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 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 degrees head-up tilt and increased with 18 degrees 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.


CONCLUSIONS
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.
Lovell
A T
AT
Department of Surgery, University College London Medical School, The Middlesex Hospital, London, UK.
Marshall
A C
AC
Elwell
C E
CE
Smith
M
M
Goldstone
J C
JC
D000328:Adult; D000758:Anesthesia; D018686:Anesthetics, Intravenous; D001794:Blood Pressure / Q000187:drug effects; D001810:Blood Volume / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004562:Electrocardiography; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D008297:Male; D008875:Middle Aged; D009020:Morphine; D009294:Narcotics; D010100:Oxygen / Q000097:blood; D019095:Point-of-Care Systems; D011187:Posture / Q000502:physiology*; D015742:Propofol; D019265:Spectroscopy, Near-Infrared; D016683:Supine Position / Q000502:physiology; D018667:Tilt-Table Test; D014851:Wakefulness / Q000502:physiology*
D018686:Anesthetics, Intravenous; D006454:Hemoglobins; D009294:Narcotics; D009020:Morphine; D010100:Oxygen; D015742:Propofol