Venous Biomechanics Poisson Effect

Venous Biomechanics of Angioplasty and Stent Placement: Implications of the Poisson Effect

Clinical question
Characterize the poisson effect following venous angioplasty and stents placement in veins.

Take-away point
There is a proportional longitudinal foreshortening and narrowing of adjacent venous segments with radial expansion of venous angioplasty and stents, which is more dramatic in veins than arteries.

Reference
Li N., Mendoza F., Rugonyi S., Farsad K., Kaufman J.A., Jahangiri Y., et al. Venous Biomechanics of Angioplasty and Stent Placement: Implications of the Poisson Effect. J Vasc Interv Radiol. 2020 Aug; 31 (8): 1348-1356.

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Study design
Prospective in Vivo swine model Testing with Real-Time Venous Wall Motion Monitoring and finite-element computational vascular model analysis

Funding source
No reported funding

Setting
Academic setting. Oregon Health and Science University, Portland, Oregon, USA







Figure
A visual synopsis of the 2 study parts and summary of results.

Summary

Venous angioplasty and stent placement for treating venous outflow obstruction has demonstrated improved flow with clinical symptom relief. However, adjacent segment narrowing after stent placement, based on the poisson effect, may carry a negative impact on the quality of venous flow. Research on venous biomechanics for venous-specific stent designs is lacking. The author performed a prospective 3 adult swine in vivo testing with real-time venous wall motion monitoring and finite-element computational vascular model analysis to characterize the poisson effect during angioplasty and stent placement. Statistical analysis including Welch t test, analysis of variance, Pearson correlation, linear regression, and polynomial curve fit were performed using Prism software.

1- Real-Time Venous Wall Motion Monitoring:

Study veins were accessed and baseline venography was obtained followed by RF ablation induced stenosis and repeat venography. Balloon angioplasty, with diameter matching the naive vein(8mm), was performed on the in the stenotic regions of one side. Bare metal stents oversized by 50% (12mm) were deployed in the contralateral stenotic region. Iron particles were injected on the venous endothelium proximal to the intended vessel segment and categorized into near group (mid-point to balloon) and far group (peripheral to angioplasty area). Neodymium magnet was placed externally for iron attraction allowing real-time visualization of the vessel wall under fluoroscopy.

· During balloon angioplasty, the mean venous stenosis decreased by 43.7% ± 2.1. There was significant longitudinal displacement of the iron particles toward the balloon midpoint(1.5% ± 0.2 of the balloon length P <.001) affecting the far group more (2.2%) compared to near group (0.9%)(t test, P< .001). The adjacent luminal collapse correlated with diameter increase of the angioplasty-treated segment (relative collapse % . 0.772 * [relative dilatation %] . 0.255; R2 . 0.8920.

· During bare metal stent deployment the mean venous stenosis reduced to 20.8% ± 0.8, with further reduction to 1.6% ± 1.2 with balloon angioplasty of the stent. There was significant longitudinal displacement toward the midpoint of the balloon(2.0% ± 0.5 of the balloon length P< .01). The longitudinal displacement increased with distance to the midpoint of the balloon in a direct linear relationship (R2 . 0.87).

2- Finite-Element Computational Vascular Model (using ADINA software)

Several parameters were obtained from the literature to simulate a healthy common iliac vein and artery. During angioplasty simulation, the reduction of the adjacent-segment diameter had a significant correlation to the degree of dilation of the treated segment. Angioplasty simulation using arterial wall biomechanical properties showed significantly less adjacent segment narrowing compared with the model using venous properties. Variations of Young moduli in the longitudinal direction had a positive relationship with the amount of adjacent segment collapse, while with circumferential direction had the opposite effect and with the radial direction had no significant impact.

The findings explain the decrease of external iliac vein diameter after stent placement in the common iliac vein. Such phenomenon is due to the nonlinear elastic property of veins secondary to the nonaligned collagen fibers in the predominant adventitia. In contrast, arteries have coaligned collagen fibers in the dominant media layer. Previous and current study showed that veins are more extensible in the circumferential direction than in the longitudinal direction since they have a much higher longitudinal young modulus and a higher longitudinal vs circumferential Poisson ratio (opposite to arteries). With radial outward force applied to venous segments, there is a significant redistribution of forces and venous tissue, resulting in a pronounced Poisson effect.

