Cardiovascular diseases (CVDs) are among the main causes of death in developed countries and they remain the leading cause of illness and professional disability  , due to increased longevity, urbanization, and lifestyle habits . In Brazil, they also represent the first cause of mortality and the second largest cause of hospitalization  , with important consequences such as physical disability and invalidity, which contribute to the increase in health expenses  .
Among the forms of treatment for CVDs, cardiac surgery (CS) stands out, including myocardial revascularization, valve repairs and exchanges, and the correction of congenital heart diseases as the main procedures . CS and postoperative control (PO) have added, in the last decades, greater knowledge and scientific experience, with high-tech equipment and state-of-the-art drugs, making procedures faster, less invasive, and less harmful to cardiac patients. Minimally invasive interventions, without cardiopulmonary bypass (CPB) and with less surgical time are examples of this technological advance . Although it is an effective procedure and it has evolved over the years, its endpoint may have cardiac, pulmonary, renal, neurological, infectious, hematological, digestive, and electrolyte complications  .
Within the set of postoperative complications related to CS, several studies report that pulmonary dysfunction is prevalent        and its etiology may result from the association of multiple surgical risk factors   . The pre-surgical risk factors listed are age, smoking, previous medical history, and preoperative pulmonary function . Among the intra- and post-surgical risk factors are anesthesia, CPB time, cardiac arrest, surgical trauma, median sternotomy, pleurotomy, phrenic nerve palsy, time on mechanical ventilation, number and position of chest drains, and post-operative pain  .
Most patients undergoing CS develop postoperative pulmonary dysfunction with significant reduction in lung volumes, impaired respiratory mechanics, decreased lung compliance, and increased work of breathing . The reduction in lung volumes and capacities contributes to changes in gas exchange, resulting in hypoxemia and decreased diffusion capacity  . Atelectasis, pleural effusion, and pneumonia are pulmonary complications frequently found   . However, other complications such as hypercapnia, reintubation, and respiratory failure are also observed . These pulmonary dysfunctions and complications are responsible for prolonging hospital stay, increasing hospital costs, and an important cause of morbidity and mortality in patients undergoing CS .
Studies related to pulmonary dysfunctions in the postoperative period of CS until hospital discharge are well established in the literature     . Nonetheless, few studies report the permanence of the deleterious effects on lung function resulting from CS after hospital discharge     . According to Belle et al.  and Johnson et al. , little is known about the outcome of abnormalities in lung function in the weeks following hospital discharge. Westerdahl et al.  stated that the reduction in lung function and pain caused by median sternotomy are common and well documented until hospital discharge. However, few studies have assessed the deleterious effects of CS on long-term lung function.
In this context, the objective of the study was to verify in the databases the presence of investigations that show the permanence of pulmonary dysfunctions after hospital discharge of patients undergoing CS.
This study was characterized as a literature review with an integrative approach, which selected articles through the databases National Library of Medicine (Pubmed) and Scientific Electronic Library Online (Scielo). The search was carried out in March 2019, using the Health Sciences Descriptors (DeCS) in Portuguese: “cirurgia torácica”, “cirurgia cardíaca”, “teste de função pulmonar”, “teste de função respiratória”; and in English: “thoracic surgery”, “cardiac surgery”, “respiratory function test”, “pulmonary function test”.
As an inclusion criterion, articles published between January 1978 and March 2019, published in Portuguese and English, which presented abstracts available online and digital access in full, were selected. Articles that did not meet the inclusion criteria and were found to be repeated and/or duplicated in the databases were excluded.
The studies were selected first by reading the titles for each database, following the eligibility criteria. After this selection, the study abstracts were read and then the articles that were available in full were analyzed. Those that did not correspond to the basic question of the study and that did not meet the inclusion criteria were excluded (Figure 1).
Figure 1. Flowchart for the selection of studies. Source: Prepared by the authors; n: number of articles.
The main characteristics of the articles included in this review are described in Table 1. The analyzed data were distributed in columns, with the following description: author/journal/year of publication, title, objective, method, and endpoint.
3. Results and Discussion
The results found demonstrated the persistence of pulmonary dysfunctions in patients who underwent MRS and/or valve reconstruction surgery, in a period of time that varied from three weeks to one year of PO, showing that the impairment of lung function may continue beyond hospital discharge. Some authors have demonstrated long-term changes in lung function through the analysis of static and dynamic volumes         and the diffusing capacity of the lungs for carbon monoxide (DLCO)  . Other investigators reported the persistence of pulmonary dysfunctions through the observation of diaphragmatic disorders    . In turn, Landymore et al.  associated the presence of pulmonary dysfunctions with the types of grafts used in CS and Johnson et al.  correlated the dysfunctions with respiratory symptoms such as cough, discharge, wheezing, and dyspnea.
Westherdal et al.  and Braun et al.  demonstrated a decrease in static and dynamic volumes and DLCO, in relation to preoperative values after four months and one hundred and sixteen days of MRS, respectively. Shapira et al.  also found significant reductions in the values of static and dynamic volumes after three months of MRS. Similarly, Van Belle et al.  and Berriztbeitia et al.  showed a decrease in the values of static and dynamic volumes after six weeks and six to eight weeks of MRS, respectively. Shenkman et al.  noticed a greater decrease in both lung volumes after three weeks of MRS or valve surgery. Following three months of surgical intervention, Shenkman et al.  reported an increase in the studied variables without reaching preoperative values.
