Medtronic’s Hybrid Closed Loop System: Getting More Bang For Your Buck

By, Gary Scheiner MS, CDE, Jennifer Smith RD, LD, CDE, Alicia Downs RN, MSN, CDE, Annette Valle RN, CDE

It’s been about a year and a half since we started using and training/managing patients on the Medtronic 670G “hybrid closed loop” system.  And we’ve learned a lot.  We’ve learned that 670G is beneficial for some, but it clearly isn’t for everybody.  Yes, for the “average” person with diabetes, it can produce improvements in glucose control while helping reduce the risk of dangerous hypoglycemia.  But there are limits to the degree of glucose control that can be achieved, and there are many hassles and extra tasks involved with using the system.

There are other hybrid closed loop systems that are already in use, despite not being on the “FDA approved” list of systems.

• Loop and OpenAPS systems are highly effective for improving glucose control, but they require special equipment and an “app build” to get them up and running.
• Tandem’s T:Slim with Basal IQ is easy-as-pie to use, but it only turns off basal insulin to help prevent lows.
• Other systems are coming to market soon:  Tandem’s Control IQ, OmniPod’s Horizon, Tidepool Loop and BigFoot Biomedical’s system are all slated to make automated basal adjustments (similar to Medtronic’s 670G, but with less work on the part of the user), but until the pre-launch studies are completed and the FDA signs off, all we can do is picture them in our daytime fantasies.

So for those who want 24-hour automated basal adjustment NOW that is FDA approved, that really leaves just one option:  670G.

Medtronic has taking steps to cut down on some of the quality-of-life drawbacks, such as requests for repeated calibrations, with a new-and-improved transmitter.  Still, there are a host of other downsides that don’t look like they’ll be going away any time soon.

The question then is this:

How can we squeeze better performance out of the 670G Hybrid Closed Loop System?

I think most of us would be willing to put up with the little inconveniences if it means much better glucose control, not just a slight improvement in time in-range.

To answer this question, the clinical staff at Integrated Diabetes Services put its collective brains together and came up with the following suggestions:

1. Get your settings right BEFORE starting auto mode (AM). 

True, when you’re in auto mode, the system doesn’t give a damn about what your basal settings were before.   It flattens everything out and adjusts based on the situation.  But you’re not always going to be in auto mode, and you don’t want your control to suffer when you’re in “manual” mode.   That means getting your basal rates to hold you steady when you’re in a fasting, non-bolused, not-exercising state.   Also, setting the carb ratios so that you’re back close to your target BG 3-4 hours after eating, and knowing how long your boluses truly last – something that can best be determined by analyzing CGM data following a correction bolus.

Carb Ratios and Active Insulin Time are the only parameters that are carried over directly from manual mode to AutoMode (AM), so it makes sense to have these setting dialed in before starting AM.

Having incorrect settings when you start AM can produce a rocky start for the first few days/weeks.  Yes, AM will make adjustments to basal delivery to help flatten things out, but these adjustments are temporary and subtle, so don’t expect them to solve everything.

2. Understand how AM operates.

AM updates its algorithm (the formulas used to determine basal adjustments and correction doses) at midnight every night based on your insulin usage for the previous six days.  For this reason, it is best to use the pump in MANUAL mode for a full six days before starting to use AM.

Because AM adjusts its algorithm based on the last six days of insulin use, be careful following periods of sharply increased insulin requirements, such as illness, steroid use, binge eating or prolonged stress.  AM may be more aggressive than usual following these periods, so consider using the temp target (150) for at least a few days following temporary periods of increased insulin use.

When in AM, the only two settings that are “carried over” from manual mode are your carb ratios and active insulin time.  AM doesn’t use your current usual basal rates or your correction/sensitivity factor.   Automode uses a target of 120 mg/dl (unless you are using the “temp target” feature), and will only suggest correction boluses if your glucose is above 150.  Likewise, it will only recommend a reverse correction (deduct from a meal dose) if your glucose is below 100.

