Clinical refresher
Med math & clinical refresher
CRNA programs and interviews probe whether your medication math and critical-care fundamentals are sharp. This is the refresher — the calculations and concepts you'll be expected to know cold, with worked examples and practice.
Conversions to know cold
The calculations you must know
Volume = (desired dose ÷ available concentration) × volume per unit
Example: Order 40 mg; vial is 80 mg / 2 mL → (40 ÷ 80) × 2 = 1 mL.
Dose = ordered (mg/kg) × weight (kg)
Example: Propofol 2 mg/kg for a 70 kg patient → 2 × 70 = 140 mg.
gtt/min = (volume mL × drop factor gtt/mL) ÷ time in minutes
Example: 1000 mL over 8 h with a 15 gtt/mL set → (1000 × 15) ÷ 480 ≈ 31 gtt/min.
mL/hr = (mcg/kg/min × weight kg × 60) ÷ concentration (mcg/mL)
Example: Norepinephrine 0.1 mcg/kg/min, 80 kg, bag 4 mg/250 mL (= 16 mcg/mL) → (0.1 × 80 × 60) ÷ 16 = 30 mL/hr.
A 1% solution = 10 mg/mL (1 g per 100 mL)
Example: Lidocaine 2% = 20 mg/mL; so 5 mL holds 100 mg.
Practice problems
Work each one on paper first, then check yourself.
1. Order 25 mg; the vial is 50 mg/mL. How many mL do you draw up?
Show the work
25 ÷ 50 = 0.5 mL.
2. Succinylcholine 1.5 mg/kg for a 60 kg patient. What dose?
Show the work
1.5 × 60 = 90 mg.
3. Infuse 1000 mL over 6 h with a 10 gtt/mL set. Rate in gtt/min?
Show the work
(1000 × 10) ÷ 360 ≈ 28 gtt/min.
4. Dopamine 5 mcg/kg/min, 90 kg, bag 400 mg/250 mL (1600 mcg/mL). Rate in mL/hr?
Show the work
(5 × 90 × 60) ÷ 1600 = 27,000 ÷ 1600 ≈ 16.9 mL/hr.
5. How many mg of lidocaine are in 3 mL of a 2% solution?
Show the work
2% = 20 mg/mL → 20 × 3 = 60 mg.
6. Convert 0.5 g to mcg.
Show the work
0.5 g × 1000 = 500 mg × 1000 = 500,000 mcg.
Critical-care quick reference
Interview-ready- CO = HR × SV (normal ≈ 4–8 L/min)
- MAP = DBP + ⅓ (SBP − DBP) — target ≥ 65 mmHg
- SV depends on preload, afterload, and contractility
- Pressors (raise SVR): norepinephrine, phenylephrine, vasopressin
- Inotropes (raise contractility): dobutamine, milrinone, epinephrine
- Dopamine is dose-dependent across receptors
- 1) pH — acidemia < 7.35, alkalemia > 7.45
- 2) PaCO₂ (respiratory) — normal 35–45 mmHg
- 3) HCO₃⁻ (metabolic) — normal 22–26 mEq/L
- 4) Compensation — which system is correcting?
- K⁺ 3.5–5.0 — hyperkalemia → peaked T waves; hypokalemia → arrhythmias
- Na⁺ 135–145 — correct slowly to avoid shifts
- Mg²⁺ and Ca²⁺ matter for rhythm and contractility
Turn knowledge into answers
Clinical questions are a big part of CRNA interviews. Practice articulating your reasoning out loud — browse the clinical question bank, then get feedback from Coach or a full mock with Ana.
For study and interview prep only. Doses, formulas, and reference ranges are summarized for review and may differ by source and institution — always confirm against an authoritative clinical reference and your program's protocols.