Calculating IV drug dosages typically involves determining the correct volume of medication to administer based on the prescribed dose, the concentration of the drug, and the patient's weight or other factors. This is usually expressed as the amount of drug (mg, mcg, units) per volume of solution (mL).
Here's a breakdown of how to approach IV drug calculations:
1. Understand the Prescription:
- Identify the drug name, prescribed dose, and route of administration (IV).
- Note any specific infusion rate or duration specified by the doctor.
2. Determine the Drug Concentration:
- Check the medication label or ampoule to find the drug concentration. This is usually expressed as mg/mL (milligrams per milliliter), mcg/mL (micrograms per milliliter), or units/mL. For example, an ampoule might contain 100mg of a drug in 2mL of solution.
3. Calculate the Required Volume:
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Use the following formula to calculate the required volume:
Desired Dose / Concentration on Hand = Volume to Administer- Desired Dose: The dose prescribed by the doctor.
- Concentration on Hand: The concentration of the drug as found on the label.
- Volume to Administer: The amount of solution you need to draw up or administer.
Example 1: Simple Volume Calculation
A patient is prescribed Pethidine I.V 75mg. The ampoule contains 100mg in 2ml. How much should you administer?
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Desired Dose: 75mg
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Concentration on Hand: 100mg/2mL
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Volume to Administer: 75mg / (100mg/2mL) = 1.5mL
4. Consider Infusion Rate (for IV Drips):
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If the drug is to be infused over a specific time period, you need to calculate the infusion rate. Common units for infusion rate are mL/hour or drops/minute.
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mL/hour Calculation:
Total Volume (mL) / Infusion Time (hours) = mL/hour -
Drops/minute Calculation:
- You'll need the "drop factor" of the IV tubing (e.g., 10 drops/mL, 15 drops/mL, 20 drops/mL, or 60 drops/mL for microdrip).
(Volume (mL) / Time (minutes)) x Drop Factor (drops/mL) = Drops/minute
Example 2: Infusion Rate Calculation
Administer 500mL of normal saline IV over 4 hours using an IV set with a drop factor of 15 drops/mL.
- mL/hour: 500mL / 4 hours = 125 mL/hour
- Drops/minute: (500mL / 240 minutes) x 15 drops/mL = 31.25 drops/minute (round to 31 drops/minute)
5. Weight-Based Calculations (Common in Pediatrics):
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Some drugs are dosed based on the patient's weight (e.g., mg/kg).
- Convert the patient's weight to kilograms (if necessary).
- Calculate the total dose: Dose (mg/kg) x Patient Weight (kg) = Total Dose (mg).
- Proceed with the volume calculation as described above.
6. Important Considerations:
- Double-check all calculations. Medication errors can have serious consequences.
- Use a calculator to ensure accuracy.
- Know your hospital's policies and procedures for IV drug administration.
- Consult with a pharmacist or experienced nurse if you are unsure about any calculation.
- Be aware of potential drug interactions and contraindications.
- Always use aseptic technique when preparing and administering IV medications.
7. Resources:
Calculating IV drug dosages requires careful attention to detail and a thorough understanding of the principles involved. Accurate calculations are crucial for patient safety.