Commentary

The authors in this study captured an important consequence of venous angioplasty and stent placement. They characterized its dramatic Poisson effect in veins and showed the resulting longitudinal foreshortening and adjacent segment collapse. Although the study has limitations including the small sample size, the possible collagen contraction after RF ablation and the lack of survival period to examine chronic changes. The study can still provide innovative solutions on vein-specific flow optimization. Current stent designs place heavy emphases on maximizing radial resistive force and chronic outward force, which may be counterproductive in venous interventions. Therefore applying and examining this study phenomenon on novel innovative venous stents, with emphasis on overcoming force and tissue redistribution related to the Poisson effect, might maximize venous flow leading to better patient outcomes.

Post Author
Tarig Elhakim, MD
PGY-2 Internal Medicine Residency,
Kendall Regional Medical Center

@TarigElhakim

Ablation Therapy for Advanced Stage NSCLC

Ablation Therapy for Advanced Stage Non-Small Cell Lung Cancer: A National Cancer Database Study

Clinical question: 

In patients with clinical stage IIIB or stage IV non-small cell lung cancer (NSCLC), is overall survival (OS) prolonged in patients who undergo ablation therapy (AT) compared to patients who do not receive AT?

Take away point: 

In patients with advanced stage NSCLC, AT as the sole therapy other than chemotherapy may be associated with prolonged overall survival.

Reference: 

Halsey, Kasey et al. (2020). “Ablation Therapy for Advanced Stage Non-Small Cell Lung Cancer: A National Cancer Database Study.” Journal of Vascular and Interventional Radiology 31 (8): 1210-1215.

Click here for abstract

Study design: 

Retrospective review

Funding source: 

National Natural Science Foundation of China grant

Setting: 

Multi-center, at cancer-accredited medical centers, utilizing the National Cancer Database (NCDB).

Summary:

Ablative Therapy (AT) is progressively utilized in the treatment of inoperable and early-stage NSCLC due to well-demonstrated prolonged survival rates, however little data exists on the efficacy AT in the treatment of late-stage NSCLC. Here, the authors retrospectively reviewed overall survival among patients with stage IIIB or stage IV NSCLC who were treated with AT as the only treatment aside from chemotherapy compared to patients who did not undergo AT.

Patients who underwent any other treatment modality for NSCLC were excluded. This resulted in a total of 140,819 patients (mean age of 69 years, 54.7% male) that were included in the study, of which 249 received AT. OS between these two cohorts was then calculated from the time of diagnosis to the time of death.

The median OS for all included patients was 5.0 months. Using a multivariate cox regression the cohort receiving AT had longer OS, however this was not a statistically significant difference. Propensity matching based on demographic and clinicopathologic characteristics was performed which resulted in a statistically significant OS of 5.9 months in the AT group compared to 4.7 months in the control group.

Additional propensity matching for tumors less than or equal to 3 cm resulting in statistically significant OS of 14.0 months in the AT group compared to 4.7 months in the control group.

Commentary:

Results of this retrospective review with propensity matched groups suggests that AT prolongs OS in patients with stage IIIB or IV NSCLC compared to AT, especially when pre-treatment tumor size is 3 cm or less.

As the efficacy of AT in early stage NSCLC is well-established (2-year survival rates exceeding 90%), this important data also suggests that AT is efficacious in more advanced stages of NSCLC. Importantly, AT can offer a treatment option for patients with late stage NSCLC especially in those who are poor surgical candidate or for those whom palliative care is indicated. The increase in OS in patients who underwent AT with tumor sizes of 3 cm or less is consistent with existing literature supporting excellent outcomes in smaller NSCL tumor sizes.

Limitations in the data presented here include relatively small AT cohort size compared to total included patient size, lack of details of cause-specific mortality, possibility for selection bias in that those who received AT in the setting of advanced stage NSCLC may be patients more clinically favorable for the procedure, and lack of detail regarding the specific ablation techniques performed.


Post Author:
Demetrios Geanon
MD Candidate, Class of 2021
Rush University Medical Center

David M. Tabriz, MD
Assistant Professor
Rush University Medical Center

@DrDaveTabriz

Feasibility and Procedural Safety of alfapump System Implantation

Feasibility and Procedural Safety of alfapump System Implantation by IR: Experience from the MOSAIC Study, a Multicenter, Open-Label Prospective Study in Cirrhotic Patients with Refractory Ascites

Clinical question
Is alfapump implantation by IR technically feasible and procedurally safe?

