Insulin pumps are designed to deliver boluses at a controlled rate. But what speed is right for you?

• If it infuses too quickly then (depending on your choice of cannula type and various physiological factors) you may feel discomfort.
• If it takes too long you can be left standing around waiting for it to complete.
  Either because you need to make another change, or maybe you’re about to disconnect for exercise (and yes, some exercise types benefit from pre-bolus).
• Some evidence (such as this 2019 paper) shows that faster infusion can result in better/faster insulin absorption.
• But anecdotal evidence says that (depending on the cannula type) above a certain threshold bolus volume, delivering too fast can result in leaking around the cannula (lost insulin).

Tandem complaints?

I hear a lot of people commenting that the Tandem t:slim X2 delivers insulin very slowly. Some people transitioning to the pump have been wondering if the slow delivery is behind their need for higher basal rates, etc.

From an engineering point of view, the reason behind the t:slim’s slower speed is clear. The pump does not have a “piston”/”syringe”-style reservoir. Instead it has a micropump that transfers tiny packets of insulin from the bladder reservoir to the infusion tubing. This lets it be very precise with the volume of insulin being delivered, but there is a limit to how fast it can deliver those packets!

The hard numbers

For the pumps in the Australian market I’ve listed below their bolus delivery speeds, sorted from fast to slow. This data has been extracted from the official product documentation.

Some pumps can have their bolus delivery speed set to 2 or 3 choices. Some pumps change their delivery speed based on the size of the bolus.

Pump	Bolus speed
Animas Vibe (“NRML” speed)	60 U/min
YpsoPump	33.3 U/min
Medtronic 640G/670G (“Quick”)	15 U/min
Animas Vibe (“SLOW”)	12 U/min (1U every 5s)
Accu-Chek Combo, Accu-Chek Insight (“Standard”)	12 U/min
Accu-Chek Insight (“Moderate”)	9 U/min
Accu-Chek Insight (“Slow”)	6 U/min
DANA R	5 U/min
DANA RS (“fast”)	4 U/min
Accu-Chek Insight (“Very slow”)	3 U/min
Tandem t:slim X2 (25U bolus)	2.97 U/min typical
Accu-Chek Solo	1-2.5 U/min
DANA RS (“medium”)	2 U/min
Medtronic 640G/670G (“Standard”), Medtronic Paradigm, Veo, Omnipod	1.5 U/min
Tandem t:slim X2 (2.5U bolus)	1.43 U/min typical
DANA RS (“slow”)	1 U/min
Medtronic Veo 554/754 (<1U bolus)	0.75 U/min

As you can see, the Tandem pump is definitely a lot slower than the Animas pump it replaced (in Australia). It’s understandable that people transitioning from the Vibe to the t:slim would comment on the change.

But it’s not dramatically slower than some of the other pump models!

My own experience

I started out on an Animas Vibe, and had it turned down to “SLOW” fairly quickly (for comfort with the cannulae I was using on my first pump, not because of leaking sites).
Then I used Medtronic pumps for a while, and the slow-down was quite noticeable.
I’ve also used a DANA R. These days I’m mainly using Combo and YpsoPump pumps.

When I say “used” I mean used as my primary pump: living with it for extended periods. I’ve “used” (or rather “played with”) most pumps in the lab, but that’s probably not the same as using it in real life.

Personally I have never noticed a need to change my insulin doses dramatically across these pump models.

But YDMV (Your Diabetes May Vary).