Think Like a Pancreas
Page 16
5 320–360 18–20
7 220–260 12–14
10 155–185 8.6–10.3
12 125–155 6.9–8.6
15 95–125 5.3–6.9
18 80–110 4.4–6.1
20 70–1 00 3.9–5.5
25 60–80 3.3–4.4
30 50–70 2.8–3.9
35 40–60 2.2–3.3
40 35–50 2.0–2.8
45 30–45 1.7–2.5
50 30–40 1.7–2.2
60 25–35 1.4–2.0
70 20–30 1.1–1.7
80 20–25 1.1–1.4
100 15–20 0.8–1.1
1 20 1 3–1 7 0.7–1.0
1 40 1 1 –1 5 0.6–0.8
1 60 10–12 0.5–0.7
180 9–11 0.5–0.6
200 8–1 0 0.4–0.6
If Stan’s blood sugar is 210, we get (210 – 110) / 65, or 1.5 units. He needs 1.5 extra units to correct his blood sugar down to 110. If he needs 3 units for his meal and has a premeal blood sugar of 210, he would increase the dose to 4.5.
If Stan’s blood sugar is 76, we get (76 – 110) / 65, or –0.5 units. He needs to take away .5 unit from his meal dose. If he needs 3 units to cover the carbs in his meal and has a blood sugar of 76, he should decrease the dose to 2.5.
Let’s look at another example. Nalani takes a total of 75 units of insulin daily. Each unit should lower her blood sugar 23 mg/dl (1700 / 75 = 23), so we’ll round up to 25.
If Nalani’s target blood sugar is 120, she will add 1 full unit for every 25 points over 120 and subtract 1 unit for every 25 points below 120. Her correction formula is:
(current blood sugar – 120) / 25
If Nalani’s blood sugar is 171, she will need (171 – 120) / 25, or 2 units, in addition to the insulin she gives to cover her food. If her blood sugar is 310, she will need (310 – 120) / 25, or approximately 8 extra units.
Sensitivity Factors May Vary
You may have noticed the word daytime used above when describing the sensitivity factor derived from the 1700 rule. That’s because sensitivity to correction boluses can vary based on the time of day, just like I:C ratios often vary from meal to meal. In particular, don’t be surprised if each unit of insulin lowers your blood sugar more at night and the early morning than it does during the rest of the day. In the evening many people experience a drop-off in hormones that counteract insulin. As a result, each unit of insulin can lower the blood sugar more at bedtime and in the middle of the night. From my experience, sensitivity factors tend to run 30 to 50 percent higher at night than during the day.
So if you’re just getting started using correction boluses and have determined that your daytime sensitivity factor is 40 mg/dl (2.2 mmol/l), you might consider increasing it to 60 (3.3) at night—at least until you have a chance to try it out a few times to see how it works.
Sensitivity factors may also change over time. With weight gain, most people lose some sensitivity to insulin, so the sensitivity factor tends to decrease. Changes in physical activity levels can also affect insulin sensitivity. Prolonged periods of inactivity due to illness, injury, travel, or sedentary occupations may lower insulin sensitivity and require a reduction in the sensitivity factor. Likewise, long-term increases in physical activity can produce the opposite effect.
Verifying Your Sensitivity Factor
You can verify the accuracy of your sensitivity factor by doing the following:
1. Test your blood sugar at least four hours after your most recent bolus of rapid-acting insulin.
2. If the blood sugar is elevated, calculate and give the appropriate dose of insulin. Go about your usual activities, but do not eat or exercise for the next several hours.
3. Test your blood sugar four hours later.
4. Calculate how much your blood sugar came down and then divide by the number of units you gave. This should come close to your sensitivity factor. If it does not, repeat the process the next day. If the results are similar to those from the first day, adjust your sensitivity factor accordingly.
