# Situation: A 45-year-old man with epilepsy is brought to the emergency department having a generalized tonic-clonic seizure. He weighs 60 kg. His family says he ran out of his antiseizure medication four days ago and drinks alcohol heavily. Seizures continue despite benzodiazepine treatment, and fosphenytoin 20 mg phenytoin equivalents (PE)/kg is ordered. What is the shortest time over which the nurse may safely infuse this loading dose?

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> subject: Nursing Practice V — Care of Clients with Maladaptive Patterns of Behavior; Care of Clients with Life-Threatening Conditions, Acute Multi-Organ Problems, High Acuity and Emergency Situations

## 문제

Situation: A 45-year-old man with epilepsy is brought to the emergency department having a generalized tonic-clonic seizure. He weighs 60 kg. His family says he ran out of his antiseizure medication four days ago and drinks alcohol heavily.

Seizures continue despite benzodiazepine treatment, and fosphenytoin 20 mg phenytoin equivalents (PE)/kg is ordered. What is the shortest time over which the nurse may safely infuse this loading dose?

## 보기

1. 24 minutes
2. 12 minutes
3. 8 minutes **✔ 정답**
4. 48 minutes

**정답: 3**

## 해설

The dose is 20 mg PE/kg × 60 kg = 1,200 mg PE. Fosphenytoin may be given at up to 150 mg PE/min, so the shortest infusion time is 1,200 ÷ 150 = 8 minutes, with cardiac and blood pressure monitoring.

## 심화 해설

Dose calculation
The prescribed loading dose is 20 mg PE/kg. For a patient weighing 60 kg, the total dose is 1,200 mg PE. Fosphenytoin is dosed in phenytoin equivalents (PE), meaning the number reflects the amount of phenytoin that will be released after the prodrug is converted in the body. This is not the same as the raw milligram weight of fosphenytoin sodium, so the PE label matters when calculating both dose and rate.

Maximum safe infusion rate
Fosphenytoin can be administered intravenously at a maximum rate of 150 mg PE/min. This is substantially faster than the maximum rate for intravenous phenytoin, which is typically limited to 50 mg/min in adults. The faster rate is possible because fosphenytoin is a phosphate ester prodrug that is highly water-soluble and does not require propylene glycol as a vehicle. Propylene glycol is the component in traditional phenytoin formulations responsible for many infusion-related adverse effects, including hypotension, arrhythmias, and local vein irritation. Because fosphenytoin avoids this vehicle, it can be infused more rapidly and with fewer local injection-site reactions [1][2].

Shortest safe infusion time
To find the shortest time, divide the total dose by the maximum rate: 1,200 mg PE divided by 150 mg PE/min equals 8 minutes. This is the fastest rate at which the entire loading dose may be delivered without exceeding the recommended maximum. Infusing over a shorter period would mean exceeding 150 mg PE/min, which increases the risk of cardiovascular adverse effects.

Why the rate limit exists
Even though fosphenytoin is better tolerated than intravenous phenytoin, rapid infusion is not without risk. Transient paresthesias and pruritus, particularly in the groin and perineal area, are common when fosphenytoin is infused at high rates. These sensations are generally self-limited and do not indicate an allergic reaction, but they can be distressing to the patient [1][2]. More importantly, hypotension can still occur during or shortly after intravenous fosphenytoin loading, especially in older patients, those with preexisting cardiovascular disease, or those receiving rapid infusions. A retrospective review found that hypotension was a clinically relevant adverse event in a subset of patients receiving intravenous fosphenytoin loading doses, reinforcing the need for rate control and hemodynamic monitoring . For this reason, the nurse must monitor blood pressure and cardiac rhythm during and after the infusion, regardless of the rate selected.

