Pharmacology and Drug Calculations Flashcards

A patient weighing 88 kg is prescribed dopamine at 7 mcg/kg/min. The pharmacy dispenses dopamine 400 mg in 250 mL of D5W. What is the correct infusion rate in mL/hour?

Answer: 23.1 mL/hr

Concentration = 400 mg ÷ 250 mL = 1.6 mg/mL = 1600 mcg/mL. Required dose = 7 mcg/kg/min × 88 kg = 616 mcg/min. Rate = (616 ÷ 1600) × 60 = 23.1 mL/hr. This is a standard ICU vasopressor calculation in SCFHS-accredited Saudi hospitals. Always verify the concentration label before programming the infusion pump.

A nurse is preparing IV vancomycin 1.5 g in 250 mL NS to infuse over 90 minutes using a 15 gtt/mL IV set. What is the correct manual drip rate in drops per minute?

Answer: 42 gtt/min

Rate = 250 mL ÷ 90 min = 2.78 mL/min. Drip rate = 2.78 × 15 gtt/mL ≈ 42 gtt/min. Vancomycin must be infused slowly to prevent Red Man Syndrome — a rate-related adverse reaction (not a true allergy) characterized by flushing and erythema of the face and neck. SFDA and CBAHI-accredited pharmacy protocols in Saudi Arabia mandate a minimum 60-minute infusion for doses ≤ 1 g and 90 minutes for doses > 1 g.

A Saudi ICU patient has a creatinine clearance (CrCl) of 25 mL/min and weighs 70 kg. The standard gentamicin dose is 5 mg/kg/day for CrCl ≥ 60 mL/min. Using proportional dose reduction, what is the adjusted daily dose?

Answer: 146 mg/day

Standard dose = 5 mg/kg × 70 kg = 350 mg/day. Adjustment factor = 25 ÷ 60 = 0.417. Adjusted dose = 350 × 0.417 ≈ 146 mg/day. In Saudi CBAHI-accredited facilities, therapeutic drug monitoring (TDM) of aminoglycosides is mandatory for renal-impaired patients; target peak 5–10 mcg/mL, trough < 2 mcg/mL to avoid nephrotoxicity and ototoxicity.

Under SFDA regulations, a nurse prepares heparin 5,000 units subcutaneously from a vial labeled 10,000 units/mL using a 1 mL syringe. What volume should be drawn up?

Answer: 0.5 mL

Volume = 5,000 units ÷ 10,000 units/mL = 0.5 mL. Heparin is a high-alert medication per SCFHS and ISMP Saudi guidelines. Vials are available in multiple concentrations (1,000, 5,000, and 10,000 units/mL), making concentration verification before calculation critical. Saudi MOH VTE prophylaxis clinical pathways commonly prescribe 5,000 units every 8–12 hours subcutaneously.

A patient is on a morphine PCA pump: demand dose 1.5 mg, lockout interval 8 minutes, 4-hour limit 24 mg. The patient presses the button 18 times in 4 hours but the pump logs only 12 successful deliveries. What total morphine dose was administered?

Answer: 18 mg

Total delivered = 12 successful doses × 1.5 mg = 18 mg. Six attempts were denied by the lockout interval. The 4-hour ceiling of 24 mg was not reached, so it did not restrict delivery. SCFHS nursing standards require nurses to review PCA attempt-to-delivery ratios; a high ratio of failed attempts may indicate undertreated pain and warrants reassessment of the PCA prescription. Saudi MOH pain management guidelines align with WHO analgesic ladder principles.

A Saudi hospital pharmacist contacts the nurse regarding a potential drug interaction for a patient receiving warfarin (INR currently 2.4, target 2–3). The physician has now prescribed fluconazole 200 mg daily for 7 days for an oral candidiasis. What is the PRIMARY pharmacokinetic mechanism behind this interaction, and what nursing action is most appropriate?

Answer: Fluconazole inhibits CYP2C9, decreasing warfarin metabolism and raising bleeding risk — monitor INR more frequently and notify the prescriber

Fluconazole is a potent inhibitor of CYP2C9, the primary enzyme responsible for warfarin (S-enantiomer) metabolism. Inhibition leads to elevated warfarin plasma levels and a significantly increased INR, raising the risk of serious bleeding. The nurse's priority action is to flag the interaction, notify the prescriber for INR monitoring frequency adjustment (e.g., checking INR within 2–3 days of starting fluconazole), and document the safety alert. In Saudi CBAHI-accredited facilities, nurses are within scope to initiate medication reconciliation alerts. Warfarin dose adjustment decisions rest with the physician or clinical pharmacist.