Safer Prescribing Workbook
SECTION 5
CALCULATIONS AND INTRAVENOUS PRESCRIPTIONS
The objectives of this section are:
•
To understand the rationale behind the rates, dilutions and compatibilities of
intravenous medicines
•
To be able to carry out simple calculations
•
To be able to calculate infusion rates for commonly prescribed medicines
• To be able to write prescriptions for intravenous infusions
• To be aware of national and local policies concerning certain intravenous
drugs
Rules for calculations
Units
Abbreviations
g
gram
mg
milligram
l
litre
ml
millilitre
mmol millimole
Microgram and nanogram must be written out in full, abbreviations such as mcg or µg
for microgram or ηg for nanogram must not be used. The terms are singular and
plural so mg not mgs and ml not mls.
1 gram
=
1 milligram =
1 microgram =
1000 milligram
1000 microgram
1000 nanogram
To convert gram into milligram multiply by 1000
e.g. change 0.35 grams to mg = 0.35 x 1000 = 350 mg
To convert milligram into gram divide by 1000
e.g. change 45 mg to gram = 45 / 1000 = 0.045g
To convert milligram into microgram multiply by 1000
e.g. change 0.025mg to microgram = 0.025 x 1000 = 25 microgram
Concentration of drug in solution
The concentration of drugs in solution may be expressed in several ways.
1. Weight dissolved in the fluid, e.g. mg/ml.
2. As percentages (%) which is the same as the number of grams of drug in
100ml of fluid.
3. As ratios e.g. 1 in 1000 which is the same as 1g in 1000ml
How to calculate doses from concentration
Drug dosages may be worked out using the following formula:
Volume needed = What you want x Volume it is in
What you've got
e.g. the required dose of a drug is 30 mg. The injection contains 40 mg in 10 ml
Volume needed = 30 mg x 10 ml =
40 mg
7.5 ml
Check that both the dose prescribed and the drug solution being used are in the
same units before using this equation (e.g. milligram and millilitre in this case).
If the concentration is expressed as percentage, first work out how many milligrams
there are per ml.
e.g. the drug is at a concentration of 2%.
This means that there are 2 grams in 100ml. 2 grams x 1000 = 2000mg
That is 2000mg in 100ml, so the concentration in mg per ml is 2000 mg/ 100ml,
which is 20mg/ml.
You can then work out the volume of the product you need for a mg dose as above.
If the concentration is expressed as a ratio, to work out how many ml of a product to
give, in order to deliver a certain number of mg, work out how many mg there are per
ml.
e.g. Adrenaline 1 in 10,000
1 in 10,000 = 1 gram in 10,000ml
1gram x 1000 = 1000mg in 10,000ml
Therefore 1mg is in 10ml and 0.1mg is in 1ml or you could say that 100microgram
are in 1ml
To deliver 500microgram you would need 5ml of this product.
Rules for writing doses
It is best to always write doses so that there is never a trailing zero. Missing out a
vital decimal point, or one that could be misread can result in overdosing. So never
write 5.0mg for example, which could result in 50mg being given.
If a decimal point must be used, it must be carefully and clearly written.
For example, a dose of 2.5mg morphine is written, without a clear decimal point a
dose of 25mg could be given which could kill the patient.
e.g. If digoxin is written you must write the whole number of micrograms.
e.g. 500 microgram not 500 mcg or µg which could be misread as milligram resulting
in 1000 x overdose, killing the patient (This HAS happened!)
and for a dose of 62.5 microgram never write 0.0625mg as this can result in a 10x
overdose as people get confused with conversion of the dose to the amount of
commercially available preparations giving 625 microgram by mistake.
Workshop
Scenario 1
Mr POD is a 35 year old gentleman who has suffered personal problems recently,
including a divorce and redundancy. He has been admitted via the Emergency
Department to the Acute Medical unity having taken a paracetamol overdose.
1. What factors would determine whether treatment is required with an antidote?
.
2. Write up an antidote regimen on the intravenous prescription chart for this patient
asking suitable questions to determine an appropriate dose.
3. You are on the ward the next evening; another paracetamol overdose patient has
been admitted and is due to receive the final part of the antidote regime that has
been prescribed by your colleague. The nurse is on duty on the ward alone and asks
you if you will check the drug with her. The following is on the tray, how would you
confirm if it is correct: Can you suggest a safer practice?
Dextrose 5% two 500ml bags
Parvolex (NAC) 7 ampoules of 10ml
Needles
Syringes
Giving set
Scenario 2
You are working on a medical ward. Mrs BS is a 49 year old patient who has come in
for an acute exacerbation of asthma.
She has received nebulised salbutamol 5mg every hour and ipratropium 500
micrograms every 4 hours. She has already received 200mg of IV hydrocortisone.
1. She is still extremely short of breath and your SPR wants you to commence IV
aminophylline. Ask relevant questions in order to ascertain an appropriate dosing
regimen for this patient, and write a prescription on the IV prescription sheet for
this patient.
2. The patient has received 8 hours of aminophylline and has adequate levels of
12mg/l (normal therapeutic range 10-20mg/l). She is still very wheezy and the
consultant wants the salbutamol to be given IV. Write a safe prescription on the IV
sheet for this patient.
3. The patient is noted to be tachycardic the following day with a heart rate of 145
bpm. Consider the possible reasons for the tachycardia in this patient and how
they might be corrected.
4. You are working in the Acute Medical Unit where a 64 year old man has been
admitted via the Emergency Department. He presented with atrial fibrillation, this
is the first onset of AF, he is considered to be stable. He has had all the relevant
tests. All possible reversible causes of AF have been corrected or excluded. The
consultant wants you to prescribe IV amiodarone. Write a clear, safe prescription
for this patient on the IV prescription form.
Additional Practice Questions
1. Write a prescription for IV erythromycin, including all instructions on how to
prepare and administer the dose.
2. Write up dobutamine for a patient who weighs 65Kg at a dose of
5micrograms/Kg/min. make sure that you include how to dilute the drug, the type
of access required and the rate that the nurse should set the pump at to
administer the correct dose (pumps are set in ml per hour).
3. Write up adrenaline for a patient who weighs 81Kg at a dose of
0.1micrograms/Kg/min. make a 1:10,000 dilution using 1mg/ml amps; specify
diluent, the type of access required and the rate that the nurse should set the
pump at to administer the correct dose (pumps are set in ml per hour).
4. Write a prescription for IV Rifampicin for a staphylococcus infection. Include the
instructions for preparation of the drug, the rate, the type of access required and
finally minimise the volume administered.
5. Write a first day prescription for a 50 kg patient who requires Fungizone. They
have normal renal function. Include in your answer instructions for preparation of
the dose and how you would monitor the treatment. What prophylactic agents
might you prescribe to prevent any reaction? Also consider how you would
increase the dose if it was tolerated.
Day 2 The patient’s renal function deteriorates and your consultant asks you to
prescribe AmBisome, what is the starting dose and what is the full treatment
dose? What would you use to flush the line?
6. Write a prescription for nimodipine for a 90kg patient who has had a subarachnoid
haemorrhage and who is not absorbing feeds. How would you assess if the drug
is adversely affecting the patient and how would you titrate the dose if tolerated.
Include in your answer IV access issues and the method of dilution.