Changes in cerebral blood volume with changes in position in awake and anesthetized subjects

Changes in posture affect cerebral blood volume (CBV), and moderate head-up tilt is used as a therapeutic maneuver to reduce CBV and intracranial pres sure. However, CBV is rarely measured in the clinical setting. Near-infrare d 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 th e awake subjects, changes in CBV were correlated with the degree of table t ilt. CBV decreased with 18 degrees head-up tilt and increased with 18 degre es 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 posi tion, 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 ti me measurement of changes in CBV at the bedside. Implications: Near-infrare d spectroscopy, a bedside technique, has been used to measure changes in ce rebral 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 h ead up position, the decease in cerebral blood volume is attenuated, relati ve to normal subjects.