Coronary artery bypass grafting (CABG) is a type of cardiac surgery where one or more impaired coronary arteries receive saphenous or mammary grafts and whose surgical goal is to reestablish a connection to areas that have been affected in the heart. Studies show that, in Brazil, in 2010, 21,000 coronary artery bypass grafting surgeries were performed and they have been some of the most performed cardiac procedures in recent years.1

Coronary artery bypass grafting (CABG) improves the patients’ quality of life, but there are associated factors that may compromise respiratory function. There is a great chance that these patients will develop postoperative complications, including the following most common ones: pulmonary edema, pneumothorax, pleural effusion, atelectasis and pneumonia.2

Pulmonary function is significantly reduced in the immediate postoperative period and the reason for this is multifactorial and is not fully understood.3 Operative conditions such as cardiopulmonary bypass (CPB), surgical incision, anesthesia, the patient’s hemodynamics, type and duration of the procedure, length of drainage or pain can cause lung disorders. All of these components seem to lead to decreased pulmonary volumes and capacities, thoracic expansion and respiratory muscle dysfunction.2^,4

The efficacy of respiratory physiotherapy is as an important marker for cardiac rehabilitation both inside and outside the intensive care unit.5 The physiotherapist contributes directly to the best prognosis, prevention and treatment in patients who undergo coronary artery bypass grafting, where rehabilitation treatments are applied, such as: manual, respiratory and pulmonary reexpansion maneuvers, postural advice and bed positioning.6

There is still little information on the behavior of lung function after cardiac surgery related to the time required to reestablish the lungs to preoperative values. Therefore, the objective of this study was to describe the behavior of pulmonary function after hospital discharge in patients undergoing CABG.

This is a prospective study involving patients from a reference hospital in Feira de Santana - BA, Brazil, from August 2017 to April 2018. This study was approved by the research ethics committee of Faculdade Nobre (FAN) of Feira de Santana-Bahia under opinion no. 2.088.639. All patients signed an Informed Consent Form.

The inclusion criteria were individuals of both sexes, aged equal to or above 18 undergoing coronary artery bypass grafting procedure via median sternotomy and extracorporeal circulation. The exclusion criterion were patients who were readmitted to the hospital, patients who did not return for the review, who found it hard to understand or to collaborate, who present post-surgical complications, hemodynamic instability, previous cardiac surgery, previous neurological symptoms, cardiac arrhythmia, and who remained at least 6 days in hospital.

After meeting all inclusion criteria, the patients had their pulmonary function assessed preoperatively. This evaluation consisted of maximal inspiratory pressure (MIP) and maximum expiratory pressure (MEP), and vital capacity (VC).

Respiratory muscle strength was tested with the patient sitting on a chair. Initially, the patient was asked to breathe close to the current volume and, after three breaths, to perform a maximal forced expiration (residual volume) and then a maximal static inspiration sustained for 3 seconds with nasal occlusion in order to get the MIP measure.

Subsequently, to measure the PEM, the patient had to breathe close to the current volume for three cycles and perform a maximal inspiration (total pulmonary capacity) followed by a maximum sustained expiration for 3 seconds. Both maneuvers were repeated at least three times with an interval of one minute and the highest value, which cannot be the one from the last measurement, was recorded. These measurements were performed using an analogue Instrumentation Industries manovacuometer model MV - 120, with a range of 0 to 120 cmH₂O.

Vital capacity was measured using the Wright Mark 8 analogue ventilator (Ferraris) with a 35 mm display, two 0-1 l/min and 0100 l/min dials. The patients are required to perform a maximal inspiration up to the total lung capacity and then a slow maximal expiration until the residual volume is reached. All patients should be sitting and perform three measurements, with a one-minute interval between them, adopting the highest value obtained as a reference.

After the initial evaluations, the patients were referred to the surgical center where they were always handled by the same medical team and were later sent to the intensive care unit (ICU) where they were managed according to the unit’s routine, which consists of non-invasive ventilation, breathing exercises and positive end-expiratory pressure. Finally, they were admitted to the ward after discharge from the ICU.

Upon hospital discharge, the patients had their lung function evaluated again. Note that no researcher has influence on the procedures adopted during the hospitalization or decision on hospital discharge.

One month after the surgery, the patients were evaluated for respiratory muscle strength and vital capacity upon their return to medical reevaluation. The evaluations at the two moments are always carried out by the same examiner.

Between August 2017 and April 2018, 30 patients who were admitted for coronary artery bypass grafting at Instituto Nobre de Cardiologia were evaluated. Of these, 18 (60%) were men and the average sample age was 62 ± 9. Other data related to the patient’s clinical characteristics are presented in table 1.

The patients had CPB time of 88 ± 15 minutes and average MV time of 7 ± 3 hours. Other data related to the patients’ surgical characteristics are shown in table 2.

