The present study demonstrated that cardiac surgery patients who received MI-E therapy had better lung function compared to those who received IPPB therapy. The incidence of post-operative pulmonary complications including pneumonia, atelectasis, and pleural effusion, were similar in both the groups. However, the number of subjects who experienced chest pain was higher in the MI-E group, compared to the IPPB group.
Compatible with previous reports , the current study observed significant decline in the values of effort-dependent lung function tests, such as FVC, FEV1, and peak expiratory flow rate, after surgery in both groups. The study subjects developed a proportional decrease in lung volume without change in the FEV1/ FVC ratio; indicating a restrictive pulmonary defect after surgery. The normal activity of most of the respiratory muscle groups is impaired after a major surgery, including the airway muscles, abdominal muscles, and diaphragm . Factors contributing to this dysfunction include anesthetic agents and neuromuscular blocking agents, drugs used for post-operative analgesia, pain, disturbed sleep patterns and the inflammatory response to surgery. In addition to the simple muscle weakness, the etiology of decreased lung function may involve poor sputum clearance, poor co-ordination between muscle groups and failure of the normal physiological reflexes, which control the effort-dependent lung function. Theoretically, MI-E therapy may simulate coughing and facilitate better sputum clearance in patients. In the present study, the percentage of improvement of segmental and lobar atelectasis in the MI-E group was observed to be higher, compared to the IPPB group. However, the difference was not statistically significant; probably due to the inadequate sample size. Therefore, further research is mandatory to elucidate the effects of MI-E therapy on the improvement of atelectasis and lung function.
Several studies have demonstrated that pulmonary complications are more common than cardiac complications, in patients undergoing cardiac surgery [2, 5]. In patients who undergo cardiac surgery, the diminution in lung volume provokes atelectasis. Discoid atelectasis is apparently unavoidable, in the immediate postoperative phase. Pleural fluid has been shown to predispose to atelectasis . The high occurrence of pleural effusions and discoid atelectasis leads to their coincidental appearance. In the present study, development of postoperative complications were observed to be similar between the two study groups; indicating that both types of respiratory therapy devices, which were used in the study, are equally effective in preventing post-operative complications. Several devices have been used to reduce post-operative pulmonary complications. IPPB was first introduced in the 1940s . It became particular popular in 1980s and 1990s, before other devices such as CPAP, BiPAP and high-flow nasal cannula therapy became available in clinical practice. However, the role of IPPB is still controversial [6, 16,17,18,19,20]. Literature reports that the incidence of atelectasis in patients who received post-operative IPPB therapy ranges from 40 to 55% [17, 20]. The present study showed that 50% of subjects in the MI-E group and 42.9% of subjects in the IPPB group developed atelectasis, which was concurrent with the previous studies. The incidence of chest pain was significantly higher in the MI-E group, compared to the IPPB group. Due to similar peak airway pressure target and tidal volume setting in both the groups, a probable explanation for the increased incidence of chest pain may be the active negative-pressure exsufflation in MI-E. The MI-E therapy design may simulate a cough or even induce cough and move the secretions towards the mouth. Although the pain could be controlled by remedial medical treatment, caregivers should pay more attention to pain control in cardiac surgery patients, who receive post-operative MI-E therapy.
As is the case with any mechanical positive-pressure device, potential complications of in-exsufflation include abdominal distention, aggravation of gastroesophageal reflux, hemoptysis, chest and abdominal discomfort, acute cardiovascular effects, and pneumothorax. However, rarely have these been reported in literature . Physiological effects on the cardiovascular system were studied during the early phases of development of in-exsufflation. The effects on cardiovascular system include an increase in the peripheral venous pressure (about one third more than during normal coughing), and slight increase in blood pressure . Pulse can increase or decrease with in-exsufflation. Severe bradyarrhythmia has been reported in patients with high spinal cord injury and premature ventricular contractions have been reported in an adolescent with Duchenne muscular dystrophy [21, 22]. Prudent measures to avoid the complications of in-exsufflation include short rest breaks between applications of in-exsufflation; avoid hyperventilation and administration of in-exsufflation before meals or feedings, vigorous medical treatment of gastroesophageal reflux, and adequate treatment of any airway inflammation.
A major limitation of the present study is its retrospective nature, which may have led to bias in the selection of study subjects. Additionally, the sample size of the study is small, and therefore the results should be interpreted with caution. Lastly, a prospective study with a larger sample size is warranted to further confirm the clinical outcomes in patients receiving IPPB or MI-E therapy. Emerging evidence suggests that MI-E therapy is useful in a selected group of patients, such as those with muscle weakness and without airway instability . Nevertheless, before its routine use in clinical practice, more studies are needed to investigate the safety and utility of MI-E therapy in the clearance of secretions from central and peripheral airways, as well as the impact on clinically relevant outcomes, including the incidence of ventilator-associated pneumonia, improvement of atelectasis, length of ICU/ hospital stay, and mortality. If mechanical ventilator day were used as primary aim point, the sample size will be 142 for each group with desired power 80% and α = 0.05.