DISCUSSION

Selecting the appropriate PDA stent size is essential to prevent post stenting complications such as stenosis, stent migration and in this case, over shunting. Alwi et al. in their study recommends PDA stent diameters of 3.5mm for patient weighing less than 3kg, 4mm for those 3-5kg and 4.5mm in those weighing more than 5kg.¹ In our case, LL weighed 2.88kg at the point of stenting. However, as the PDA measured 3.5mm in diameter, there were concerns regarding stent migration and thus a 4mm stent was selected, which may have contributed to the overshunting.

Medical management of symptomatic over-circulation in post-PDA stent patients may sometimes be inadequate whereas surgical reduction such as ligation and clips carry the risk of additional sternotomy as well as stent distortion. Transcatheter option using modified MVPs has been well described to reduce blood flow across branched pulmonary arteries but has yet to be sanctioned for the use in downsizing PDA stents.² Maschietto N et al has described a novel technique placing a modified Diabolo stent within the PDA stent but this requires a 6F delivery system which may not be suitable in smaller babies.³

Our case highlights the positive outcome of using a modified MVP as a flow restrictor in PDA stent overshunting. It can be modified easily, requires only low profile delivery catheters, easily retrievable and reposition and offers immediate mechanical obstruction/restriction to the flow rather than relying on thrombus formation allowing us to see its immediate effects post deployment and make necessary modifications if necessary prior to release.⁴ To date, the modified MVP has been largely found to be successful as a flow limiter in the main and branched pulmonary arteries in multiple case series and animal studies.⁵ To our knowledge, there is only one other report of a modified MVP being used as a flow restrictor in PDA stent overshunting. Lauren N et al had described similar success in using a modified MVP as a flow restrictor.⁶

There has been very limited data as to what extent of predeployment modification is needed to achieve desired flow restriction. We first created three fenestrations to the 5Q MVP using a 23G hypodermic sterile needle, targeting a flow reduction of approximately 50% through the stent’s crosssectional area, though mathematically, removing 3 out of 8 covered cells will only result in 37.5% reduction of cross sectional area, and found the post deployment results to be desirable. Creation of limited fenestration initially will allow for further modification to the same device until the desired results are obtained as there is no quantitative measurement pre-deployment that reflects the exact amount of flow reduction. Rather, surrogate readings of improvement in diastolic blood pressure, reduction in oxygen saturations and subjective flow reduction from ECHO and angiogram pre and post deployment of the implant serve as a guide as to whether the modifications made was adequate prior to releasing the device.^{6, 7}

Few techniques of modifications have been mentioned, including using blade scalpel, eye Bowie and coronary balloons for more uniform dilatation.^{6, 7} But neither has proven superior to the other in creating a desired fenestration mainly because it is almost impossible to determine the ideal size of fenestration to achieve the desired outcome. In our case, the 23G hypodermic sterile needle was chosen as it is readily available, the needle hub provided sufficient grip and easy maneuverability and the uniform needle shaft with a short bevel avoided unintentional over-dilatation of the MVP PTFE sections. The end results were comparable to other modification techniques but further studies needs to be done in the future to determine the optimal modification technique.

There were no immediate complications related to the procedure or placement of the MVP within the stent. Risk of thrombus formation at the MVP and complete occlusion of the PDA stent can be minimized with adequate antiplatelet therapy.⁷