OVINE FETAL LARYNGEAL CHEMOREFLEX THRESHOLDS AND RESPIRATORY EFFECTS

In newborn infants, laryngeal contact with solutions of low chloride c oncentration or pH evokes swallowing, laryngeal adduction, and respira tory inhibition (laryngeal chemoreflex). To determine whether the lary ngeal chemoreflex is present during fetal life and its effect on fetal respiratory activity, eight time-bred ewes (128 +/- 2 days) were prep ared with fetal electrocortical diaphragm and esophageal electrodes an d a nasopharyngeal catheter. After a 60-minute control period, increas ing volumes (0.1 to 1.0 ml/kg) of 0.15 mol/L NaCl or distilled water ( 0.05 to 1.0 ml/kg) and decreasing concentrations of NaCl (0.15 to 0.02 mol/L) at a fixed volume (0.3 ml/kg) were sequentially administered t hrough the nasopharyngeal catheter (38 degrees C). The minimum water v olume that stimulated swallowing was significantly less than the minim um 0.15 mol/L NaCl volume (0.10 +/- 0.02 vs. 0.70 +/- 0.05 ml/kg). The maximum NaCl concentration that stimulated swallowing was 0.04 +/- 0. 01 mol/L. During the control period, respiratory activity averaged 14. 6 +/- 0.7 breaths/minute and did not change during absent swallow resp onses or isotonic saline-induced swallows. However, respiratory activi ty significantly decreased during wafer (4.7 +/- 0.6 breaths/minute) a nd hypotonic saline-induced swallow responses (3.7 +/- 0.7 breaths/min ute). Fetal electrocortical activity did not change during absent or s timulated swallows. We conclude that laryngeal water or hypotonic sali ne solution may stimulate fetal swallowing and suppress fetal respirat ory activity, similar to the newborn laryngeal chemoreflex. We specula te that an exaggeration of the laryngeal chemoreflex apnea response in the newborn may predispose to sudden infant death syndrome.