Hypoxia and hypercapnia in asphyctic differentiation of regional sympathetic activity in the anesthetized rabbit

Summary The contributions of hypoxia and of hypercapnia to the differentiated changes of regional sympathetic activity during asphyxia were investigated in anesthetized, paralyzed rabbits. Under artificial ventilation with gas mixtures of various O₂ and CO₂ contents, the discharges of a postganglionic nerve twig accompanying the retroauricular artery (cutaneous sympathetic) and of a splanchnic nerve branch (visceral sympathetic) were recorded. PaO₂, PaCO₂, pH, arterial pressure, and heart rate were measured.Moderate hypoxia (PaO₂ 27.2 Torr) induced a differentiated response of regional sympathetic activity similar to that observed during moderate asphyxia, i.e. increase of visceral and decrease of cutaneous sympathetic activity. During severe hypoxia (PaO₂ 16.4 Torr) both visceral and cutaneous sympathetic activity increased, the latter after a temporary decrease.—During hypercapnia (PaCO₂ 59.3 Torr) only a slight increase of visceral sympathetic activity was observed, while cutaneous sympathetic activity did not change.—It is concluded that the differentiated responses of the sympathetic nervous system during asphyxia are caused mainly by hypoxia.