Six Minutes-Walk Test and Respiratory Function Tests Predict Complications and Mortality in Coronary

Method: Longitudinal observational studies with patients who will perform elective of coronary artery bypass grafting at the cardiology Hospital. Were included clinically stable patients of both sexes and older than 18 years old. The data collection was divided into preoperative, intraoperative and postoperative period. For quantitative variables was used the Student’s t-test or the Mann-Whitney test. The verification of the association between categorical variables was used the Fisher or Chi-square test, and the Odds ratio estimate was calculated. The level of significance adopted was 5%.


INTRODUCTION
In the last decades, the frequency of surgical procedures has been increasing, especially coronary artery bypass grafting (CABG) [1]. CABG is considered one of the treatment options to alleviate symptoms, improve quality of life and decrease mortality in patients with coronary artery disease (CAD).
In individuals older than 70 years is estimated that 70% of the CAD incidence is submitted to CABG [2]. In addition, in 2012 in Brazil, more than 102.000 cardiac surgeries were performed, demonstrating a severe public health problem [3]. The surgical procedure is considered of high complexity, affecting functional capacity and lung function. The need for sternotomy, capture of the internal thoracic artery graft [4], pleurotomy for insertion of the drains [5], and the use of extracorporeal circulation (EC) [6], lead to deleterious effects on pulmonary function. In addition, the procedure can lead to neurological, renal and cardiac complications [7], reduced functional capacity [8] and lead to death [6].
It is known that CABG leads to postoperative pulmonary complications, but it is still uncertain whether functional and pulmonary conditions prior to the surgical procedure have a significant impact on the incidence of complications and intra-hospital mortality. To assess functional capacity, the 6-minute walk test (6MWT) in the cardiopathy has been shown to be a good predictor of cardiovascular events. It is a method that is simple, safe, reproducible and easy to apply.
According to the usual scientific literature, the 6MWT was considered a independent prognostic marker of mortality in patients with heart failure (HF) [9]. Regarding pulmonaryrespiratory evaluation, the use of manovacuometry aims to identify changes in respiratory muscle strength and respiratory muscle training has already been shown to reduce the incidence of complications in the postoperative period of CABG [10]. Spirometry can also be used to assess lung function and aims to identify changes in volumes and capacities commonly present in the postoperative period, since the surgical procedure may generate a restrictive pattern related to the reduction of pulmonary functional residual capacity (FRC) [11]. Due to the features of the surgical procedure, intra-and postoperative complications are present, and some available mortality scores and studies associate the clinical and operative characteristics with the increased incidence of complications and death. However, there is still a gap regarding the usefulness of functional and respiratory evaluation to assist in the identification and prevention of possible complications in order to guide a more appropriate management in order to prevent significant functional losses and complications in order to reduce the time of hospitalization. Thus, this study goal was to verify the relationship between clinical characteristics, functional capacity, spirometric values and respiratory muscle strength with possible intra and postoperative complications of CABG. for observational studies [12]. The patients were evaluated in the period of hospital admission for CABG and received follow-up in the pre and postoperative period (intensive care unit and ward) until hospital discharge.

Eligibility criteria
We included clinically stable patients of both sexes over the age of 18, who were literate and referred for elective coronary artery bypass surgery through the specialty outpatient clinic, who signed the Informed Consent Form (ICF). Patients with psychiatric or neurocognitive conditions who were unable to obtain reliable clinical data and/or musculoskeletal problems that interfered walk test evaluations were excluded from the study.

Sample
Ninety-seven patients met the eligibility criteria and signed the ICF. The sample was calculated from the study by Riedi et al. [13] using as main outcome the inspiratory muscle strength (IMS), whose standard deviation was found to be 50 cmH2O [15]. Assuming this standard deviation, for a significant difference of 50 cm H 2 O with 80% test power and 5% alpha (error type 1) performed by the Mann Whitney test, the target sample size was 40 subjects.

Data collection and following
After receiving the medical instructions to schedule the surgery, the patients were screened and referred to the multiprofessional clinic, where they were guided about the research and after signing the ICF, were screened according to the hospitalization forecast.
A specific evaluation form was used to collect the variables of interest. This evaluation was divided into three stages: preoperative, intraoperative and postoperative periods. In

Preoperative stage
In this stage, the clinical features related to the preoperative condition of the patients, collected from the chart were identified: sex, age, weight, height, date of hospital admission, personal history, European System for Cardiac Operative Risk Evaluation (EuroScore) [14], and left ventricular ejection fraction (LVEF) according to the echocardiogram, classified by the Simpson method [15].  The following were considered as complications in the ICU and ward: death, stroke, delirium, respiratory failure, increased bleeding, cardiorespiratory arrest, arrhythmias, shock and surgical wound infection.

6-minute walk test
The test was performed following the American Thoracic Society Guidelines [16] in a 30 meter corridor with flat surface; the patient was instructed to walk as fast as he could without running. If there was any sign or symptom of exertion intolerance, the patient could stop in the hallway and lean/sat in the chair. The test was performed only once.
At the beginning of the test were evaluated: blood pressure (BP), measured with sphygmomanometer (Premium®), heart rate (HR) and peripheral oxygen saturation (SpO 2 ) with pulse oximeter (Contec Medical Systems®) and perceived exertion using the adapted Borg Scale [16].
At the end of the walk the distance traveled was measured, and the HR, SpO 2 and the adapted Borg scale were repeated at 6 minutes and at 8 minutes of recovery. The examination was performed under the guidance of the same physiotherapist, who did not go along with the patient during the walk [16].

