Effect of morphine on the bispectral index during isoflurane anesthesia in dogs

To assess the effect of morphine on the bispectral index (BIS) in dogs during isoflurane anesthesia maintained at a constant end-tidal concentration. Prospective, randomized, experimental trial. Eight adult Beagle dogs, weighing between 7.1 and 9.8 kg. Anesthesia was induced with isoflurane via a face mask. Dog's tracheas were intubated and anesthesia maintained with isoflurane at a constant end-tidal concentration (e'Iso) of 1.81% for a 30-minute equilibration period. Pulmonary ventilation was controlled to normocapnia. After equilibration, baseline values were recorded prior to intravenous administration of morphine sulfate (0.5 mg kg(-1)) (MT) or an equal volume of saline (CT). Measurements for heart rate, systolic, diastolic and mean arterial pressure (SAP, DAP and MAP) were recorded at 10, 20, 30, 45, 60, 75, 90, 105 and 120 minutes after treatment. Bispectral index was recorded every 10 seconds for 3 minutes for each time measurement. Venous blood samples were collected at baseline, 10, 20, 30, 45, 60 and 120 minutes for determination of morphine serum concentrations. Anesthesia was discontinued after the last measurement and dogs were allowed to recover. Baseline BIS for MT and CT at 1.81%e'Iso were 63 +/- 10 and 58 +/- 9, respectively. Bispectral index in MT was 4-8% lower at 20, 75, 90 and 105 minutes compared with CT. There were no differences in BIS between baseline and any subsequent measurement within either MT or CT. Heart rate, SAP, MAP, and DAP decreased after morphine administration. Intravenous administration of 0.5 mg kg(-1) morphine sulfate did not cause clinically significant changes in the BIS of unstimulated dogs during isoflurane anesthesia at an e'Iso of 1.81%.