Does anyone understand the nitty-gritty of looping?

[mattrix]

I've been sitting on this question for a while, but I cant think it out.

I really like the idea of an IV insulin pump that reads near IV plasma glucose and makes immediate adjustments accordingly - count me in.

However pumps are not IV, but inject into an infusion depot. Consequently they suffer the same vagaries that we all have to deal with.
"If I inject x units now, how much will it affect me and when?"
"What may change between now and then?"

Sure a loop can see your plasma glucose dropping and make a quick correction, but that correction is not going to do anything for some hours.

Also the infusion depot acts as a buffer, the insulin released from the buffer at a point in time is proportional to the amount of insulin in the buffer (half-time decay). So to double the insulin released you have to double the insulin stored in the buffer.

That is to say the insulin released into the body fluids is not proportional to the insulin entering the buffer at a given time. Think about MDI, if I inject 10 units at time 't' it does not release 10 units into my blood at time 't', but instead 10 units are released into the blood over the upcoming hours ( in my case about 4.5 hours).

Maybe I am underestimating the 'smarts' of the pump? If you set a pump to a basal of 1 unit an hour is that a) the amount of insulin released into your blood or b) is it the amount of insulin put into the infusion depot? These will be different things at different times.

*** deleted a lot here as it was repetitive **

 

[EllieM]

I notice a difference according to where on my body I place my infusion set....

I don't think it's a magic bullet, but it is preferable to MDI (for me anyway).

Also, different pumps have different algorithms?

I get a lot less hypos now I am on a pump (tandem tslim)

 

[evilclive]

By "insulin depot", do you mean the area of subcutanous fat where the infusion bit sits?

The pump is pretty straightforward - X amount of insulin comes out of the sharp bit. That's what it knows about. If it's giving you 1U/hour, it'll do that in drips over the hour. Eg Omnipod doses in multiples of 0.05u, every 5 minutes, so it'll give you .1, .1, .05, .1, .1, .05, .1, .1, .05, .1, .1, .05.

Yes, absorbtion rates from the body after it's come out of the pointy bit will vary and will be important. I think the pumps have an "Insulin on board" idea to help with that. So if you're dropping and you've got insulin on board, yes, it'll stop dripping in for a bit. Sometimes really quite a long time.

If you're dropping fast and you've got insulin in you, yes, the pump won't be able to do anything about that. If it's going too fast, isn't going to stop, and you don't already have food in you, you'll need to eat.

If you inject 10u and it takes 4.5 hours to release, consider why you're doing it. This will be a bolus dose for food - and the sugar from eating doesn't all come at once, it also spreads out over time. So it's not a problem, it's a feature that the delivery ends up being slow - the requirements aren't that it's instant.

Yes, if you're throwing a lump in as a correction, that will suffer from delay - but the hope is that the pump/sensor has noticed it going up higher sooner than you did, and has already been giving you slightly more, so it shouldn't need as much correction.

So everything you've written is correct - but the algorithms have been designed to cope with this, and they do generally work. Not perfectly, and probably not for everybody, but IME they're better than being on the fixed basal dose that slow acting injectable insulin gives you.