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Difficulty: MediumPharmacological and Parenteral Therapies

A client who weighs 165 lb165\text{ lb} is experiencing prolonged seizure activity. The healthcare provider prescribes an intravenous (IV) loading dose of phenytoin at 15 mg/kg15\text{ mg/kg}. To prevent severe hypotension and cardiac dysrhythmias, institutional policy dictates that IV phenytoin must not be administered faster than 50 mg/min50\text{ mg/min}.

What is the minimum time, in minutes, required to safely infuse this loading dose? Record your answer using one decimal place.

Answer: 22.5 minutes

Answer

22.5
The calculation requires a sequential three-step dimensional analysis: first converting the patient's weight to kilograms (165÷2.2=75 kg165 \div 2.2 = 75\text{ kg}), then calculating the total dose required (75×15=1125 mg75 \times 15 = 1125\text{ mg}), and finally determining the time required to administer that dose at the maximum safe rate (1125÷50=22.5 minutes1125 \div 50 = 22.5\text{ minutes}). Administering the medication any faster would risk severe hypotension and arrhythmias.

Step-by-Step Solution

1
Convert the client's weight from pounds to kilograms.
165 lb÷2.2 lb/kg=75 kg165\text{ lb} \div 2.2\text{ lb/kg} = 75\text{ kg}
The prescribed dose is weight-based in kilograms, so an accurate conversion is required first.
2
Calculate the total prescribed loading dose in milligrams.
75 kg×15 mg/kg=1125 mg75\text{ kg} \times 15\text{ mg/kg} = 1125\text{ mg}
The total amount of medication must be known to determine how long the infusion will take.
3
Calculate the minimum infusion time based on the maximum safe rate of 50 mg/min50\text{ mg/min}.
1125 mg÷50 mg/min=22.5 minutes1125\text{ mg} \div 50\text{ mg/min} = 22.5\text{ minutes}
Phenytoin administration must not exceed 50 mg/min50\text{ mg/min} to prevent cardiovascular toxicity. Dividing the total dose by the maximum rate provides the shortest allowable infusion time.

Key Concept

Safe administration of intravenous phenytoin requires accurate weight-based dosing and strict adherence to maximum infusion rates to prevent life-threatening cardiovascular side effects.
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Topics

Pharmacological and Parenteral Therapies

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