PDA (Patent or Persistent Ductus Arteriosus)

Despite multiple trials of treatment, there is still no consensus approach to prolonged patency of the ductus arteriosus. For a list of papers tackling this thorny issue see our Resources section.

But no matter what your view on how interventionist to be when it comes to ductal treatment, we would still argue that a comprehensive echo assessment should be central to any assessment.

The echo assessment has 2 key elements:

1 - Exclusion of structural heart disease. In particular coarctation of the aorta. If coarctation is present the duct will be patent to attempt to conserve lower body blood flow. Closing it would be disastrous. See the structural assessment page.

2 - Assessment of 'haemodynamic significance'. This is in itself a difficult area, but there is clearly a difference between small, tightly constricted PDAs (generally with continuous high velocity flow, forward diastolic flow in the descending aorta and normally sized left-sided chambers) and large non-restricted PDAs (generally with low velocity flow in diastole, reversed diastolic flow in the descending aorta and dilated left-sided chambers).

As ever, when performing an echo to assess ductal patency, one should always conduct a full structural assessment.

When the first 5 views have been achieved it's time to examine the duct. Using a high sagittal view, try to visualise the proximal pulmonary trunk and the distal aortic arch. The duct joins these two structures.

If large it can be seen on 2D imaging:

But colour Doppler is extremely useful:

The PDA shown above appears to be very large. Some other ducts are clearly tiny:

The difficulty with assessment comes with moderate ducts. Which of these two infants has the larger PDA?:

Of course it's a trick question. This is the same infant, with the same PDA. The two images were taken 20 seconds apart. The only difference between the two is the colour gain setting on the echo machine. By altering the gain the PDA can be made to appear to be either 1.4mm:

or 2.4mm:

Standard advice is to adjust gain until flow cannot be seen outside blood vessels, but this decision is clearly arbitrary. To try to clarify which ducts are 'haemodynamiclly significant' one needs to examine other parameters:

1 - Flow profile within the duct. A constricting PDA tends to have high velocity flow throughout the cardiac cycle:

A non-restrictive duct will have lower flow during cardiac diastole.

As a rule of thumb if the end-diastolic flow through the PDA is less than 50% of the peak systolic velocity this suggests poor constriction.

2 - Flow pattern in the descending aorta (DAo). Our recent MRI assessments of shunt volume through the PDA suggested that descending aortic flow reversal is the most reliable marker of high volume PDA shunt. Read the full paper here.


Image the DAo at around the level of the diaphragm. The easiest way to do this is to tilt caudally from your standard sagittal ductal view, look for the vertebrae, and then tilt slightly to the left. Using pulsed wave Doppler and angle correction the flow profile can be seen. An infant with a constricted PDA will have forward diastolic flow in the DAo. An infant with a non-restrictive duct will have reversed diastolic flow in the DAo as blood which advanced forwards in systole is 'sucked' back up through the PDa and into the lungs in diastole:

3 - Dilatation of left-sided cardiac chambers. High volume through the duct passes through the pulmonary vasculature and returns to the left atrium. Particularly with prolonged shunting the left atrium and even the left ventricle become dilated. Dilatation can be assessed by 'eyeballing' the difference between normally filled chambers and dilated chambers.

But can also be quantified by measuring the left atrium to aortic root ratio:

An LA:Ao > 1.4 suggests diltatation.