an important determinant of functional capacity after CABG.

Key Words: Exercise, Rehabilitation

List of Abbreviations: CABG, coronary artery bypass graft, CPET, cardiopulmonary exercise testing, EPAP, expiratory positive airway pressure, FEV₁, forced expiratory volume in 1 second, FVC, forced vital capacity, PEmax, maximal expiratory pressure, PImax, maximal inspiratory pressure, 6MWT, six-minute walk test, Vo₂peak, peak oxygen uptake

IN MOST INSTITUTIONS, respiratory physical therapy is routinely offered after CABG with the expectation that it will reduce postoperative complications and reduce length of hospital stay. Traditionally, prevention and treatment of pulmonary complications have included early mobilization and physical therapy interventions, including a variety of respiratory maneuvers such as deep breathing, coughing, incentive spirometry, or application of positive end-EPAP.¹ In 2006, Hulzebos et al² demonstrated that preoperative intensive inspiratory muscle training was able to prevent pulmonary complications in high-risk patients undergoing CABG, suggesting that the maintenance of inspiratory muscle strength after surgery is important in preventing postoperative morbidity. However, there remains debate concerning the prevention of reduction in respiratory muscle strength after cardiac surgery.³, ⁴, ⁵

Therefore, the present study was conducted to evaluate the effects of a 6-day postoperative cardiopulmonary rehabilitation program on inspiratory muscle strength and its possible association with submaximal and maximal functional capacity after CABG.

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Methods 

Patient Population 

This prospective, randomized controlled trial was carried out in patients who were recruited from the waiting list for a first CABG intervention at the Hospital de Clínicas de Porto Alegre. Entry criteria for the study included a previous history of more than 1 vessel coronary artery disease, lifelong abstinence from tobacco, and formal indication for CABG, according to established guidelines.⁶ Patients older than 75 years of age; those with chronic renal failure (dialysis for more than 3mo); or those with unstable angina in the 48 hours prior to CABG, moderate or severe valve disease, complex cardiac arrhythmias, stroke, and/or inability to exercise the lower limbs were excluded. We also excluded subjects with previous pulmonary disease (FVC<80% of predicted and/or FEV₁<70% of predicted),⁷ and those with a history of asthma. The protocol was approved by the institution committee for ethics in research, and all subjects signed an informed consent form.

Design and Intervention 

Eligible patients were randomized to a cardiopulmonary rehabilitation program (rehabilitation; n=10) or to usual care (control; n=10). The randomization was done by the Graphpad StatMate computerized program (version 1.01i).^a Those patients assigned to control were followed by their own physicians, received routine nursing assistance, and were visited daily by the 1 of the investigators (C.P.M.), but were not exposed to any specific respiratory or motor physical intervention. Those randomized to rehabilitation received the same medical and nursing care but followed a structured physical therapy program twice a day. The program consisted of bronchial hygiene characterized by detachment and removal of secretions, and respiratory exercises, which were applied in the respiratory muscles in order to strengthen and increase the resistance. Patients were instructed to maintain diaphragmatic breathing at a rate of 12 to 18 breaths a minute during EPAP mask use, and the expiratory pressure was increased progressively by 3 to 8cm water during 3 to 12 minutes. Also, they performed flexion/extension of hip and knee, active free exercises for upper limbs, ambulation exercise, and ascent/descent of stairs (Table 1, Table 2). Preoperatively, both groups were instructed about coronary risk factors and stress management. Before and 7 days after surgery, patients were evaluated by resting electrocardiogram, 2-dimensional echocardiogram, chest radiograph, pulmonary function, and respiratory muscle testing by investigators who were blind to group assignment. On the seventh day after CABG, the 6MWT was performed, and 30 days after CABG operation, all patients had a maximal CPET. Neither group participated in a phase 2 rehabilitation program, nor was either group oriented to exercise after hospital discharge until performing the CPET.

