Effects of phenytoin therapy on bispectral index and haemodynamic changes following induction and tracheal intubation

Laryngoscopy and tracheal intubation (LTI) increase blood pressure and heart rate (HR). Intensity of these changes is influenced by the anaesthetic depth assessed by the bispectral index (BIS). We determined the effect of phenytoin on anaesthetic depth and its influence on haemodynamics following LTI. Fifty patients of ASA grades I and II on oral phenytoin 200 to 300mg per day for more than one week were compared with 48 control patients. Standard anaesthesia technique was followed. BIS, non invasive mean blood pressure (MBP) and HR were recorded 30, 60, 90 and 120 sec after LTI. Phenytoin group needed lesser thiopentone for induction, 5 mg (1.1) vs. 4.3 mg (0.7) [p=0.036]. BIS was significantly lower in the phenytoin group vs. the control 30, 60, 90 and 120 sec after LTI [43.1 (16.0) vs. 48.9 (14.9), p=0.068, 56.3 (16.7) vs. 64.3 (14.4), p=0.013, 59.8 (15.8) vs. 67.5 (12.1), p=0.008, 62.6 (14) vs. 68.9 (11.2), p=0.017, and 64.2 (11.3) vs. 69 (11.7), p=0.033], respectively. MBP was also lower in the phenytoin group 30, 60, 90 and 120 sec after LTI [112.8 mmHg (13.8), vs. 117.9 mmHg (18) p=0.013, 108.6 (12.8) vs. 117.5 (16) p=0.003, 106.1 mmHg (14.1) vs. 113.2 mmHg (14.9), p=0.017, 101.8 mmHg (13.8) vs. 109.5 mmHg (14.1), p=0.007], respectively. HR was lower in phenytoin group at 30 sec. (p=0.027), 60 sec (p=0.219), and again at 120 sec (p=0.022). Oral phenytoin therapy for over a week results in greater anaesthetic depth as observed using BIS, which also attenuated haemodynamic response of LTI.