Learn — Endocrine & hormonal drugs

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Insulin — types, onset, peak, duration

Insulin preparations are classified by how quickly they start to work, when they peak and how long they last, because that profile decides when the dose is given in relation to food and when hypoglycaemia is most likely. The onset-peak-duration table is not trivia at the bedside: it tells the nurse the exact hour at which to expect a hypo and the time at which the blood sugar must be checked.

Mechanism: insulin binds a tyrosine-kinase receptor and moves glucose into muscle and fat through GLUT4, suppresses hepatic gluconeogenesis and glycogenolysis, and drives potassium into cells — the last effect is why insulin with dextrose treats hyperkalaemia and why potassium must be watched during any insulin infusion.
Rapid-acting analogues (lispro, aspart, glulisine) begin in 10 to 15 minutes, peak at 1 to 2 hours and last 3 to 5 hours, so they are given immediately before or just after the meal and are the insulins used in pumps.
Regular or soluble human insulin begins in 30 to 60 minutes, peaks at 2 to 4 hours and lasts 6 to 8 hours, so it is given about 30 minutes before food; it is the only preparation that may be given intravenously and is therefore the insulin of diabetic ketoacidosis and of hyperkalaemia.
NPH or isophane insulin is intermediate — onset 1 to 2 hours, peak 4 to 10 hours, duration 10 to 18 hours — so it is given twice daily and its peak explains the classic mid-afternoon or 2 to 3 in the morning hypoglycaemia; it is cloudy and is gently rolled between the palms, never shaken, to resuspend.
Long-acting analogues (glargine, detemir, degludec) are essentially peakless and last about 20 to 24 hours or longer, cover the basal requirement, are given once daily at the same time each day, and must never be drawn into the same syringe as another insulin.
Premixed preparations such as 70/30 combine an intermediate with a short or rapid insulin, give two peaks a day and suit a patient who cannot manage a basal-bolus regimen, but they demand fixed meal times and cannot be adjusted component by component.
Administration: when two insulins are mixed, draw the clear regular insulin before the cloudy NPH so the clear vial is never contaminated; inject subcutaneously at 90 degrees into abdomen, thigh, upper arm or buttock, rotate sites within one region, and remember the abdomen absorbs fastest and exercise of the injected limb speeds absorption further.
The adverse effect that decides nursing action is hypoglycaemia — sweating, tremor, palpitations, hunger and irritability first, then confusion, seizure and coma; the others are weight gain, lipohypertrophy at unrotated sites, local allergy and hypokalaemia during an infusion.
The reversal is glucose: 15 to 20 g of fast-acting oral carbohydrate if the patient is conscious and can swallow, repeated after 15 minutes if the value is still low, and intravenous dextrose or intramuscular glucagon 1 mg if the patient is unconscious or cannot swallow, followed by a complex carbohydrate once awake.
Monitoring: capillary glucose before meals, at bedtime and at the expected peak of the insulin in use, with usual non-pregnant adult targets of a pre-meal glucose of 80 to 130 mg/dL, a post-meal value under 180 mg/dL and HbA1c below 7%, relaxed in the frail elderly; unopened insulin is stored at 2 to 8 degrees Celsius and never frozen, while the vial or pen in use may stay at room temperature away from heat and light for about a month.

Insulin sliding scale

A sliding scale is a written order that gives a dose of short-acting or rapid-acting insulin according to the capillary blood glucose measured at that moment, with no insulin below a threshold and rising increments above it. It matters because it is purely reactive — it treats a sugar that has already risen — and current inpatient guidance holds that a sliding scale used alone is inadequate; the standard is scheduled basal-bolus insulin, with the sliding scale surviving only as the correction component.

A typical correction scale reads: no insulin below 150 mg/dL, 2 units for 150–200, 4 units for 201–250, 6 units for 251–300, 8 units for 301–350, and 10 units with a call to the doctor above 350 mg/dL. The scale actually used is always the institution's written order, never one recalled from memory.
Basal-bolus is the recommended inpatient regimen: a total daily dose of about 0.4–0.5 units/kg/day (0.3 units/kg/day in the elderly, in renal impairment or in the underweight), roughly half as basal insulin such as glargine or NPH and half as prandial regular or rapid-acting insulin split across the three meals, with the correction scale added to the prandial dose.
Sliding scale alone swings the patient between hyperglycaemia and hypoglycaemia without lowering the mean glucose; it is acceptable only briefly, in a patient who is eating nothing, or while the basal requirement is being worked out.
Inpatient glycaemic targets: 140–180 mg/dL for most patients, critically ill or not; a tighter 110–140 mg/dL only in selected patients if it can be reached without hypoglycaemia; treat any value below 70 mg/dL as hypoglycaemia.
Timing matters more than the number: regular (soluble) insulin is given 30 minutes before the meal and a rapid analogue such as lispro, aspart or glulisine within 15 minutes of it. Never give a prandial dose to a patient who is nil by mouth or whose meal tray has not arrived.
Monitoring: capillary glucose before each meal and at bedtime in a patient who is eating, every 4–6 hours in a patient who is not, and every 1–2 hours during an intravenous insulin infusion.
Route rule: only regular (soluble) human insulin and the rapid-acting analogues may be given intravenously; NPH, premixed and long-acting analogue insulins are never given by the intravenous route.
Hypoglycaemia below 70 mg/dL: if the patient is conscious and can swallow, give 15 g of oral glucose and recheck in 15 minutes (the 15–15 rule); if unconscious, give 25 mL of 50% dextrose intravenously, or glucagon 1 mg intramuscularly where there is no line, then recheck and follow with a complex carbohydrate.
Basal insulin is never stopped in a person with type 1 diabetes, even when nil by mouth — withholding it precipitates diabetic ketoacidosis within hours.

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