CAROTID SINUS REFLEXES TO THE RESPIRATORY CENTER

Abstract

When both common carotids of dogs or cats are closed, hyperpnea is usual but not as constant as hypertension; the former is transitory, the latter persistent; endosinusal pressure is lowered by about 35% (dog), cerebral blood-flow by about 45%; sinus dener-vation abolishes hypertension of occlusion, but hyperpnea is not always prevented: hyperpnea of carotid occlusion is not always entirely reflex in origin, though it is usually so in great part. In perfusion experiments, rise in endosinusal pressure regularly causes respiratory depression or apnea in dogs, cats, and rabbits; hyperpnea upon reduction of pressure is common but not constant. Respiratory reflexes probably arise in end-organs distinct from those involved in circulatory effects. Reflexes aroused in sinuses by changes in chemical composition of blood are responsible for most or all of hyperpnea of anoxemia, but that of CO2 excess is mainly central in origin. In dogs, receptors that respond to chemical stimuli are mainly located in structures receiving blood from the first 5 mm. or so of the occipital arteries, and not in the sinuses proper.[long dash]II. Sensitivity of respiratory center to changes in its blood-supply is shown by apnea upon release of occluded carotid and vertebral arteries, upon reduction of high intracranial tension, and upon injection of adrenalin, in animals with sinus and aortic reflexes excluded. In cerebral perfusion experiments (rabbits, dogs, cats) respiration varies inversely with change in central blood-flow although sinus and aortic reflexes are excluded. Reflexes are therefore not the sole cause of respiratory responses to changes in cephalic or systemic blood-pressure: the respiratory and vaso-motor centers are also directly influenced by changes in blood supply. Under favorable circumstances direct sensitivity of the center may be at least as great as sensitivity of reflex mechanisms, and direct effects may be more powerful than the reflex ones; under other circumstances the center is much less sensitive. These variations may be due to differences in tone of finer cerebral vessels. Sinus respiratory reflexes are necessary only to hyperpnea of anoxemia, and their discovery ex:-plains differences between stimulant effects of CO2 and anoxemia. Otherwise they do nothing that cannot be accomplished as well without them.