For instance, yesterday I checked my blood sugar four hours after lunch, and it was 205 mg/dl (11 mmol/l) (darned undercounted hoagie!). Applying my daytime formula of (BS – 100) / 40, I gave 2.6 units using my insulin pump. Several hours later, just before dinner, my blood sugar was down to 112. I dropped 93 points (205 – 112). Dividing by 2.6 units, I come up with a sensitivity factor of 36 points per unit—not exactly 40, but close enough!
CGM can also be used to verify whether your sensitivity factors are set correctly. In Figure 7-4, a nighttime correction bolus for high blood sugar brought the level down below target. This indicates that the sensitivity factor may be set too low, which means that each unit drops the blood sugar more than anticipated.
Figure 7-4. CGM demonstrating the results of a nighttime correction bolus
(Food Dose + Correction Dose – Insulin On-Board) × Activity Adjustment
Part 3. Bolus Insulin On-Board
Patience is a virtue. Unfortunately, we all know people who refuse to abide by that philosophy. They want everything right away—no time to putz around with something that isn’t working: Fix it, change it, replace it now.
Sometimes, things have a way of working themselves out—if given a chance. Take, for instance, insulin. The fastest insulin on the pharmacy shelves still takes about four hours to complete its job. Four hours. Not ten minutes. Not one hour. Not even two hours. Blood sugar measurements taken within a few hours of a bolus will usually still be elevated because the bolus has not yet finished working. So what is one to do? Blast away at the elevated blood sugar with a fully loaded correction bolus? Or sit there helplessly, hoping and waiting for it to come down?
For those of us who want to manage our blood sugar aggressively but prefer not to be thrust into hypoglycemic seizures, the best option lies somewhere in the middle. And this is all about accounting for the insulin that is still working from previous boluses.
This still-working insulin actually goes by many different names, depending on who you’re talking to: IOB (Insulin On-Board), BOB (Bolus On-Board), Active Insulin, Insulin Remaining, and so forth. For those of you unfamiliar with the concept of IOB (let’s just call it IOB because that saves me a few keystrokes), it refers to the amount of bolus insulin that was previously delivered but is still active (working) in your body. This is important to know because it prevents “stacking” insulin when bolusing for high blood sugars within a few hours of a previous bolus.
For instance, I—like many of you, I’m sure—don’t like the feeling of being in the 200s. I used to check my blood sugar a few hours after eating, and if it was elevated, I would apply my usual correction bolus formula. And sure as sugar, I’d wind up low a few hours later.
Nowadays, insulin pumps calculate IOB for you and deduct it, at least in part, from subsequent boluses (see Table 7-5). You might say that IOB is what puts the “smart” in today’s smart pumps. By taking IOB into account, pumps make it safe to correct high readings at almost any time. However, different pumps have different ways of calculating IOB as well as applying it to bolus calculations. Should you trust the pump’s IOB estimate? Will using it improve or hinder your control? Understanding how your pump handles IOB will help you answer these questions.
Table 7-5. How different pumps handle IOB
Pump type How it calculates IOB What it does with IOB
Accu-Chek/Roche Based only on correction boluses given, using a true curvilinear action profile Calculates correction bolus required to reach predicted/desirable glucose level, based on time since bolus was given
Animas Based on all boluses given, using a true curvilinear action profile Deducts from correction bolus only if BG is above target
Deltec Based on all boluses given, using linear action profile Deducts full IOB from total correction plus meal dose if BG is below target
Insulet/OmniPod Based only on correction boluses given, using a true curvilinear action profile Deducts full IOB from total correction plus meal dose
Medtronic Based on all boluses given, using a true curvilinear action profile Deducts from correction bolus only
For those who don’t own a smart pump or don’t bother to use the pump’s bolus calculator features, you will need to make the IOB adjustment on your own.
IOB should be based on the typical absorption and action patterns that you see when you take rapid-acting insulin. Keep in mind that insulin absorption and action can vary from person to person and from situation to situation. In some cases rapid insulin can be finished in just over two hours, and in other instances it can take as long as five or six. However, in most cases the actions of rapid-acting insulin follow the pattern shown in Figure 7-5 and Table 7-6.
