Revisiting the topic of filling pump cannulae

I’ve recently been using Medtronic Quick-set infusion sets with my pump, and made a surprising discovery which I’ll get to below. But first some introduction:

Insulin pump users will be familiar with the advice that teflon cannulae should be replaced every 3 days (steel cannulae every 2 days) to avoid long-term damage such as lipohypertrophy (warning: disturbing images in Google search). The build up of fat and internal scar tissue can cause problems with insulin absorption and frequency of getting “bleeders”. I used to let my cannula stay in for longer, but after realising I was noticing some of those issues I now stick to a regular 3-day schedule.

Changing cannula and insulin on different schedules

Some people were only taught how to change the infusion set at the same time as refilling their pump reservoir, and after 3 days often still have insulin in the reservoir. But instead of throwing that out (and trying to fine-tune how much insulin they put in to be exactly 3 days worth, in a world of variable insulin needs) I have found it’s a lot more flexible to be able to put a new cannula in and keep using the same insulin.

But doesn’t the insulin need to be replaced after 3 days?

The official statements on this vary. The default labels for Humalog and Novorapid in Australia mention changing after 48 hours(!), while the US FDA labels mention 7 and 6 days respectively. At least one pump in Australia has approval for 7 days in their reservoir.

I explored this a year ago in “Have I cooked my insulin?“. When using Medtronic, Accu-Chek, and DANA pumps I regularly have each reservoir last 6-8 days. But I still change the cannula in my body every 3 days!

Different types of infusion sets

If I’m not refilling the pump and just inserting a new cannula, I usually keep the filled tubing connected to the pump, and just connect it to the new cannula. The details vary for each model of infusion set.

For the current list of infusion sets available in Australia through NDSS, check their IPC Order Form.

I did mention some of this a year ago in the “Filling a cannula” and “Not wasting infusion set tubing” articles. But here’s a recap for each type of infusion set, along with the information I’ve discovered for the Medtronic Quick-set.

TenderLink, VariSoft, Silhouette, Comfort

Accu-Chek TenderLink, Tandem VariSoft 30, Medtronic Silhouette, Animas Comfort: these are all the same except for the connector at the pump end of the tubing.

When priming the pump line the cannula is not connected to the tubing, and after connecting to a freshly-inserted cannula you need to fill with 0.7U (7 microlitres or µl) to fill the void left by the introducer needle.

The sealed package of the infusion set contains both the cannula and the tubing, so rather than replacing the tubing if you’re not replacing the reservoir (and then having to waste insulin priming the new line) it’s usual to just discard the new tubing.

Accu-Chek TenderLink are available through NDSS in a cannula-only version. This means you don’t have to discard the tubing and also saves a little money. These can be used with the Silhouette/Comfort/VariSoft tubing with no reconfiguration.

Note that the Sure-T/TruSteel/Contact sets described below also share compatible tubing with these, so it’s possible to switch cannula type on the same tubing.

AutoSoft 30, Inset 30

These Tandem and Animas infusion sets (in the USA Medtronic also has the identical Mio 30) are shipped with the site and the tubing packaged separately in the box. After insertion the primed tubing is connected to the site and a fill of 0.7U is required.

Note that the tubing from these sets is compatible with the TenderLink et al. I’m still using up tubing from the Inset 30 with TenderLink cannula sites.

Sure-T, TruSteel, Contact Detach

Whether from Medtronic, Tandem, or Animas these steel cannulae are again all the same.

We can simply attach the new cannula to the old pump tubing, fill it until drops come out the end, and insert. Then we can discard the extra tubing. No cannula fill is required.

Note that the TenderLink/Silhouette/Comfort/VariSoft sets described above also share compatible tubing with these, so it’s possible to switch cannula type on the same tubing.

Rapid-D

Accu-Chek’s steel infusion set can be dealt with in the same way the Sure-T et al.

Actually these are purchased in two parts: “cannula only” (which includes the 10cm tubing to the stress-relief site) and “transfer set” (which is the tubing from the pump). So you don’t have to even waste a transfer set by opening that package.

