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Fluids
Fluids are primarily prescribed to maintain adequate hydration or electrolyte balance in situations where a patient is unable to do this on their own. Examples may include severe dehydration from vomiting or diarrhoea. The choice of fluid to prescribe is dependent on the clinical scenario and how patients present. This will be covered in more detail below.
There is no need to be put off by seeing a question on fluid prescribing in the exam. In fact, these are often much easier marks to be gained by simply memorising some rules of thumb.
We have created a table below to help you memorise these. Note that there are only a handful of options for fluids prescribing!
Adults
| Clinical Scenario | Management |
|---|---|
| Emergency resuscitation | Sodium chloride 0.9%, 500 mL over 15 mins |
| Emergency hypoglycaemia | Glucose 20%, 100 mL over 15 mins |
| Emergency hypokalaemia | Sodium chloride 0.9% / Potassium chloride 0.3%, 1L over 4 hours |
| Emergency hypercalcaemia | Sodium chloride 0.9%, 1L over 4 hours |
| Maintenance fluids ONLY (no loss) | 25-30 mL/kg/24 hrs of water 1 mmol/kg/24 hrs of Na and K 50-100 g/24 hrs of glucose 1L over 8-12 hrs |
| Maintenance fluids (WITH loss) | 30 mL/kg/24 hrs of water Replace electrolytes lost 1L over 4-6 hrs |
Note:
Potassium is always mixed with other components, such as NaCl.
Children
| Clinical Scenario | Management |
|---|---|
| Emergency resuscitation | Sodium chloride 0.9%, 10 mL/kg over 10-15 mins |
| Maintenance fluids ONLY (no loss) | 100 mL/kg/24 hrs for first 10 kg 50 mL/kg/24 hrs for subsequent 10 kg 20 mL/kg/24 hrs for any kg >20 kg eg. A 25 kg child requires: (100 mL × 10) + (50 mL × 10) + (20 mL × 5) = 1.6 L/24 hrs |
Example Question
Case Presentation: A 3 year old girl is brought into A&E with a 4-day history of diarrhoea and vomiting. She is becoming increasingly drowsy. PMH: Asthma. DH: Salbutamol.
O/E: Temperature 37.4°C. HR 170/min. Sinus rhythm. BP 100/60. RR 42/min. O2 sats 98% on room air. Dry mucous membranes. Weight 12 kg.
Investigations: Na 144 mmol/L. K 3.7 mmol/L. U 7.8 mmol/L. Cr 50 µmol/L
Prescribing Request: Write a prescription for one IV fluid appropriate for this patient.
Daily Requirements
| Component | Requirement |
|---|---|
| Water | 25-30 mL/kg/24 hrs |
| Sodium (Na) | 1 mmol/kg/24 hrs |
| Potassium (K) | 1 mmol/kg/24 hrs |
| Glucose | 50-100 g/24 hrs |
Fluid Content
| Fluid Type | Content |
|---|---|
| Sodium chloride 0.9% 1L | 150 mmol of Na |
| Potassium chloride 0.3% 1L | 40 mmol of K |
| Potassium chloride 0.15% 1L | 20 mmol of K |
| Glucose 5% 1L | 50g of Glucose |
Note:
Potassium cannot be replaced faster than 10 mmol/hr. This is why:
- Potassium chloride 0.3% must be given over at least 4 hours (40 mmol).
- Potassium chloride 0.15% must be given over at least 2 hours (20 mmol).
More difficult questions may ask you to calculate how much more fluids a patient needs to be prescribed. In these questions, follow the steps below:
- Work out weight-specific daily requirements.
- Determine how much was given in the first bag.
- Calculate the remaining requirements for the second bag.
- Decide on the appropriate next bag of fluids.
Example Question
Scenario:
A 100 kg adult patient has been admitted with dehydration. The patient's daily fluid requirement needs to be calculated, and you are to determine how much fluid to give in the second bag, considering sodium chloride 0.9% / potassium chloride 0.3% 1L bag has already been administered.
Calculate Daily Requirements Based on Weight
For a 100 kg patient, the daily requirements are as follows:
- Water:
- Requirement: 25-30 mL/kg/24 hours
- Calculation: 100 kg×27.5 mL/kg= 2750 mL
- Sodium (Na):
- Requirement: 1 mmol/kg/24 hours
- Calculation: 100 kg×1 mmol/kg=100 mmol
- Potassium (K):
- Requirement: 1 mmol/kg/24 hours
- Calculation: 100 kg×1 mmol/kg=100 mmol
- Glucose:
- Requirement: 50-100 g/24 hours
- Calculation: Use middle value: 75 g
Determine Amount Given in the First Bag
The first bag administered was 1L of sodium chloride 0.9% / potassium chloride 0.3%. So, the patient has already received:
- 150 mmol of Sodium (Na)
- 40 mmol of Potassium (K)
- 1000 mL of Water
Calculate How Much More Needs to Be Given
Water Requirement:
- Required: 2750 mL
- Given: 1000 mL
- Remaining: 2750 mL - 1000 mL = 1750 mL
Sodium (Na) Requirement:
- Required: 100 mmol
- Given: 150 mmol (already exceeds the requirement)
- Remaining: 0 mmol (no more sodium needed)
Potassium (K) Requirement:
- Required: 100 mmol
- Given: 40 mmol
- Remaining: 100 mmol - 40 mmol = 60 mmol
Glucose Requirement:
- Required: 75 g
- Given: 0 g
- Remaining: 75 g - 0 g = 75 g
Select Appropriate Fluids for the Second Bag
Based on the remaining requirements, the second bag should address the remaining needs for water, potassium, and glucose:
- Water: 1750 mL needed.
- Potassium (K): 60 mmol needed.
- Glucose: 75 g needed.
Suggested Second Bag:
To meet these needs, you could administer the following:
-
1L of Potassium Chloride 0.3% (40 mmol K, 1000 mL water), mixed with sodium chloride (as potassium should not be given alone).
-
1L of Glucose 5% (50 g glucose, 1000 mL water).
Either option is correct.