The Fetal Cardiac Function

Abstract

Despite significant progress in the prenatal diagnosis of congenital heart disease and the postnatal management, the prenatal evaluation of fetal heart function remains difficult. The unique characteristics of the fetal circulation have a significant impact on its cardiac function. Commonly used physiological concepts about the function of the heart can be misleading when applied to the intrauterine situation. Most noninvasive parameters of cardiac function are not validated in the fetus. In addition, unlike structural defects that can be easily confirmed after delivery, functional hemodynamic abnormalities diagnosed in utero cannot be verified postnatally with certainty as the neonatal circulation defers considerably from the fetal circulation. This review attempts to describe commonly used methods of assessment of fetal cardiac function, their physiological basis and, their utility in clinical practice. THE FETAL CARDIAC FUNCTIONThe heart's main function is to deliver sufficient oxygenated blood to meet the metabolic requirements of the tissues [1]. The overall cardiac performance reflects the interaction of the heart, blood volume and blood vessels. The unique characteristics of the fetal circulation [2] have a significant impact on its cardiac function and distribution of cardiac output. The fetal circulation differs considerably from the adult and neonatal circulation. The placenta, a large extracorporal organ designed for disposal at birth, performs the functions of many organ systems in utero. Human placenta receives about 30% of the combined cardiac output [3,4] and the total peripheral resistance in the fetus is mainly determined by the placental-vascular resistance [5]. The placental volume blood flow normalized for fetal weight decreases with advancing gestation after 25 weeks [6].Both sides of the fetal heart under normal conditions have similar pressures [7] and work in parallel, and the fetal heart rate is much faster than the adult. As a result the weight-indexed combined cardiac output is higher in the fetus and the outputs of the left and right ventricles can be different. Right ventricular dominance develops towards term, and the output of the right ventricle constitutes about 60% of the combined cardiac output [8]. The parallel disposition of two ventricular pumps confers the aortic isthmus (Fig. 1), i.e. the vascular segment located between the origin of the left subclavian artery and the junction of the aorta with the ductus arteriosus, a special function of maintaining communication and balance between two parallel arterial outlets [9].The pulmonary circulation has a high vascular resistance and is sensitive to the changes in arterial pH and partial Fig. (1). Color Doppler image of fetal aorta demonstrating the isthmic area (arrow) and typical pulsed-wave (inverted) Doppler velocity waveforms (lower panel) obtained at the aortic isthmus in a near term fetus. A brief small reverse component during early diastole (arrow-head) is normal in late gestation.pressure of oxygen (PO 2 ) and carbon di...

Reference values for variables of fetal cardiocirculatory dynamics at 11–14 weeks of gestation

“…Fetal cardiac output, the product of stroke volume and heart rate, is known to increase with advancing gestation during the first half of pregnancy29. Although in the lamb, variations in cardiac output are explained mainly by changes in heart rate30, in the human fetus variations in heart rate (within the normal range) are not necessarily associated with a change in fetal cardiac output31, and even fetuses with congestive heart failure may have normal heart rate32.…”

Aortic Isthmus and Cardiac Monitoring of the Growth-Restricted Fetus

“…2 The extent of aortic isthmus flow reversal may help in assessing the severity of circulatory compromise in fetuses with cerebral arteriovenous fistula. Measurement of the fetal cardiac output and Q ai provides the possibility of assessing blood flow distributed to the brachiocephalic circulation.…”

Technical aspects of aortic isthmus Doppler velocimetry in human fetuses

“…However, during prenatal life, it plays an important role in maintaining an adequate balance between the brachiocephalic circulation supplying the upper body (including the brain) and the subdiaphragmatic circulation supplying the lower body and placenta1. The parallel arrangement of the fetal circulation allows for unequal right and left ventricular outputs as the placental, pulmonary and lower body vascular resistances act mainly on the right ventricle, whereas the upper body resistance acts mainly on the left ventricle2. Under physiological conditions and in the absence of structural cardiovascular malformations, such as hypoplastic left heart syndrome, critical aortic stenosis and interrupted aortic arch, the flow in the aortic isthmus is forward during the whole cardiac cycle.…”