Table 1. Summarized Cardiopulmonary Rehabilitation Program

Table 2. Summarized Active Exercises During Cardiopulmonary Rehabilitation Program

NOTE. All patients performed the same sequence of exercise described in the 1st postoperative day from the 2nd to the 6th postoperative days.

Pulmonary Function and Respiratory Muscle Testing 

Measurements of FVC and FEV₁ were obtained with a computerized spirometer,^b as recommended by the American Thoracic Society,⁷ and results were expressed as percentage of predicted.⁸

PImax and PEmax were evaluated as previously described⁹ and expressed as percentage of predicted.¹⁰ Both PImax and PEmax were obtained using a pressure transducer^c connected to a system with 2 unidirectional valves.^d The highest pressure obtained in 6 measurements was used for analysis.

Measures of Functional Capacity 

The maximum distance covered during the 6MWT was used to assess submaximal functional capacity, and patients self-graded their degree of effort during the test using the 1 to 10 modified Borg rating of perceived exertion.¹¹ Maximal functional capacity was evaluated with an incremental CPET on a treadmill,^e as previously described.⁹ Twelve-lead electrocardiograph tracings were obtained every minute.^f Blood pressure was measured every 2 minutes with a standard cuff sphygmomanometer. Metabolic and ventilatory variables were measured by a computer-aided and previously validated system.^12,g

Statistical Analysis 

Data were analyzed on the Statistical Package for Social Sciences^h and SigmaStat Statisticalⁱ checked for completeness, and tested with the Kolmogorov-Smirnov for normality. Based on the results of a previous study,¹³ we estimated that a sample size of 10 subjects in each group would have a power of 90% to detect a 10% difference in respiratory muscle pressure, for alpha equal to .05. Descriptive data are presented as mean ± SD. Baseline data were compared by the Student t test for continuous variables or by the Fisher exact test for categorical variables. The Pearson correlation coefficient was used to evaluate associations. The effects of the interventions on respiratory variables were compared by 2-way analysis of variance for repeated measures, and post hoc analysis was conducted by the Tukey test.

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Results 

Patients 

During the 12-month recruitment period, 186 CABG surgeries were performed in our institution (fig 1). Of these, 25 patients were screened and fulfilled the inclusion and exclusion criteria. Five patients were excluded after surgery because of mediastinitis (n=2), resuscitated cardiac arrest (n=1), stroke (n=1), and death during surgery (n=1). Therefore, 20 patients were analyzed, and table 3 summarizes baseline demographic, clinical, and surgical characteristics of both groups. Groups were similar for age, sex distribution, body mass index, New York Heart Association class, left ventricular ejection fraction, previous myocardial infarction, beta-blocker use, number of grafts, and prevalence of hypertension and diabetes. Preoperative arterial blood gases, pulmonary function, and ventilatory muscle function were similar and within normal limits for the 2 groups. Likewise, preoperative chest radiographs were normal in all patients. Extubation time was less than 8 hours after CABG in all patients. For those randomized to the usual care group, 6 patients presented pleural effusion, 1 had lobar atelectasis, and 2 had pneumonia during the postoperative period. Four patients randomized to the rehabilitation had mild pleural effusion. There were no adverse events during the week of in-hospital rehabilitation.

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• Fig 1. 

  Flowchart of patients through the study.

