Regional brain blood flow and O2 delivery during severe exertion in the pony

Regional distribution of brain blood flow (radionuclide labelled 15 microns diameter microspheres) and O2 supply were studied in 11 healthy adult grade ponies at rest and during severe exercise (SE) performed on a treadmill (heart rate = 220 +/- 4 beats X min-1; VO2 = 126 +/- 9 ml X min-1 X kg-1). During SE, the mean aortic pressure increased to 169 +/- 4 mm Hg and the pHa, PaCO2 and PaO2 were 7.213 +/- 0.010, 30 +/- 1 mm Hg and 85 +/- 4 mm Hg, respectively. The hemoglobin concentration increased by 59.6% with SE. Whereas blood flow increased in the cerebellar gray matter (96%), pons (39.5%) and medulla (55.6%), in none of the other brain regions blood flow was different from rest. However, vascular resistance decreased only in the cerebellar cortex. Due to a 58% increment in arterial O2 content with SE, the O2 supply to all regions of the brain increased (P less than 0.01). The latter may be suggestive of an overall increase in brain metabolism during heavy exertion in ponies.