Boluses are short spurts of insulin given to cover the carbs in meals and snacks and to lower any high readings that may occur. Unlike basal rates, boluses are not programmed ahead of time but are given in the appropriate amount when they are needed. Once a pump BC is programmed with a carb factor and a correction factor by the user and health care provider, the user simply enters how many carbs they plan to eat and their current glucose. The bolus recommendation from the BC can always be adjusted by the wearer as needed for exercise, illness, or stress.
The more carbs in a meal or snack, the larger a carb bolus needs to be. Accurate carb counting or a very consistent diet is critical to obtain accurate bolus doses. Although carbs are the primary reason that the glucose rises after a meal, meals with large amounts of protein or fat may require larger bolus doses or a combo bolus.
A personal carb factor (CarbF) programmed into the pump makes carb coverage easier and more accurate. The CarbF determines the size of the bolus that will be recommended for the grams of carb in a meal. Calculated with the help of your health care provider, a CarbF is how many grams of carb 1 unit of insulin will cover. Like basal rates, the CarbF can be adjusted as needed to improve control, and different CarbFs can be entered for different times of the day. The pump divides the total carbs in a meal or snack by the CarbF to determine the carb bolus it recommends for the food. Some pumps offer a built-in carb database to simplify carb counting.
Carb Bolus Types
With a standard carb bolus, all of the bolus is given over a short period of time. A pump is also flexible and can also delay some or all of its bolus delivery to accomodate low glycemic index or high fat meals. Combo or extended boluses can be used with medications like Symlin (pramlintide) or GLP-1 inhibitors like Byetta (exenatide) that delay food. They are also helpful when digestion has been slowed by gastroparesis.
A combination or dual-wave bolus gives some of the bolus now and the rest over time, while an extended or square-wave bolus is like a temp basal increase where all of the bolus is given over a period of time. For example, a combo bolus may best match the digestion of specific foods with part of the bolus given immediately and the rest over the next 90 or 120 minutes.
Carb Bolus Timing
Even on an insulin pump, insulin action is still relatively slow. Carb boluses are not delivered directly into the blood like the insulin delivery from a pancreas. Instead, the pump infuses insulin into fat below the skin from where it is gradually absorbed into the bloodstream.
This delay in uptake means that carb boluses have to be given well before a meal begins to have their best effect. Boluses are ideally given 15 to 20 minutes before eating for better postmeal glucose levels unless the glucose is low. When carbs are accurately counted and matched with a premeal bolus using an accurate carb factor, the post-meal glucose will ideally rise no more than 40 to 80 mg/dL (2.2 to 4.4 mmol) and return to the target glucose within 4 to 5 hours.
If a meal bolus is given right before eating for convenience, the glucose often spikes after the meal and raises A1c levels. This spiking can easily be seen on a CGM. Faster insulins are under development that may eventually better match the digestion times typically seen with carbs. This would allow boluses to be given closer to meal time with less risk of delayed hypoglycemia.
Someone with a normal pancreas and counter-regulatory system never sees a high or low glucose reading. With diabetes, a high glucose will occur whenever there is a deficit in a basal rate or a carb bolus dose. Other things like menses, stress, pain, or infection can also cause high readings. Even those with excellent glucose levels average about two correction boluses a day.12
The higher the glucose, the larger the correction bolus that will be needed to bring it down. A correction factor (CorrF), called an insulin sensitivity factor or ISF in some pumps) is how many points the glucose will fall per unit of insulin. It is used to determine how much bolus insulin to take to bring a high reading down to target without going low. Once the correction factor is programmed into a pump BC, correction boluses will be automatically calculated, with reductions made as needed for any active BOB.
The infusion set and pump should be thoroughly checked for leaks anytime there is an unusual or unexplained high glucose. If an infusion site failure or mechanical problem causes a high glucose, a correction dose should be given by syringe or insulin pen to ensure delivery. Always take action to lower your glucose before getting involved in troubleshooting the pump.