Chemoreceptor responsiveness in fetal sheep.

Area of Science:

• Fetal Physiology
• Cardiovascular Regulation
• Respiratory Control

Background:

• Quantification of fetal peripheral chemoreceptor responses to oxygen saturation and carbon dioxide partial pressure is lacking.
• Understanding these responses is crucial for assessing fetal well-being during various physiological conditions.

Purpose of the Study:

• To quantitate fetal peripheral chemoreceptor responsiveness to acute hypoxemia and hypercapnia.
• To investigate the relationship between resting oxygen saturation and chemoreceptor sensitivity.
• To determine the interdependence of oxygen and carbon dioxide as stimuli for fetal chemoreceptors.

Main Methods:

• Measurements in 24 chronically instrumented fetal sheep.
• Induction of acute hypoxemia via uterine arterial occlusion at varying oxygen saturations.
• Calculation of heart rate change relative to oxygen saturation change (delta HR/delta sat) as an index of responsiveness.
• Acute hypercapnia was induced in a subset of animals.

Main Results:

• Chemoreceptor responsiveness (delta HR/delta sat) was inversely related to resting oxygen saturation below 65%.
• Increased resting oxygen saturation above 65% decreased responsiveness, suggesting adaptation.
• Decreased resting oxygen saturation below 65% increased responsiveness, indicating a lack of adaptation.
• Acute hypercapnia induced bradycardia, directly related to the increase in fetal arterial PCO2.
• No interdependence between O2 and CO2 stimuli was observed during hypoxemia-induced chemoreceptor activation.

Conclusions:

• The fetal peripheral chemoreceptor is sensitive to both hypoxemia and hypercapnia.
• Hypoxemia response is accentuated with decreasing initial oxygen saturation, suggesting limited adaptation.
• Peripheral chemoreceptors demonstrate sensitivity to hypercapnia, leading to bradycardia.