Principles of insulin treatment

Injections:

Insulin is delivered either via a plastic syringe or a pen-injection device

Injections are given to a pinch of skin on the:

• Thighs
• Abdomen
• Upper arm

The injection site should be changed regularly to prevent areas of lipohypertrophy.

Rate of insulin absorption depends on local subcutaneous blood flow and is accelerated by:

• Exercise
• Local massage
• Warm environment

Rate of absorption (fastest to slowest):

• Abdomen
• Upper arm
• Thigh

Soluble insulin:

Forms a clear solution which is short-acting when injected.

Prolonged acting insulins:

Soluble insulin can also be formulated with zinc or protamine to retard its actions; insulin prepared in this way has a cloudy appearance.

Insulin analogues:

These are modifications of the human insulin molecule which allow the hormone a much more rapid effect than the short-acting preparations.

Insulin in clinical use:

In normal subjects, a sharp increase in insulin occurs after meals; this is superimposed on a constant background of secretion. Insulin therapy attempts to reproduce this pattern, but ideal control is usually impossible to achieve for 4 reasons:

1. In normal subjects, insulin is secreted directly into the portal circulation and passes directly to the liver in a high concentration. In contrast, subcutaneous insulin passes into the systemic circulation before the portal circulation, therefore resulting in a higher concentration in the systemic circulation than the portal circulation.
2. Subcutaneous insulin takes 60-90 mins to achieve peak plasma levels, so the onset and offset of action are too slow
3. the absorption of subcutaneous insulin into the circulation is variable
4. Basal insulin levels in the normal individual are constant, but injected insulin invariably peaks and declines, with resulting swings in metabolic control.

Treating young patients:

• Can be started on injections of an intermediate-insulin at a dose of 8-10U BID
• Some recovery of endogenous insulin secretion may occur over the first few months (the ‘honeymoon’ period) and the insulin dose may need to be reduced or even stopped for a period
• Requirement s rise thereafter and a multiple injection regimen is then appropriate for most younger patients

Treating patients with NIDDM:

• There is no consensus for insulin therapy in NIDDM.
• Injections of premixed soluble and isophane insulins (e.g: Mixtard) BID are widely used and reasonably effective
• Outside the UK, combinations of insulin with sulphonylureas or Metformin are popular

Multiple injections:

If glycaemic control is inadequate with the standard approach, the alternatives are:

• Multiple insulin injections
• Continuous subcutaneous insulin infusion (CSII)

Multiple injection regimens and infusion devices have the advantage of flexibility concerning mealtimes, which is of great value to patients with busy jobs, shift workers and those who travel regularly.

More about infusion devices:

Continuous subcutaneous insulin infusion (CSII) is delivered by a small pump strapped around the waist that infuses a constant trickle of insulin via a needle in the subcutaneous tissues. Mealtime doses are delivered when the patient touches a button at the side of the pump.

Disadvantages include:

• Inconvenience of being attached to the pump
• Skin infections
• Risk of ketoacidosis if the flow of insulin is broken (since these patients have no protective reservoir of depot insulin)