Figure 7-5. Typical action profile of rapid-acting insulin
Table 7-6. IOB based on time since bolus was given
For example, if you gave yourself 6 units of insulin for a 3 p.m. snack and then check your blood sugar at 5 p.m., you still have 35 percent of your bolus remaining (6 units × .35 ≅ 2 units). If you gave 12 units at dinner and check your blood sugar three hours later, you still have approximately 1.2 units (12 × .10) remaining.
For the sake of simplicity, some people choose to assume that their insulin lasts a certain number of hours and figure a linear run-off of the insulin. Those who like to be aggressive with their dosing can assume a three-hour action time and figure that one-third of their insulin is used up each hour, as shown in Table 7-7.
Table 7-7. Simplified IOB based on three-hour action time
A more traditional approach assumes that the bolus takes four hours to finish working, and 25 percent is used up each hour. (See Table 7-8.)
Table 7-8. Simplified IOB based on four-hour action time
For those who want to be more conservative with their dosing (in an effort to prevent hypoglycemia), assume that the insulin takes five hours to finish, and 20 percent is used up each hour.
Table 7-9. Simplified IOB based on five-hour action time
Figuring IOB More Precisely
Whether you use a pump to calculate your IOB or figure it out yourself, the calculation hinges on how long you think it takes for your insulin to finish working. You know what they say about people who make ASSumptions—so sometimes it’s better to get the facts straight. Why? Because if you underestimate how long your insulin lasts, you will also underestimate how much IOB you have. And when you deduct too little IOB from your boluses, you open yourself up to more bouts of hypoglycemia. Conversely, if you overestimate how long your insulin lasts, you will also overestimate how much IOB you have. This can lead to excessive bolus deductions and more frequent hyperglycemia.
Confused yet? Don’t worry. If you can determine your insulin action time properly, it won’t matter.
There are a few ways to figure out how long it really takes for your bolus insulin to finish working. One way is to label your insulin with radioactive dye and see how long it takes for your body to stop glowing. Okay, maybe that’s not all that practical. Another way is to check your blood sugar every thirty minutes after giving a correction bolus and then see how long it takes for the blood sugar to stop dropping. Once the correction bolus is given, you should not eat, exercise, or give any more boluses until you reach the point at which the blood sugar flattens out. Here is an example:
Time Blood sugar
8 a.m. 238 mg/dl (13.2 mmol/l)
Correction bolus given
8:30 235 (13.1)
9:00 222 (12.3)
9:30 174 (9.8)
10:00 141 (7.8)
10:30 125 (6.9)
11:00 118 (6.6)
11:30 111 (6.2)
12:00 p.m. 112 (6.2)
It appears that the blood sugar flattened out at 11:30, which was three and a half hours after the bolus was given. So the duration of insulin action (also called “active insulin time”) is three and a half hours.
A better way to measure how long it takes for your boluses to finish working (short of exposing yourself and your insulin to plutonium) is to watch the trend graph on a continuous glucose monitor after giving a correction bolus. Here are some examples:
Figure 7-6. Using CGM to determine insulin action time
In the example on the left, a correction bolus was given, and the blood sugar stopped falling after three hours. In the middle example, the correction bolus took four hours to finish. In the example on the right, a meal bolus was given, and the blood sugar stopped dropping at five hours.
What to Do with IOB
Regardless of the approach you use to calculate your IOB, taking IOB into account before giving any more boluses is very important. Whether you deduct IOB from correction boluses only or meal boluses as well is entirely up to you. Those who choose to deduct IOB only from correction boluses (i.e., high blood sugar doses) are assuming that any previous meal boluses are still covering undigested food. Although this may be true in some cases, the vast majority of carbohydrates digest pretty rapidly. That’s why I prefer to deduct IOB from any boluses, whether they are being given to cover food, high blood sugar, or both.