Clinical context in status epilepticus
This patient is in generalized convulsive status epilepticus that has not responded to benzodiazepines. Fosphenytoin is a standard second-line agent in this setting because it achieves therapeutic free phenytoin concentrations rapidly after intravenous administration [2]. The patient’s history of running out of antiseizure medication and heavy alcohol use is relevant to the underlying seizure threshold, but it does not change the fosphenytoin dosing or rate calculation. The loading dose remains weight-based, and the maximum infusion rate remains 150 mg PE/min.

Comparison with intramuscular administration
Fosphenytoin can also be given intramuscularly when intravenous access is unavailable. Intramuscular loading doses are well absorbed, and therapeutic plasma concentrations are achieved reliably . However, in status epilepticus, the intravenous route is preferred because it provides the most rapid and predictable delivery. The intramuscular route is more relevant when intravenous access cannot be established or when a patient is being transitioned to maintenance therapy outside the emergency setting.

| Parameter | Fosphenytoin IV | Phenytoin IV |
| --- | --- | --- |
| Maximum adult infusion rate | 150 mg PE/min | 50 mg/min |
| Vehicle | Water-soluble prodrug, no propylene glycol | Propylene glycol and ethanol, pH adjusted |
| Local infusion reactions | Less pain, phlebitis, and tissue injury | Higher risk of pain, phlebitis, purple glove syndrome |
| Common rapid-infusion effects | Transient paresthesia, pruritus, hypotension possible | Hypotension, bradycardia, arrhythmias |
| Dose expression | Phenytoin equivalents (PE) | Phenytoin sodium |

Key point! The dose is calculated in PE, not in milligrams of fosphenytoin sodium. Always confirm that the ordered dose and the infusion pump rate are both expressed in PE to avoid a dosing error.

Watch out! A rate of 150 mg PE/min is the maximum, not the default. In older adults, patients with cardiac disease, or those who develop hypotension or paresthesias during infusion, the rate should be slowed and the infusion time extended. The shortest safe time of 8 minutes applies only when the patient is hemodynamically stable and the maximum rate is clinically appropriate [1].References (research sources)

- [1]Fosphenytoin: clinical pharmacokinetics and comparative advantages in the acute treatment of seizures.Research articleFischer JH, Patel TV, Fischer PA (2003) · DOI: 10.2165/00003088-200342010-00002

- [2]Safety of fosphenytoin sodium.Research articleFierro LS, Savulich DH, Benezra DA (1996) · DOI: 10.1093/ajhp/53.22.2707

## 임상 시나리오

Fosphenytoin Loading Dose InfusionSafe Maximum Rate for Status Epilepticus
Calculate the total dose in phenytoin equivalents: 20 mg PE/kg × 60 kg = 1,200 mg PE. Fosphenytoin is dosed as PE, not raw drug weight.

The maximum safe infusion rate for fosphenytoin is 150 mg PE/min, faster than IV phenytoin (50 mg/min) because it is water-soluble and does not contain propylene glycol.

Shortest infusion time: 1,200 mg PE ÷ 150 mg PE/min = 8 minutes. Infusing faster exceeds the recommended maximum rate.

CautionMonitor cardiac rhythm and blood pressure continuously during and after the infusion. Do not confuse fosphenytoin's rate limit with that of IV phenytoin.

## 핵심 개념

- **Fosphenytoin** — A water-soluble phosphate ester prodrug of phenytoin that is converted to phenytoin in the body and can be infused faster than phenytoin.
- **Phenytoin equivalents (PE)** — A dosing unit that reflects the amount of phenytoin released after fosphenytoin conversion, not the raw weight of fosphenytoin sodium.
- **Status epilepticus** — A seizure lasting longer than 5 minutes or recurrent seizures without return to baseline, requiring rapid antiseizure medication administration.
- **Maximum infusion rate** — The fastest rate at which a medication can be safely administered intravenously without exceeding recommended safety limits.
- **Propylene glycol** — A vehicle in traditional IV phenytoin formulations responsible for hypotension, arrhythmias, and local vein irritation; absent in fosphenytoin.

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