Cardiac Changes in the Intrauterine Growth-Restricted Fetus

The fetal heart starts functioning at around the fourth week of gestation and completes its formation by the sixth gestational week. As opposed to the postnatal circulation, which is a serial system, the fetal circulation is a parallel system which has two major connections between the right and left sites of the heart/circulation: (1) foramen ovale and (2) ductus arteriosus (DA).

Approximately 50% of the highly oxygenated blood carried by the umbilical vein returning from the placenta passes

Fetal Heart in IUGR Fetuses

Increased placental impedance is common in IUGR pregnancies and may be a way to distinguish the growth restricted from the constitutionally small but healthy fetus. Changes in the placental impedance mainly affect the peripheral circulation; however, fetal cardiac function has been shown to be affected by these changes.

Due to its physiological and anatomical nature, it is technically challenging to reliably assess the fetal cardiac systolic and diastolic function. Different methods have been

Systolic Function

Compared with AGA fetuses, both aortic and pulmonary peak systolic velocities and pulmonary time to peak velocity are reduced, and aortic time to peak velocity is increased in IUGR fetuses.⁸

Ejection force of both ventricles below the 5th percentile has been associated with poor perinatal outcome. In AGA fetuses, left and right ventricular ejection force values significantly increase with advancing gestation and the two ventricles exert similar force. Conversely, the ventricular ejection force

Diastolic Function

IUGR fetuses have significantly lower left and right ventricular diastolic filling compared with AGA fetuses, without significant changes in diastolic function.¹¹ In AGA fetuses, the E/A ratio increases progressively during pregnancy in both mitral valve and tricuspid valve waveforms, approaching 1 at term. Earlier in pregnancy, IUGR fetuses have similar E/A ratios to AGA fetuses.¹² However the E/A ratio does not increase later in pregnancy and is significantly lower than in AGA fetuses.¹³ The

Coronary Circulation and Cardiac Sparing

IUGR fetuses have increased heart size proportional to their body weight due to free wall hypertrophy without ventricular dilation.¹⁶ Coronary blood flow (CBF), which is essential in myocardial metabolism, plays a significant role in adaptation of the fetal heart. Changes in the coronary blood flow can be observed in affected fetuses. Increased CBF is seen in IUGR, fetal anemia, ductus arteriosus constriction, and fetal bradycardia. In contrast to IUGR fetuses, it is not possible to observe the 

Fetal Heart in Animal Models of IUGR

Studies in newborn piglets showed that there is a marked linear correlation between body weights and various organ weights in normal newborn piglets. However, in IUGR piglets, the ratio of brain weight to liver weight was increased more than 2.5 times that of normal newborns (P < 0.05). Similar findings were observed for the fetal heart.¹⁹

Myocardial contractility is preserved in IUGR fetuses. When AGA and IUGR piglets were compared under baseline conditions, arterial blood pressure, cardiac

Conclusion

IUGR fetuses demonstrate progressive hemodynamic changes. Umbilical artery pulsatility index is the first variable to become abnormal, followed by the middle cerebral artery, right diastolic indices (right E/A, ductus venosus), right systolic indices, and finally, both diastolic and systolic left cardiac indices. It appears that there is an earlier and more pronounced right than left and diastolic than systolic fetal cardiac function deterioration in growth-restricted fetuses.