Maintenance intravenous fluids — OSCE prescribing station

The stem

You are the foundation doctor on the surgical ward. Mr Khalil is nil by mouth while he waits for surgery tomorrow. He is not vomiting, has no drains and is clinically well hydrated. Prescribe his fluids for the next 24 hours.

Patient details

Patient: Mr Ahmad Khalil, 58 years
Weight: 70 kg
Allergies: None known
Sodium: 139 mmol/L
Potassium: 3.9 mmol/L
Urea / creatinine: 5.1 mmol/L / 82 µmol/L
Glucose: 6.2 mmol/L
Heart and kidneys: Normal

Drug chartTo write

Drug (generic name)	Dose	Route	How often / rate	Notes

Write the drug, dose, route and frequency, and sign every entry.

The examiner’s questions

Question 1Prescribe his maintenance fluids for 24 hours.Show the model answer

He needs about 25 to 30 mL/kg/day of water (1,750 to 2,100 mL), about 1 mmol/kg/day each of sodium, potassium and chloride (about 70 mmol), and 50 to 100 g/day of glucose to limit starvation ketosis.

Drug (generic name)	Dose	Route	How often / rate	Notes
Sodium chloride 0.18% with glucose 4% and potassium chloride 0.15% (20 mmol/L)	1,000 mL	IV	Over 12 hours	Bag 1
Sodium chloride 0.18% with glucose 4% and potassium chloride 0.15% (20 mmol/L)	1,000 mL	IV	Over 12 hours	Bag 2

This gives 2,000 mL of water, about 60 mmol of sodium, 40 mmol of potassium and 80 g of glucose. Use premixed bags of potassium; never add concentrated potassium on the ward.

Question 2Why not give 0.9% sodium chloride for maintenance?Show the model answer

Two litres of 0.9% sodium chloride contain 308 mmol of sodium and chloride, more than four times his daily need. Used for maintenance it causes sodium and water overload and hyperchloraemic acidosis. 0.9% sodium chloride and balanced solutions such as Hartmann’s are for resuscitation and replacing losses.

Question 3How would you change the prescription for a frail 80-year-old woman weighing 50 kg?Show the model answer

Use 20 to 25 mL/kg/day, about 1,000 to 1,250 mL, because older patients, and those with heart failure, kidney disease or malnutrition, are at risk of fluid overload. Hypotonic fluids also carry a risk of hyponatraemia, so check her sodium daily.

Question 4How do you monitor him?Show the model answer

Daily review of fluid balance and weight, urea and electrolytes at least daily while he is on intravenous fluids, and a new prescription each day. Stop the fluids as soon as he can drink. If he starts vomiting or has nasogastric losses, add replacement fluid on top of maintenance.

After you answerThe teaching pointShow

Every intravenous fluid prescription answers three questions: is this for resuscitation, maintenance or replacement? Write a new prescription every day rather than letting the drip run on.

Related: Results station 2 →

The patients are written for teaching and are not real. Doses are typical adult doses; always check the current formulary and your hospital’s guidelines before prescribing.