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LETTER TO THE EDITOR

Transient but Profound Reduction of Bispectral Index Values After Tourniquet Deflation: Did the BIS Detect An Alteration of Brain Electrocortical Activity?

Japanese Red Cross Society of Wakayama Medical Center; simosimo_7{at}ybb.ne.jp (Shimogai, Iranami, Yamazaki) Department of Anesthesiology; Wakayama Medical University; Wakayama City; Wakayama, Japan (Hatano)

To the Editor:

We (1) previously reported the occurrence of astroglial cell damage immediately after tourniquet deflation from bone cement. In a recent arthroscopic knee repair, tourniquet deflation was followed by a large, transient decrease in bispectral index (BIS), suggesting the release of circulating substances that altered the brain’s electrocortical activities.

A 27-year-old softball player underwent video-assisted repair of the left anterior cruciate ligament. The surgery was aided by a pneumatic tourniquet on the proximal left thigh, inflated to a pressure of 250 mm Hg. When the tourniquet was deflated, the BIS abruptly decreased to lower values, <10, for approximately 10 min (Figs. 1a and b). Then, over the next 10 min, the BIS value gradually increased, until it returned to the previous value of approximately 60. His arterial blood pressure (110–130/70–80 mm Hg), heart rate (60–90 bpm), and pupil size (5–6 mm) were unchanged during this period. End-tidal CO₂ transiently increased to 58 mm Hg upon deflation, and then returned to the baseline value of 28–35 mm Hg. Anesthetic emergence was rapid and unremarkable, with no suggestion of neurological deficit.

View larger version (15K): [in this window] [in a new window]   Figure 1. (a) BIS values plotted against time during the anesthesia. The horizontal bar denotes the duration (approximately 5 min) of low BIS values. The closed circle denotes the deflation of pneumatic tourniquet. (b) Recorded bipolar electroencephalograph (1–2 Hz) during the low BIS period. The configuration of waves does not show any apparent artifact components.

Tourniquet-induced limb ischemia and exsanguination lead to production of H₂O₂ and free radicals (2,3). Skeletal muscle is highly vulnerable to ischemia (4). Increased pressure within an osteofascial compartment, found in both acute trauma and exercise-induced elevated compartment pressure, may result in circulatory compromise (5). Indicators of skeletal muscle ischemia, including interstitial concentrations of glucose, lactate, and hypoxanthine, increase more prominently after limb exsanguination than after circulatory occlusion alone (6). Tourniquet-induced exsanguination and ischemia during limb surgery may cause more severe ischemic changes in muscular, athletic patients than in patients of average physical condition.

Glial cells and oligodendrocytes may be injured by ischemia-reperfusion events (7). These cells’ electrocortical activity is negatively correlated to the concentration of hypoxantine (8). We speculate that the transiently low BIS in this case was caused by the brain’s response to toxic substances, such as hypoxantine, produced during the tourniquet-generated exsanguination and ischemia.

REFERENCES

1. Kinoshita H, Iranami H, Fujii K, et al. The use of bone cement induces an increase in serum astroglial S-100B protein in patients undergoing total knee arthroplasty. Anesth Analg 2003;97:1657–60.[Abstract/Free Full Text]
2. Mathru M, Dries DJ, Barnes L, et al. Tourniquet-induced exsanguination in patients requiring lower limb surgery. An ischemia-reperfusion model of oxidant and antioxidant metabolism. Anesthesiology 1996;84:14–22.[Web of Science][Medline]
3. Friedl HP, Till GO, Trentz O, Ward PA. Role of oxygen radicals in tourniquet-related ischemia-reperfusion injury of human patients. Klin Wochenschr 1991 15;69:1109–12.
4. Blaisdell FW. The pathophysiology of skeletal muscle ischemia and the reperfusion syndrome: a review. Cardiovasc Surg 2002;10:620–30.[Web of Science][Medline]
5. Awbrey BJ, Sienkiewicz PS, Mankin HJ. Chronic exercise-induced compartment pressure elevation measured with a miniaturized fluid pressure monitor. A laboratory and clinical study. Am J Sports Med 1988;16:610–15.[Abstract/Free Full Text]
6. Korth U, Merkel G, Fernandez FF, et al. Tourniquet-induced changes of energy metabolism in human skeletal muscle monitored by microdialysis. Anesthesiology 2000;93:1407–12.[Web of Science][Medline]
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8. van Os S, de Abreu R, Hopman J, et al. Purine and pyrimidine metabolism and electrocortical brain activity during hypotension in near-term lambs. Biol Neonate 2006;89:35–41.[Web of Science][Medline]

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