A nurse is preparing to administer phenytoin 100 mg PO every 8 hours.
If the available phenytoin suspension is 125 mg/5 mL, how many mL should the nurse administer per dose?
The Correct answer and Explanation is:
To calculate how many milliliters of phenytoin suspension the nurse should administer, we can use the formula:Dose to be administered (mL)=(Prescribed dose (mg)Available concentration (mg/mL))\text{Dose to be administered (mL)} = \left(\frac{\text{Prescribed dose (mg)}}{\text{Available concentration (mg/mL)}}\right)Dose to be administered (mL)=(Available concentration (mg/mL)Prescribed dose (mg))
Given:
- Prescribed dose: 100 mg
- Available concentration: 125 mg per 5 mL
First, determine the concentration in mg/mL:Concentration=125 mg5 mL=25 mg/mL\text{Concentration} = \frac{125 \text{ mg}}{5 \text{ mL}} = 25 \text{ mg/mL}Concentration=5 mL125 mg=25 mg/mL
Next, calculate the required volume:Dose to be administered=100 mg25 mg/mL=4 mL\text{Dose to be administered} = \frac{100 \text{ mg}}{25 \text{ mg/mL}} = 4 \text{ mL}Dose to be administered=25 mg/mL100 mg=4 mL
Thus, the nurse should administer 4 mL of the phenytoin suspension per dose.
Explanation
This problem is an example of dosage calculation, a critical skill in nursing to ensure patient safety and effective treatment. The calculation hinges on understanding the relationship between the prescribed dose and the concentration of the medication available. In this case, the phenytoin suspension has a known concentration of 125 mg per 5 mL, which simplifies to 25 mg per mL. The prescribed dose is 100 mg, meaning the nurse must determine how many milliliters of the 25 mg/mL solution will deliver 100 mg.
By dividing the prescribed dose (100 mg) by the concentration (25 mg/mL), we find that 4 mL of the suspension contains the required 100 mg. This process ensures that the patient receives the exact dose needed for therapeutic effectiveness without risking underdosing or overdosing, both of which could lead to adverse outcomes.
Furthermore, accuracy in dosage calculations is vital because errors can have severe consequences. For instance, administering too much phenytoin can lead to toxicity, manifesting as symptoms like nystagmus, ataxia, and confusion. Conversely, too little might not achieve the desired therapeutic effect, leaving the patient at risk for uncontrolled seizures.
In summary, the nurse should administer 4 mL of phenytoin suspension, ensuring the correct dosage by accurately calculating the volume needed based on the concentration available.