However, if you recently had a meal or snack that you expect to digest slowly, only deducting IOB from correction boluses is reasonable because the original bolus is probably still offsetting undigested food. This includes times when you just had a very large quantity of food, a prolonged/drawn-out meal, or a meal consisting mainly of lowglycemic-index foods (these will be discussed later in this chapter under the subject of Bolus Timing).
Otherwise, simply subtract IOB from any bolus you plan to give. This helps to prevent stacking boluses and hypoglycemia. If you would normally take 4 units to cover a meal and/or high reading, but 1 unit is still active from an earlier bolus, it would be wise to give only 3 units. If you would normally take 2 units for an elevated blood sugar but 2.5 units are still active from a previous bolus, you should not take anything. In fact, if the IOB exceeds your correction bolus by a relatively large amount, you might need to snack in order to prevent a low blood sugar from occurring in the next couple hours.
(Food Dose + Correction Dose – Insulin On-Board)
x Activity Adjustment
Part 4. Adjustment for Physical Activity
Okay, let’s see what we have so far. Bolus insulin is calculated based on the amount of carbohydrate, the blood sugar level, and the amount of insulin still working from previous boluses. So that’s it, right? Well, not exactly.
You see, a unit of insulin is not always a unit of insulin. Let me put that another way: A unit of insulin given in one situation may work more or less effectively than a unit given in another situation. Something called insulin sensitivity determines insulin’s effectiveness. The more sensitive we are to insulin, the more each unit will lower the blood sugar, and the more carbohydrate it will cover. A unit that normally lowers the blood sugar by 50 mg/dl (2.9 mmol/l) might lower it by 75 (4.4). A unit that usually covers 10 grams of carb might cover 15 or 20.
A number of factors can affect our sensitivity to insulin, but the most significant factor on a daily basis is physical activity. Not just exercise, but any form of physical activity, including cleaning, shopping, yard work, playing, sex, and anything else that has us using our muscles and breathing heavily.
Muscles are one of the main targets for insulin. With increased work, muscle cells become much more sensitive to insulin. This enhanced insulin sensitivity may continue for many hours, depending on the extent of the activity. The more intense and prolonged the activity, the longer and greater the enhancement in insulin sensitivity.
With enhanced insulin sensitivity, insulin exerts a greater force than usual. Thus, you will need to adjust boluses for upcoming and, in some cases, previous physical activity. Some forms of physical activity, most notably high-intensity/short-duration exercises and competitive sports, can produce a short-term rise in blood sugar levels. This is due primarily to an adrenaline surge. Adjustment for these types of activity will be discussed in the next chapter.
A
fter-Meal Exercise
Most daily activities and aerobic exercises (exercises performed at a submaximal level over a period of twenty minutes or more) will cause blood sugar levels to drop due to enhanced insulin sensitivity and sugar metabolism. To prevent low blood sugar, you can reduce your insulin dose, increase your food intake, or both.
When exercise is going to be performed within two hours after a meal, the best approach is usually to reduce the mealtime bolus. Because physical activity influences both aspects of the bolus (the food part and the blood sugar correction part), both need to be reduced. To accomplish this, use an activity multiplier. Calculate your mealtime bolus as usual (based on the food, blood sugar level, and IOB) and then multiply the bolus by a factor that results in a dosage reduction (see Table 7-10).
Table 7-10. Bolus multipliers for physical activity
Exercise multipliers are based primarily on the duration and intensity of the activity. Whereas duration is fairly easy to measure, intensity is, shall we say, in the eye of the beholder. The “Rating of Perceived Exertion” chart in Figure 7-7 is a simple yet accurate way to assess the intensity of your workout. It takes into account the speed/pace of the activity, your current physical condition, number/size of muscles utilized, your skill/familiarity with the activity, and the workout environment.
For example, if Jan is planning a leisurely twenty-minute bike ride after dinner (she considers it “fairly light”), she would multiply her dinner bolus by .90, which would reduce her dose by 10 percent. If she plans a much more intense sixty-minute ride up and down hills (which she considers “very hard”), she would multiply her dinner dose by .50, which would reduce her dose by 50 percent.