What is the dose and drop rate of Phenytoin (antiepileptic medication) for a 7kg boy when administered in 100ml of normal saline (NS) over 30 minutes?

From the FDA Drug Label

A loading dose of 15 to 20 mg/kg of Phenytoin Sodium Injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL) The drug should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.

For a 7 kg boy, the dose of Phenytoin would be:

• Loading dose: 15 to 20 mg/kg, so the dose would be 15 mg/kg x 7 kg = 105 mg or 20 mg/kg x 7 kg = 140 mg
• Administration rate: The rate of administration should not exceed 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower. For a 7 kg boy, the rate would be 1 to 3 mg/kg/min, which is 1 mg/kg/min x 7 kg = 7 mg/min or 3 mg/kg/min x 7 kg = 21 mg/min. Since 50 mg/min is greater than both of these rates, the slower rate of 1 to 3 mg/kg/min (7 to 21 mg/min) should be used.

To administer the dose over 30 minutes in 100 ml of normal saline, we need to calculate the total amount of Phenytoin to be administered and the rate of administration in drops per minute.

• Total dose: Let's use the higher end of the loading dose range, 140 mg.
• Concentration: Since the label does not specify the concentration of Phenytoin in the solution, we will assume that the dose is directly administered.
• Administration volume: 100 ml
• Administration time: 30 minutes
• Drop rate: 1 ml = 20 drops, so 100 ml = 2000 drops
• Drops per minute: 2000 drops / 30 minutes = approximately 67 drops per minute

However, we must ensure that the administration rate does not exceed the recommended rate of 1 to 3 mg/kg/min or 50 mg per minute. Since we are administering 140 mg over 30 minutes, the rate would be 140 mg / 30 minutes = 4.67 mg/min, which is within the recommended range for a 7 kg boy (7 to 21 mg/min).

So, the dose would be 140 mg and the drop rate would be approximately 67 drops per minute 1, 1, 1.

From the Research

For a 7kg boy, I recommend administering 140 mg (20 mg/kg) of Phenytoin in 100ml of normal saline over 30 minutes, which translates to a drop rate of 200 ml/hour or approximately 67 drops per minute using a standard administration set (20 drops/ml). This dosage is based on the most recent and highest quality study available, which suggests that a loading dose of 20 mg/kg is effective and safe for pediatric patients 2. The study published in 2014 in the Archives of Disease in Childhood found that the use of intravenous loading doses of phenytoin at 20 mg/kg achieved therapeutic plasma concentrations rapidly and with minimal side effects.

To calculate the dose, we use the recommended loading dose of 20 mg/kg, which for a 7kg boy equals 140 mg. Administering this dose in 100ml of normal saline over 30 minutes requires careful consideration of the drop rate to ensure safe and effective delivery of the medication. Given that 1 ml is equivalent to 20 drops, the total number of drops in 100ml is 2000 drops, and over 30 minutes, this translates to approximately 67 drops per minute.

It's crucial to administer Phenytoin slowly to prevent cardiovascular side effects like hypotension and arrhythmias, but not too slowly as to delay achieving therapeutic levels for seizure control 3, 2. The medication should be administered through a large vein using an in-line filter, and the IV line should be flushed with normal saline before and after administration since Phenytoin is incompatible with many other medications. Continuous cardiac monitoring is recommended during infusion, and the infusion should be slowed or temporarily stopped if the patient develops hypotension, arrhythmias, or signs of local irritation at the IV site.

Key considerations in the administration of Phenytoin include:

• Using the correct dosage based on the patient's weight
• Administering the medication at the appropriate rate to avoid side effects
• Monitoring the patient closely for signs of toxicity or other adverse effects
• Ensuring compatibility of the medication with other substances and the administration equipment.

By following these guidelines and the recommendations based on the most recent and highest quality evidence, healthcare providers can safely and effectively administer Phenytoin to pediatric patients, including a 7kg boy, to achieve therapeutic levels for seizure control while minimizing the risk of adverse effects.