Smart Pumps and Sports: Using Insulin On Board
The Sports Corner Column will feature different aspects of insulin pump therapy and sports participation. Through short concise articles, one particular aspect of pump therapy and its relationship to sports activity will be discussed. Smart pumps have a feature that every single pump wearer should use. This feature is called "insulin on board" and needs to be activated through the setup menu of your pump. Insulin on board calculates the decay of insulin in the body after a bolus (not basal) of insulin is given. This article will discuss what insulin on board is and how to use it around sports participation.
Insulin On Board (IOB)
Insulin on board is probably the most valuable feature in the latest smart pumps. When insulin is administered via a bolus from the pump, it has a timeframe where it can effectively lower blood sugars. For example, if a 10 unit bolus is given the first hour, 30% is used up in the body (minus 3 units leaving 7 units still working in the body), the second hour another 30 % (minus another 2.1 units leaving 4.9 units still working in the body) and the third hour another 30% (minus 1.4 units leaving 3.4 units still working in the body) and finally the fourth hour the last 10% (minus .3 units leaving 2.97 units still working in the body).
The only problem with this analogy is that not everyone fits this profile. Like basal rates, not everyone have the same rates in a 24-hour period. The latest smart pumps have adjustable time frames to accommodate these differences. Some individuals may set their insulin on board feature at 3.5 hours or 5 hours rather than 4 hours as mentioned in the example above. One way to see what works best for each individual is to check blood sugars each hour after giving a bolus to see how blood sugars react.
Stacking of Insulin
Before the insulin on board feature was available, many individuals would have a high blood sugar (i.e., 250 mg/dl) 1-2 hours after eating (postprandial) and consistently give a correction bolus to bring their blood sugar down to a normal range. The problem with this technique is not accounting for the insulin on board in the body and often going low by overcorrecting. Now, after inputting a blood sugar of 250 into the pump, the pump will account for the insulin on board and possibly suggest a different correction bolus, which will help avoid a low blood sugar. For example, a person who has a target of 100 mg/dl and an insulin sensitivity factor of 50 (1 unit of insulin will drop the blood sugar 50 points) in the example above would normally give a correction bolus of 3 units to get to 100 mg/dl. Now, using the insulin on board feature on the pump, the pump may only suggest 1 unit based on how much insulin is still working in the body rather than 3 units.
Insulin on board calculation can also work when a blood sugar is only 95 mg/dl an hour or two after eating. It is common for blood sugars to rise 60-80 points higher than the pre- meal blood glucose reading even when a correct dose is given. If a blood sugar is taken 90 minutes after eating showing a 95mg/dl that may be a red flag knowing it is common for levels to go up somewhat after eating. Looking at the insulin on board feature on the pump may show 4 units still working in the body, which may indicate the need for carbohydrates to head off a low blood sugar.
Using IOB around Sports
Most athletes understand that aerobic exercise can lower blood sugar so it is prudent to raise blood sugar higher than normal (e.g., 150mg/dl) prior to starting the competition. Some athletes may experience high blood sugar (e.g., 300 mg/dl) prior to a game from a miscalculated bolus earlier in the day or often due to frayed nerves about the competition. This anxiety can cause the body to release stress hormones (i.e., epinephrine, norepinephrine, cortisol) which causes the liver to release glucose (the stored form of glucose - glycogen), raising blood sugar to an unwanted 300 mg/dl. After inputting the 300 mg/dl into the pump set the target for exercise (150 mg/dl) instead of a normal range of say 100 mg/dl. The exercise target blood sugar can be calculated in the pump by having the correction factor to the individual already determined through the setup menu.
Following the example above of a blood glucose reading of 150 mg/dl two hours after eating a pre event meal and just prior to competition the plan may seem to be on target but in reality it may not after checking the insulin on board feature. With increased activity, insulin may get ahead of the food eaten causing blood sugar to plummet. The exercise can increase blood flow and ultimately increase the sensitivity to insulin. Therefore, if there are 4 units of insulin on board it may be a sign to eat some carbs to avoid a low blood sugar. How much depends on many factors but it is recommended that individuals eat 15-30 grams of carbs for every 30-60 minutes of exercise.
The examples above are easy numbers to calculate but most individuals do not have easy numbers to calculate (e.g., 1 unit drops the blood sugar 38 points) so using insulin on board helps fine-tune adjustments rather than guessing and hoping for the best.
When starting to use the insulin on board feature it is vital to use the doctor and/or diabetes team to set the parameters on the pump. The smart pumps today help with calculations but they should not take the place of someone's brain when making decisions on insulin adjustments. There are so many issues to be considered it is important not to rely strictly on the calculations of the pump. Intensity, duration, and type of activity can have overriding factors that may offset the suggested action of insulin on board.
Rick Philbin, MBA, M.Ed., ATC
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