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NEUROSURGICAL ANESTHESIA

Changes in Cerebral Blood Volume with Changes in Position in Awake and Anesthetized Subjects

A. Timothy Lovell, MB, BS, FRCA^*, Alan C. Marshall, MB, BS, FRCA, Clare E. Elwell, PhD, Martin Smith, MB, BS, FRCA, and John C. Goldstone, MD, FRCA^*

*Division of Academic Anaesthesia, Department of Surgery, University College London Medical School, The Middlesex Hospital; Department of Neuroanaesthesia, The National Hospital for Neurology and Neurosurgery; Department of Medical Physics and Bioengineering, University College London, London, United Kingdom

Address correspondence and reprint requests to Dr. J. C. Goldstone, Division of Academic Anaesthesia, UCL Medical School, Room 103, The Middlesex Hospital, Mortimer St., London W1N 8AA, UK.

Changes in posture affect cerebral blood volume (CBV), and moderate head-up tilt is used as a therapeutic maneuver to reduce CBV and intracranial pressure. However, CBV is rarely measured in the clinical setting. Near-infrared spectroscopy allows real-time bedside monitoring of cerebral hemodynamics, and we have used this technique to measure changes in CBV with changes in posture in 10 normal subjects and 10 propofol-anesthetized patients. In the awake subjects, changes in CBV were correlated with the degree of table tilt. CBV decreased with 18° head-up tilt and increased with 18° head-down tilt (P < 0.0001, r = -0.924). In anesthetized patients, there were differences between head-up and head-down tilt. In the head-down position, CBV was also correlated with the degree of table tilt (P < 0.001, r = -0.782), whereas there was a clinically insignificant reduction in CBV in the head-up position. Near-infrared spectroscopy allows continuous, real time measurement of changes in CBV at the bedside.

Implications: Near-infrared spectroscopy, a bedside technique, has been used to measure changes in cerebral blood volume in normal subjects. We have used the same technique in anesthetized patients and have shown that, when a patient is placed in the head up position, the decrease in cerebral blood volume is attenuated, relative to normal subjects.

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