Dialysis is a treatment for kidney failure, where the kidneys can no longer effectively filter waste and excess fluid from the blood. Hemodialysis, the most common form of treatment, requires the patient’s blood to be continuously circulated outside the body through a filter called a dialyzer. Cannulation is the process of inserting specialized needles into a patient’s bloodstream to connect them to the dialysis machine. This procedure must be performed precisely at the start of every treatment session to ensure an efficient flow of blood for cleaning.
Vascular Access: The Necessary Foundation
A successful hemodialysis treatment requires a robust access point, which must be created surgically before dialysis begins. This vascular access must handle high blood flow rates, typically between 300 to 450 milliliters per minute, required by the dialysis machine.
The two primary types of permanent access used for cannulation are the Arteriovenous (AV) Fistula and the AV Graft, both commonly placed in the arm. An AV Fistula is created by directly connecting an artery to a vein, causing the vein to enlarge and thicken over several months, a process known as maturation. This natural vessel is durable, has the lowest risk of infection, and can withstand repeated cannulations.
An AV Graft is used when a patient’s natural veins are unsuitable for a fistula, involving a synthetic tube connecting the artery and vein. While a graft can be used sooner than a fistula, it carries a higher risk of clotting and infection. A third option, the Central Venous Catheter (CVC), is a flexible tube inserted into a large vein in the neck or chest for immediate, temporary use. The CVC does not require cannulation with needles, as the dialysis tubing connects directly to external ports.
The Cannulation Process
Cannulation involves inserting two separate needles into the vascular access for each treatment session. One needle, the arterial needle, draws blood out of the body to the dialyzer. The second, the venous needle, is positioned further along the vessel to return the filtered blood to the patient’s circulation.
Needle size, measured by gauge, is selected based on the prescribed blood flow rate. Larger needles, such as 14 or 15 gauge, are needed for higher flow rates to prevent excessive pressure on the access. The needle is inserted at a shallow angle, typically between 20 to 35 degrees, and advanced until a flash of blood confirms entry into the vessel.
To preserve the health of the access site, the “rope ladder” method is recommended. This involves systematically rotating the cannulation sites along the entire length of the vessel for each treatment. This rotating technique prevents repeated trauma to a single area that can lead to the formation of weak spots or aneurysms.
A poorly executed insertion can compromise the integrity of the access, potentially shortening its functional lifespan. Another method, the “buttonhole” technique, uses a single, established track for insertion with a dull needle. While this can reduce pain, it requires careful sterile technique due to a higher risk of infection.
Patient Experience and Site Management
Since cannulation is repeated multiple times per week for years, minimizing patient discomfort is a major focus of care. Many patients use topical anesthetic creams or sprays applied to the site up to an hour before the procedure. Distraction techniques, such as deep breathing or conversation, are also employed to help manage anxiety and pain during insertion.
Immediate complications must be managed quickly to prevent long-term damage to the vascular access. Infiltration occurs when the needle punctures the back wall of the vessel, allowing blood to leak into the surrounding tissue. This causes immediate pain, swelling, and the formation of a hematoma. If infiltration occurs, the needle must be removed immediately, and a new site selected to ensure the required blood flow rate is achieved.
After treatment, the needles are removed, and hemostasis (stopping the bleeding) is achieved by applying gentle, direct pressure over the puncture sites. Pressure should be applied only enough to stop the bleeding without completely compressing the vessel, which could cause clotting. After bleeding stops, a clean dressing is applied to protect the site from infection. Patients must monitor the access daily, checking for the characteristic vibration or “thrill” that confirms proper blood flow.