VENTRAL MEDULLARY SURFACE-ACTIVITY DURING HYPOXIA IN AWAKE AND ANESTHETIZED GOATS

The rostral ventrolateral medullary surface (VMS) plays a major state- dependent role in the control of breathing; its role during hypoxia re mains speculative. We therefore assessed activity within the rostral V MS by measuring reflectance of scattered light in 5 goats during normo xia, hypoxia, and hyperoxia in awake and halothane anesthetic states. Within the first minute of hypoxia, light reflectance began to decreas e in the awake state; reflectance reached a stable nadir within 30 min about 10 and 17% below control values (P < 0.01), at 12 and 10% inspi red O-2, respectively. In the anesthetized state, reflectance decrease d (P < 0.01) by 6% at 10% inspired O-2. After 30 min in the awake stat e, reflectance returned (P < 0.01) toward control values, reaching a s table level at 7 and 11% below control at 12 and 10% inspired O-2, res pectively (P < 0.05). Hyperoxia resulted in a 1% increase (P < 0.05) i n reflectance. Changes in reflectance during hypoxia did not consisten tly parallel changes in breathing, heart rate, or arterial blood press ure. We conclude that, a) decreased reflectance during hypoxia results , in part, from increased neural activity, and b) state exerts a subst antial effect on the response of VMS areas to hypoxia.