Overlap refers to the period of time when the effect of one insulin shot is diminishing and the next insulin shot is taking effect. Caregivers can purposefully manage overlap to increase the effect of insulin on their pets' blood glucose levels and thus hold the curve of their blood glucose levels fairly flat.
Suppose a cat gets an insulin shot every 12 hours, gets 15 hours duration from each shot, and is on an insulin that has a three hour onset. During the 3 hours after each insulin shot, there will be two insulin shots working to reduce blood glucose levels: the diminishing previous shot and the rising current shot. This may mean that the ongoing insulin dose should be less than if no overlap was used.
Using overlap safely and effectively requires significant information about and analysis of how a particular insulin works in your pet (its duration, onset, and peak) and how feeding and other forces (such as exercise or sleep) affect your pets' blood glucose levels. Using overlap may result in diminished insulin needs as your pet's blood glucose levels are reduced from the diminishing effect of the previous shot, and possibly also from the carryover effects of previous shots.
Purposeful use of overlap is considered an advanced approach because it usually requires the administration of insulin at blood glucose levels below the renal threshold, or at least lower than some people are willing to administer the next shot. Maximum use of overlap involves administering the next insulin shot at blood glucose nadir, when the effect of the previous shot is at its peak. This technique, ideal in theory, can be dangerous in practise -- you must be sure that the previous shot is about to diminish, which depends on the insulin's action profile in this animal being very predictable. Waiting until the glucose reading has started to rise a bit is more reliable.
- The link below is to a discussion on the Feline Diabetes Message Board about the use of overlap.
- See cases of cats and dogs using overlap deliberately for flatter curves
- See also the discussion of tight regulation. Also, overlap is not the same as carryover.