Altres ajuts: We are thankful to Jonathan McFarland for his editing aid and to Mireia Admetllo and Camino Fernández for their help in collecting clinical data. This project was funded in part by SAF, CIBERES, BRN-Pla Armengol 2014, SEPAR 2015, and SEPAR Becario 2015.

Hypercapnic encephalopathy is relatively frequent in severe exacerbations of COPD (ECOPDs), with its intensity usually being evaluated through clinical scales. Bispectral index (BIS) is a relatively new technique, based on the analysis of the electroencephalographic signal, which provides a good approximation to the level of consciousness, having already been validated in anesthesia. The objective of the study was to evaluate the utility of BIS in the assessment of the intensity of hypercapnic encephalopathy in ECOPD patients. A total of ten ECOPD patients were included, and the level of brain activity was assessed using BIS and different scales: Glasgow Coma Scale, Ramsay Sedation Scale (RSS), and Richmond Agitation-Sedation Scale. The evaluation was performed both in the acute phase and 3 months after discharge. BIS was recorded for a total of about 600 minutes. During ECOPD, BIS values ranged from 58.8 (95% CI: 48.6-69) for RSS score of 4 to 92.2 (95% CI: 90.1-94.3) for RSS score of 2. A significant correlation was observed between values obtained with BIS and those from the three scales, although the best fit was for RSS, followed by Glasgow and Richmond (r =−0.757, r =0.701, and r =0.615, respectively; P <0.001 for all). In the stable phase after discharge, BIS showed values considered as normal for a wake state (94.6; 95% CI: 91.7-97.9). BIS may be useful for the objective early detection and automatic monitoring of the intensity of hypercapnic encephalopathy in ECOPD, facilitating the early detection and follow-up of this condition, which may avoid management problems in these patients.