Medtronic 670G Auto-mode and carb-ratios


(Jess) #1

After using the 670G auto-mode for a few weeks now and looking over my Carelink data, I realized that the insulin pump pauses the background “microbolusing” for about 1.5 hours after a large meal bolus. Because my body still needs “background” insulin during this time, now some of my meal bolus is being consumed as “basal”.

Now my meal-boluses have to do double-duty; Bolus for carbs, AND cover background insulin needs, so using my manual-mode carb ratios, I don’t get enough insulin to cover my carbs, which has lead to post-meal blood-glucose spikes.

Okay, so I can adjust my carb-ratios to compensate for this, but the 670G only stores one set of carb-ratios (different ratios for different times of the day), and shares the same carb-ratios between manual-mode and auto-mode. So, when you get kicked out of auto-mode (for whatever reason), guess what? Your carb-ratios are now causing the pump to deliver too much carb bolus. Surprise!

I suspect this is how Medtronic got this pump approved by the FDA; by deliberately conflating meal-bolus insulin with background “basal” insulin, and hiding it with carb-ratio adjustments.

The real kicker is that they system didn’t need to be designed this way. A control algorithm could have been designed so that it maintained carb-ratio equivalance between manual-mode and auto-mode. Medtronic choose to make things more difficult for us.

“Thank you for choosing Medtronic.” I guarantee you, this will be the LAST TIME I choose Medtronic.


(Dennis J. Dacey, PwD) #2

Hi Jess @MrEntropy, that is a really interesting observation and deserves some exploration. No, I DO NOT have a 670G yet; my existing MiniMed Medtronic warranty expires this month and I suspect that sometime in the next 15 months Medicare will permit a new pump and maybe even the 670G.

My suspicions behind the Medtronic algorithm you describe is to get around the FDA rejection fear that too many people would be forced into hypoglycemia; probably good in that a slightly higher Body Glucose Level can always be corrected “later” by a micro-bolus.

Now I’m curious, during the few weeks that you have been using the 670G have you attempted to validate its basal by long term fasting? With my pumps, i’ve had three models, after changing basal rates I will validate basal by fasting for 18 hour periods and check my BG frequently to ascertain that I’m staying within my target ranges.


(Jess) #3

Dennis,
I Fasted for about 20 hours a few days ago (about 3 weeks into auto-mode), and my blood glucose was about 150 average over that time. Peaked about 165, and went down to about 130 minimum. My target BG is 120, not 150 either; Not impressed.

Another MAJOR GOTCHA, the 670G automode system has is that it doesn’t use your insulin-sensitivity for calculating high-blood-sugar correction boluses while in auto-mode. My BG peaked over 300 mg/dL one afternoon after a big lunch, and when I went to enter a correction bolus under the bolus sceen in automode, it calculated I only needed about a 1.3u bolus. This was at 3PM, and I had eaten at noon, so I didn’t have much active insulin left in my system either. Using my sensitivity-ratio, I really needed about 14u for the correction. What really erks me is that the pump doesn’t tell you it isn’t giving you enough insulin for the correction. Automode can handle it, but it will take about 12 hours to come down from a significant high. Issuing a proper correction bolus can get you “back to normal” within a few hours. I actually filed an FDA complaint about this, as this can trip-up a lot of people.


(Dennis J. Dacey, PwD) #4

Jess,
I can see your frustration. For me as a retired businessman, that would not be satisfactory performance.
I guess for my next pump I will need to “convince” United Health to make reimbursement for the Tandem t-Slim / Dexcom G6 closed-loop system which appears to far out-perform Medtronic; historically, I’ve been a supporter of Medtronic.


(Jess) #5

Dennis,

Yeah, I’m an Electrical Engineer, and I am appalled by the performance of this Automode system. I’ve worked with electronics instrumentation and control systems in the past. My professional opinion; Medtronic’s control algorithm is a big turd.


(Ryan) #6

I absolutely agree that the pump needs to be updated to allow different saved settings for auto and manual. As long as the sensors are terrible in day one, I will need to be in manual mode about 15% of the time. Changing all settings back and forth at least twice per week is a big pain.


(Chance) #7

I can attest from my own experience that the 670 likes to turn itself off after meal boluses until it gets a gauge for where you are going. I would certainly like a way to force it to give its background insulin unless its obvious that I’m going to trend low, so I can understand the frustration here. At the same time, I don’t think I’ve ever seen my pump stop background micro boluses after a meal for more than 45 minutes without appearing that the reduced insulin was warranted.

