A closed loop insulin delivery system consists of an insulin pump, a sensor for continuous glucose monitoring, and an algorithm that automates insulin delivery in response to glucose levels. Such a system “closes the loop” because its functions are continuously regulated by feedback (algorithm uses sensor glucose readings to calculate how much insulin the pump should deliver to bring future glucose readings into a target range, then repeats the cycle over and over based on subsequent readings).
Diabetes technology has advanced significantly in the past few years, but for now a true “artificial pancreas” that would not require input from its human operator remains elusive. Even Loop and its more advanced commercial competitors are best described as hybrid closed-loop systems since they still require the user to input meal boluses and adjust for physical activity.
What is Loop?
Loop is a specific experimental, open source, non-FDA-approved, iOS (Apple) app that relies on glucose predictions to inform temporary adjustments to the basal rate, with the purpose of bringing glucose levels into a specified target range. It can be operated in “open loop” by making recommendations that its human user has to approve before enacting them, or in “closed loop” by making its changes to basal rates automatically.
Loop does not “learn” about your insulin needs over long periods of time (it only applies short-term corrections based on blood glucose changes in the past 60 min) - it assumes you’ve provided accurate settings and uses them to forecast the impact of various known factors (i.e. meals, basal insulin, insulin on board, and recent glucose trends) on blood glucose in order to adjust how it delivers your insulin. Thus, settings are critical - any combination of settings can balance each other out, but because Loop uses predictions based on the settings entered the consequences are more serious. Settings should therefore be validated through rigorous real-world testing and reflect how things work in your body as closely as possible, to give Loop the best chance at success.
It is equally important to understand that Loop works best when you provide it all the necessary and appropriate inputs (e.g. settings, meals, exercise) and then trust the system to do its job. Because its algorithm is based off of modeling the effect of carbohydrates & insulin and predicting the trajectory of your blood glucose, manual bolusing, suggestion overrides, and general micromanagement cannot be accounted for in its equations and only interfere with the course it has mapped out ahead. In other words, tampering with the system actually makes it work worse, especially if you don’t know how the system works. In fact, once you have overcome the steep learning curve involved, the benefits of Loop are arguably as much about reducing the cognitive burden of diabetes as they are about improving blood glucose control.
The importance of evaluating your settings
“Keep an open mind if you want to keep a closed loop” says Katie DiSimone. If you are not using the settings in your bolus calculator because “they don’t ever work”, then now is the time to re-evaluate. It is absolutely possible to have two wrong settings balance each other out and look like the right settings under some circumstances. The challenge with inaccurate settings is that they won’t balance out in all situations... so all pump settings must be as accurate as possible prior to “closing the Loop.” Loop will not be successful without accurate settings.
In order to get a deeper understanding for Loop, you should study the documentation published by the Loop community: Loop Docs. You should be very familiar with Loop Docs before you “close the Loop.” This is your Bible and will guide all your questions. While the Loop FB page is a fantastic resource, the community will direct you to Loop Docs most for most of your questions.