Regional distribution of blood flow in the brain of horses at rest and during exercise.

The study explores the distribution of blood flow in different parts of a horse’s brain both at rest and during exercise. The researchers noted differences in blood flow to various regions of the brain, and found that despite changes in heart rate, oxygen levels, and blood pressure during exercise, the control of blood flow to the brain is well-maintained.

Methodology

• The research took place with nine clinically normal horses.
• Regional brain blood flow was assessed using radionuclide-labeled microspheres (tiny spheres that emit radiation) that have a diameter of 15 microns. These were injected into the left ventricle, one of the heart’s four chambers.
• The researchers obtained reference blood samples from the horse’s aorta, the main and largest artery in the body.

Results

• At rest, higher blood flow was observed in the cerebral cortex and the caudate nuclei, compared to the white matter of the brain.
• A similar pattern of perfusion or blood flow distribution was observed in the cerebellum, another part of the brain.
• In the brain stem, blood flow tapered off from the thalamus-hypothalamus region to the medulla
• During exercise, there were gradual increases in heart rate, aortic pressure, and right atrial pressure.
• Exercise also caused the horses to have reduced oxygen levels in the arteries (hypoxemia) and increased levels of carbon dioxide (hypercapnia).
• Despite these changes, the cerebral and cerebellar grey-to-white matter perfusion remained constant, suggesting that the different regions have different metabolic oxygen needs.
• Exercise did not alter the blood flow to the cerebral cortex and caudate nuclei significantly, but there was a noted decrease in blood flow to the cerebral white matter.
• All cerebral tissues showed increased vascular resistance during exercise, indicating a self-regulating mechanism in controlling blood flow to different parts of the brain.
• The cerebellar cortex saw an increase in blood flow with strenuous exercise due to vasodilation, the process of blood vessels widening.
• However, in the cerebellar white matter, vascular resistance increased during exercise.
• Blood flow in the medulla, pons, midbrain, and thalamus-hypothalamus remained relatively constant during exercise.

Conclusions

• Regardless of arterial hypoxemia, hypercapnia, and hypertension triggered by exercise, the autoregulation of cerebral and cerebellar blood flow is effectively maintained in horses during physical exertion.