Insulin Injection
Timing
Even short-acting insulins take about 30 minutes to enter the blood stream and start action. For insulin to be effective it should reach the blood stream before glucose derived from food enters it. Hence all insulins except rapid acting insulin analogues should be injected at least 30 minutes before a meal, unless the blood glucose at that time has been documented to be low. The interval between the insulin injection and the subsequent meal may even be increased to 45-60 minutes if the blood glucose at that time has been documented to be high (> 180 mg/dl). The common practice of routinely injecting insulin just before or after a meal, due to fear of hypoglycaemia is wrong.
Where can insulin be injected?
Insulin is injected subcutaneously that is, under the skin, in the layer of fat, but above the muscles.
Insulin may be injected into the subcutaneous tissue of the upper arm (front and side), thigh (front and side), and the abdomen (with an exception of a circle with a 5 cm radius around the navel) see fig.5.
The absorption of insulin from a part depends on its blood flow. The abdomen with a rich blood supply, has the fastest rate of absorption, followed by the arms and thighs in that order.
The insulin absorption is maximum if injected in muscle and least if injected in skin, with injection in fat below the skin in between. Hot bath, massage and exercise increase local blood flow and hence increase the rate of insulin absorption. On the other hand, cold hath and smoking reduce the rate of insulin absorption. In general, it is advisable to avoid injecting insulin over a muscle about to be exercised. Insulin should be injected in abdomen before exercise.
Rapid and short-acting insulins are injected in abdomen, while intermediate-acting insulins are injected into the thighs.
The rate of insulin absorption varies from day to day. This accounts for variations in blood glucose response to same dose of insulin.
Rotation of injection areas
It is very important to rotate the sites of insulin injection. Repeated insulin injections into same area injures the skin and the underlying fat tissue and results in an increase in the scar tissue. Many persons are tempted to inject insulin in same place because this makes the part numb. Insulin is not absorbed properly through the scar tissue. Rotation of sites of injection prevents swelling or lumping of the fat under the skin.
Rather than moving to a new area each day, it is better to work through the available area for several days and then move to a different area. One can inject in the same area for one or two weeks, depending on the number of daily injections. When rotating to a new area, one should move to the next best absorption area. The suggested plan for rotation of sites of insulin injections is as follows ; from abdomen to right arm, then to left arm, left thigh, right thigh and then again back to abdomen. Do not inject into the same spot at the same area within a week. Avoid an area where one cannot lift up the fat issue. Check with doctor if the site is painful or if there is lumping of fat issue.
Some people note down the injection areas on area charts. The gap between two injection sites should be at least 1.5 cm.