Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses.
OBJECTIVE
To test the hypothesis that head-down positioning in anesthetized horses increases intracranial pressure (ICP) and decreases cerebral and spinal cord blood flows.


METHODS
6 adult horses.


METHODS
For each horse, anesthesia was induced with ketamine hydrochloride and xylazine hydrochloride and maintained with 1.57% isoflurane in oxygen. Once in right lateral recumbency, horses were ventilated to maintain normocapnia. An ICP transducer was placed in the subarachnoid space, and catheters were placed in the left cardiac ventricle and in multiple vessels. Blood flow measurements were made by use of a fluorescent microsphere technique while each horse was in horizontal and head-down positions. Inferential statistical analyses were performed via repeated-measures ANOVA and Dunn-Sidak comparisons.


RESULTS
Because 1 horse developed extreme hypotension, data from 5 horses were analyzed. During head-down positioning, mean +/- SEM ICP increased to 55+/-2 mm Hg, compared with 31+/-2 mm Hg during horizontal positioning; cerebral perfusion pressure was unchanged. Compared with findings during horizontal positioning, blood flow to the cerebrum, cerebellum, and cranial portion of the brainstem decreased significantly by approximately 20% during head-down positioning; blood flows within the pons and medulla were mildly but not significantly decreased. Spinal cord blood flow was low (9 mL/min/100 g of tissue) and unaffected by position.


CONCLUSIONS
Head-down positioning increased heart-brain hydrostatic gradients in isoflurane-anesthetized horses, thereby decreasing cerebral blood flow and, to a greater extent, increasing ICP. During anesthesia, CNS regions with low blood flows in horses may be predisposed to ischemic injury induced by high ICP.
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Robert J
RJ
Brosnan
Alejandro
A
Esteller-Vico
Eugene P
EP
Steffey
Richard A
RA
LeCouteur
Irwin K M
IK
Liu
Betsy
B
Vaughan
D018685:Anesthetics, Inhalation / Q000008:administration & dosage* / Q000493:pharmacokinetics; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D001933:Brain Stem / Q000098:blood supply; D002490:Central Nervous System / Q000098:blood supply* / Q000187:drug effects; D002531:Cerebellum / Q000098:blood supply; D054022:Cerebrum / Q000098:blood supply; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006736:Horses / Q000502:physiology*; D007427:Intracranial Pressure / Q000187:drug effects / Q000502:physiology*; D007530:Isoflurane / Q000008:administration & dosage* / Q000493:pharmacokinetics; D008297:Male; D010477:Perfusion; D014159:Transducers / Q000662:veterinary
D018685:Anesthetics, Inhalation; D007530:Isoflurane