Hybrid Closed-Loop Systems for Type 1 Diabetes

This past year, I did my usual weekend traveling to present at various diabetes conferences. On a trip to North Carolina to speak at a JDRF meeting, I brought my 16-year-old daughter along so that we could visit a few colleges. I thought I might impress her by renting something a step up from the usual econo-car. What we got had a slew of gadgets I’d never even heard of, and more dashboard buttons than a 747. As we were heading down the highway jamming out to some Grand Funk Railroad (to be honest, I was jamming — she was rolling her eyes and covering her ears), the car seemed to develop self-awareness. It started beeping every time I approached another car. The steering wheel would turn on its own when I veered toward the side of my lane, and the gas and brake would come to life out of nowhere.

“Dad,” she said condescendingly, “this is one of those self-driving cars!”

“Cool,” I said. “Does that mean I can close my eyes and take a nap?”

Clearly, that was not the case. Technology can assist with things like driving, but it still can’t take the place of a competent driver.

Today, we’re in the midst of a similar revolution in diabetes management: Systems that act kind of like “assisted insulin delivery.” Better known as “hybrid closed-loop systems,” or HCL systems, they take an insulin program and mix in some technological magic to make it work better. But don’t be fooled into thinking that these are artificial pancreases or cures for diabetes. Understanding what these systems can and (more importantly) CAN’T do will help you to decide whether an HCL system is right for you and how to get the most out of one.

HCL basics

A hybrid closed-loop system is made up of three parts:

1) An insulin pump

2) A continuous glucose monitor (CGM)

3) A computer program (called an “algorithm”) that takes data from the CGM and adjusts the pump’s insulin delivery automatically

The algorithm is what’s new and exciting. It can raise and lower the pump’s insulin delivery in an attempt to keep glucose levels within a desirable range. All commercially available HCL systems (Medtronic’s MiniMed 670G and Tandem’s Control-IQ with Dexcom G6) and do-it-yourself systems (Loop and OpenAPS) self-adjust the pump’s basal delivery. Some (Tandem and Loop) can also self-administer conservative bolus doses to correct high glucose levels. Each HCL system has its own set of advantages and drawbacks. For a detailed set of options and comparisons, visit the following page on the website of Integrated Diabetes Services (my private practice): bit.ly/HCLSystems

Automatic adjustments to insulin delivery are useful for several reasons. Basal requirements tend to vary throughout the day and can change from day to day based on an almost unlimited number of variables. And bolus doses, which remain the responsibility of the user, are sometimes miscalculated. When mealtime insulin doses come up a bit short or are too aggressive, the HCL system adjusts the basal delivery to compensate — sort of like the assisted driving features that protect us from potential accidents in case we lose focus momentarily.

There are two main advantages to using an HCL system. One is emotional and the other numerical. Emotionally, HCLs can alleviate a significant amount of mental burden on the user and their caregivers. Because the system makes minute adjustments on its own, there is less need for the user to constantly micromanage. There is also less worry about low blood sugar (particularly overnight) and less stress caused by dramatic blood sugar swings. All that mental energy that used to go into glucose management can now be used for more important things, like fantasy sports statistics and binge-watching your favorite shows.

The numeric part is a little less universal. HCL users are likely to experience a lower average glucose level, less hypoglycemia (low blood sugar) and more time spent in a desirable glucose range. Likely, but not definitely. The degree of improvement depends on the parameters of the system. And there are instances where people have exceptional glucose control before using an HCL system and find that their average actually goes up a bit after they start using it. But from the standpoint of glucose stability, things almost always improve when an HCL system is used.

You still can’t fall asleep at the wheel

HCL systems can’t do everything. They can’t predict the stock market. They can’t figure out the teenage brain. And they can’t offset rapid changes in blood sugar levels. HCL systems make relatively subtle insulin adjustments in a reactive manner, using insulin that takes hours to either clear or take effect. Think of your diabetes as a huge ship with a small rudder moving at a very fast speed. Even if you turn the rudder as far as it can go, it is going to take a while for the ship to change course. The pump’s basal adjustment just isn’t powerful or responsive enough to prevent high and low glucose levels when confronted with anything that causes blood sugars to rise or fall quickly, such as:

• Food (particularly carbohydrates)

• Physical activity (particularly heavy exercise)

• Stress (particularly sudden, unexpected crises)

• Sudden hormone changes (resulting from injuries, trauma, rebounds from lows)

• Active bolus insulin

In other words, timely/accurate bolusing on the part of the user is still necessary, as is adjustment for day-to-day lifestyle events such as exercise and stress. Although most HCL systems have temporary “overrides” to make the algorithm more or less aggressive than usual, their effects are modest and gradual, and the overrides usually need to be set well in advance.

There are also several limitations to any HCL system. There are restrictions as to how much and for how long most systems will adjust the insulin delivery. They rely on a steady stream of accurate CGM data in order to function. And any issues with insulin delivery, including infusion sets/site problems, can alter insulin absorption patterns and render the HCL algorithm ineffective.

