The amount of water used in a syringe driver depends on several factors, including the dose and concentration of the medication being administered, the flow rate or delivery time, and the total volume capacity of the syringe. While there is no universal rule for how much water to use, some general guidelines can help determine the appropriate water volume for a particular syringe driver setup.
Quick Overview
The basic steps for calculating syringe driver water volume are:
- Determine the medication dose and concentration.
- Calculate the flow rate based on the desired delivery time.
- Calculate the total volume needed based on dose, concentration and flow rate.
- Select an appropriate syringe size (e.g. 10mL, 20mL, 50mL)
- Subtract the medication volume from the total syringe volume to determine water volume.
Additional factors like dead space volume and medication compatibility may also need to be considered in some cases.
Determining Medication Dose and Concentration
The first step is to know the dose and concentration of the medication to be delivered by the syringe driver. The dose is the total amount of medication, usually measured in milligrams (mg) or micrograms (mcg), to be administered over a certain time period. The concentration is the strength of the medication solution to be used, typically measured in mg/mL or mcg/mL.
For example, a physician’s order may specify:
- Morphine dose: 10mg
- Morphine concentration: 10mg/1mL
So in this case, 10mg is the total dose of morphine and 10mg/1mL is the concentration of the morphine solution.
Calculating Flow Rate
The flow rate, or infusion rate, determines how fast the syringe driver will deliver the medication over time. It is usually measured in mL/hr. The flow rate depends on the desired duration of delivery.
For example, if a 10mg morphine dose needs to be delivered over 24 hours, the flow rate would be:
- Total dose = 10mg
- Desired delivery time = 24 hours
- Flow rate = Total dose ÷ Delivery time
- Therefore, flow rate = 10mg ÷ 24 hrs = 0.417 mL/hr (or ~0.4 mL/hr)
Faster delivery over a shorter duration requires a higher flow rate, while slower delivery over a longer duration requires a lower flow rate.
Calculating Total Volume
Once the flow rate is determined, the total volume needed can be calculated using the following formula:
- Total Volume = Dose (mg) x 1 mL / Concentration (mg/mL)
Continuing the 10mg morphine example:
- Dose (morphine) = 10mg
- Concentration (morphine) = 10mg/1mL
- Total Volume = 10mg x 1mL / 10mg/1mL = 1mL
So based on the dose, concentration and desired timeframe, the syringe driver needs to deliver 1mL of morphine solution at a flow rate of 0.4 mL/hr over 24 hours.
Selecting Syringe Size
Once the total volume is known, an appropriate syringe size can be selected. Common syringe sizes used in syringe drivers include:
- 10 mL
- 20 mL
- 30 mL
- 50/60 mL
The syringe must be large enough to accommodate the total volume of medication plus any additional water needed. In this example, a 10mL syringe would be sufficient since only 1mL of medication is needed.
Calculating Water Volume
The final step is to determine the volume of water needed to fill the rest of the syringe. This is calculated as:
- Water Volume = Total Syringe Volume – Medication Volume
For the 10mg/24 hour morphine example using a 10mL syringe:
- Total syringe volume = 10mL
- Medication volume = 1mL (from earlier calculations)
- Therefore, water volume = 10mL – 1mL = 9mL
So for this setup, 9mL of water would be drawn into the 10mL syringe, then 1mL of morphine solution would be added to create the final 10mL volume for the infusion.
Additional Considerations
Dead Space Volume
One other factor that may need to be considered when calculating water volume is dead space. Dead space refers to any extra volume in the syringe/tubing that will not be infused into the patient. Typical dead space is around 0.4mL for a syringe driver setup. To account for this, the water volume can be reduced by 0.4mL without changing the overall dose calculation.
For example, if dead space is 0.4mL, the water volume in the previous scenario would be adjusted to:
- Total syringe volume = 10mL
- Medication volume = 1mL
- Dead space = 0.4mL
- Water volume = 10mL – 1mL – 0.4mL = 8.6mL
Medication Compatibility
If multiple medications need to be mixed, it’s important to ensure they are compatible and stable together. Some drugs may interact or break down when combined. Checking compatibility charts or contacting a pharmacist can help determine if medications can be safely mixed.
Additionally, the order that medications are drawn up and mixed together may matter. For example, drawing up an antibiotic first before opioids is often recommended to prevent opioid particles from sticking to the syringe. Following best practices for medication compatibility and mixing order ensures optimal drug delivery.
Maximum Concentration
There are maximum concentration limits for certain intravenous medications, above which they may not be stable or suitable for infusion. Checking the drug monograph or other references for concentration ceilings can guide appropriate dilution and syringe filling.
For instance, morphine is generally not given at concentrations above 25mg/mL due to potential adverse reactions at higher concentrations. So if a large morphine dose was needed, the concentration would need to be diluted to stay within the safe range.
Example Calculations
Here are some examples of syringe driver water volume calculations for different medication doses and delivery times:
Example 1
- Morphine dose: 5mg
- Morphine concentration: 5mg/1mL
- Delivery time: 24 hrs
- Flow rate: 5mg ÷ 24hrs = 0.208 mL/hr (~0.2 mL/hr)
- Total volume: 5mg x 1mL / 5mg/1mL = 1mL morphine solution
- Syringe size: 10mL
- Water volume: 10mL – 1mL = 9mL
For 5mg morphine over 24hrs in a 10mL syringe, use 9mL water.
Example 2
- Hydromorphone dose: 1.5mg
- Concentration: 1.5mg/1mL
- Delivery time: 12 hrs
- Flow rate: 1.5mg ÷ 12 hrs = 0.125 mL/hr
- Total volume: 1.5mg x 1mL / 1.5mg/1mL = 1mL
- Syringe size: 20mL
- Water volume: 20mL – 1mL = 19mL
For 1.5mg hydromorphone over 12hrs in a 20mL syringe, use 19mL water.
Example 3
- Metoclopramide dose: 30mg
- Concentration: 5mg/1mL
- Delivery time: 24 hrs
- Flow rate: 30mg ÷ 24 hrs = 1.25 mL/hr (~1.3 mL/hr)
- Total volume: 30mg x 1mL / 5mg/1mL = 6mL
- Syringe size: 60mL
- Water volume: 60mL – 6mL = 54mL
For 30mg metoclopramide over 24hrs in a 60mL syringe, use 54mL water.
Syringe Driver Water Volume Table
Here is a table summarizing example water volumes needed for different medication doses/delivery times using 10mL syringes:
| Medication | Dose | Delivery Time | Water Volume |
|---|---|---|---|
| Morphine | 5mg | 24 hours | 9mL |
| Hydromorphone | 1.5mg | 12 hours | 8.5mL |
| Methadone | 10mg | 24 hours | 9mL |
| Midazolam | 5mg | 8 hours | 9.5mL |
| Metoclopramide | 30mg | 24 hours | 4mL |
Key Tips for Syringe Driver Water Volumes
- Always check medication dose, concentration and delivery timeframe.
- Calculate flow rate, total volume and select syringe size.
- Subtract medication volume from syringe size to determine water volume.
- Account for any dead space in syringe/tubing setup.
- Consult compatibility charts if mixing medications.
- Double check calculations to ensure accurate volumes and dosing.
- Label syringe clearly with drug, dose and rate.
Conclusion
Determining the appropriate water volume is an essential step in preparing syringe drivers for continuous medication delivery. With some simple calculations using the prescribed dose, concentration, duration, flow rate and syringe capacity, the required water volume can be accurately derived. Considering dead space and compatibility issues can further optimize the syringe contents. Taking care with the measurements and setup helps ensure patient safety and effective medication administration via syringe driver.