Rachwalic  and Vaydia et al.  observed persistent pulmonary dysfunction through the analysis of static and dynamic lung volumes in patients who underwent valve surgery and both reported a decrease in lung volumes. Two and a half months after valve surgery, Rachwalic  indicated that there was a 20% reduction in static and dynamic volumes and, in six months, they found that these values remained 5% below preoperative levels. Vaydia et al.  corroborated these findings when they concluded that 17 of the 31 patients in their study had reduced spirometric values three months after the surgical intervention
Other investigations have shown pulmonary dysfunction through the analysis of diaphragmatic disorders    . Baltieri et al. , in a case report of a patient undergoing MRS, demonstrated partial resolution of diaphragmatic dysfunction after 10 weeks of transcutaneous electrical diaphragmatic stimulation. Dimopoulou et al.  studied 63 patients who underwent two different cardiac cooling techniques (saline and “ice-slush”) during CS and they concluded that after 30 days, eight out of 10 patients still had reduced latency time
Table 1. Results of studies on the permanence of pulmonary dysfunctions after hospital discharge of patients who underwent cardiac surgery.
TEDS—transcutaneous electrical diaphragmatic stimulation; CS—cardiac surgery; n—sample; MRS—myocardial revascularization surgery; X-ray—X-ray; PO—postoperative; TV—tidal volume; FEV1—forced expired volume in the 1st second; FEF 25%—forced expiratory flow 25%; FEF 50%—forced expiratory flow 50%; FEF 75%—forced expiratory flow 75%; IC—inspiratory capacity; FRC—functional residual capacity; TPC—total pulmonary capacity; DLCO—diffusing capacity of the lungs for carbon monoxide; MEF—peak of maximum expiratory flow; PO—postoperative; FVC—forced vital capacity; % FVC—percentage of forced vital capacity; FEF 25% - 75%—forced expiratory flow 25% - 75%; % FEF 25% - 75%—percentage of forced expiratory flow 25% - 75%; IMA—internal mammary artery; MVV—maximum voluntary ventilation; VC—vital capacity; IMA—internal mammary artery; SVC—slow vital capacity. * ice-slush—topical myocardial cooling technique used during cardiac surgery.
of phrenic nerve conduction. Efthimiou et al.  verified 100 patients, divided into two groups: 50 individuals in Group I, with “ice-slush” and 50 in Group II, without “ice-slush”. After one month and one year of PO of CS, 12 and 5 patients in Group I, respectively, had diaphragmatic paralysis. In turn, Curtis et al.  observed diaphragmatic elevation in 79 patients in the first month and in 14 patients after one year; in both situations, patients were submitted to CS with “ice-slush”.
Landymore et al.  related pulmonary dysfunction to the type of graft used in CS, dividing 106 patients into three groups: Group I (control)—saphenous graft; Group II—internal mammary artery graft (IMA) with dissection of the pleura; and Group III—IMA graft without dissection of the pleura. The authors concluded that 53% of Group II patients had loss of left lung volume related to atelectasis and left pleural effusion after three months of CS. When analyzing the work by Johnson et al.  conducted with 138 patients who underwent MRS and valve surgery, it was possible to notice the presence of atelectasis and reduced respiratory muscle strength correlated with the presence of cough, discharge, wheezing, and dyspnea, after eight weeks of PO.
As for the observation time of pulmonary dysfunctions after CS, a large variation was found between the researchers analyzed. Most investigations have verified these endpoints over a period of three weeks to four months  -   and only two studies have extended the follow-up time of patients by up to one year  . Due to the heterogeneity of the methodological design of the studies in relation to the observation time of the endpoints, it was not possible to establish a comparative analysis to determine which type of pulmonary dysfunction could have persisted longer in the PO of CS.
Regarding the period of publication of the articles, nine selected works (64, 3%) are from the 1990s, with the most recently published article referring to 2012. No data were found in the researched literature that allowed to explain the reason for the accumulation of investigations in the 1990s and the lack of studies on the subject in the last fifteen years.
With regard to the quantity and origin of publications, this study revealed that almost all of the selected articles (92.8%) were produced internationally. Only one case report  was found, written in Portuguese, a fact that suggests that there is a shortage of similar publications in Brazil and/or in Portuguese that deal with the permanence of pulmonary dysfunctions after hospital discharge in patients who underwent CS.
This study found that patients who underwent CS develop reduced pulmonary function and that these events may persist beyond hospital discharge. Therefore, it is necessary to assess the importance of continuing respiratory physical therapy protocols started in the hospital phase. Westerdahl et al.  claim that no method of postoperative therapy has been used to prevent and treat changes in lung function in the long term, and it is also unclear how long after CS breathing exercises should be recommended. Likewise, Forshag and Cooper  suggest that an acceptable endpoint for breathing exercises will be when the patient’s lung function returns to preoperative levels or to a stable baseline.
The study concluded that pulmonary disorders remained beyond hospital discharge in patients who underwent CS. Such dysfunctions were observed in all selected studies and their duration varied from three weeks to one year of PO of CS. There seems to be a lack of research on the late effects of CS on lung function; there, more studies are needed to elucidate the mechanisms that lead to persistent changes in lung function after discharge from patients who underwent CS.
The authors developed and organized all parts of the text in conjunction.
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