Integrated pulmonary index can predict respiratory compromise in high-risk patients in the post-anesthesia care unit: a prospective, observational study

Background Respiratory compromise (RC) including hypoxia and hypoventilation is likely to be missed in the postoperative period. Integrated pulmonary index (IPI) is a comprehensive respiratory parameter evaluating ventilation and oxygenation. It is calculated from four parameters: end-tidal carbon dioxide, respiratory rate, oxygen saturation measured by pulse oximetry (SpO(2)), and pulse rate. We hypothesized that IPI monitoring can help predict the occurrence of RC in patients at high-risk of hypoventilation in post-anesthesia care units (PACUs). Methods This prospective observational study was conducted in two centers and included older adults (>= 75-year-old) or obese (body mass index >= 28) patients who were at high-risk of hypoventilation. Monitoring was started on admission to the PACU after elective surgery under general anesthesia. We investigated the onset of RC defined as respiratory events with prolonged stay in the PACU or transfer to the intensive care units; airway narrowing, hypoxemia, hypercapnia, wheezing, apnea, and any other events that were judged to require interventions. We evaluated the relationship between several initial parameters in the PACU and the occurrence of RC. Additionally, we analyzed the relationship between IPI fluctuation during PACU stay and the occurrences of RC using individual standard deviations of the IPI every five minutes (IPI-SDs). Results In total, 288 patients were included (199 elderly, 66 obese, and 23 elderly and obese). Among them, 18 patients (6.3 %) developed RC. The initial IPI and SpO(2) values in the PACU in the RC group were significantly lower than those in the non-RC group (6.7 +/- 2.5 vs. 9.0 +/- 1.3, p < 0.001 and 95.9 +/- 4.2 % vs. 98.3 +/- 1.9 %, p = 0.040, respectively). We used the area under the receiver operating characteristic curves (AUC) to evaluate their ability to predict RC. The AUCs of the IPI and SpO(2) were 0.80 (0.69-0.91) and 0.64 (0.48-0.80), respectively. The IPI-SD, evaluating fluctuation, was significantly greater in the RC group than in the non-RC group (1.47 +/- 0.74 vs. 0.93 +/- 0.74, p = 0.002). Conclusions Our study showed that low value of the initial IPI and the fluctuating IPI after admission to the PACU predict the occurrence of RC. The IPI might be useful for respiratory monitoring in PACUs and ICUs after general anesthesia.