Driving HCL safely and successfully

Remember, this system can’t drive itself. It is NOT your personal diabetes chauffer. When using an HCL system, your diabetes management skills will come into play each and every day. Will you have to work at it less than before using an HCL? Almost certainly. Will you achieve better glucose control? Almost definitely. But only if you think of it as a PARTNERSHIP between you and your technology.

That said, here are some strategies that can help you to gain the most benefit from your hybrid closed-loop system.

Get your pump settings right BEFORE getting started.

Having the correct basal and bolus settings will shorten the HCL “learning curve” and ultimately get you optimal results. That means doing basal testing/fine-tuning and analyzing your post-meal and post-correction data so that the various aspects of your bolus settings are properly configured, including the insulin-to-carb ratios, correction factors and duration of bolus action.

Count your carbs like a pro.

Even with the correct insulin-to-carb ratios, inaccurate carb counts will ultimately cause you (and the system) to come up short in terms of glycemic control. Use labels, look up non-labeled foods, measure your portions and work with a registered dietitian to develop your carb-counting skills (if you’d like to test your skills, check out “Counting Carbohydrates Like a Pro”)

Don’t hide anything.

The more the algorithm knows, the better it can perform. Enter EVERYTHING you eat — including the tiniest of snacks (unless you’re snacking prior to exercise to prevent a low) and treatments for lows.

Pay attention to bolus timing.

Bolusing late will cause the HCL system to ramp up your basal insulin, and may produce hypos a few hours after the bolus is finally given. And since extended bolusing is not an option with most HCL systems, it will be necessary to alter the timing of boluses for slowly digesting meals. This can be accomplished by entering the carbs in two parts or letting the system know that your meal will take many hours to fully digest (in HCL systems that accept this type of information). A sound understanding of the glycemic index can be helpful.

Plan WELL ahead for physical activity.

Remember, HCLs have their limitations. They can’t reverse a sharply declining blood sugar caused by exercise. Setting a temporarily higher target or less-aggressive algorithm can help, but it needs to be set at least an hour prior to the activity. Even then, it is often necessary to also cut back on any boluses given within two hours prior or consume rapid-acting carbs before and perhaps during the exercise session.

Change the way you treat your lows.

Because the HCL will be cutting back on basal insulin delivery before, during and (sometimes) after a low occurs, you won’t need as much carb as usual to treat your hypoglycemia.

Suspend when disconnecting.

If you use a pump with tubing that can be disconnected at the site, be sure to let the pump know when you are not connected. This way, the HCL system knows that you are not receiving any insulin and can adjust the basal delivery accordingly when you reconnect.

Manage your sites properly.

This can be the weak link of the entire system…if you let it. For an HCL system to perform as intended, it is imperative that insulin absorb in a consistent, predictable manner. Change sites often, rotate carefully and troubleshoot at the earliest signs of trouble. And choose a type of set that performs well. For many people, flexible angled or steel needle sets produce better outcomes than flexible 90-degree sets.

Optimize your CGM performance.

Just about everything the HCL algorithm does is based on CGM data. If you use a sensor that requires calibration, do so on schedule using a clean finger and a high-quality meter. Even if your CGM does not require calibration, it is worth spot-checking your sensor performance with periodic finger-sticks and calibrating if necessary. Also, it is best not to use your sensors beyond their intended lifespan, as the accuracy can suffer.

Override with care.

In the vast majority of situations, it is best to trust the HCL’s dosing recommendations. However, if you feel the need to override, do so by changing carb entries rather than falsifying your glucose values. Entering incorrect glucose readings into your bolus calculator can negatively affect the performance of the CGM or algorithm.

Use the system overrides.

All HCL systems offer the opportunity to make the algorithm temporarily more or less aggressive. Some also allow you to switch to a different basal program or insulin delivery profile. This is sort of like shifting gears when driving (when using a manual transmission). If you know your insulin needs will be higher or lower than usual, let the system know! Examples include illness, stress, high or low levels of physical activity, high-fat meals, alcohol consumption and sleep times.

Know when to shut it down.

Recognize that there may be times when you can manage your diabetes better manually than using the HCL’s algorithm, such as days with unusual levels of physical activity, strange foods, stress or any time your CGM is not operating optimally. It is perfectly fine to temporarily turn off the HCL algorithm in these situations.

Be patient.

It takes at least a few weeks to really understand how the HCL algorithm operates and to see if adjustments to your basic pump settings are needed. With some systems, the basal and correction bolus algorithms adapt based on your daily insulin use. So if you don’t see great results right away, hang in there.

A good mechanic is worth his weight in gold

I have always found that the most successful people surround themselves with the right experts and make effective use of their strengths. Such is the case with diabetes management. Building and utilizing your own team of specialists can mean the difference between great quality of life and just wearing a bunch of expensive gadgets. From establishing the right insulin delivery settings to choosing the right HCL system to optimizing its performance, it pays to work with experts who know these systems inside and out.