When conducting in vivo work with catheters, you can’t see beneath the skin. We wanted to show what is actually happening at the tip of the catheter, where blockages can occur, and what can be done to prevent them.
Blood in the tip of a catheter can clot and form blockages, preventing sampling or infusion. Best case, an intervention is needed to clear the clot; worst case, the animal must be pulled off the study. But where and how do these blockages typically occur?
Rat Catheters (3Fr)
Test 1: Traditionally, a hemostat is used to access an externalized catheter with a blunt needle and plug. When the hemostat is released, blood pulls back as the catheter springs back to its normal shape. With a 3Fr rat catheter this pull back can be about 1µl.
Test 2: In Test 2, we show the results when using a miniature external port. These ports, such as the PinPorts™ used in Instech Vascular Access Buttons™, limit pullback to the volume vacated when the injector tip is removed from the port; this 2mm of 25ga tubing equates to about 0.5µl.
Test 3: In our third test, we demonstrate the way to eliminate the pullback completely. We show that if you apply slight pressure to the syringe plunger as you remove the injector from the port - known as positive pressure technique - you can replace the volume of the injector tip with sterile flush or lock solution and can eliminate the blood pullback. This technique is well known to ICU nurses who deal with central venous catheters, but is not always practiced in laboratory animal research.
3Fr Rat Catheter Results
Test 1: Standard externalization with traditional technique - 6mm of blood in the catheter.
Test 2: Vascular Access Button™ with no finger pressure - 3mm of blood in the catheter.
Test 3: VAB™ with positive pressure technique - no blood in the catheter.
Mouse Catheters (1Fr)
While positive pressure technique can improve patency of rat catheters, it can mean the difference between success and failure with mouse catheters.
Test 1: A 1Fr mouse catheter, typically used to access the mouse carotid artery or femoral vein, has a volume per cm one-tenth that of a rat catheter. As you’ll see in the video above, the results are dramatic. Even with a PinPort™, blood can be pulled back 30 to 50mm into the catheter if positive pressure technique is not used.
Test 2: Using the same setup as Test 1 with the 1Fr catheter and a VAB™, we applied the positive pressure technique and were again able to eliminate blood pullback.
1Fr Mouse Catheter Results
Test 1: VAB™ with no finger pressure - 50mm
Test 2: VAB™ with positive pressure technique - 0mm
A researcher reported anecdotally that the technique changed the success rate of mouse carotid artery catheterizations from 25% to 80%.
Catheter blockages can have a negative impact on sampling and infusion studies, requiring more animals than necessary. Positive pressure technique is difficult, if not impossible, when catheters are clamped with hemostats and closed with plugs. It may have been one of the factors that contributed to the 5x increase in patency of mouse jugular vein catheters in Charles River’s recent study of standard exteriorizations versus transcutaneous access ports1. Our tests have shed some light on what happens beneath the surface, and amplified the importance of using a vascular access port for mice in combination with a solid positive pressure technique.
To minimize the amount of lock solution that is infused into the animal, practice the technique on the benchtop using the exact catheter and equipment you will use in vivo before moving to a live animal.
With mice it is even important to use two part syringes that do not have rubber plungers as the compliance of the plunger can pull blood into the catheter after your finger is off the plunger.
1Mallett S, Murray T, Karicheti V et al, Charles River Laboratories, “Patency of Jugular Vein Catheters in CD-1® IGS Mice: Evaluation of Three Catheter Maintenance Schedules in Standardly Externalized Catheter and Transcutaneous Buttons,” AALAS poster #P65, October 2017.