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Closed Loop

Closed-Loop

After you learn what you need from open-loop, this page provides suggestions to smooth the transition to closed loop.

Timing

Consider transitioning in steps. Some loopers start closed-loop when there are fewer distractions, possibly on weekends. It can be easier to transition at a time that does not involve food, possibly overnight.

Maximum Basal Rate

When starting closed-loop, it is important to be conservative. Start with the "Temp Basal Only" dosing strategy and limit the maximum basal rate. If your Meal Entries or Therapy Settings (basal rates, CR, ISF) are incorrect, this approach limits the risk of getting too much insulin. Typically, experienced loopers set their max closed-loop basal rate at no more than 3-4 times their average basal rate. Wait until you are comfortable with the slower corrections in "Temp Basal Only" before transitioning to "Automatic Bolus".

Temp Basal Only vs Automatic Bolus

Both Dosing Strategy methods update the prediction with each CGM or glucose reading, typically every 5 minutes, and use the updated prediction to generate a recommended bolus or recommended dosing restriction.

  • "Temp Basal Only" provides no more than 17% (per 5 minute interval) of that recommended bolus using temporary basal
  • "Automatic Bolus" mode provides 40% of that recommended bolus as an immediate bolus
  • When Loop recommends restricting insulin, both methods use temporary basals that are less than the scheduled basal, often commanding a temp basal of 0 U/hr

Glucose Correction Range

If your basal, ISF, or carb ratios are not correct, Loop may give you more insulin than you need to reach the correction you selected. Setting the correction range slightly higher at first helps prevent unexpected low glucose as you adjust your settings.

Watch the IOB

Watch whether Loop accumulates positive or negative IOB while holding your glucose steady when no food is present. If you consistently have positive or negative IOB, review whether to adjust your basal rate or ISF.

Expert Tip

In the absence of food, glucose trends should flatten out when positive or negative IOB trends to zero.

  • If glucose drops below the correction range and continues to drop while IOB goes negative, basal rates may be too high
  • If glucose remains above the correction range while IOB remains positive, basal rates may be too low

The ISF is also important, but basal should be evaluated first.

Meals

Start with meals that you know well. If Loop suggests less or more insulin than expected as a bolus before the meal - consider why this may be true.

  • If glucose is trending down, Loop may be trying to prevent a low glucose event
  • If glucose is trending up, Loop may be trying to add a correction to the meal dose
  • In any case:
    • You can adjust the absorption time and carb amount to see if that modifies the suggested bolus
    • You can override the Loop suggestion
    • Do not be surprised if Loop immediately suspends basal
    • Loop needs to see glucose start to rise before deciding you need more insulin after the initial meal bolus recommendation
    • Loop will not automatically provide more insulin until your glucose is above the lower range of the correction range (but will recommend a manual bolus)

This is definitely an area where YDMV (your diabetes may vary), so don't expect or accept that what works for others will work for you. Test, observe, and adjust as needed.

Automated Dosing

Loop calculates a predicted glucose curve based on your programmed settings for carb ratio (CR) and insulin sensitivity factor (ISF), using your glucose, insulin and carb history.

Two scenarios are given below to help illustrate the closed-loop automatic actions of Loop. A more typical scenario is to enter carbs and then use Loop's recommendation for an appropriate bolus.

  1. Enter a bolus with no carb entry
  2. Enter a carb entry without a bolus

Bolus with No Carbs

If you enter a bolus without entering carbs, the prediction will be for your glucose to go low. (The Loop model calculates a negative number for recommended bolus.) For this case, Loop issues a Temp Basal to prevent the low, typically 0.0 U/hr but always less than your scheduled basal rate.

COB and IOB

  • COB is the carbohydrates (g) that Loop expects to be absorbed
  • IOB is the current active insulin (above or below the scheduled basal rate)

Carbs with No Bolus

If you enter carbs and select Save without bolusing, you have COB without associated IOB. In that case, Loop predicts your glucose will start rising and updates the recommended bolus, which includes consideration of your Glucose Safety Limit, Correction Range and Maximum Bolus . If that recommended bolus is positive, Loop might deliver some part of that bolus automatically - the exact percentage and timing of that delivery depends on your Dosing Strategy. At each loop cycle (new glucose reading), Loop updates the prediction and calculates a new recommended bolus. When you enter carbs without bolusing, Loop may start delivering some insulin, but if your glucose doesn't start rising as Loop expects, it revises the recommended bolus with each new glucose value.

When does Automatic Dosing Happen

Automatic dosing only happens when Closed Loop is enabled in the settings screen.

The Loop app generates a glucose prediction over the next 6 hours (the duration of insulin action), which is why the predicted glucose plot is included on the bolus screen. The Loop app considers glucose prediction with respect to your scheduled Correction Range over the full DIA, weighting closer predictions more than later predictions, when calculating Recommended Bolus.

It is actually easier to answer when Loop will not automatically increase insulin delivery.

In the situations listed below, the prediction at the end of the DIA can be significantly higher than your Correction Range but no automatic increase in insulin delivery will occur:

  • If at any time in the next 3 hours, the Loop app predicts glucose below Glucose Safety Limit, Temp Basal is immediately set to 0.0 U/hr and recommended bolus is set 0 U
  • If the prediction dips below the low-end of your Correction Range, there is no automatic increase over scheduled basal
  • If the current glucose is less than the low-end of your Correction Range, there is no automatic increase over scheduled basal
  • If the current IOB is greater than or equal to two times the Maximum Bolus setting, there is no automatic increase over scheduled basal

Even in cases where Loop does not automatically increase insulin delivery, the recommended bolus might be positive, which you see if you tap on the bolus icon manually.