Table 3. Demographic, Clinical, and Surgical Characteristics

Characteristics	Control (n=10)	Rehabilitation (n=10)	P
Age (y)	63±6	64±7	.45
Sex (M/F)	5/5	6/4	.87
Body mass index (kg.m−2)	27±2	28±3	.43
Systemic hypertension	7	7	.84
Diabetes mellitus	2	3	.76
Ejection fraction (%)	61±7	63±8	.80
New York Heart Association class	I	I	NS
Pulmonary function (% predicted)
FEV1	96±12	99±9	.58
FVC	93±10	96±6	.47
Respiratory muscle testing
PImax (cmH2O)	65.0±16.0	72.0±17.0	.35
PImax (% predicted)	60.0±1.8	68.0±1.9	.42
PEmax (cmH2O)	68.0±36.0	72.0±34.0	.79
PEmax (% predicted)	82.0±3.5	89.0±3.2	.66
Blood gas analysis
pH	7.38±0.03	7.39±0.04	.57
Paco2 (mmHg)	40.6±4.0	41.0±4.0	.82
Pao2 (mmHg)	95.0±1.3	95.0±1.3	.87
β-Blocker use (%)	90	90	NS
No. of grafts
2	1	1
3	6	6
4	3	3
Previous myocardial infarction (%)	40	50	NS
Time with orotracheal tube (h)	7.2±2.2	7.4±2.4	.78
Borg RPE for 6MWT	7	7	NS

NOTE. Values are mean ± SD or as indicated.

Abbreviations: Borg RPE, Borg rating of perceived exertion (Borg scale was evaluated from 1–10); F, female; M, male; NS, nonsignificant; Paco₂, partial pressure of carbon dioxide, arterial; Pao₂, partial pressure of oxygen, arterial.

Pulmonary Function and Respiratory Muscle Testing 

Figure 2 presents the results for pulmonary function testing and respiratory muscle testing. For both groups, there was a significant reduction in FVC and FEV₁ by the 7th postoperative day. By the 30th postoperative day, however, the rehabilitation group had a significantly better recovery of FVC and FEV₁. As also shown in figure 2, PImax and PEmax presented a smaller reduction in the rehabilitation group than the control group 7 days postoperation. By the 30th postoperative day, both measures of respiratory muscle strength showed a significantly better recovery in the rehabilitation group.

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• Fig 2. 

  Mean ± SD FVC, FEV₁, PImax, and PEmax, expressed as percentage of predicted (pred) in the preoperative period, 7 and 30 days postoperation in patients randomized to cardiopulmonary rehabilitation (Rehab) or usual care (Control). Two-way analysis of variance for repeated measures revealed time and interaction effects in the 4 variables (P<.05). *P<.05 for the Tukey test comparing groups.

Measures of Functional Capacity 

Seven days after CABG, patients in the rehabilitation group covered a significantly longer distance in the 6MWT (323±67 vs 415±78m; P<.05). There were significant correlations between the distance covered in the 6MWT and PImax (r=.75; P<.01) as well as PEmax (r=.73; P<.01). Table 4 summarizes results from the CPET for the groups. By the 30th postoperative day, Vo₂peak was significantly higher in the rehabilitation group. There were significant correlations between Vo₂peak 30 days after CABG and PImax (r=.90; P<.001) (fig 3) as well as PEmax (r=.69; P<.01).

Table 4. Results From Maximal Cardiopulmonary Exercise Testing for Controls and Rehabilitation Patients 30 Days After Coronary Artery Bypass Graft

NOTE. Values are expressed as mean ± SD or as otherwise noted.

Abbreviations: R, gas exchange ratio; V̊_E, expired volume per unit time; V_E/Vco₂peak, relationship between change in V_E and Vco₂ uptake during incremental exercise; V_E/Vo₂peak, relationship between change in V_E and Vo₂ uptake during incremental exercise.

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• Fig 3. 

  Scatterplot of the association between PImax and VO₂peak at 30 days after CABG. There was a highly significant correlation between the variables (r=.90; P<.001). Abbreviation: Rehab, rehabilitation.

