Foramen ovale blood flow and cardiac function after main pulmonary artery occlusion in fetal sheep

New Findings

What is the central question of this study? At near term gestation, foramen ovale blood flow accounts for a significant proportion of fetal left ventricular output. Can foramen ovale increase its volume blood flow, when right ventricular afterload is increased by main pulmonary artery occlusion? What is the main finding and its importance? Foramen ovale volume blood flow increased during main pulmonary artery occlusion. However, this increase was attributable to a rise in fetal heart rate, because left ventricular stroke volume remained unchanged. These findings suggest that foramen ovale has a limited capacity to increase its volume blood flow.

Abstract

Foramen ovale (FO) accounts for the majority of fetal left ventricular (LV) output. Increased right ventricular (RV) afterload can cause a redistribution of combined cardiac output between the ventricles. To understand the capability of FO to increase its volume blood flow and thus LV output, we mechanically occluded the main pulmonary artery in seven chronically instrumented near term sheep fetuses. We hypothesised that FO volume blood flow and LV output would increase during main pulmonary artery occlusion. Fetal cardiac function and haemodynamics were assessed by pulsed and tissue Doppler at baseline, 15 and 60 min after occlusion of the main pulmonary artery and 15 min after occlusion was released. Fetal ascending aorta and central venous pressures, and blood gas values were monitored. Main pulmonary artery occlusion initially increased fetal heart rate (p < 0.05) from 158(7) to 188(23) bpm and LVCO (p < 0.0001) from 629(198) to 776(283) ml/min. Combined cardiac output fell (p < 0.0001) from 1524(341) to 720(273) ml/min. During main pulmonary artery occlusion, FO volume blood flow increased (p < 0.001) from 507(181) to 776(ml/min). This increase was related to fetal tachycardia, because LV stroke volume did not change. Fetal ascending aorta blood pressure remained stable. Central venous pressure was higher (p < 0.05) during the occlusion than after it was released. During the occlusion fetal pH decreased and pCO₂ increased. LV systolic dysfunction developed while LV diastolic function was preserved. RV systolic and diastolic function deteriorated following the occlusion. In conclusion, FO has a limited capacity to increase its volume blood flow at near term gestation.

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