Overview of Dose Calculation
Taking medications is not just about knowing which medicine to take and when to take it, but also understanding the right amount to take. Determining the correct dosage is as crucial as choosing the right medicine. Medicines come in various forms, but working out the proper dosage is typically most challenging with liquids. Liquids and fluids are provided in a variety of containers and strengths, but by using straightforward math, we can easily calculate the correct dosage and reduce the chance of making mistakes. There are three formulas that can help with calculating dosages. These include formulas for infusions (gradual introduction of the medicine into the bloodstream), boluses (a single, large dose), and continuous infusions (a consistent, ongoing introduction of the medication).
Equipment used for Dose Calculation
When giving someone fluids through a needle, the type of equipment needed can change based on how the fluids are being given. Let’s first talk about using a needle and syringe. You would use these to take medicine or fluid out of a small glass container known as a vial. However, when a doctor or nurse gives medication through a tube inserted into your vein (also called an intravenous or IV port), they use a method known as a “flush” to send the medication into the tube or into a bag of safe fluid, like normal saline water.
Now, when they want to measure these fluid infusions, it’s very crucial that they use a drip chamber and know how many drops are needed to measure out one milliliter (mL) of the fluid. You might wonder what a drip chamber is. It’s a part of an IV administration set where fluid accumulates so it can then drip at a pace set by the healthcare provider. There are two types of drip sets used – a macro set and a micro set.
Here’s how they work: a macro set makes larger drops, so it takes fewer drops to make up 1 mL. With most macro sets, you would need 10, 15, or 20 drops for 1 mL. On the other hand, a micro set uses many more small drops for each mL, typically 45 or 60. When giving most medicines in a continuous flow, this micro drip set is the better choice. One easy way to tell the difference between the two sets is by looking at the drip chamber. With a micro set, you’ll see a small silver pin sticking out from the end of the chamber that’s connected to the bag. This pin helps make the drops smaller – and that’s why it’s called a micro set.
Also, some medicines that are given in this way come in a glass container. If that’s the case, the healthcare provider will use a specific type of tubing that has a vent port, which lets air into the bottle. This is important because without the air, the fluid coming out would eventually create a vacuum, stopping the medication from flowing.
Preparing for Dose Calculation
Calculating the right dose of medicine is crucial and can be especially stressful in emergency situations. This is why it’s often recommended to use a calculator. A small mistake, like a misplaced decimal point, can lead to giving a patient 10 times the intended dose, potentially causing severe complications. To prevent such errors, health professionals should be familiar with three standard calculation methods. These methods serve as a backup to check the result from the calculator.
Creating a chart with pre-calculated standard doses can also help, especially in confused or high-stress situations. However, this step should be taken carefully. Medication companies sometimes change the concentration of a drug without informing providers, which could lead to incorrect dosing and harm to the patient. Consistent verification and updates to the chart can prevent these problems.
It is always suggested to calculate medicine dosage before mixing the medicine, especially for treatments involving infusions or drips. Doing the maths beforehand makes it easier to adjust the numbers and achieve an easier-to-manage rate of drug delivery, helping ensure a safer and more effective treatment.
How is Dose Calculation performed
Fluid Infusion
There are times when a person needs to be given a specific amount of liquid through an IV drip over a certain period of time. This fluid can either be on its own or can have medication mixed in. The goal is to make sure that the fluid is dispensed over a specific period, usually measured in minutes. The calculation to figure out the right drip rate ultimately results in drips per minute.
To figure this out, you multiply the fluid volume needed (in milliliters) by the gtt/set (a measure of how fast the fluid drips), then divide that by the time (in minutes) to get drips per minute.
For example, let’s say a patient needs 250 mL of normal saline solution over 30 minutes, and the gtt/set is ten. If you put those numbers in the formula, you’ll find that the clinician will need to set the drip rate to about 83 drips per minute (or almost 1.5 drips per second).
Now let’s consider another example where 150 mg of a medicine called amiodarone is mixed with a 100 mL bag of another liquid called D5W. If we want to deliver all the 150 mg in 10 minutes, it doesn’t matter that there’s medicine in the bag. The goal is to empty the 100 mL bag in 10 minutes. Using the formula, we’ll find that the drip rate should be set at 100 drips per minute (or about 1.5 drips per second).
Bolus
This is a method used when there’s a need to obtain a specific amount of medication from a container. The calculation here will give the quantity in milliliters (mL).
If you want to deliver 0.2 mg of a medicine called epinephrine (or adrenaline) to a child, and the ratio available is 1:10,000 (meaning that there’s 1 mg of epinephrine in 10 mL of liquid), the formula will tell you that from the vial, you need to draw 2 mL.
Continuous Infusion
This method is useful when there’s a need to deliver a specific amount of medicine continuously for an extended period. Suppose the clinician calculates the required amount of medicine drips per minute, and there’s a need to change the dose; all they need to do is rework the calculation with the new numbers.
Consider this example: a patient with low blood pressure needs to be given an epinephrine (also known as adrenaline) infusion at a rate of 2 mcg/min. One vial of epinephrine with a ratio of 1:1,000 is placed into a 250 mL bag of normal saline solution using a rate of 60 gtt/set. With the formula applied, the drips per minute will need to be approximately 30.
Lastly, consider this example: a 43-year-old man who weighs 100 kg needs to receive a dopamine dose at a rate of 5 mcg/kg/min. With a 500 mL bag of normal saline solution, 400 mg of dopamine in a vial, and a rate of 60 gtt/set, drip rate per minute should be approximately 37.5. Given that it’s not possible to achieve exactly 37.5 drips per minute, setting the rate at 1 to 1.5 seconds per drip will get you close to the target.
What Else Should I Know About Dose Calculation?
Understanding and applying these formulas can help medical professionals ensure they are giving you the correct amount of medicine. Keeping up-to-date with the latest medical practices and studies helps them make sure they are administering medication as safely as possible. Often, these professionals may need to make quick decisions during your treatment.
