Pumping Like a Pancreas

Using the advanced bolusing features.

Theresa Garnero By Theresa Garnero, APRN, BC-ADM, MSN, CDE

Q: I'm told the insulin pump has advanced bolusing features that can imitate my pancreas. How so?

A. Insulin pumps have many features that allow you flexibility to match your insulin requirements with your lifestyle. Let's review some basic concepts then get to advanced bolusing specifics.

The cornerstone of any advanced insulin therapy program is anticipation—projecting the body's insulin needs for the next several hours. Once your diabetes care team experienced in pump management has determined your pump settings (based on weight, total daily doses of insulin used and past observations of glucose patterns, concepts of which will be explained in greater detail below), you can lean on the advanced bolus features to add a layer of sophistication to anticipating your future insulin needs.

Insulin pump therapy maximizes flexibility and improves your chances of accurately predicting insulin requirements using a basal/bolus approach, the foundation to any intensive insulin therapy program. That's providing you count carbohydrates fairly well and your basal rates are set correctly. Let's review this basic concept:

Basal: small amounts of insulin released continuously through the day and night.
Bolus: on-demand insulin for mealtime coverage or correction of high glucose.

One way to compare this concept is to think of insulin like your bank account—you always need a little in reserve. Basal insulin is like your paycheck that comes in regularly to provide for expected expenses, whereas bolus insulin is like a debit or credit card to cover quick, small purchases. Basal insulin controls glucose when not eating and keeps the liver from putting extra glucose into the system. Bolus insulin corrects high glucose readings prior to a meal and covers the glycemic surge expected from consuming the meal in front of you.

How are bolus rates calculated?
Many methods exist. To get to the bolus amounts, the basal rate must first be calculated. (People may need several basal rates based on work patterns, dawn phenomenon, weekend activities, menstruation, exercise, etc. something a healthcare provider skilled in pump therapy will help determine.)

As a guideline (source:Joslin's Insulin Deskbook, 2008), the basal rate is about 50% of a person's total daily dose (TDD) of insulin units distributed throughout 24 hours.

For bolusing, two approaches are used:
1. Use set doses (rarely used). Of the remaining 50% — of an average of 4 to 5 days worth — of TDD, take 40% for breakfast (since insulin resistance is higher in the morning requiring more insulin), and 30% for lunch and dinner. This approach works best when someone is consistent with meal content and timing (carb counting is not a requirement).

For example, if a person needs 40 units per day total, 20 units would be used for basal rate and 20 units for boluses. To get the units per hour needed for the basal rate, divide the 20 units by 24 hours in a day to equal 0.8 units/hour. To get the units per meal needed of the remaining 20 units, divide 20 units by 40% to equal 8 units for breakfast, and divide 20 units by 30% to equal 6 units for lunch and dinner.

2. Use flexible doses (commonly used). Correct the high and cover the meal at the same time. This combines a correction dose for any high premeal glucose along with the projected insulin needed for pending carbohydrate consumption. This requires knowing your correction factor (how many points your glucose will drop when you take 1 unit of insulin) and of course, advanced skills at carbohydrate counting (how many carbs 1 unit of insulin will cover). The clear advantage to this method is its attention and resolution of historical glucose values leading to the current value with projected insulin needs to cover the meal.

To estimate insulin-to-carbohydrate ratios, use the "450-rule".
Take 450, divide by average TDD. In this example, 450 divided by 40 units equals 11.25, or 1 unit will cover about 11 grams of carbohydrate. To test if this was the right ratio, check glucose 2 hours later. If the rise was less than 40 to 80 points above the premeal reading, it is the correct amount. If the glucose rose more than 80 points, you'd need a lower carb-to-insulin ratio (like 1 unit covers 10 grams of carb). If glucose did not rise after the meal, you may need a higher carb-to-insulin ratio to avoid hypoglycemia (like 1 unit covers 15 grams of carb).


To estimate correction or sensitivity factor, use the "1500-rule"*
Take 1500, divide by average TDD. In this example, 1500 divided by 40 units equals 37.5, or 1 unit will lower glucose about 40 points (round the number to keep you sane). To test if this was the right ratio, check glucose 2 to 3 hours after a correction bolus (when you are not planning on eating for a few hours). If the glucose is within 30 points of your target, the correction factor is appropriate.

*The 1500-rule is for Regular insulin; the 1800-rule is for rapid acting insulin.

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Last Modified Date: February 16, 2013

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