Bispectral Index Decreased to “0” in Propofol Anesthesia after a Cross-clamping of Descending Thoracic Aorta

THE Bispectral Index (BIS) monitor (BIS® monitor; As- pect Medical System, Inc., Newton, MA) integrates elec- troencephalographic data via a proprietary algorithm based on empirical data into a nominal 100 to 0 scale, in which 100 represents the awake state and 0 represents the isoelectric electroencephalogram. 1 The BIS® moni- tor is highly capable of correctly predicting both the loss and recovery of consciousness and has been reported to be more reliable in assessing the level of sedation and hypnosis during surgery than other available processed electroencephalogram algorithms. 2- 4 It also has been used clinically to guide anesthetic drug administration 5 and to quantify the pharmacokinetic and pharmacody- namic action of anesthetic drugs in the laboratory setting. 6 We report a case of a rapid decrease in the BIS score during total intravenous anesthesia using propofol after cross-clamping the descending thoracic aorta for elec- tive thoracic aortic aneurysm repair and suggest that the BIS® monitor provides useful information on altered propofol pharmacokinetics after aortic cross-clamping. advanced between the inferior vena cava and right atrium. The tip was confirmed to be correctly positioned by transesophageal echocardiog- raphy. A standard roller pump and oxygenator to circulate oxygenated blood from the long venous cannula to the femoral artery were avail- able for partial cardiopulmonary bypass of distal perfusion. During surgery, the patient was hemodynamically stable. A cross-clamp of the descending thoracic aorta at the level of Th 7 was performed, followed by the start of partial cardiopulmonary bypass of distal perfusion at a perfusion rate of 3-3.2 l/min under normothermic conditions. Guided by transesophageal echocardiographic images that revealed the left ventricular cavity size and function, perfusionists could regulate the rate of distal perfusion and level in the venous reservoir for adequate preload in the upper body. Approximately 15 min after cross-clamping, the patient's BIS level decreased rapidly from its steady-state value of 60 to a value of 0 (infusion rate of propofol, 6 mg · kg 1 ·h 1 ). Mean arterial pressure was 72 mmHg, and continuous cardiac output moni- toring indicated 1.2-1.4 l/min proximal to the aortic cross-clamp. Pulsation of bilateral carotid arteries was clearly palpable, suggesting adequate blood supply to the brain. Although depth of anesthesia was assessed as very deep, infusion of propofol was discontinued until the BIS started to increase. Fifteen minutes after infusion of propofol was stopped, the BIS began to increase by degrees. At a BIS score of 30, infusion of propofol was begun at 6 mg · kg 1 ·h 1 . Subsequently, the BIS score began to decrease again and was less than 10 at approxi- mately 25 min after restart of propofol infusion. This phenomenon could be reproduced by stopping and restarting of propofol infusion (Fig. 1). An aortic graft was inserted, with a cross-clamp time of 105 min. Approximately 15 min after declamping, the patient's BIS level increased slowly from a value of 20 to more than 30. Surgical proce- dures were completed uneventfully, and this patient was admitted to the intensive care unit, with an anesthetic time o f7h4 5min. The patient had recovered completely from anesthesia 6 h after admission to the intensive care unit and manifested no neurologic deficits. All blood samples drawn simultaneously from the radial artery and the left femoral artery, including those before cross-clamping of the aorta, at 3 time points during aortic cross-clamping, and 15 min after declamping, were retained for high-performance liquid chromatography (MC Med- ical, Inc., Osaka, Japan) analysis. High-performance liquid chromatog- raphy analysis revealed that the propofol concentrations of samples from the radial artery (proximal to the aortic clamp) during cross- clamping of the aorta were significantly elevated compared with pre- cross-clamping values. Also, propofol concentrations in samples from the radial artery were approximately 4 -5 times higher than those from the femoral artery (Fig. 1).