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Discussion 

This randomized controlled trial shows that, in stable patients with coronary artery disease submitted to CABG, a 6-day program of rehabilitation significantly attenuated the expected postoperative reduction in inspiratory and expiratory muscle strength, and also improved recovery of functional capacity. Moreover, inspiratory muscle strength 30 days after CABG strongly correlated with maximal functional capacity as determined by Vo₂peak. Our data provide evidence that a postoperative in-hospital program, which included the use of EPAP mask and bronchial hygiene techniques, coupled with progressive distance walking and calisthenics, results in maintenance of respiratory muscle strength and supports the concept that inspiratory muscle strength is an important determinant of functional capacity during the postoperative period.²

Previous studies on the effects of postoperative physical therapy after CABG on respiratory muscle strength have shown conflicting results. Crowe and Bradley¹³ showed no benefit of respiratory physical therapy on pulmonary function, while Borghi-Silva et al¹⁴ found that the combination of EPAP and physical therapy was more effective in minimizing changes in the spirometric variables than it was in inspiratory muscle strength on the 5th postoperative day.

As demonstrated previously, postoperative respiratory muscle dysfunction may contribute to reductions in FVC, tidal volume, and total lung capacity,², ¹⁵, ¹⁶ and these changes may augment the incidence of atelectasis in the basal lung segments, increasing ventilation/perfusion mismatch.¹⁵ Data recently published from our hospital demonstrated that incentive spirometry with EPAP reduces pulmonary complications and improves pulmonary function and six-minute walk distance in patients with a previous history of tobacco use undergoing CABG.⁴ As also shown by Haeffener et al,⁴ our postoperative in-hospital rehabilitation approach partially prevented the reduction in FVC and FEV₁ in individuals who never smoked. Our in-hospital rehabilitation results in marked improvement in inspiratory and expiratory muscle strength, in addition to clinically relevant improvements in submaximal and maximal functional capacity and pulmonary function in postoperative CABG surgery patients. Thus, we postulate that the use of EPAP might enhance alveolar expansion and reduce the size of pleural effusion, with consequent improvements in both restrictive and obstructive components of pulmonary function.

The mechanisms by which CABG induces impairment of respiratory muscle function are probably multifactorial. Johnson et al¹ demonstrated a 30% decrease in PImax and PEmax after CABG and suggested that the respiratory muscle impairment observed after this surgical intervention might be a result of postoperative decline in skeletal muscle strength and concomitant pain. Respiratory muscle performance can improve with inspiratory muscle training in patients with heart failure and inspiratory muscle weakness⁹ and in high-risk patients who have undergone CABG.² Our rehabilitation intervention did not include specific respiratory muscle training; thus, the results found on PImax and PEmax 1 week after CABG might be partly attributable to EPAP use and walking practices. Considering that Vibarel et al¹⁷ showed that an aerobic exercise program had no significant effect on PImax in patients with chronic heart failure, we can only speculate that the use of an EPAP mask could have contributed to the preservation of respiratory muscle strength.

Despite the fact that our study was not powered to evaluate the postoperative incidence of complications, patients randomized to rehabilitation tended to have less pulmonary complications than controls, irrespective of time with the orotracheal tube immediately after CABG or number of grafts. Recently, preoperative cardiopulmonary rehabilitation has been shown to reduce complications after CABG.², ¹⁸, ¹⁹ However, because of logistic limitations, it is not possible to offer preoperative intervention in most institutions. In 2008, Hirschhorn et al⁵ reported that a physiotherapy-supervised, moderate-intensity walking program in the in-patient phase after CABG improves walking capacity at discharge from the hospital. However, the addition of respiratory and musculoskeletal exercises conferred no additional benefit to exercise capacity. When we compared our 6MWT results with those of Opasich et al²⁰ and Herdy et al,¹⁹ the distance walked was superior in our trial (416 vs 296 vs 299m, respectively). Compared with our randomized usual care patients, the rehabilitation group walked 416m versus the control group's 324m (29% higher).

The rehabilitation group was strongly encouraged to walk as soon as possible, and a progressive increase in the intensity of the pace and distance was the goal. Considering that the baseline characteristics of both groups were similar and all subjects underwent CABG, it seems to us that an approach in which these individuals were encouraged to exercise may have had a marked impact on postdischarge functional capacity. In addition, early mobilization of the saphenous venectomy lower limb may have attenuated pain during the 6MWT.

