Validating the sedation-agitation scale with the bispectral index and visual analog scale in adult ICU patients after cardiac surgery

Objective: To validate the Sedation-Agitation Scale (SAS) with the Visual Analog Scale (VAS) and Bispectral Index (BIS) in adult ICU patients after cardiac surgery. Design: Prospective study comparing blinded evaluations of the SAS, VAS and BIS. Setting: Forty-two-bed multidisciplinary ICU. Patients and participants: Convenience sample of 39 adults after cardiac surgery. Measurements and results: Bispectral Index 3.2 was continuously recorded using the Aspect A-1000 and evaluators were blinded to this value. The bedside nurse and a trained researcher independently rated wakefulness using a 100 mm VAS upon patient arrival on the ICU, at first awakening, when ventilator weaning was started and after extubation; the researcher also evaluated patients using SAS. Upon arrival on the ICU, the median SAS score was 2 (interquartile range = 1-3), the mean VAS was 26 +/- 30 and the mean BIS was 70 +/- 16. Twenty-four patients underwent a trial of weaning from mechanical ventilation with a SAS of 4 (IQR = 4), VAS of 86 +/- 12 and BIS of 87 +/- 10. SAS correlated well with VAS performed by one researcher (r = 0.91, p < 0.001) or by 19 different bedside nurses (r = 0.43,p < 0.001) and with BIS 3.2 (r = 0.60, p < 0.001). The correlation between SAS and BIS was reduced in patients with above average electromyogram (EMG) power. As a measure of construct validity, significant differences were noted for the BIS, SAS, VAS and EMG between ICU arrival and extubation (all p < 0.001). Conclusions: Sedation-Agitation Scale and BIS are valid measures of wakefulness after cardiac surgery, but EMG interference may affect the accuracy of BIS for a small percentage of patients not receiving neuromuscular blockade.