Comparison of rapid shallow breathing index (RSBI) calculated under direct and indirect form in the postoperative period of cardiac surgery

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Objectives: To compare and analyze whether the values of rapid shallow breathing index (RSBI) determined by means of a ventilator display and a digital respirometer were correlated. Methods: Twenty two adult patients (17 males and 5 females) in the postoperative period of cardiac surgery and in mechanical ventilation were studied. Before data collection, each patient was evaluated, received physiotherapy, in order to promote hygiene bronchial and pulmonary reexpansion, care and was positioned in elevated dorsal recumbent at forty-five degrees. After these procedures, minute ventilation (MV) and respiratory rate (RR) obtained from mechanical ventilator display and digital respirometer were recorded. The RSBI was calculated by the ratio of RR and tidal volume (VT). Paired t-test to compare related variables. The intra-class correlation coefficients (ICCs) were determined. Results: A significant difference was found between the RSBI obtained from the ventilator and by the digital respirometer (p=0.011). The two methods exhibited high interrelation. There was a close correlation between RSBI (ICC=0.86), a moderate concordance related to MV (ICC=0.74) and high concordance related to RR (ICC=0.8) and VT (ICC=0.79) were observed. To all variables the p-value was <0.05. Conclusions: There were a significant concordance between the RSBI obtained from the ventilator display and the digital respirometer. However, the sample was small and more studies are needed to establish a model of correction of obtained values, as long as many authors have reported that pressure support ventilation can overestimate the RSBI. Key words: ventilator weaning, mechanical ventilation, rapid shallow breathing index, respiratory therapists cardiac surgical procedures