Maximal exercise capacity, as evaluated by Vo₂peak, was significantly higher in the rehabilitation group at 30 days. As indicated by the findings of Hirschhorn et al,⁵ spontaneous recovery of functional capacity may occur during this period, but the higher Vo₂peak in the intervention group is most likely associated with the adaptive processes caused by in-hospital rehabilitation. Dall'Ago et al⁹ observed a direct relationship between oxygen uptake and inspiratory muscle strength in patients with heart failure. Interestingly, we also identified a high correlation between improvement in inspiratory muscle strength and the increase in functional capacity after rehabilitation (r=.90; P<.001) in patients with preserved left ventricular ejection fraction. While respiratory muscle strength in our study was not specifically trained, intensive respiratory exercise—with positive pressure—probably generates a smaller reduction in inspiratory and expiratory strength after surgery. Finally, it is worth noting that to minimize any bias when applying CPET on the 30th day after discharge, neither group participated in a phase 2 rehabilitation program, nor were they oriented to exercise after hospital discharge.

Clinical Implications 

After a major surgery such as CABG, in-hospital rehabilitation can promptly benefit cardiorespiratory and muscular systems. Considering the postoperative convalescent stage, in which patients are limited in their physical autonomy, the significant improvement in functional, respiratory, and muscular parameters may be evidence of an improvement in a patients' ability to perform daily activities—a very important end point.

The current findings and those from other recent investigations⁴, ⁵, ¹⁴, ¹⁹ warrant attention for clinicians who treat patients post-CABG. If early rehabilitation can attenuate disability and protect against high-risk cardiopulmonary complications such as atrial fibrillation, major atelectasis and/or pleural effusion, and pneumonia, then nonpharmacologic post-CABG management, including exercise programs, should be considered.

Study Limitations 

Although the sample size estimation has provided this study statistical power to randomize 10 subjects in each group, we believe that it would be important for subsequent studies to gather larger samples, allowing efficacy data to be transformed into effective findings. This study was not designed to compare differences in pulmonary complications or other adverse events; thus, we do not have information on outcomes. We did not perform presurgical functional capacity measurements in either group. However, in patients with coronary artery disease waiting for CABG, functional capacity can be limited by ischemia and/or angina, and this may be corrected by surgery. Because we did not test the groups before the surgical intervention and both groups were quite similar at baseline, the different results observed in submaximal and maximal functional evaluations post-CABG are most likely not explained by baseline differences.

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Conclusions 

This controlled trial demonstrated that a 6-day, postoperative, in-hospital rehabilitation program attenuated the postoperative reduction in respiratory muscle strength and also improved the ability of functional capacity to recover after CABG. The association between PImax and Vo₂peak during the late postoperative period suggests that inspiratory muscle strength is an important determinant of functional capacity after CABG.

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• a GraphPad Software Inc, 5755 Oberlin Dr, Ste 110, San Diego, CA 92121.
• b Erich Jaeger; Cardinal Health Corporate Headquarters, 7000 Cardinal Place, Dublin, OH 43017.
• c MVD-500 V.1.1 Microhard System; Global Med, Rua Pedro Chaves Barcelos 571, CEP 90450-010, Porto Alegre/RS, Brazil.
• d DHD Inspiratory Muscle Trainer; available at: www.Dhdmedical.com.
• e INBRAMED 10200; Rua Santos Dumont, 1766 Bairro São Geraldo CEP 90230-240, Porto Alegre/RS, Brazil.
• f Nihon Kohden Corp, 1-31-4 Nishiochiai, Shinjuku-ku, Tokyo 161-8560, Japan.
• g Total Metabolic Analysis System; TEEM 100, Aero Sport, Ann Arbor, MI.
• h Version 14.0; SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.
• i SigmaStat 3.1; 225 W Washington St., Ste 425, Chicago, IL 60606.