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Materials in Medicine: Hemodialysis and Prosthetic Vascular Grafts

Posted on March 20th, 2017 by in New Materials & Applications

Materials in medicine 1

Depiction of an ePTFE Arteriovenous Fistula Graft in a Human Arm (left); and Needle inserted in a Graft (right) (Source: W. L. Gore & Associates, www.goremedical.com)

Human kidneys are very important because they keep the composition, or makeup, of the blood stable, which lets the body function normally (Source: The National Institute of Diabetes and Digestive and Kidney Diseases, www.niddk.nih.gov). The functions of kidneys include:

  • Prevention of the buildup of wastes and extra fluid in the body
  • Keeping levels of electrolytes stable, such as sodium, potassium, and phosphate
  • Making hormones that help
    • Regulate blood pressure
    • Make red blood cells
    • Bones stay strong

Kidney Failure is defined as kidney function dropping down to 15% or less.  If someone’s kidneys fail or he/she has chronic kidney disease and experiences severe symptoms such as swelling, shortness of breath, fatigue, muscle cramps, nausea or vomiting.  When kidneys fail, dialysis keeps the body in balance by:

  • removing waste, salt and extra water to prevent them from building up in the body
  • keeping a safe level of certain chemicals in blood, such as potassium, sodium and bicarbonate
  • helping to control blood pressure

There are two major forms of dialysis:

  • Hemodialysis: Fluid and toxin removal from blood by diffusion and convection across a semipermeable membrane by sending the patient’s blood through an external filter
  • Peritoneal dialysis: Fluid and toxin removal by diffusion and convection by instilling and removing fluid into the peritoneal cavity through an abdominal catheter

Hemodialysis is the most effective method among the available dialysis options.  But it requires chronic access to the circulation to flow the blood out of a vein, dialyze the blood and then return it through an artery.  In essence the method works as an external kidney function that removes waste and excess water from the blood (Figure 1).  After trials with a few access techniques with mixed results the early difficulties were overcome by the introduction of the subcutaneous arteriovenous fistula in 1966 (Source: Brescia MJ, Cimino JE, Appel K, Hurwich BJ (1966): Chronic hemodialysis using venipuncture and a surgically created arteriovenous fistula. New Eng J Med 17; 275(20): 1089-1092).

The indications for prosthetic grafts include: lack of suitable vessels particularly in elderly and diabetic patients, need for immediate access to the vein and in children who cannot tolerate multiple painful vein punctures.

Materials in medicine 2

Figure 1 Schematic of a Hemodialysis Machine attached to Patient

 

 

 

Subsequent development of the bridge fistula, initially with reversed saphenous vein and later with synthetic materials, further expanded the availability of hemodialysis to a larger group.  The great saphenous vein, also called the long saphenous vein, is a large, subcutaneous, superficial vein of the leg. It is the longest vein in the body running along the length of the lower limb.  The introduction of expanded polytetrafluoroethylene (ePTFE) for bridge fistulae in 1976 was a milestone that allowed dialysis clinics to grow and cover significant numbers of additional patients (See a past post /new-materials-applications/eptfes-simplicity-and-sophistication).

Expanded PTFE bridge is a tube, seen in Figure 2 (left), that is placed subcutaneously in the interior forearm of a patient as seen in Figure 2 (right).  Expanded PTFE is a microporous material made of polytetrafluoroethylene that can be made into tubular shape that forms the bridge. The white tube is an ePTFE bridge called Arteriovenous Fistula (AVF) Graft.  Fistula refers to the surgically made passage between the artery and the vein in Figure 2 (right).  The fistula is made between the artery above the wrist and the largest available vein in close proximity.

Materials in medicine 3

Figure 2 Examples of ePTFE Bridge (left) (Source: W. L. Gore & Associates, www.goremedical.com); and Diagram of an ePTFE Graft inserted in a Human Arm (right)

An example of vascular graft offered by W.L. Gore & Associates is called ACUSEAL® (Figure 3).  It has a multilayer construction consisting of:

  • An elastomer membrane between the inner and outer layers of expanded ePTFE
  • Surface of the ePTFE layer (luminal layer) in contact with blood has a special treatment called CBAS. This treatment is a durable, non-leaching heparin-coating technology with endpoint attachment that enhances blood compatibility and provides thrombo-resistant (non-clotting) blood contacting surfaces for cardiopulmonary bypass circuit devices.
  • Surface of the other ePTFE layer (abluminal layer) is untreated
Materials in medicine 4.png

A.

 

Materials in medicine 5

B.

Figure 3 (a) Tri-Layer Construction of a GORE® ACUSEAL Vascular Graft (b) Scanning electron micrograph of an expanded polytetrafluoroethylene graft with heparin immobilized on its inside surface (it is in contact blood)

The microporous structure of ePTFE allows in-growth of the tissues surrounding the graft thus integrating and stabilizing the graft.  The catheter needle can be inserted into the graft (cannulated) within 24 hours after implantation.

Hemodialysis is the far more common type of dialysis—about 90% of all dialysis patients.

While effective in the short term, hemodialysis has drawbacks:

  • It is confining and inconvenient; patients are tethered to a machine for three- to four-hour sessions in a clinic three times per week.
  • Dialysis is exhausting for patients and fraught with morbidity and eventual mortality. 35% of hemodialysis patients remain alive after five years of treatment. (Source: University of California, San Francisco, School of Pharmacy and Medicine, https://pharm.ucsf.edu/kidney/need/statistics)

An important issue for people on dialysis is controlling their diets.  They may have to be on a special diet. The patient may not be able to eat all foods and may have to control fluids intake.  A patient’s diet depends on the type of dialysis.  Most patients need to limit the amount of sodium, potassium, and phosphorus in in their diet.  A Registered Dietitian Nutritionist (RDN) works with dialysis patients on the diet management.

Medicare coverage is extended to a person of any age who requires either dialysis or kidney transplantation to maintain life. The 650,000-plus people who live with end-stage renal disease are 1% of the U.S. Medicare population but account for roughly 7% of the Medicare budget.  Hemodialysis treatment costs an average of $89,000 per patient annually in the United States. This amounts to a total annual hemodialysis cost in the United States of $42 billion; $34 billion of this is absorbed through the Medicare budget. The remainder is either covered by Medicaid, private insurance, or paid out of pocket. (Source: U.S. Renal Data System, www.usrds.org, 2014).

Ms. Ghazale Dastghaib, MSD RDN, LDN is the co-author of this post.


 

All opinions shared in this post are the author’s own.

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