FlexLink

Accu-Chek’s 90˚ teflon sets are inserted before the primed tubing is connected. They then need 1U of fill. This fills both the void left by the needle and the dead space within the head/connector of the site.

For a second site you can either discard the tubing that was in the package, or use one of the cheaper “cannula only” packages.

Cleo 90

Each box of Cleo sets includes separately packaged sites (with inserters) and tubing. So you can leave the tubing in its package and just do the regular 0.2U fill for the 6mm version, or 0.3U for the 9mm cannula.

Orbit

These are the Ypsomed Orbit soft, Orbit micro, and DANA Soft Release O sets. I’ve written about these before. The package usually includes tubing and site, but there are cheaper “Universal cannula” versions without tubing.

The tubing is connected to the site after insertion, so it’s an easy process to just discard extra tubing if you need to.

The teflon (“soft”) sites need a fill of 0.3U for the 6mm versions and 0.4U for the 9mm.
The steel (“micro”) sites still need 0.2U to fill the deadspace in the site.

Mio, AutoSoft 90, Inset II

The Medtronic Mio, Tandem AutoSoft 90, and Animas Inset II are all the same except for the pump connector.

But the tubing comes connected to the cannula, inside the integrated inserter. It can only be disconnected from the site after insertion.

The traditional instructions describe priming the tubing from the pump until drops come out the end of the cannula, and then after insertion the 6mm sites need 0.3U fill while the 9mm sites need 0.5U.

However, on recent versions of these sets Unomedical (the manufacturer) altered the fill volumes: 0.1U for 6mm and 0.2U for 9mm.

However, the deadspace in the head of the site was filled along with the tubing, so different fill amounts are needed if you insert one without having first connected it to a pump and primed the line. From observing sites not connected to my body (using a microscope to watch the fill) I found that an extra 0.2U of insulin was required.

Thus with these sites I usually fill my reservoir with approximately enough for 6 days, and insert a cannula using the regular process. Then after 3 days I take a new site, unwind the tubing, insert into my body, and immediately disconnect and discard the new tubing. I transfer the tubing from the old site to the new one and fill with 0.3U for a “bare” 6mm site, or 0.4U for a bare 9mm site.

In doing so I have not noticed any BG “spike” after a new site (unless there was lipohypertrophy involved). Note that I leave the old site inserted for a few hours until I’m sure the new one is working properly (so if needed I can temporarily switch back to it to check).

Incidentally, when using Inset II sets I sometimes used Mio sets for the alternate sites as I could simply discard the tubing. They really are identical.

Quick-set

Only available from Medtronic (but with both luer-lock and “Paradigm” versions of the tubing) these sites come with integrated tubing which can only be disconnected after insertion.

As with the Mio, the regular process fills the site’s deadspace as part of the priming of the line, and then after removal of the needle a 0.3U (for 6mm) or 0.5U (for 9mm) fill is required.

But just as with the Mio it’s possible to insert a new site and immediately discard the tubing. Recently doing the same observations as with the Mio, I found that an extra 0.8U is required! Obviously the deadspace in the connector is quite a bit larger. Thus I fill with 1.1U for “bare” 6mm sites, and expect that 1.3U will be needed for bare 9mm sites.

For my sensitivity, if I forgot to increase the cannula fill by the right amount, missing out on the next 0.8U of insulin my pump thought it was delivering (whether basal or bolus) would result in a noticeable BG spike!

Conclusions

I’m not recommending anyone keeps using anything longer than safe, but in my own use have had a lot of success with continual pumping and often only using a new tubing set with each new reservoir rather than each new cannula.

It seems a waste throwing away new tubing (and better when we have the option to not do that) but probably less wasteful than throwing away the insulin in our tubing (which depending on the tubing length can range from 9U up to more than 22U!).

At the same time, inserting a new site without having to also refill the pump (and vice versa if possible) does mean that each change event becomes quicker and easier.