Fluid balance chart — OSCE chart station

The stem

Mrs Sara Nasser, 64 years, weight 70 kg, is on day 1 after a laparotomy and division of adhesions for adhesional small bowel obstruction. No bowel was resected. She is nil by mouth, with a nasogastric tube on free drainage, a urinary catheter and an abdominal drain. It is 20:00 and the nurse calls you because her urine output has been falling all afternoon.

Observations	Before surgery	Now, 20:00
Pulse /min	84	108
Blood pressure mmHg	132/78	104/68
Respiratory rate /min	16	20
SpO₂	97% air	96% on 2 L O₂
Temperature °C	36.8	37.4
Consciousness	Alert	Alert
Capillary refill	2 s	3 s

Blood results	Before surgery	This morning
Sodium mmol/L	139	136
Potassium mmol/L	4.1	3.3
Urea mmol/L	5.2	9.8
Creatinine µmol/L	74	112
Haemoglobin g/L	128	121

On examination her tongue and lips are dry and the neck veins are not visible lying flat. The abdomen is soft and appropriately tender around a clean wound. The drain contains serous fluid.

Fluid balance chart

Fluid balance, day 1 (24 hours)	mL
Input
IV 0.9% sodium chloride	1,000
IV Hartmann’s solution	1,000
Oral sips	100
Total input	2,100
Output
Urine (catheter)	600
Nasogastric aspirate	900
Abdominal drain	150
Total output	1,650
Recorded balance	+450

Hourly urine, last 6 hours	mL
15:00	30
16:00	25
17:00	20
18:00	20
19:00	15
20:00	15

The chart runs from 20:00 yesterday, when she returned from theatre, to 20:00 today. Insensible losses are not recorded.

The examiner’s questions

Question 1What is her recorded balance, and is it reassuring?Show the model answer

Input 2,100 mL minus output 1,650 mL gives +450 mL. It is not reassuring. Insensible losses from the skin and breathing, about 10 mL per kg a day (roughly 700 mL for her) and more with fever, are not recorded. After a laparotomy for obstruction, more fluid is lost into the swollen bowel and the tissues. Her true balance is negative.

Question 2Is her urine output adequate?Show the model answer

No. 600 mL in 24 hours is about 0.36 mL per kg per hour, and over the last 6 hours it has fallen to 15 mL per hour, about 0.2 mL per kg per hour. The target is at least 0.5 mL per kg per hour. Less than 0.5 mL per kg per hour for 6 hours, together with a creatinine that has risen 1.5 times from 74 to 112, means acute kidney injury, stage 1.

Question 3What do her observations and examination tell you?Show the model answer

She is hypovolaemic: a rising pulse of 108, a blood pressure well below her own baseline, a capillary refill of 3 seconds, a dry mouth and empty neck veins. The urea has risen more than the creatinine, which also points to dehydration. Her NEWS2 score is 4 (oxygen 2, pulse 1, systolic pressure 1). A falling blood pressure is a late sign; act before it falls further.

Question 4How should the nasogastric losses and her potassium be managed?Show the model answer

Gastric fluid is rich in hydrogen, chloride and potassium, which is why her potassium has fallen to 3.3. Replace the nasogastric losses volume for volume with 0.9% sodium chloride containing potassium chloride, on top of her maintenance fluid, and recheck the potassium. Low potassium also slows the return of bowel function.

Question 5What do you do now?Show the model answer

Check the catheter is not blocked. Give a fluid challenge: 250 to 500 mL of crystalloid, such as Hartmann’s solution, over 15 minutes, then reassess pulse, blood pressure, capillary refill and urine output, and repeat if she responds. Prescribe maintenance plus nasogastric replacement for the next 24 hours. Repeat urea and electrolytes, stop nephrotoxic drugs such as non-steroidal anti-inflammatory drugs, and keep hourly urine measurements. Inform your senior. If she does not respond, look for bleeding, sepsis or an unrecognised bowel injury.

After you answerThe teaching pointShow

Low urine output after surgery is usually a sign of too little fluid in the circulation. Read the chart with the patient: the pulse, blood pressure, capillary refill and blood results tell you what the recorded balance cannot. Do not give diuretics to make the number look better.

The patients are written for teaching and are not real.