There are limits to how much AM will increase basal insulin.  This varies from person to person.  Based on our experience, it seems to be roughly 2.5 times your “average” basal rate.  So AM will not be able to fix high readings quickly; use correction boluses instead.   Likewise, AM will not be able to “fix” lows once they have already occurred.  Reducing or turning off basal will produce a gradual glucose rise starting an hour or two later.  Lows (or near-lows) still need to be treated with rapid-acting carbohydrate.

3. Do all you can to ensure consistent insulin absorption. Problems with insulin action or delivery will interfere with AM’s ability to regulate your blood sugar levels. 

• Change your infusion set frequently – every 2-3 days
• Rotate your sites properly (avoid using the same spot twice in a month)
• Change your infusion set at the first sign of site irritation or ineffectiveness
• Store your insulin properly and avoid using past the expiration date
• Remove air bubbles from your reservoir and tubing before using them

4. Know when to take yourself OUT of auto mode. 

There are situations when auto mode has the potential to… shall we say… get in the way of optimal glucose control.  For example:

• When consuming a slowly-digesting meal, a normal bolus may peak and finish working too early. Because the extended (square/dual) bolus feature is is not available when using AM, it might be best to temporarily switch to manual mode and bolus using a square or dual delivery. b. Temporary basal
• Switching out of auto mode allows you to set temporary basal rates for situations such as sick days, stress, prolonged exercise, alcohol consumption, and high-fat meals.
• Steroids, such as cortisone or prednisone, can raise your blood sugar significantly for several days. AM may not be able to keep up with the increased insulin need.  Switching to manual mode with a temp basal increase or a secondary basal pattern tends to work better.
• On days with intense or prolonged physical activity, both basal and bolus insulin needs can be markedly reduced. AM limits the length of time at which very low basal delivery is permitted, and bolus doses calculated while in AM will likely be too high when exercising.
• If you have elevated blood sugar due to an infusion site issue, it is best to switch to Manual Mode, change out your infusion set, and deliver a bolus via injection to correct the high BG. Once the BG is back in-range, switch back to AM.

*Oh, by the way:  Don’t forget to reactivate “suspend before low” when turning AM off, as this feature is deactivated every time AM is turned on.

5.  Don’t hide things from AM.

When disconnected, put your pump into suspend so that the system knows you’re not receiving any insulin.  If the system thinks you’re receiving AM-driven basal insulin when you’re actually not, it will tend to under-dose you for elevated or rising blood sugars for the next several hours.    Likewise, if you give yourself an injection of insulin or use inhaled insulin (Afrezza), turn off AM for the next 2-4 hours.  Otherwise, the system may over-deliver insulin, thinking that you have less insulin-on-board (active insulin) than you really have.

If you eat, enter the carbs and bolus accordingly.  AM is not designed to “cover” most meals and snacks.  Small, slowly-digesting snacks may be an exception.

And do not, under any circumstances, enter a glucose value (for a bolus calculation or correction) that is not true a true value based on meter or CGM data.  Doing so may cause AM to alter your insulin delivery patterns for several hours, and the discrepancy between the glucose entry and the sensor value may force you to perform extra calibrations.

Some people prefer to enter “ghost carbs” (carbs that they didn’t actually eat) in order to get the pump to deliver a bolus to correct elevated readings.  While this practice is certainly better than entering a fictitious blood sugar value, it is generally discouraged because of the effect it will have on AM for the next several hours.

6. Get your sensor to work as well as possible.

AM (and your peace of mind) depend on reliable sensor data.  Calibrate your sensor three to four times every day, ideally when glucose levels are steady in order to avoid discrepancies related to lag time.  Use the Contour Next meter that came with your pump if at all possible.  This is the most accurate meter on the market, and will provide the truest readings for calibration purposes.

Follow the correct procedures for inserting your sensor, including all of the taping procedures. A secure/stable sensor will perform better than one that is often peeling loose.

Whenever there is a significant discrepancy between the sensor and calibration values, you may be asked to calibrate again right away.  DON’T DO IT.  Wait at least 30 minutes so that the values will match more closely.  This can help you to avoid a sensor error or AM failure.

7.  Plan ahead for exercise. WAY ahead.

Don’t forget that rapid-acting insulin typically takes 3-4 hours to clear.  Basal adjustments made by AM a few hours ago are still affecting your blood sugar.   If exercise tends to make your blood sugar drop, set the temporary target (150) at least an hour before starting exercise.  Keep the temp target running after exercise if you tend to drop following your workouts.