Table 3 shows the three evaluation phases in the variables. There was a significant reduction in pulmonary function behavior at hospital discharge compared to the preoperative period and surgical revision. At hospital discharge, MIP (cmH₂O) dropped to 88 ± 9 and the preoperative value was 116 ± 5 (p < 0.001), as well as in the surgical revision with 109 ± 5 (p < 0.001). In MEP (cmH₂O), the variables also dropped compared to the preoperative period. At hospital discharge, the value was 67 ± 10 (p < 0.001) and, at surgical revision, it was 90 ± 8 (p < 0.001).

As for the variables of vital capacity (ml/kg), the preoperative period obtained 45 ± 12, and there was as a decrease in the hospital discharge figures (31 ± 9, p < 0.001) and surgical revision figures (39 ± 7, p = 0.33). At the peak expiratory flow (L/min), the preoperative period was 430 ± 40. At hospital discharge, the value dropped to 310 ± 59 (p < 0.001) and, at surgical revision, it was 390 ± 32 (p < 0.001).

Based on the results reported, it can be seen that coronary artery bypass grafting surgery may lead to reduced pulmonary function and it is not reestablished even after one month of the surgical procedure.

According to Barros et al.,7 inspiratory and peripheral muscle strength is often described as reduced after cardiac surgery. As analyzed in this study. Patients undergoing cardiac surgery repeatedly develop postoperative pulmonary dysfunction, always presenting severe reduction in pulmonary volumes, impaired respiratory mechanics and increased respiratory work.8

According to Cavenaghi et al.,9 pulmonary dysfunction after cardiac surgery is normal, in which the patient has severely reduced pulmonary volumes and capacities, decreased lung compliance, poorer respiratory mechanics and increased respiratory work. For Soares et al.,10 some intraoperative factors that may interfere with and justify the impairment of pulmonary function, such as the use of CPB, the degree of sedation, the intensity of surgical manipulation and the number of pleural drains, are indicated as some responsible for altering respiratory mechanics.

The results obtained in this article confirm these data and corroborate the current literature, since a significant drop was observed in all spirometric variables analyzed, comparing the values obtained in the postoperative period with those from the preoperative period, hospital discharge and surgical revision. These findings were evidenced in other studies on pulmonary function after cardiac surgeries, which showed that pulmonary function remains 25% to 30% lower even after 3.5 months of surgery.11^,12

Urell et al.,13 state that respiratory muscle strength is not compromised two months after cardiac surgery. In the study by Jonsson et al.,14 it was evidenced that after two months of cardiac surgery there is an increase in pulmonary function (vital capacity, functional residual capacity and total lung capacity) associated with increased physical activity level and, consequently, functionality. These results were not replicated in this study and may be related to the smaller sample size than the study by Jonsson et al.,14 Another cause for this difference in results may lie in the possibility of pain impacting lung function for up to 45 days due to the scarring process. As the patients in this study were evaluated after thirty days, there may be some interference, as opposed to the study by Jonsson et al.14

Pain has a significant negative correlation with the decreased values ​​in the variables of the study of Baumgarten et al.15 In this study, pain was not evaluated, but this may be a possible explanation for a reduction in the variables of this study.

Mueller and Lima16 state in their study that most patients that underwent cardiac surgery complain of intense postoperative pain, being one of the main causes of pulmonary complications due to the lower thoracic expansion, thus generating a shallow breathing.

Static and dynamic complacencies that reflect pulmonary function may be significantly reduced even after one year of surgery.17 In the study by Annoni et al.,1 patients undergoing coronary artery bypass grafting presented increased expiratory muscle strength and, consequently, peak expiratory flow, in addition to showing improvement in quality of life. As opposed to the results of this study, which shows inconsistent responses, since the PEF, MIP, MEP and VC values were smaller, thus showing loss of muscle strength.

These differentiated results can be associated with several factors. In the study by Annoni et al.,1 all patients were followed up by physiotherapy that prescribed individualized exercises at least twice a day. All patients were encouraged to perform a 6-minute walk test (6MWT) over 50 meters with medium intensity in the preoperative period. According to Laizo,18 the 6-minute walk test (6MWT) is a method that has been used to evaluate functional capacity and is also used as a predictor of morbidity and mortality in patients with cardiovascular and respiratory diseases.

Another justification may be the sample size, which was only twelve patients undergoing coronary artery bypass grafting, as opposed to this study, in which thirty patients were evaluated.

The limitations of this study include the lack of a sample calculation, failure to evaluate confounding variables such as pain and lack of correlation between pulmonary function behavior and clinical and functional outcomes.

It can be concluded that patients undergoing coronary artery bypass grafting surgery present significant worsening of pulmonary function in the postoperative period, causing significant reduction in respiratory, expiratory and peak expiratory flow, not returning to normal even after one month of the procedure.