EFFECTS OF HYPOXAEMIA ON FOETAL HEART RATE, VARIABILITY AND CARDIAC RHYTHM

Abstract

1. Experiments were carried out in 30 chronically catheterized foetal sheep (128-144 days; term 150 days) and in seven of these foetuses before, during and after acute hypoxaemia. The extent to which changes in sympathoadrenal activity and cardiac vagal activity affected the foetal cardiac response to hypoxaemia was measured. Three measurements were used: foetal heart rate (FHR), heart rate variability (HRV; measured as the coefficient of variation in pulse interval) and power spectral density (PSD; measured over the frequency ranges of 0.04-1.3 Hz). Cardiac vagal activity was blocked by atropine, beta-adrenoceptor activity was blocked by propranolol. 2. Under normoxaemic conditions, cardiac vagal blockade caused a rise in mean arterial pressure (MAP; P < 0.001), an increase in FHR (P < 0.001), a decrease in HRV (P < 0.001) and a decrease in PSD (P < 0.001). beta-adrenoceptor blockade caused a rise in MAP (P < 0.001), a fall in FHR (P < 0.01), a decrease in HRV (P < 0.001) but no change in PSD. 3. During mild hypoxaemia (PO2 = 12-14.5 mmHg) and moderate hypoxaemia (PO2 = 10-11.9 mmHg), foetal MAP (P < 0.001, P< 0.001), HRV (P < 0.01, P < 0.001) and PSD in the frequency range 0.04-0.45 Hz increased (P < 0.05-P < 0.001). Foetal heart rate decreased when foetuses became moderately hypoxaemic (P < 0.001). 4. After cardiac vagal blockade, hypoxaemia was associated with an increase in FHR compared with non-blocked hypoxaemic foetuses (P < 0.01, P < 0.001). The increase in HRV was abolished (P < 0.001, P < 0.001) as was the increase in PSD (P < 0.01-P < 0.001). 5. After beta-adrenoceptor blockade, the bradycardia that occurred during hypoxaemia was enhanced (P < 0.01, P < 0.05), the increase in HRV was not affected and neither was the increase in PSD. 6. As FHR and HRV of normoxaemic foetal sheep were affected both by atropine and propranolol, it would seem that both cardiac vagal and sympathoadrenal activity modulate the foetal heart under resting conditions. The lack of any effect of beta-adrenoceptor blockade on PSD under these conditions suggests that power spectral analysis (PSA) is not as sensitive as the other two methods in detecting sympathetically mediated modulation of the heart. 7. Because the hypoxaemia induced bradycardia and increase in HRV and in PSD were abolished by atropine (P < 0.01-P < 0.001), it is concluded that during hypoxaemia foetal HRV is mainly modulated by changes in cardiac vagal tone. Propranolol had no effect on foetal HRV, although it reduced it under normoxaemic conditions; therefore, it is concluded that cardiac sympathetic neural activity was not increased in acute hypoxaemia uncomplicated by acidosis. However, there was strong evidence of increased sympathoadrenal tone on the foetal heart in hypoxaemia, that is, there was a rise in FHR in hypoxaemic atropinized foetuses and a greater fall in FHR in beta-adrenoceptor blocked hypoxaemic foetuses. Therefore, this increased sympathetic influence on the foetal heart during hypoxaemia must be predominantly the result of increased adrenomedullary secretion of catecholamines. 8. Maintenance of foetal cardiac output depends on the chronotropic and ionotropic effects of catecholamines. Therefore, this adrenomedullary influence on the foetal heart during hypoxaemia is important to offset the opposing effects of increased cardiac vagal tone.