DISCUSSION
Noninvasive ventilation performed immediately appeared as an effective resource in decreasing the loss of functional capacity, the rate of reintubation, improvement of oxygenation up to one day after its completion and reduction of hospital stay in patients undergoing coronary artery bypass grafting.
In our study, patients who underwent immediate NIV presented a reduction in the loss of functional capacity, as evidenced by the distance covered in the 6MWT, a similar result was verified by Araújo-Filho et al.16 in patients in the postoperative period of valve replacement. Possible justification for this decrease is due to the meta-reflex. The realization of NIV increases pulmonary capacity and oxygenation, thus attenuating the metaborreflex causing improvement in the perfusion of peripheral muscles, causing an increase in functional performance17-19.
Performing the 6MWT raises the patient’s metabolic rate, requiring greater blood flow to the peripheral muscles. The muscle fibers recruited during the 6MWT are type I, that is, it depends on oxygen, so when better lung function and greater blood flow rich in oxygen, the better the functional capacity of these patients. In the present study, the difference in the 6MWT was 44 meters between the conventional and immediate groups. Gremeaux et al.20 show that the difference in the 25-meter result becomes clinically important for this patient profile.
Another fundamental point in this discussion is that NIV tends to improve the performance of the left ventricle, optimizing cardiac output and improving tissue perfusion21. This contributes to improving the functional capacity of these patients. It is important to understand that the application of NIV immediately after extubation is a resource that optimizes lung function, but the improvement in performance in the walking test can be achieved with an increase in cardiovascular function and peripheral muscles. Probably the second did not influence the result since there was no difference in the groups’ MRC. There is a need to verify, through echocardiogram, myocardial behavior such as ejection fraction, stroke volume and ventricular mass.
Shoji et al.22 found a high rate of reintubation among patients undergoing cardiac surgery. They attribute this result to comorbidities such as hypertension and diabetes mellitus and to complications such as pneumonia and renal dysfunction. Therefore, our study brings NIV immediately, as a preventive factor for these complications, reducing the risk of extubation failure.
According to Wu et al.23 the role of NIV remains controversial, since the rate of reintubation does not present a significant difference, however, some authors indicate immediate NIV to avoid complications and reduce hospital stay24,25. One possibility for the divergent results is the fact that the duration of the application of NIV, the selection of patients and the protocols performed.
According to the Brazilian guideline on mechanical ventilation, the use of NIV is indicated in obese, elderly and patients with more than one comorbidity12. As a result, we realized that the patients in our study were older, overweight and had two or more comorbidities, with satisfactory results after using immediate NIV, being able to reduce the reintubation rate.
Liu et. al 26 in their study show that the prophylactic use of NIV significantly reduced the rate of post-surgical complications, also showing improvement in gas exchange. The immediate use of NIV significantly reduced the rate of atelectasis in our study. The main effect of positive pressure at the end of expiration during NIV is to reopen collapsed alveoli and keep the lung aerated. This reversal of alveolar collapse tends to improve the ventilation / perfusion ratio, generating an increase in gas exchange, which was found in the present study.
In addition, it is worth noting a higher PaO2 / FiO2 ratio in patients who underwent immediate NIV even after 24 hours of the intervention. Despite the lack of significance in arterial oxygen pressure, recruitment of the alveoli generated less need for supplemental oxygen, which reflected in the relationship between the effectiveness of gas exchanges.
This way, it was possible to keep the patient with oxygenation level for a longer time, with a lower O2 supply and decreasing the toxicity related to the use of this gas. Therefore, in line with our result, Landoni et al.17 demonstrate that noninvasive ventilation seems to be a useful tool to decrease respiratory work, reduce atelectasis, prevent respiratory failure and improve gas exchange.
According to Laizo et al.27, complications related to respiratory function are the main causes to increase the length of hospital stay. Since in our study the rate of respiratory complications was low, in the group that performed NIV immediately after extubation, it may be a justification for reducing the length of hospital stay. This decrease can contribute to lower hospital costs and as a preventive factor for future complications associated with prolonged hospital stay, such as infections and loss of muscle mass.
Systematic reviews found that immediate NIV did not achieve a significant result in terms of length of stay in the ICU or hospital17,28. This can be justified by the profiles of the patients studied, who had low ejection fractions, hypoactivity and important deficits in muscle strength associated with heart failure. Contrary to our study, the patients evaluated did not present any hemodynamic instability before NIV, did not need surgical reintervention and mainly obtained positive results on the functionality scale.
One of the limitations of this study is the fact that it did not present the sample calculation, which would be effective for reaching a conclusion with a lower percentage of error. Other limitations were the fact that the study did not use a scale to assess patients’ pain level, such as Visual Analogue Scale (VAS), where through it the patient can present the degree of pain at the moment and the absence of a blind examiner for variables as blood gas analysis.