MVP (Micro Vascular Plug®) Embolization Experience on 104 Patients: How to Select to Proper Model and Avoid Technical Failures

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Primary clinical success was considered as hemodynamic stability in emergency setting and resolution of the underlying vascular pathology in elective cases; secondary clinical success was considered clinical success after a second embolization session.

Results:
Vessel occlusion was achieved in 78 patients after MVP release, while in 26 subjects additional embolics were required; the overall technical success was 75%. Primary clinical success was 96.1%.
MVP was oversized between 30% and 40% compared to the target vessel caliper; MVP-3 and MVP-5 were the most frequently adopted in this sample No statistical differences in terms of effectiveness were observed among patients assuming anticoagulation. A straight and longer landing zone were statistically associated with higher technical success compared to a curved and a shorter ones, respectively.
No clinically adverse events directly related to MVP device occurred; in 3 cases migration was registered without clinical complications.

Conclusion:
MVP is a safe and effective embolic device. Oversizing is recommended. While eventual concomitant anticoagulation therapy did not in uence the technical outcome, straight course and length of the landing zone are essential parameters to evaluate before deployment.
Background Page 3/11 The Micro Vascular Plug (MVP) (Medtronic, USA) is a mechanical embolic device with control detachment. The rst experience description in literature is relatively recent, dated 2014 (Pellerin et al. 2014).
Structurally, MVP is made of a self-expanding nitinol skeleton ovoid-shaped covered with a Polytetra uoroethylene (PTFE) coating and soldered to a pusher wire (Giurazza et al., 2018). The detachment is mechanical by anticlockwise torqueing. Each MVP presents two radiopaque markers at proximal and distal extremities, allowing visualization at uoroscopy and CT scan.
This study aims to describe a three years experience of arterial embolization with MVP in both emergent and elective scenarios; patients characteristics and landing zone features have been analyzed to understand how to select the proper model and prevent eventual technical failures.

Materials And Methods
This is a multicenter retrospective observational study; the local ethical committees approved the study.
All patients treated in elective conditions gave their written informed consent to the procedure; those managed in emergency signed a written consent in case their clinical conditions allowed.
Local electronic records have been analyzed to detect all patients that underwent to a transarterial embolization using MVP between 1 January 2018 and 31 December 2020. Both emergent and elective procedures have been considered.
The following parameters have been investigated: age, sex, underlying pathology, angiographic appearance of the vascular injury, anticoagulation therapy at time of procedure, district, caliper of the target artery, course of the landing zone, additional embolics, technical and clinical success. Technical success was considered as complete vessel occlusion at nal DSA (digital subtraction angiography) without deployment of other additional embolics.
Primary clinical success in emergent procedures was intended as hemodynamic stability with increased or stabilized hemoglobin values, while in elective procedures it was considered resolution of the underlying vascular pathology; secondary clinical success was considered clinical success after a second embolization session. In 85 patients the embolization has been performed in emergency while in the other 19 patients the procedure was scheduled.
Among the emergent embolization procedures, 57 were for post-traumatic hemorrhages, 19 were for spontaneous bleeding in patients under anticoagulation therapy, 5 were for bleeding neoplasms and 4 were for bleeding duodenal ulcers; among the elective embolization procedures, 11 were for arteriovenous stula, 6 were for splenic aneurysms and 2 were presurgical embolizations for tumor resection.

Release technique
The MVP have been always released with the pull-back technique (Giurazza et al. 2018) avoiding the pushing technique ( Fig. 1): the MVP is rst advanced in order that the distal radiopaque marker is positioned in correspondence of the distal tip of the catheter; then the catheter is withdrawn up to the detachment zone and nally the MVP is released. In case the operators judged necessary and feasible, a uoroscopic check has been performed to verify MVP expansion before detachment.
All attempts have been applied to position the pusher wire in axis with the MVP in order that the detachment zone was straight and not angulated. MVP-3 and MVP-5 were released through a 2.7 Fr microcatheter while MVP-7 and MVP-9 through a 5 Fr diagnostic catheter.
Before introducing the MVP through the hub, the whole catheter/microcatheter dead space has been ushed with saline in order to avoid clot formation that could hinder the release.
The target vessel caliper was measured at preprocedural CT scan in arterial phase or at diagnostic DSA.

Statistical analysis
Chi-square test was employed to investigate the relationship between technical success and ongoing anticoagulant therapy, MVP landing zone course and MVP landing zone length respectively; p-value was considered signi cant if < 0.05.
ROC curves analysis were performed to assess possible cut-off values of vessel diameter which could predict technical success for MVP-3 and MVP-5, according to the area under the curve (AUC); this was unfeasible for MVP-7 and MVP-9 due to the low number of cases.
Electronic database was conducted with Excel® Micosoft Corp. (USA); descriptive and inferential statistical analysis were performed using SPSS® v.22 IBM (USA).