With that said, I doubt the problem will be fixed anytime soon, (It’s not like Medtronic can pull a Tandem and release a remote update, after all). As a person who likes solutions though, I believe I remember hearing that reducing your active insulin time under your delivery settings makes the pump act more aggressively in its micro boluses and can have a significant difference.

You also mentioned that auto mode doesn’t use the insulin sensitivity number you provide in its calculations. Does this imply that it generates its own number through its learning process? It just seems unlikely, but also like a classic Medtronic move. I’d love it if you could explain it a bit better because I’m a bit lost there @MrEntropy.


(Tawnya) #8

There is a setting for active insulin time that can be adjusted. This setting tells the pump how long the bolus lasts in your system before basal needs to kick back in. I believe default is around 4 hours. I adjusted mine down to 2.5 and it made a big difference, but I totally agree that auto mode does not give me the tight control I had with the 630g. My a1c went up from 6.2 on the 630g to 6.9 in auto mode. I decided to just turn auto mode off and run it like my old pump and my bg averages 110 now.


(Nancy) #9

@MrEntropy - thanks for the early adopter report! I’m on the fence about moving to a closed-loop (I really, really don’t want to wear multiple peripheral devices all the time!) although I am most interested in the auto-sensing feature. How is the sensor? Is it VERY much improved over the first gen?

I’m off topic. I wanted to ask if you and @Chancey have provided the same feedback to Medtronic? It’s great you’re warning your community, but the Minimed manufacturer needs to know so they can improve the next gen.

Thanks again, I want to gather as much real life feedback (I’ve already chatted with a couple of the Minimed ambassadors who use the 670), you’ve added a balance to my data.

I wish you continued good health!


(doug) #10

Several things about the 670G annoy me. When I have high bg after a meal, I turn of auto-mode, do a calculated bolus, and turn auto-mode back on.


(Jess) #11

Chancey,

That’s correct. The 670G does not use your insulin sensitivity to calculate a correction bolus. Your insulin-sensitivity might factor into the calculation somehow, but when I called Medronics and asked about this, they said auto-mode doesn’t use your insulin sensitivity. Medtronics recommended I exit auto-mode, to do a correction bolus, then re-enter auto-mode. But, auto-mode will dial-back your microbolusing if it sees a lot of active-insulin, so I have found I need a more aggressive (lower) sensitivity factor to compensate. If you ever switch back to manual-mode for an extended period of time, you’ll be overcorrecting highs if you use the sensitivity factor that compensates for automode. You can only store one sensitivity factor in the pump, so this is a pain.

Auto-mode will bring you down from a high, but it will take about 12 hours (personal observation) vs 3 hours with a normal correction bolus.


(ceolmhor) #12

Why do you turn off auto mode to do this? Under these circumstances – unless I misunderstood what you’re saying – I just do the calculated bolus.


(doug) #13

Because of the length of time Auto Mode takes to correct high BS. I do this only if BS is over 300 after a meal. I go back to Auto Mode after the bolus is complete.


(ceolmhor) #14

But can’t you do the bolus without exiting auto mode? I do this all the time.


(doug) #15

Yes. But Auto Mode bolus doesn’t reduce BG fast enough for me.


(Patricia) #16

HOW?! I get “too many” requests as is for Auto Mode BG and even when the finger stick BG is still on the screen no less. How do you manually enter a BG into auto mode without a new finger stick or do a correction bolus bypassing auto mode without exiting it?


(Ryan) #17

In either mode, one can select “Enter BG” and manually input your current blood sugar reading.


(Patricia) #18

I do that often but it only works with calibration. It doesn’t populate Auto Mode, which keeps demanding a new BG even when one is just done and on the meter to view. That is my complaint/ frustration!


(Elisha) #19

I have the tslim pump it’s the cheapest model and I’ve one go out and not give me any basal for 2 weeks, ( I figured this out by not using it but letting it think that I was). When I called tandem they sent another pump and I got it in 3 days( hq to Southwest Missouri) but they have been upgrading the sites they use and they are better than the ones before. But don’t take my advice DO RESEARCH. It also has a 7 day li ion rechargeable battery. 4 out 5 stars


(smckinn2001) #20

To all, WOW! I have the 670, but have never used auto-mode because the pump reads low 95% of the time, but at least 20 points (e.g., says I’m at 110 when I’m really at 130). Sometimes the discrepancy is greater and only once in a while it’s less. Combining that with everything I read about insulin sensitivity does NOT make me want to try it now. Hmm…guess I’ll just continue using it manually.