Restrictive vs Unrestrictive

The definition of "restriction" in the context of patent ductus arteriosus (PDA) remains somewhat ambiguous, as no universally accepted criteria exist. However, it is generally understood that a "restrictive" structure limits the transmission of flow or pressure. This concept can be simplified as follows:

• Unrestrictive PDA: This allows pressure to equalize across both sides of the connected structure, similar to two boxes connected by a large channel.

• Restrictive PDA: This limits the amount of pressure or flow, akin to nearly closing off a garden hose.

• The "Most" Restrictive PDA is theoretically the one that is closed. Hence, there is no flow through it and no pressure transmission across it. Sometimes, the duct is on the verge of closing, where minimal or no blood flow is observed, especially during diastole when pressure in the aorta and pulmonary artery decreases. During systole, there may be some "stenting" of the lumen of the duct with some flow, which closes of in Diastole. We often call this "closing pattern". 

Article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7717963/ mentions: "Non-restrictive shunts have a low peak systolic velocity with a high systolic to end-diastolic velocity gradient while restrictive shunts have a high peak systolic velocity and a low systolic to diastolic velocity gradient. If the ratio between peak systolic and end-diastolic velocity is >2 then it is considered as a pulsatile flow pattern while a ratio of <2 is described as restrictive shunt suggestive of a closing PDA".

Important caveats:

• "High-Velocity Flow": A restrictive PDA typically shows high-velocity flow due to the narrowed ductal lumen. Doppler studies often demonstrate peak systolic velocities greater than 2.5 m/s and sometimes exceeding 3.5 m/s. 

• "Continuous Flow Pattern": The flow across a restrictive PDA is typically continuous throughout the cardiac cycle, with a high-velocity systolic jet and less pronounced diastolic flow. A restrictive PDA often presents with a significant pressure gradient between the pulmonary artery and the aorta, which can be calculated using the peak Doppler velocity (using the modified Bernoulli equation). A high gradient (e.g., >20 mmHg) suggests a restrictive flow.

• The velocity criteria depend on the pressure (and/or resistence) differences between the aorta (AO) and the pulmonary artery (PA). For example, you can have a very small duct that restricts red blood cells from passing through, but if the AO and PA pressures are nearly equal, the flow will be of low velocity. Consider a situation where systolic blood pressure (sBP) is 100 mmHg and systolic pulmonary arterial pressure (sPAP) is 98 mmHg. In this case, very little flow will pass through the ductus (even if it is very tiny), but since sBP is slightly higher than sPAP, the flow will still be left-to-right. However, with only a 2 mmHg difference between the two arteries, the flow will be minimal. If the PDA is extremely narrow, such as (hypothetically) 0.001 mm, it becomes "restrictive" in that it doesn't transmit flow or pressure effectively, but the flow remains of very low velocity. Typically, at this stage, the flow contour would be continuous and non-pulsatile due to the Windkessel Effect. 

  • The gradient between PA and AO also depends on their respective pressures. For instance, consider a PDA connecting an aorta with pressures of 50/30 mmHg and a pulmonary artery with pressures of 20/10 mmHg. The pressure gradient across the PDA would be a peak of 30 mmHg and an end value of 20 mmHg. The flow would appear continuous and non-pulsatile, yet with a relatively high velocity, such as during systole, where the velocity might reach 2.7 m/s (30 mmHg ~ 4V^2).

  • Let's consider an unrestrictive scenario: in this case, all pressure is transmitted across both ends, and the flow will depend on the pulmonary vascular resistance (PVR) to systemic vascular resistance (SVR) ratio. If the aortic pressure is 45/25 mmHg and the pulmonary artery pressure is also 45/25 mmHg, the flow will be from left to right. Unlike the previous example of 50/30 mmHg, this pressure is slightly lower, likely due to the loss of a few mmHg as the flow needs to fill a much larger vascular network, with both the pulmonary and systemic systems being connected.

In life, very few things are absolute. The PDA is a dynamic structure that gradually becomes more restrictive as it closes. As it narrows, it increasingly limits pressure transmission, much like a hose that becomes progressively occluded, restricting the flow.

Some Criteria for a restrictive ductus:

• Peak Doppler velocity > 2 m/s

• Ductal diameter < 1.5 mm 

• High pressure gradient across the ductus (>20-30 mmHg) 

• Continuous Doppler flow with a “sawtooth” appearance on spectral Doppler.

This paper: https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.105.592063 says "If the ductus is large and offers minimal resistance to flow (nonrestrictive), the degree of shunting depends on the status of the pulmonary vascular resistance. In many children with moderate or large patent ductus, pulmonary vascular resistance remains modestly elevated, which limits the shunting sufficiently to lessen the physiological impact and permit survival and growth"

This paper https://onlinelibrary.wiley.com/doi/10.1097/MPG.0000000000002420 says: "Ductal flow pattern was evaluated from the parasternal short axis using continuous-wave Doppler and, based on the ratio of end-diastolic to peak-systolic velocity, was defined as pulsatile (≥0.5) or restrictive (<0.5)"

This paper https://jamanetwork.com/journals/jamapediatrics/article-abstract/2375129 says: "For a PDA to cause significant shunting from systemic to pulmonary circulation, the flow needs to be unrestrictive and completely or almost completely left to right; the latter is sometimes referred to as a growing pattern. The PDAs of less than 1.5 mm in diameter are considered small because they are commonly restrictive, cause a mild increase in pulmonary circulation, and are rarely associated with echocardiographic markers of a high-volume shunt.12,13 Further subclassification of PDAs of at least 1.5 mm as moderate or large is based on incremental probability of a high-volume shunt. Although the diameter correlates well with shunt volume, using an absolute cutoff to describe hemodynamic significance may be misleading given the interobserver measurement variability (10%-15%) and its dynamic nature."

This paper https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1720677/ says: "The closing pattern suggests a restrictive PDA small enough to prevent a clinically significant left to right shunt. The growing and pulsatile patterns reflect falling pulmonary artery resistance in the presence of a large ductus with clinically significant left to right shunt, and are an indicator for risk of developing clinically significant PDA. Clinicians need to recognise that once pulmonary artery resistance has fallen, if the closing pattern has not developed, the ductus is large enough to create problems."

Closing left to right

Small left to right restrictive

Bidirectional

Right to left

Some Key Articles