Results
Vessel occlusion was achieved in 78 patients after MVP release, while in 26 subjects additional embolics were required; therefore the overall technical success was 75%. Primary clinical success was 96.1%.
Concerning anticoagulation therapy at the time of the procedure, no statistical differences (p-value = 0.6) were observed in terms of technical success between patients assuming and not assuming.
A straight landing zone was statistically associated with higher technical success compared to a curved one (p-value < 0.001); similarly, a landing zone longer than the unsheated MVP length was associated with higher technical success compared to a shorter one (p-value = 0.048).
Technical success was obtained in 23/34 patients (67.6%); in 11 cases, other embolics were required to achieve the vessel embolization (coils in 6 patients, spongel slurry in 4 cases and glue in 1 case).
The ROC curve analysis showed a slight trend to technical success in case the MVP was deployed in a vessel with a caliper < 2.1mm (AUC: 0.326).
Technical success was obtained in 35/57 patients (61.4%); in 22 cases, other embolics were required to achieve the vessel embolization (coils in 12 patients, spongel slurry in 7 cases and glue in 3 cases).
Primary clinical success was obtained in 50 patients (98%), secondary clinical success in the other one (2%).
The ROC curve analysis showed a slight trend to technical success in case the MVP was deployed in a vessel with a caliper < 3.1mm (AUC: 0.405).
Technical success was obtained in 11/12 patients (91.7%); in 1 case other embolics were required to achieve the vessel embolization (coils).
Primary clinical success was obtained in all patients (100%).
Technical success was obtained in 9/13 patients (69.2%); in 4 cases other embolics were required to achieve the vessel embolization (coils in 3 patients and glue in one patients).
Primary clinical success was obtained in all patients (100%).
According to CIRSE classi cation system for complications (Filippiadis et al. 2017), no clinically adverse events directly related to MVP device occurred. Three MVP migrated distally after release; in one patient the landing zone was shorter than the unsheated MVP (Fig. 5), while in the other two the landing zone was curved in a splenic artery.

Discussion
The main technological improvement provided by MVP is the possibility to release a plug peripherally; especially models MVP-3 and MVP-5 present the relevant advantage of microcatheter compatibility. Apart from their size, this is possible thanks to the high exibility of nitinol skeleton and pusher wire, which allow navigability into diagnostic catheters and microcatheters without creating tension. Empirically, even in thin and tortuous vessels, MVP does no induce retreat and instability of the delivering catheter.
In order to obtain a correct release of the device, the pullback technique preceded by saline ush of the catheter dead space is mandatory. Another relevant aspect to consider is the landing zone evaluation; in this sample a straight landing zone was signi cantly associated with technical success. This is related to the nitinol skeleton proper expansion, producing effective adhesion of the PTFE covering to the vessel wall; furthermore, the torqueing detachment is facilitated because the detachment point is on the same long axis of the MVP. Also the length of the landing zone proof to be signi cantly related to technical success; this should be longer than the MVP length to allow the expansion of the full device without risk of distal migration, as in the case described in Fig. 5.
In this study the choice of MVP model was empirically oversized between 30% and 40% compared to the target vessel caliper, especially in bleeding patients where vasospasm underestimate the vessel diameter measurement. For MVP-3 and MVP-5, the most frequently adopted models in this sample, a trend to technical failure has been analyzed for target vessel calipers higher than 2.1mm and 3.1mm respectively.
Concerning patients assuming anticoagulation therapy, in this study the embolic property of MVP was not in uenced by this factor; compared to coils, this advantage should be related to the PTFE covering which create a full lumen physical barrier to the blood ow. Another advantage of this device compared to other metallic embolics is the absence of artifacts at follow-up CT. Even if a comparison with coils was not part of the aim of this paper, MVP should not be considered as an alternative to them. Apart from a different higher mean cost, it presents speci c features: its predictive landing zone makes it particularly interesting in case deployment is close to vascular bifurcation or to spare healthy vessels. In patients assuming anticoagulation, PTFE covering makes it an effective embolic. Finally, if the size is properly selected, a single MVP allows effective and immediate occlusion; usually to obtain embolization with coils, more than one are released and time to obtain clot formation needs to be waited.
This paper presents some limitations. First, its design is retrospective observational: having analyzed a lapse of three years, the operators have improved their skills in that time and so technical failure would be more frequent in the rst part of the study period. Moreover, heterogeneity in clinical practice among the 2 centers may represent a confounding factor in interpreting the results. Then, the sample is disomogeneous because included emergent and elective conditions; this may induce bias in the proper choose of the model size because vasoconstriction during hemorrhage is a parameter to consider for oversizing; however, the study aims to report an overall experience of embolization with this device rather than focusing on a speci c scenario.

Conclusions
In this sample MVP proof to be a safe and effective embolic device, able to achieve de nitive vessel occlusion without additional subsequent agents in 75% of the cases. The most frequently adopted models were in order MVP-5 and MVP-3 in bleeding patients: compared to other plugs, the main technological improvement of these devices is the possibility of releasing peripherally through a microcatheter. Oversizing between 30% and 40% is highly recommended. While eventual concomitant anticoagulation therapy did not in uence the technical outcome, straight course and length of the landing zone are essential parameters to evaluate before deployment.