Intracranial elastance in isoflurane-anesthetized horses.
OBJECTIVE
To determine whether high intracranial pressure (ICP) during spontaneous ventilation (SV) in anesthetized horses coincides with an increase in intracranial elastance (ie, change in ICP per unit change of intracranial volume).


METHODS
6 adult horses.


METHODS
Anesthesia was induced and maintained in each horse for 5 hours with isoflurane at a constant dose equal to 1.2 times the minimum alveolar concentration. Direct ICP measurements were obtained by use of a strain gauge transducer inserted in the subarachnoid space, and arterial blood pressure was measured from a carotid artery. Physiologic responses were recorded after 15 minutes of normocapnic controlled ventilation (CV) and then after 10 minutes of SV. Aliquots (3 mL) of CSF were removed from each horse during SV until ICP returned to CV values. Slopes of pressure-volume curves yielded intracranial elastance.


RESULTS
Intracranial elastance ranged from 0.2 to 3.7 mm Hg/mL after removal of the first aliquot of CSF Slopes of pressure-volume curves were largest following removal of the initial CSF aliquot, but shallow portions of curves were detected at relatively high ICPs (25 to 35 mm Hg). A second-order relationship between SV ICP and initial intracranial elastance was found.


CONCLUSIONS
In horses anesthetized with isoflurane, small changes in intracranial volume can cause large changes in ICP Increased intracranial elastance could further exacerbate preexisting intracranial hypertension. However, removal of small volumes of CSF may cause rapid compensatory replacement from other intracranial compartments, which suggests steady-state maintenance of an increase in intracranial volume during isoflurane anesthesia in horses.
Brosnan
Robert J
RJ
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
LeCouteur
Richard A
RA
Steffey
Eugene P
EP
Imai
Ayako
A
Farver
Thomas B
TB
D000704:Analysis of Variance; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002245:Carbon Dioxide / Q000097:blood / Q000134:cerebrospinal fluid; D015170:Cerebrospinal Fluid Pressure / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects; D006736:Horses / Q000502:physiology*; D007427:Intracranial Pressure; D007530:Isoflurane / Q000494:pharmacology*; D010100:Oxygen / Q000097:blood / Q000134:cerebrospinal fluid; D012123:Pulmonary Ventilation / Q000187:drug effects
D018685:Anesthetics, Inhalation; D002245:Carbon Dioxide; D007530:Isoflurane; D010100:Oxygen