Giussani, Dino Antonio; (1993) The role of the carotid chemoreceptors in the control of the fetal cardiovascular system during hypoxaemia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In term fetal sheep short (ca. 60 minutes) episodes of reduced oxygenation in the blood (acute hypoxaemia) produce a number of cardiovascular and endocrine responses. Amongst the cardiovascular responses are a rapid initial bradycardia and a gradual increase in arterial blood pressure. In addition, the combined ventricular output (CVO) of the fetus is redistributed favouring the cerebral, myocardial and adrenal vascular beds at the expense of the gastrointestinal tract, renal, pulmonary, cutaneous and skeletal muscle circulations. Amongst the fetal endocrine responses to acute hypoxaemia are increased plasma concentrations of arginine vasopressin (AVP), adrenocorticotropic hormone (ACTH) and cortisol. There are several reasons for suggesting that aspects of both cardiovascular and endocrine responses to acute hypoxaemia are reflex in nature. Recent investigations have suggested a possible role for the peripheral chemoreceptors, in particular the carotid chemoreceptors, in mediating some of these responses. I have tested the hypothesis that the initial cardiovascular responses of the sheep fetus to acute hypoxaemia are initiated principally via the carotid chemoreceptors. Secondly, I have examined other possible aspects of the reflexes by using a combined de-afferentation and de-efferentation technique. Finally I have investigated the role of the carotid chemoreceptors in mediating increases in plasma AVP, ACTH and cortisol concentrations during hypoxaemia. My results suggest that: 1) The initial cardiovascular responses of the sheep fetus to acute hypoxaemia have a powerful carotid chemoreflex component, employing muscarinic efferents to the heart and α-adrenergic efferents to the peripheral circulation; 2) fetal survival during hypoxaemia is dependent on these reflexes and on the release of catecholamines from the adrenal medulla; 3) the carotid chemoreceptors may mediate the release of additional humoral vasoconstrictors which contribute to fetal survival during a hypoxaemic challenge; and 4) increased plasma cortisol concentrations during hypoxaemia may be partly mediated by an ACTH-independent carotid chemoreflex.

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