Aortic isthmus blood flow in fetuses of diabetic mothers

The Aortic Isthmus: A Significant yet Underexplored Watershed of the Fetal Circulation

The aortic isthmus (AoI) is a unique fetal watershed with a waveform reflecting its complex haemodynamic physiology. The systolic component represents left and right ventricular systolic ejection, and the diastolic component represents comparative downstream vascular impedance between the brachiocephalic and subdiaphragmatic fetal circulations. Several indices have been devised to quantify different components of the waveform, including the pulsatility index, resistance index, isthmic flow index, and recently the isthmic systolic index. There have been promising preliminary studies applying these indices to both cardiac (congenital) and extracardiac pathologies, including intrauterine growth restriction and twin-twin transfusion syndrome. However, the waveform's multifactorial origin has proven to be challenging, and the difficulty in separating various components of the waveform could explain that AoI evaluation does not have a clear clinical utility. Further research is underway to realise the full potential of this vessel in fetal cardiac and haemodynamically compromised pathological conditions. In this review article we outline the physiological origin of this Doppler waveform, describe in detail the various published indices, summarise the published literature to date, and finally outline potential future research and hopefully clinical applications.

Myocardial hypertrophy and dysfunction in maternal diabetes

Diabetes in pregnancy, both pre-gestational and gestational, is a frequent cause of fetal myocardial hypertrophy, partly due to fetal hyperinsulinism. In fetal life, cardiac function may be impaired, especially during diastole, as a result of decreased left ventricular distensibility and altered left atrial dynamics secondary to myocardial hypertrophy. In neonates, the hypertrophy is a transient disorder, with spontaneous regression of the increased myocardial thickness during the first months of life. Nevertheless, cardiac hypertrophy may be associated with neonatal cardiomegaly and respiratory distress secondary to poor left ventricular compliance. The development of a number of new echocardiographic parameters discussed in this article, and primarily based on the pathophysiological consequences of myocardial hypertrophy, highlight an area of research priority: the assessment of diastolic function in fetuses of diabetic mothers with (and without) myocardial hypertrophy. A score for grading the severity of fetal diastolic dysfunction in these fetuses is proposed.