Blood glucose curves
In-hospital blood glucose curves
Blood glucose curves serve two very useful purposes that other monitoring parameters do not. They identify clinically undetectable hypoglycemia so that the insulin dose can be decreased before clinical signs of hypoglycemia develop. Thus, a periodic BGC is recommended for seemingly well-controlled patients. More importantly, although other techniques and clinical signs may suggest control is lacking, multiple reasons for poor control exist, including too low and too high an insulin dose. The only way to know how to appropriately change an insulin dose is to perform a BGC.
There are several situations when a BGC should be performed:
- After the first dose of a new kind of insulin;
- At 7–14 days after an insulin dose change;
- At least q 3 mo even in well-controlled diabetics;
- At any time clinical signs recur in a controlled patient; and
- When hypoglycemia is suspected.
To construct a BGC, BG is generally measured q 2 hr for one interval between injections (i.e., for 12 hr if insulin is administered twice daily and for 24 hr if insulin is given once daily). When using glargine (Lantus) in cats, BG should be monitored every 3–4 hr. However, when BG is <150 mg/dL in both cats and dogs during any curve, BG should be measured hourly.
The AlphaTrak 2 may be the most accurate BG meter (glucometer) for veterinary patients because it has been calibrated in dogs and cats.32-34 Although human glucometers are readily accessible to pet owners, the Task Force does not recommend their use due to inaccuracies when reading canine and feline blood.
A normal insulin treatment and feeding schedule must be maintained as much as possible during the BGC. Unless patients eat their normal amount of the normal food at the normal time, a BGC should probably not be obtained. When first regulating a diabetic patient, assessment of owner technique is crucial. Therefore, it is ideal if the feeding and insulin injection are done in the hospital so the injection can be observed. Obtaining a fasting blood sample for BG measurement prior to insulin injection can also aid in appraisal of glycemic control. However, this may not be possible if normal feeding time occurs before a hospital opens or if a dog or cat will not eat in the hospital. If an owner’s technique is suspect, the injection time can be changed to occur in front of the veterinarian. Clearly, cooperation between client and veterinarian is necessary to optimize the information obtained with minimal disturbance to routine.
A BGC should establish duration of treatment effect and the lowest BG (i.e., the nadir). The ideal nadir is a BG of 80–150 mg/dL. The highest BG should be close to 200 mg/dL in dogs and 300 mg/dL in cats.
In assessing a BGC, whether it is the first curve performed on a patient or the most recent of many, two basic questions need to be asked. First, has the insulin succeeded in lowering BG? And second, how long has the BG been controlled? By answering these questions, logical changes in dosing regimen can be made.
The first aim in regulating a diabetic is to achieve an acceptable nadir. If an acceptable nadir is not achieved, the insulin dosage should be adjusted (see below). An acceptable nadir with good clinical control may not be obtained if the insulin used has a short duration of activity. Hypoglycemia must always be avoided. No matter what other BG concentrations are during the day, if BG is ever <80 mg/dL, the insulin dose must be reduced.
Once an acceptable nadir is achieved, duration of action, roughly defined as the amount of time BG is controlled, can be determined. Duration cannot be evaluated until the nadir is optimized. The BG should be controlled for as close to 24 hr per day as possible.
The Somogyi or overswing phenomenon, also called hypoglycemia-induced hyperglycemia, refers to hypoglycemia followed by marked hyperglycemia. It results from a physiological response when an insulin dose causes BG to be <60 mg/dL or when BG concentration decreases quickly. In either case, counter-regulatory hormones, which act to increase BG (e.g., cortisol, epinephrine, and glucagon), are released. Hyperglycemia usually occurs rapidly and can be followed by a period of insulin resistance. In cats, however, hypoglycemia does not always trigger a Somogyi phenomenon and resistance may not occur.35 The same is likely true for dogs. If a Somogyi phenomenon is observed, insulin dosage must be decreased. Once the nadir is <80 mg/dL, counter-regulatory hormones will no longer interfere and the true duration of effect will become apparent.
Glucose curves are not perfect and must always be interpreted in light of clinical signs. Blood glucose curves vary from day to day and can be affected by deviation from the patient’s normal routine.36,37 Stress hyperglycemia falsely elevates results. See the Online Resource Center for examples of interpreting various glucose curves.
At-home blood glucose curves
Obtaining a BGC at home is strongly recommended both for dog and cat owners, but even more so in the case of feline patients due to the chance of stress hyperglycemia in a hospital setting. For home BGC, capillary blood is suitable.38 Commonly used sites of blood collection are the ear, gums, non–weight bearing or accessory foot pads, or elbow callus. If using devices designed for pricking human fingertips, one with a variable needle depth should be chosen. A hypodermic needle can also be used, especially if the marginal ear vein is the site of blood collection.39
Not all owners are suited to the task of obtaining a home BGC, something that takes time and patience to master. The most frequent problems encountered by owners are the need for more than one puncture to obtain a blood drop, obtaining a sufficient volume of blood, the need for assistance in restraining a pet, and the pet’s resistance to obtaining a blood sample.39 Curves can vary from day to day even when done at home and must always be interpreted in light of clinical signs.40 Practice team members can refer to the Online Resource Center for more detailed information and resources for pet owners on at-home monitoring utilizing BGCs.