If the 150 target is not enough to prevent a low blood sugar, consider consuming rapid-acting carbs prior to the workout without bolusing.   And as mentioned previously, if you disconnect from your pump during exercise, place the pump into suspend mode.

8. Re-think the way you treat hypos

When glucose levels are approaching a low level, AM will usually reduce basal insulin significantly.  This reduction may continue until the glucose levels returns to a safe level.   As a result, treatment for a low or pending low will require LESS carbohydrate.   The combination of your usual carb treatment and reduced basal may result in a rebound high blood sugar.

Also, be careful about “trusting” your sensor when recovering from a low.  In many instances, the sensor will take 30 minutes or more to detect the blood sugar rise after you have treated.  If you trust the sensor under these conditions, you will over-treat.  Best to check your glucose with a fingerstick 10-15 minutes after treating the low in order to verify that your blood sugar has (or has not) risen to a safe level.

9. Pump setting adjustments

Switching into AM may require some adjustments to your usual pump settings.  IF your pump’s settings were not fine-tuned prior to using AM, you are likely to find that your carb ratios were too conservative and your pump’s usual basal settings were too aggressive.  If you make changes to your carb ratios after examining your data from AM, consider adjusting your pump’s basal settings at those times as well.  That way, your glucose will be less likely to go high or low when revering back to manual mode.

Although rapid insulin is known to have an active time of about 3-5 hours, it can be helpful to set a shorter time (2 to 2.5 hours) when in AM.   This causes AM to consider less “active” insulin when calculating correction doses.  This can be helpful to those looking to make the algorithm more aggressive.

10. Avoid grazing!

Eating and bolusing too frequently can get in the way of optimal system performance.  AM permits more aggressive basal adjustments when there is little-to-no active bolus insulin present.   Grazing makes the system less aggressive and leads to more AM “kick-outs” due to prolonged maximum or minimum basal delivery.

11. Analyze your data

If a tree falls and nobody hears it, does it make a sound?  If your blood sugar is chronically out-of-range and nobody does anything about it, what good is it?  Download your pump and look at your data regularly; weekly at the beginning and monthly after things settle down.

Carelink is Medtronic’s designated data software.  It can be a bit confusing and doesn’t offer all features we want (no sensor overlay report, for example), but it does provide some useful insights.  In particular, check the Meal Bolus Wizard report to see if your carb ratios are in need of adjustment, or if you need to bolus earlier (spikes right after eating are usually a sign of bolusing too late).  Physical activity and other lifestyle events don’t show up in Carelink, so keep track of these on your own, and see how your glucose responded in the Weekly Review Report.

Another option for reviewing/analyzing your data is a web-based program called Tidepool.   Unlike Carelink, Tidepool allows notes to be taken and uploaded into the reports.  It also allows you to actually SEE what the basal rate was when AM was ticking it up and down throughout the day, and individual day reports are magnified for easier interpretation.

So what can realistically be accomplished with 670G?

The pivotal trials completed by Medtronic a few years ago showed that users of 670G achieved an average A1c of approximately 6.9% (avg glucose of about 150 mg/dl) with about 70% of time in the 70-180 range.

The patients we have worked with who apply the strategies described above have managed to achieve the following:

• Average blood sugars from 130-145. Lower averages are not realistic with current target setting and correction parameters in AM. Users who are used to using lower targets should set their manual mode target to 120 before starting to use AM in order to become accustomed to the pending changes.  This corresponds to an average A1c of 6.5%.
• Time in-range (70-180) approximately 80-90% of the time.
• Fewer than three hypoglycemic events per week.

Not too shabby!

The idea here is not to sell you on any particular product.  We’re proud of the fact that we give our patients a fair rundown on everything available so that they can make an educated choice.  But if you already have the Medtronic 670G or are giving it strong consideration, hopefully these suggestions will help you glean the most possible value from it.

Feel free to reach out to our team for additional guidance.  We are happy to work with anyone via phone and the internet to help get the best possible results from any system.