That insulin preparations should be both precise and of uniform strength all over the world is vital to the health and safety of millions of diabetics. Advances in technology now allow biological standardisation in animal insulin to be replaced by physicochemical methods (high performance liquid chromatography: HPLC).
Soluble insulin inj. (neutral, regular insulin) is an aqueous solution of insulin. It is simple to use, being given s.c. 2-3 times a day, 30 min before meals. There is little risk of serious hypoglycaemic reaction if it is used sensibly. If a meal must be delayed, then the insulin injection should be delayed. The dose can easily be adjusted according to self-performed blood glucose measurements.4 For these reasons it is often used initially to balance diabetics needing insulin and always for the treatment of diabetic ketoacidosis. The biggest disadvantages of soluble insulin for long-term use are the need for frequent injections, and the occurrence of high blood glucose before breakfast.
Soluble insulin is neutral, adjusted to pH 7.0. Acid formulations of soluble insulin are no longer available.
Intravenous soluble (neutral) insulin is used in diabetic ketoacidosis. It may be given intermittently (i.v. or i.m.) but continous infusion is preferred. If the insulin is infused by drip in physiological saline (40 units/l) as much as 60-80% can be lost due to binding to the fluid container and tubing. It is necessary to take this into account in dosing. Polygeline (Haemaccel) may be added to bind the insulin in competition with the apparatus and so carry it into the body.
Use of a slow-infusion pump with a concentrated solution (insulin 1.0 unit/ml) is recommended. Insulin loss is minimised and control of dose is more accurate than when more dilute solutions are used. (For i.v. doses see diabetic ketoacidosis, below.) Insulin is suitable for adimistration by continuous i.v. infusion because its short t/2 (5 min) means that the plasma concentration rapidly reaches steady state after initiating the infusion or altering its rate (5 x t'/2, see p. 101). Long-acting (sustained-release) preparations must not be given i.v.
(see Fig. 35.1) are sustained-release formulations in which rate of release is controlled by modifying particle size. Neutral pH, soluble insulin can be mixed with them without altering the time course of effect of either and these formulations can be a great convenience.
Duration of action. Patients live by a 24-hour cycle and plainly insulins having a duration of action exceeding 24 hours can cause problems, especially early morning hypoglycaemia.
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