Effects of body position on intracranial and cerebral perfusion pressures in isoflurane-anesthetized horses

Inhalant anesthetics may interfere with normal cerebrovascular autoregulation. It was, therefore, hypothesized that intracranial pressure (ICP) and cerebral perfusion pressure (CPP) in isoflurane-anesthetized horses would be especially sensitive to body and head position because of the potential for large hydrostatic gradients between the brain and heart in this species. Anesthesia was induced and maintained in six clinically healthy, unmedicated geldings with 1.57% isoflurane in O(2); mechanical ventilation was used to maintain normocapnia. ICP was measured by using a subarachnoid strain-gauge transducer. Blood gases and carotid arterial, right atrial, and airway pressures were also measured. Five body positions were studied in semirandomized order: dorsal recumbency (DR) with head down (HD), DR with head level (HL), lateral recumbency (LR), sternal recumbency (SR) with HL, and SR with head up (HU). Data were analyzed by using paired t-tests. ICP and CPP values, respectively, are as follows (means +/- SD): 36 +/- 4 and 55 +/- 18 mmHg (DR-HD); 34 +/- 6 and 51 +/- 32 mmHg (DR-HL); 24 +/- 5 and 48 +/- 4 mmHg (LR); 19 +/- 11 and 87 +/- 12 mmHg (SR-HL); and -14 +/- 7 and 71 +/- 10 mmHg (SR-HU). Significant differences were found among all positions, except for SR-HL vs. LR. Significant increases in CPP were observed only in sternal positions. In conclusion, ICP in isoflurane-anesthetized horses changes inversely with the brain-to-heart hydrostatic gradient. DR may also cause increases in ICP, irrespective of head position.