Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses.

This research investigated how head-down positioning in anesthetized horses affects blood flow to their brain and spinal cord. The results show that positioning a horse this way increases the pressure within its skull, decreases blood flow to certain areas of the brain, and does not affect spinal cord blood flow. The findings suggest that during anesthesia, certain areas of a horse’s nervous system, due to their low blood flow, could be more susceptible to damage from increased pressure inside the skull.

Hypothesis and testing methods

• The study was conducted to test the hypothesis that positioning an anesthetized horse with its head down would increase the pressure inside the skull and decrease blood flow to the brain and spinal cord.
• To test this, six adult horses were anesthetized using ketamine hydrochloride and xylazine hydrochloride, with an anesthesia was maintained with 1.57% isoflurane in oxygen.
• An intracranial pressure (ICP) transducer was placed in the subarachnoid space, an area between layers of tissue covering the brain and spinal cord. Catheters were also placed in the left cardiac ventricle and in multiple vessels for measurement purposes.
• Measurements of blood flow were made by using a fluorescent microsphere technique while each horse was placed in horizontal and head-down positions.

Results of the study

• Out of six, data from five horses were considered for analysis as one horse developed extreme low blood pressure (hypotension).
• It was found that during head-down positioning, the mean ICP increased to 55(+/-2) millimeters of Mercury (Hg), compared with 31(+/-2) mm Hg during horizontal positioning. However, the blood pressure in the brain (cerebral perfusion pressure) was not affected by the positioning.
• Compared to when the horse was in a horizontal position, blood flow to the cerebrum, cerebellum, and the topmost part of the brainstem decreased significantly by about 20% during head-down positioning.
• However, blood flow within the pons (a part of the brain involved in sleep and consciousness), and medulla (an area responsible for breathing and circulation) were only mildly decreased and not significant.
• Spinal cord blood flow was low and not affected by the horse’s position.

Conclusion of the study

• The research concluded that head-down positioning increases the pressure gradient between the heart and brain in isoflurane-anesthetized horses, which decreases cerebral blood flow and, to a greater extent, increases ICP.
• Therefore, during anesthesia, certain areas of a horse’s nervous system already with low blood flows could be more prone to suffer ischemic injury (damage that happens when blood flow to an area of the body is blocked) due to increased ICP.