Respiratory Effects of Division of the Carotid Sinus Nerve in the Lamb Soon After the Initiation of Breathing

Abstract

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

The Influence of Acute Hypoxia and Carotid Body Denervation on Thermoregulation During Non‐Rapid Eye Movement Sleep in the Developing Lamb

“…The time course of such events is both critical and informative, in that death occurred between 19 and 24 days of age, irrespective of whether lambs were denervated at 2 weeks or 2 days of postnatal age (Bureau et al 1985). It has been shown that newborn lambs will establish effective breathing after birth following antenatal CBD and that the cool of the extra-uterine environment is an important thermal stimulus to breathing (Harned et al 1968;Harned & Ferreio, 1973). A likely hypothesis arising from these studies is that an increased metabolic rate acts to maintain respiratory drive in the younger lamb, but when older the metabolic stimulus declines thereby making them more vulnerable to the combined respiratory and metabolic effects of CBD.…”