⚙ Action Profiles of Common Insulins
Crucial Timing of Insulin vs. Meals
- Rapid-Acting (Lispro/Aspart/Glulisine): Give 10 to 15 mins before eating. (May give immediately after eating if oral intake is highly unpredictable).
- Short-Acting (Regular): MUST be given 30 to 45 mins before the meal. Giving it at the time of eating causes a post-prandial spike (before the insulin peaks) followed by delayed hypoglycaemia.
- Intermediate (NPH) / Premix: Give 30 mins before breakfast and dinner.
- Basal (Glargine/Degludec): Can be given at any time of day, but must be given at the exact same time every day.
Clinical Diagnostics & Alerts
⚠ Hypoglycaemia Begets Hypoglycaemia Unawareness
Repeated hypoglycaemic events blunt the autonomic warning signs (sweating, palpitations, tremors). Patients may suddenly develop severe neuroglycopenia (confusion, seizures, coma) without any prior symptoms. Action: If unawareness is suspected, strictly avoid hypoglycaemia for 2 to 3 weeks by raising target GRBS to restore autonomic warning mechanisms.
Somogyi Effect (Rebound Hyperglycaemia): High morning GRBS caused by an undetected nocturnal hypoglycaemic event (usually around 3 AM) which triggers counter-regulatory hormones (glucagon, cortisol, epinephrine) to spike the sugar. Treatment: Decrease the night NPH/Premix dose or give a bedtime snack.
Dawn Phenomenon: High morning GRBS caused by a normal physiological surge of growth hormone and cortisol in the early morning hours, creating insulin resistance. 3 AM sugars will be normal or high. Treatment: Increase the night NPH/Premix or shift it closer to bedtime.
Rule of 15 for Hypoglycaemia: If GRBS < 70 mg/dL: Administer 15 grams of fast-acting carbohydrate (e.g., 3 tsp sugar in water, 1/2 cup juice). Recheck GRBS in exactly 15 minutes. Repeat if still < 70 mg/dL. Once > 70, follow with a complex carb snack to prevent recurrence.
Specialised Inpatient Scenarios
Tube Feed Insulin Strategy: Match the insulin profile to the feed delivery pattern. Continuous feeds: NPH q6h or basal q24h (each covering ~1 U per 10 to 15 g of carbohydrate). Bolus feeds: Regular insulin SC 30 min before each feed (1 U per 10 to 15 g). Cyclic / nocturnal feeds: NPH at feed start; stop / taper 1 h before feed ends to prevent post-feed hypoglycaemia. If the feed is interrupted unexpectedly > 2 h, start D10 IV at the prior feed rate until feed resumes.
TPN Insulin Rule: Add insulin directly to the TPN bag. Start at 0.1 U Regular per g of dextrose (e.g., 250 g dextrose → 25 U in bag). Add a separate SC sliding scale for breakthrough hyperglycaemia. Never use a separate IV insulin infusion alongside TPN — if TPN stops abruptly, profound hypoglycaemia results. If TPN must be stopped suddenly, run D10 IV at the prior TPN rate × 1 to 2 h.
U-500 Regular (Severe Insulin Resistance): Indicated when TDD > 200 U/day (or > 2 U/kg/day). U-500 is 5× more concentrated than U-100, so dose in volume units (e.g., 200 U of U-100 = 40 "volume units" of U-500). Dosing errors are catastrophic. Use only dedicated U-500 syringes or U-500 pens — never standard U-100 syringes. Endocrine consult mandatory. Avoid in ICU / critical illness (use U-100 infusion there).
Postpartum Insulin Cliff: Within 1 to 2 hours of placental delivery, insulin requirements drop by ~50% (loss of placental hPL, cortisol, oestrogen). Continuing pregnancy-era doses is the commonest cause of postpartum DKA in T1DM. Action: Reduce basal to 50% of pre-pregnancy TDD immediately. Breastfeeding reduces requirements by a further 10 to 20%. For GDM, most patients normalise within 24 to 48 h and can stop insulin entirely with monitoring.
Sick Day Rules (T1DM): NEVER stop basal insulin during illness, even when not eating. Counter-regulatory hormones raise insulin needs by 10 to 20% during febrile illness. Check ketones if GRBS > 250 mg/dL or if unwell with normal GRBS. Frequent correction doses (q3-4h) of Regular insulin. Hospitalise if vomiting plus moderate/large ketones — this is impending DKA.
Active Labour Glucose Target: Maintain capillary BG 70 to 110 mg/dL throughout labour using IV insulin infusion + D5NS at 100 to 125 mL/h. Tight intrapartum control reduces neonatal hypoglycaemia at delivery. Hourly capillary BG. For elective C-section: hold all SC insulin morning of surgery, use IV protocol until eating.
