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ExtractIncreased activity of the carotid chemorecep tors from hypoxia and hypercarbia, induced by occlusion of the umbilical cord, has importance in initiating effective respiration of the term lamb. The state of hypoxia and hypercarbia continuing after birth should also evoke strong stimulation from the carotid glomi. The present experiments measure the effects on ventilation resulting from interruption of carotid chemoreceptor stimulation to the respiratory center by division of each carotid sinus nerve of the lamb during the newborn period.Ewes were given spinal anesthesia and each lamb was partly removed from the uterus to permit an operative approach through the neck. The carotid sinus nerves were exposed and tagged with loose threads to facilitate division of the nerve at a later stage. The lamb was then delivered, its umbilical cord clamped, and its inspiratory volume measured by pneumotachometric techniques.Each of the seven lambs studied was permitted to establish respiration before its first carotid sinus nerve was divided. The time intervals between birth and division of the first nerves varied from 5.5 to 38.5 minutes after birth. The second nerves were divided several minutes later when respiration had recovered from the effects of division of the first nerve. A significant fall in ventilation was observed within 50 seconds after division of each carotid sinus nerve. Improved ventilation began to occur in some animals 150 seconds later, and occurred in all animals within several minutes after division of each nerve. Levels of Pc>2> pH and PCO2 i n carotid artery blood before nerve division revealed hypoxia, hypercarbia and acidosis; this aberrant state became worse during the period of diminished ventilation. The minimal changes in heart rate and blood pressure resulting from interruption of baroreceptor fibers were insufficient to influence respiration. Speculation

Respiratory patterns after deliberate cord occlusion of four fetal lambs with bilaterally divided carotid sinus nerves were compared with those of four control lambs subjected to similar surgery without actual division of the nerves. The animals with divided carotid sinus nerves did not develop sustained rhythmic respiration and showed fall in Po2, which persisted, and progressive fall in pH and rise in Pco2 during the 10-minute period of observation after cord occlusion. The mock-operated animals developed sustained rhythmic respirations and showed rise in Po2, fall in Pco2 and less fall in pH than the animals with divided carotid sinus nerves. This striking inability of the lambs deprived of carotid chemoreceptor function to initiate effective respiration indicates that this mechanism has an important role in the initiation of breathing under these conditions.

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