Before using any medication for catheter clearance, ensure you have ruled out other causes of the occlusion. VADs without a blood return should not be used, but make sure your technique is not the problem. Use a slow, gentle technique to aspirate. Use a smaller diameter syringe (e.g. 5 mL or 3 mL) as these cause LESS pressure on aspiration. Look at the history of the problem to determine if this is truly a thrombotic occlusion or if it could be caused by contact between incompatible medications producing a precipitate or a total nutrient admixture that causes a lipid buildup on the intraluminal catheter walls. Mechanical problems such as pinch-off syndrome should also be considered for subclavian insertion sites. Also check for external problems such as closed clamps or kinked tubing.
The internal volume of the entire catheter system can be difficult to determine. The design of the port body and lumen size of the catheter varies. The manufacturer provides the internal volume or priming volume of each brand of implanted port. However, the catheter is cut to a patient-specific length, which decreases the original internal volume.
Several interventions to manage this problem should be considered. Obtaining the internal lumen volume could be done by simple aspiration. First flush the catheter with normal saline. Attach an empty syringe, and slowly aspirate until the blood has reached the connection of the syringe to the needleless connector on the port access extension set. Note the space in the syringe as this should approximate the catheter’s internal volume. But remember, this has to be done and documented before an occlusion occurs. This volume would determine the amount of tPA required to fill the entire length of the system.
The other option is to instill the tPA dose of 2 mg in 2 mL, and then follow it be a small amount of normal saline such as 0.5 to 1 mL to move the tPA to the distal end of the catheter and into direct contact with the location of the thrombus.
Dilution of the tPA to less than 2 mg in 2 mL by adding more volume is not recommended as this may not be as effective.
I would recommend reading the 2016 Infusion Therapy Standard of Practice #48 Central Vascular Access Device Occlusion for more details. This question is not addressed because there are no studies reporting these variations, but there can be concern among nurses doing this procedure for implanted ports.