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Split (Combo,Dual) Bolus

Pizza Night? Key techniques to reduce blood sugar spikes (and your stress!) using your insulin pump

If you have not done so already, we recommend that you read the following WaltzingTheDragon.ca pages on insulin pumps as background for the information that follows:
A Basal-Bolus Approach
Getting More from Your Insulin Pump: 7 Basic Pump Features
Getting the Most from Your Insulin Pump: 5 Advanced Pump Features

Getting the Most from Your Insulin Pump: 5 Advanced Features:

Advanced Pump Feature #3: Split Bolus
a.k.a. Combo Bolus, Dual-Wave Bolus, Extended Bolus

We'll get to some practical techniques for taming those blood sugar spikes soon, but first the theory behind this advanced insulin pump feature...

Insulin pumps allow you to alter how much of a bolus is delivered and when, resulting in different bolus shapes:

1. Normal Bolus delivers the entire dose immediately. For example, if 6 units (U) of insulin is the desired dose, all 6U will be delivered within moments of pressing the "Okay" button. This is most similar to giving insulin by injection; the insulin acts as quickly as possible for its type, because the entire dose is being delivered at once, and so will return high blood sugar to target more quickly than the other bolus shapes (2 and 3 below). 

When should I use a Normal Bolus?

For most people, a Normal Bolus will be used most often. It may be helpful for carb-containing snacks/meals with low amounts of protein and fat. As a correction for high blood glucose, it's the only logical choice, since you want your blood sugar to return to target as quickly as possible.

2. Extended Bolus (for Accu-Chek, Omnipod and Animas pumpers), or Square Wave Bolus (for Medtronic pumpers), spreads the dose out over a specified period of time with 0.0 U given up front; there is no initial "dump" of insulin which may lead to a low if the insulin gets in before the food is digested. For example, if 6U is the desired dose, you could choose to extend the bolus delivery over 3 hours, meaning that equal fractions of the total dose will be delivered every few minutes so that the 6U of insulin will be delivered over the 3-hour period.

When should I use an Extended/Square Wave Bolus?

It may be helpful for snacks/meals that contain low amounts of carbohydrate, with moderate to high amounts of protein and/or fat, such as steak and a salad alone. It can also be helpful when your child comes into mealtime with a blood glucose reading that is on the lower side, and you want to ease into the insulin action to allow blood sugar to rise before insulin affects it. It also may be appropriate for individuals who have a medical condition that causes slow or delayed digestion (such as gastroparesis).
tips from the trenches of type 1 diabetes
If Paul has a blood glucose that is on the lower side (and he doesn’t want to eat extra, high-GI carbs to bring his blood glucose up), if we give an extended bolus over 0.5hrs for a moderate GI meal, most times his blood sugar will balance out nicely without going low first.

3. Split Bolus (a.k.a. Combo Bolus, Dual Wave Bolus, or Multi-wave Bolus, for Animas, Medtronic and Accu-Chek users, respectively; Omnipod uses the same "Extended Bolus" term as in #2 above) is a hybrid of the first two insulin delivery modes: some specified portion of the total insulin bolus is delivered upfront, as a normal/straight bolus, while the rest is delivered over a specified period of time as an extended/square wave bolus. For example, given a 6U dose delivered as a 60/40 combo/square wave bolus over 3 hours: 60% of the total dose (or 3.6U) will be delivered within seconds of pressing the "Okay" button; the remaining 40% (or 2.4U) will be delivered (in equal fractions) every few minutes over the next 3-hour time period. The result is an initial dose to deal with faster-digesting foods, plus an extended tail of insulin action to deal with the slower-digesting foods.
When should I use a Split Bolus?

For snacks/meals that are predominantly low on the glycemic index (such as plain pasta, spaghetti with tomato sauce, barley, or pulses like chick peas or black beans) OR for foods that contain a mix of a moderate to high amount of carbs plus moderate to high amounts of protein and/or fat (such as pizza, pasta with a heavy cream or cheese sauce including home-made macaroni and cheese, a burger with fries, cheesy / meaty / deep-fried Mexican food, popcorn with butter). A split bolus may also be helpful for "grazing", when the person with diabetes may be eating small amounts of food over a few hours (for example, movie night at a friend's house; a cocktail party; a kids' birthday party with a free-access snack table).

For other examples of foods that may be managed well using a Split Bolus, and for more information on how different foods break down more slowly or quickly than others, check out WaltzingTheDragon.ca's page Glycemic Index Explained.  

The benefit of having options in terms of the shape of the bolus is that the rate of insulin delivery can be better matched to how quickly or slowly certain foods are digested: quick delivery can be used to deal with quickly-digested foods, mediating the blood glucose spike that may strike after eating starch and carbs; a prolonged delivery can be used for slower digesting foods, avoiding the lows that often set in when insulin acts more quickly than the slowly digested foods do. As a result, we can even out the blood sugar peaks and valleys that may follow certain meals, leading to more successful blood glucose management... better feeling, lower A1C's, better health!

How Do I Know If I Should Split the Bolus?

