Umbilical artery, ductus venosus and middle cerebral artery

Evaluation of the umbilical artery Doppler by PW-Doppler. The Resistive index is calculated as the (Peak Systolic Velocity-End Diastolic velocity)/Peak Systolic Velocity. More on the umbilical arterial profile here. 

More information on fetal MCA Doppler here. 

Venous flow Dopplers of IVC and Ductus Venosus

More on ductus venosus Doppler here. 

A-Wave Reversal in the Ductus Venosus: A Sign of Elevated Right Atrial Pressure

A-wave reversal in the ductus venosus is a Doppler finding that may indicate increased right atrial pressure, particularly in the context of fetal distress or heart failure.

Doppler Flow Dynamics in the Ductus Venosus

Under normal fetal conditions, ductus venosus flow shows a triphasic pattern:

• S-wave: forward flow during ventricular systole

• D-wave: forward flow during early diastole

• A-wave: diminished, but still antegrade flow during atrial contraction

However, in the setting of elevated central venous pressure, especially with increased end-diastolic pressure in the right atrium and right ventricle, this pattern becomes disrupted:

• The A-wave may diminish, reach the zero-flow baseline, or even reverse, reflecting impaired forward flow during right atrial contraction (atrial systole which is during ventricular diastole/filling phase).

• A-wave reversal is best appreciated in the ductus venosus (also known as the canal of Arantius) and is an important marker of cardiac decompensation in the fetus.

Structural Characteristics of the Ductus Venosus

Unlike other systemic veins, the ductus venosus has thick, less distensible walls, which makes it more resistant to retrograde flow during atrial contraction. As such:

• It typically maintains forward flow even when right atrial pressure rises slightly.

• Significant A-wave reversal suggests a marked increase in right atrial pressure or the onset of venous congestion.
  
  

Clinical Significance

A-wave reversal in the ductus venosus is a highly specific marker of:

• Fetal hemodynamic compromise (e.g., heart failure, hydrops fetalis)

• Elevated right atrial pressure or impaired ventricular compliance

• Impending fetal decompensation in high-risk pregnancies
  
  

Recognizing this Doppler pattern—and understanding how the structural properties of the ductus venosus affect flow interpretation—is crucial in fetal echocardiography and prenatal surveillance of compromised fetuses.

Differential diagnosis of reversal of flow in the "a" wave of the ductus venosus on fetal echocardiogram

1. Increased Right Atrial Pressure / Increased Right Ventricular End-Diastolic Pressure: This is the direct hemodynamic explanation for the A-wave reversal. When the pressure in the right atrium increases significantly during atrial contraction, it can overcome the resistance to backflow in the ductus venosus (which has thicker walls that normally resist reflux) and cause flow reversal. This can occur in situations like tricuspid atresia with restrictive foramen ovale, cor triatriatum dexter with obstruction of the systemic venous return.

2. Fetal Heart Failure or Impending Heart Failure: The presence of ductus venosus A-wave reversal can be a sign observed in cases of fetal distress or insufficiency (heart failure) and suggests progression of heart failure, especially if seen alongside increased atrial reversal in other systemic veins like the IVC. This indicates the heart is struggling to handle the venous return, leading to elevated filling pressures.

3. Significant Tricuspid Regurgitation: Severe tricuspid insufficiency is identified as a potential cause of pulsatile right atrial dilation and inversion of the atrial (a) wave in the ductus venosus. The significant backflow from the right ventricle into the right atrium during systole increases right atrial pressure, particularly impacting the diastolic filling phase leading to the increased A-wave amplitude and potential reversal.

4. Conditions Leading to Right Ventricular Volume or Pressure Overload and Subsequent Diastolic Dysfunction: Any condition causing the right ventricle to be stiff or excessively volume/pressure loaded in diastole could theoretically contribute to this finding. This can happen in situations like a massive Vein of Galen malformation. 

Article on Ductus venosus Doppler available here.  (Seravalli V, Miller JL, Block-Abraham D, Baschat AA. Ductus venosus Doppler in the assessment of fetal cardiovascular health: an updated practical approach. Acta Obstet Gynecol Scand. 2016 Jun;95(6):635-44. doi: 10.1111/aogs.12893. Epub 2016 Mar 28. PMID: 26946331.)