Pulmonary function test
Pulmonary function testing (PFT) was performed using the spirometer (Easy One Plus Portable Spirometer Machine®-ndd DOI : https://doi.org/10.35702/card.10003 Medical Technologies). The maneuver performed was forced vital capacity (FVC) with the patient in the seated position and using a nasal clip, and it was performed three times. The patient was instructed to breathe at the tidal volume level (TV), followed by a maximal inspiration from the FRC, then exhaled air to the residual volume (RV), ending with deep inspiration until lung total capacity (LTC) [17]. The values were predicted by age, height and sex according to the equation of Pereira et al. [18]. FVC, forced expiratory volume in the first second (FEV1) and the ratio of forced expiratory volume in the first second to forced vital capacity (FEV1/FVC) were evaluated.

Respiratory muscle strength
Measurement of ventilatory muscle strength was performed with the analogical manovacuometer (Comercial Médica®) with the individuals seated, the trunk at a 90º angle to the thighs and the airway occluded by a nasal clip. The patient was instructed to perform respiration at TV level, followed by an expiration to RV and a deep inspiration up to LTC for maximal inspiratory pressure (MIP) value [19,20].
The values obtained were MIP, a measure that predicts static inspiratory muscle strength, with a rest interval of 1 minute. A total of five maneuvers were performed, of which three were accepted and at least two of them should present difference values less than 10% of the highest value. To determine the predictive values of each patient, according to age and sex, we used the equations of Neder et al. [21].

EuroScore
EuroScore was calculated according to the 17-item model that assesses patient-related risk factors and cardiac and surgical factors by chart analysis. The score is divided into low mortality risk with score from 0 to 2, moderate risk from 3 to 5 and high risk with scores above 6 [22].
Patient risk factors assess age, sex, presence of chronic obstructive pulmonary disease, extracardiac arteriopathy, neurological dysfunction, previous cardiac surgery, active endocarditis, critical preoperative status, and serum creatinine values [22].
Cardiac risk factors evaluate the presence of unstable angina, left ventricular dysfunction, recent acute myocardial infarction, and the presence of pulmonary hypertension. Moreover, the surgical factors assess the presence of emergency procedure, other surgeries besides CABG, thoracic aortic surgery and the presence of post-infarction ventricular septal defect [22].

Statistical method
Quantitative variables were expressed as mean and standard deviation, and qualitative variables were expressed by absolute frequency and percentage. To evaluate the normal distribution of the data the Komogorov-Smirnov test was used.
For the comparison between the groups for the quantitative variables the Student's t-test or the Mann-Whitney test was used. Verification of the association between categorical variables was performed using Fisher's exact test or Chisquare test, and the estimated odds ratio (OR) was calculated.
The significance level was 5%.

Participants
A total of 97 participants were eligible for the study from

Characterization of the sample
The characteristics of the participants are shown in Table 1   of the predicted. In spirometry, it was possible to observe a mean FEV1/FVC ratio above 70% of the predicted and mean FVC above or equal 80% of the predicted, which values are classified as normal [18].   Legend: m=meters; %= percentage. Note : Value presented in mean ± SD; *= significance p>0.05.

General complications
The

Complications in the ward
Of the 27 participants who had complications, 29.2%

Deaths
All of the participants who died were in the ICU and presented a high BMI, indicative of obesity grade 1, which was considered a predictor of mortality in the sample evaluated (p=0.01). In addition, the predicted percentage of MIP prior to surgery was also considered a predictor of mortality (p=0.01), as these participants presented MIP on average 29.7% of predicted, a value that is 70% below the expected for a population without pulmonary disturbances. OR analysis has shown that the presence of diabetes can increase the chances of death. High BMI has a role that increases the chances of complications in the postoperative period, Dordetto et al. [25] have shown that overweight and obesity are indicators of surgical failure due to the pathogenesis of obesity that elevates inflammatory markers and further exacerbates inflammatory response when associated with DM type 2 [5].

DISCUSSION
Araújo et al. [26] analyzed the repercussion of overweight and obesity in the postoperative period of CABG and identified  [32] observed that reduced respiratory muscle strength is related to worsening pulmonary function, leading to a higher incidence of postoperative pulmonary complications.
In the evaluation of functional capacity, the 6MWT was not considered a predictor of complications and mortality in our population, despite being predictive of hospitalization and mortality in patients with HF. According to Rubim et al.
[9] the 6MWT 'has been shown to be reliable for evaluation and to predict mortality in a potent and independent way in patients with HF, whose degree of left ventricular dysfunction is related to a worse prognosis of the disease. In addition, it was considered to be of intense activity for these patients, not being representative of activities of daily living. In preevaluation of patients for the cardiac transplant queue, the 6MWT 'showed a correlation between the distance travelled, perceived exertion and heart rate when compared to surviving patients and those who died [33].
Another important finding was the incidence of non-dialytic

Limitations
We had a considerable loss of patients due to cancellation of the surgery, and others types of approach to CABG that were concomitant, such as valve replacement and aneurysm repair. Futhermore, high-risk patients or those who required emergency surgery, which could impact on the results, were not evaluated. For the future, there is a need for new observational studies that include this type of patients.

CONCLUSION
The present study identified as predictors of complications and in-hospital mortality the preoperative spirometric values below predicted (FEV1 and FVC) and reduced values of inspiratory muscle strength; the six-minute walk test was not a predictor.