Keeping in mind the above examples (and those in the Glycemic Index Explained and Adjusting for GI Level pages), check your child's blood sugar before eating and 1 hour after eating a meal like pasta / spaghetti, pizza, burgers, fries, steak and baked potato, rich cheese cake, full-fat ice cream, chick peas, buttery popcorn...). If your child's blood glucose has dropped as of the 1-hour point, this may be an indication that the meal she has just eaten is appropriate for a split bolus. Now have her check her blood glucose a few hours later... if the reading has sky-rocketed, this may confirm the need for a split bolus when those foods are eaten.

A real-life example:

Before my son had an insulin pump, and before I learned anything about the glycemic index, these were his blood sugar readings (in mmol/L) one evening after fast food chicken nuggets and fries (a high carb, high fat meal):
5:00pm (before eating): 8.2
6:30pm (1-hour after he finished eating): 5.1
9:00pm: 13.2
1:00am: 18.7 (despite the usual correction 4 hours earlier)
7:00am: 15.6 (despite previous corrections at 9pm and 1am)
This is what happens with these types of meals: there is too much insulin early into the slowly-digesting meal, so blood glucose drops. Then there is not enough insulin available later, when the majority of the food digests and enters the blood stream as glucose - that is, the insulin "tail" is too short, leaving much of the glucose in the bloodstream long after the meal, so an extra bolus of insulin is needed to get this excess glucose out of the blood and into the cells. (Plus the fat leads to insulin resistance, a pesky problem for adults and children with diabetes...but that's a whole other story.) However, if the insulin is delivered as a Split Bolus, then less insulin is given up front, and the insulin tail is extended, better matching the slower rate of digestion of the food.

Notes About the Split Bolus:

  • This typical blood glucose pattern (a drop followed by highs) indicates use of a split bolus if, and only if, the basal rates have previously been confirmed. Otherwise, it's possible that the early drop in blood glucose was due to excess basal insulin, while the steady climb post-meal could coincide with insufficient basal insulin.
  • An accurate carb count and correct insulin:carb (I:C) ratio should also be confirmed. If too much insulin was given for the meal (either because the carbs were over-estimated, or the I:C ratio is too strong) then it's possible that the early low blood sugar was simply due to excess insulin; the persistent highs that followed could then be due to the Symogi effect, or a "low rebound".
  • To help sort all this out, gather the clues together to look at the whole picture: Are the basal rates, I:C ratios, carb content confirmed? Was the meal low glycemic index, high protein*, and/or high fat? Was there an initial drop in blood glucose followed by later high blood glucose? If so, you could try a split bolus, note the results, and make adjustments from there.
  • A Continuous Glucose Monitor (CGM) is a particularly useful way of visualizing the effect of problem foods on blood glucose - and can be a very efficient way of perceiving the need for a split bolus.
  • If you plan to use a split bolus at a given meal, and your child's blood glucose is above-target before that meal, remember to separate the blood glucose correction from the food bolus, delivering all of the correction dose up front (and well in advance of the meal, whenever possible). You then program a combo/dual wave/multi-wave only for the food bolus. If you don't split the two, a portion of the correction dose will also be extended over a period of time, delaying the return back to target.

What if we tried a Split Bolus but the split needs to be tweaked?

See Waltzing the Dragon's article, Tweaking a Split Bolus.

What if we already have a Split Bolus going, and then we have to add another Split Bolus for more carbs to be eaten?

See Waltzing the Dragon's article, Splitting a Split Bolus.

What if we accidentally give a Straight Bolus when we should have used a Split Bolus?

Essentially this will result in too much insulin being delivered up front for the slower rate at which the food will be digested. If your child with diabetes doesn't want any more of the combo-worthy food (in which case the initial bolus could have served as the Normal portion of a delayed Split Bolus), you have a few options:
  • You could check blood glucose at one hour post-meal, and give fast-acting carbs to prevent a low as needed (with insulin, as this is a timing issue, not a problem with the amount of insulin itself). This can be risky, however, if you forget to check. And determining an effective amount of extra carbs can be tricky (as it varies with the amount of carbs that should have been given a split bolus).
  • Your best bet may be to bump the GI level of the whole meal toward the moderate end by adding a quickly-digested food at meal-time. When this situation arises in our home, we add a dessert of watermelon, left-over Hallowe'en (Christmas, Valentine's, Easter...) candy*, or some other high glycemic food (with insulin as usual).
*other than plain chocolate, which is actually fairly low GI
tips from the trenches of type 1 diabetes
When this happens in our family, instead of giving a normal bolus for the high GI food we are using to balance the mistake, we use a 100% extend over the amount of hours we would have done the combo for. This usually works out nicely. (Except that when we’ve had to use this, Paul is usually full beyond belief and now has to cram more food into him!)


Looking for more information on Split Bolus? Check out: Tweaking a Split Bolus and Splitting a Split Bolus
Looking for more information on Advanced Pump Features? Go back to Getting the Most from Your Insulin Pump: 5 Advanced Pump Features, or go directly to:
Advanced Pump Feature #1: Temporary Basal Rates
Advanced Pump Feature #2: Alternate Basal Programs
Advanced Pump Feature #4: Suspend / Resume
Advanced Pump Feature #5:
Delivery Speed

The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.

This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care profession.

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