Abstract
Extract: Increased activity of the carotid chemoreceptors 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 PO2, pH and PCO2 in 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: When viewed in conjunction with experiments performed in lambs immediately at birth and with those performed in older newborn lambs, these experiments reveal a continuum of physiologic activity of the carotid chemoreceptors which is important in the establishment and regulation of breathing. The consistent recovery of ventilation after the initial depression produced by division of the second carotid sinus nerve indicates that under these conditions, chemical regulatory mechanisms other than those of the carotid glomi are important in the lamb. The only peripheral chemoreceptors known to be active in the newborn are those in the carotid bodies. Accordingly, some central chemosensor mechanisms must be invoked to explain the recovery of respiration.
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Harned, H., Herrington, R., Griffin, C. et al. Respiratory Effects of Division of the Carotid Sinus Nerve in the Lamb Soon After the Initiation of Breathing. Pediatr Res 2, 264–270 (1968). https://doi.org/10.1203/00006450-196807000-00005
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DOI: https://doi.org/10.1203/00006450-196807000-00005