Take away point
Alfapump placement by an interventional radiology approach is technically feasible.

Reference
Bendel, E. C., K. Sniderman, C. Shaw, R. T. Frederick, F. Wong, A. Sanyal, S. K. Asrani, P. S. Kamath, J. Capel and Z. J. Haskal (2020). "Feasibility and Procedural Safety of alfapump System Implantation by IR: Experience from the MOSAIC Study, a Multicenter, Open-Label Prospective Study in Cirrhotic Patients with Refractory Ascites." Journal of Vascular and Interventional Radiology 31(8): 1256-1262.e1253.

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Study design
Multicenter prospective

Funding source
Sequana Medical 

Setting
Department of Radiology Mayo Clinic; Toronto General Hospital; Baylor University Medical Center; California Pacific Medical Center; Virginia Commonwealth University; Division of Interventional Radiology University of Virginia School of Medicine





Figure 1. The alfapump System implanted on the right side. (a) Schematic illustration. (b) X-ray image

Summary

The Sequana Medical alfapump is an implanted subcutaneous device that treats ascites by pumping peritonea l fluid into the bladder where it is excreted by voiding. The device consists of a pump component, a peritoneal catheter, and a bladder catheter. Current clinical data describes the device implantation procedure via a surgical approach. However, similar devices have been implanted by interventional radiologists. One aim of the MOSAIC trial (A Multicenter, Prospective, Open Label, Uncontrolled Feasibility Study to Assess the Safety and Effectiveness of an Automatic Low Flow Ascites Pump In Patients with Cirrhosis and Refractory or Recurrent Ascites) was to evaluate the technical feasibility and safety of alfapump implantation by interventional radiologists.

The authors evaluated 29 patients with cirrhotic refractory ascites from 6 different medical centers for procedural, periprocedural, and safety characteristics of IR alfapump implantation procedures. Technical success was 100%, defined as “the patient leaving the operating room with an implanted alfapump system that was actively pumping or pumping when activated or programed to do so”. 63% of the procedures were performed under general anesthesia and 37% were performed under local anesthesia. All patients received antibiotic prophylaxis.

83.3% of cases were performed according to the device instructions for use. The physician diverged from the instructions in five cases. The peritoneal catheter was implanted first in two instances to perform paracentesis during the procedure. Three cases presented procedural difficulty, of which two called for an additional incision for tunneling and one required a bladder catheter extension. One patient had a malfunctioning pump which was replaced during the procedure. One patient had post-op bleeding, which resolved after transfusion. The median hospital stay was 4 days.

27 patients were followed up at 3 months. One patient died secondary to cirrhosis and two patients had device explant because of cellulitis and wound dehiscence. 40 serious adverse events (SAE’s) were reported in 18 patients. Two SAE’s were classified as procedure related due to post-op bleeding and implant site abdominal pain and leakage. One patient developed bacterial peritonitis 26 days post-op, which was possibly procedure related. There were four re-interventions including one pump exchange the day of the procedure and three peritoneal catheter replacements for leaking, kinking, or dislodgment.

Overall, there was a 100% technical success rate, three procedure related SAE’s, four surgical revisions, and two explanations. The total adverse event rate was 31%. The study showed that alfapump implantation by IR is technically feasible. 

Commentary

The authors prospectively examined 29 cases of alfapump device implantation by interventional radiologists. The 100% technical success rate demonstrates that an IR approach is technically feasible. The overall adverse event rate was 31%. However, the authors described potential procedural changes such as increasing the initial pump volume and earlier pump initiation that may help to decrease adverse events related to pump pocket fluid accumulation. Nevertheless, the adverse event rate was comparable to or lower than reported in prior studies. As the authors mention, the study is limited by the small sample size, short follow up, and lack of control group. As the alfa pump device becomes more prevalent, subsequent larger studies will help further characterize the safety of an IR implantation and the risks and benefits compared to a surgical technique. Overall, alfa pump placement by interventional radiology is technically feasible and the alfa pump is a promising device for the treatment of recurrent ascites.

Post Author

Maxwell Cretcher, D.O.
Integrated Interventional Radiology Resident, PGY-4
Department of Interventional Radiology
Dotter Interventional Institute, Oregon Health and Science University
@MCretcher