Heart rate variability and electrocardiogram changes in the fetal lamb during hypoxia and beta-adrenoceptor stimulation

Abstract

Hypoxic changes in the ST waveform of the fetal ECG (FECG), with elevated T waves as the main response, have earlier been described in the fetal lamb as mediated via .beta.-adrenoceptor cells and initiated by catecholamine release. A similar background to the increase in fetal heart rate variability (FHRV) during hypoxemia was suggested. This study compared FHRV and changes in the ST waveform of the FECG as indicators of fetal distress. Acutely exteriorized mature lamb fetuses (26) were submitted to periods of graded hypoxemia. Fetal blood gases were measured and O2 content was calculated. The FHRV was analyzed by a computer program and calculated as the differential index (DI) and the interval index (II). Using a list of R-R intervals the DI and II were computed according to YEH et al. Hypoxemia resulted in initially strongly increased DI from 3.1 .+-. 2.5 to 17.7 .+-. 13.8 (P < 0.001) and in II from 1.88 .+-. 0.65 to 3.77 .+-. 2.06 (P < 0.001). The change in O2 content was strongly associated with the variability indices, as a strong correlation was found between ''.DELTA. O2 content/min and .DELTA.DI/min (r = 0.81). In 5 fetuses the effect of prolonged hypoxemia on DI was studied. After the initial rise from 1.6 .+-. 1.1 to 31.2 .+-. 9.0 (P < 0.02) DI decreased to 19.4 .+-. 20.7. A regression analysis showed a strong connection between DI and PaO2, (n = 93, T = -5.34), whether there was no relation between DI and pH (T = -1.85). There is strong evidence that hypoxemia and asphyxia induce an increase in the concentration of catecholamines in fetal blood. Similar changes in the FECG waveform during hypoxemia and during .beta.-adrenoceptor stimulation were observed. Both the FECG changes and the increase of FHRV during hypoxemia thus may be mediated via .beta.-adrenoceptors. However, during isoprenaline infusion no change in DI or II was seen whereas there was a significant rise in T/QRS ratio from 0.30 to 0.51 (P < 0.05). The length of the time period during which FHRV was analyzed influenced the result. The ST waveform changes during hypoxemia had a slower onset than the FHRV changes but were progressive during prolonged hypoxemia, with an increase from the prehypoxemic value 0.23 .+-. 0.18 to 0.84 .+-. 0.43 (P < 0.02) at the end of the hypoxemic periods. A significant relation was found between T/QRS and PaO2, (n = 99, T = -3.17) and also between T/QRS and pH (T = -2.67). The increase in DI during hypoxemia appears not to be mediated via the .beta.-adrenoceptors, especially as it is not accompanied by a positive chronotropic effect on the fetal heart. It was suggested that the increased FHRV instead could be an effect of .alpha.-receptor stimulation. Estimation of FHRV appears, if not influenced by other factors, to enable an early and sensitive detection of fetal hypoxemia, whereas the method was difficult to interpret during acidosis. Significant changes in the FECG were seen during prolonged hypoxemia and also correlate to metabolic disturbances.