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ABG interpretation quiz: ten surgical patients

Read each patient and gas, interpret it aloud — pH, primary disorder, compensation, anion gap — then reveal the model answer.

Written and clinically reviewed by , consultant minimally invasive and bariatric surgeon, Al-Makassed Hospital, Jerusalem, and Assistant Professor of Surgery, Al-Quds University. Last updated .

Before you start

Use the same five steps every time: the patient and the oxygen; the pH; which component explains it; whether compensation is as expected; and, in any metabolic acidosis, the anion gap. New to this? Read the step-by-step ABG guide first, and check your answers with the ABG calculator.

Case 1Persistent vomitingCase 2Pain out of proportionCase 3Drowsy on the wardCase 4A new stomaCase 5Breathless on day fiveCase 6Abdominal pain and vomiting in a young manCase 7Pre-operative assessmentCase 8The normal-looking gasCase 9After a trauma resuscitationCase 10Two problems at once

Case 1 — Persistent vomiting

The patient

A 58-year-old man has vomited after every meal for two weeks. He has lost weight, and there is a succussion splash. Endoscopy later shows a pyloric tumour.

Arterial blood gas
pH7.52
PaCO26.4 kPa (48 mmHg)
HCO338 mmol/L
Na134
K2.9
Cl82
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Alkalaemia with a high bicarbonate: metabolic alkalosis. Expected PaCO2 = 0.7 × 38 + 21 = 47.6 mmHg (46–50): the PaCO2 of 48 is appropriate respiratory compensation. With the low chloride and potassium, this is a hypochloraemic, hypokalaemic metabolic alkalosis.
Question 2How do you correct it?Show the model answer
Intravenous 0.9% sodium chloride with potassium chloride. The chloride lets the kidney excrete the excess bicarbonate. Monitor potassium closely and decompress the stomach with a nasogastric tube.
After you answerThe teaching pointShow

The urine may be acidic despite a systemic alkalosis (paradoxical aciduria): the volume-depleted kidney keeps sodium in exchange for H+ and K+. Correct the volume and chloride and it resolves.

Case 2 — Pain out of proportion

The patient

A 74-year-old woman in atrial fibrillation has severe central abdominal pain. Her abdomen is soft and only mildly tender. She is grey and sweaty.

Arterial blood gas
pH7.19
PaCO23.6 kPa (27 mmHg)
HCO310 mmol/L
Na138
Cl102
Lactate8.0 mmol/L
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Acidaemia with a low bicarbonate: metabolic acidosis. Winter’s: 1.5 × 10 + 8 = 23 mmHg (21–25). Her PaCO2 of 27 is higher than expected: she is not fully compensating, a sign of exhaustion. Anion gap 138 − (102 + 10) = 26: high anion gap, a lactic acidosis. Delta ratio (26 − 12) ÷ (24 − 10) = 1.0: a pure high anion gap acidosis.
Question 2What is the likely diagnosis?Show the model answer
Acute mesenteric ischaemia: pain out of proportion to the signs, atrial fibrillation and a high lactate. She needs urgent CT angiography and a surgical and vascular opinion.
After you answerThe teaching pointShow

A normal lactate does not exclude early mesenteric ischaemia, but a high lactate with pain out of proportion is an emergency. Do not wait for the abdomen to become peritonitic.

Case 3 — Drowsy on the ward

The patient

On the evening after an open cholecystectomy, a 66-year-old man is hard to rouse. He has a morphine PCA. His respiratory rate is 7 and his pupils are pinpoint.

Arterial blood gas
pH7.28
PaCO28.0 kPa (60 mmHg)
HCO327 mmol/L
PaO27.6 kPa on air
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Acidaemia with a high PaCO2: respiratory acidosis. The PaCO2 is 20 mmHg above 40, so acute compensation predicts an HCO3 of about 26; 27 fits an acute respiratory acidosis. The PaO2 is below 8 kPa with a high CO2: type 2 respiratory failure.
Question 2What do you do?Show the model answer
Stop the PCA, give oxygen, support the airway, and give naloxone in small titrated doses. Call for senior help and review his opioid prescription.
After you answerThe teaching pointShow

In acute hypoventilation the kidneys have not had time to compensate, so the bicarbonate is barely raised. A high bicarbonate with a high CO2 suggests the problem is chronic.

Case 4 — A new stoma

The patient

Ten days after a subtotal colectomy with an end ileostomy, a 34-year-old woman with Crohn’s disease feels weak. Her stoma has put out 2.5 litres a day for four days.

Arterial blood gas
pH7.32
PaCO24.0 kPa (30 mmHg)
HCO315 mmol/L
Na136
K3.1
Cl112
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Acidaemia with a low bicarbonate: metabolic acidosis. Winter’s: 1.5 × 15 + 8 = 30.5 mmHg (28.5–32.5): compensation is appropriate. Anion gap 136 − (112 + 15) = 9: normal anion gap (hyperchloraemic) acidosis.
Question 2Why?Show the model answer
Small-bowel effluent is rich in bicarbonate. A high-output stoma loses it, and the kidney retains chloride in its place. She also needs potassium, magnesium and fluid replacement, and her stoma output reduced.
After you answerThe teaching pointShow

A high-output stoma (more than about 1.5–2 litres a day) causes dehydration, hypokalaemia, hypomagnesaemia and a normal anion gap acidosis. Measure the output, not just the electrolytes.

Case 5 — Breathless on day five

The patient

Five days after a total knee replacement, a 61-year-old woman becomes suddenly breathless with pleuritic chest pain. Heart rate 118. Her chest is clear.

Arterial blood gas
pH7.50
PaCO23.9 kPa (29 mmHg)
HCO322 mmol/L
PaO28.4 kPa on air
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Alkalaemia with a low PaCO2: respiratory alkalosis. The PaCO2 is 11 mmHg below 40; acute compensation predicts an HCO3 of about 21.8, so 22 fits an acute respiratory alkalosis. The PaO2 is low for a patient on air.
Question 2What must you exclude?Show the model answer
Pulmonary embolism. Hypoxaemia with hypocapnia, tachycardia, a clear chest and recent surgery. Arrange a CT pulmonary angiogram and start treatment-dose anticoagulation if there is no contraindication.
After you answerThe teaching pointShow

Anxiety also causes a respiratory alkalosis, but it does not cause hypoxaemia. Never label a hypoxic post-operative patient as anxious.

Case 6 — Abdominal pain and vomiting in a young man

The patient

A 22-year-old man has diffuse abdominal pain and vomiting for a day. He is breathing deeply and rapidly and has been very thirsty. Glucose 28 mmol/L, blood ketones 5.2.

Arterial blood gas
pH7.12
PaCO22.5 kPa (19 mmHg)
HCO36 mmol/L
Na132
Cl96
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Acidaemia with a very low bicarbonate: metabolic acidosis. Winter’s: 1.5 × 6 + 8 = 17 mmHg (15–19): compensation is appropriate (the deep breathing is Kussmaul respiration). Anion gap 132 − (96 + 6) = 30: high anion gap. Delta ratio (30 − 12) ÷ (24 − 6) = 1.0.
Question 2Does he need an operation?Show the model answer
Not yet. This is diabetic ketoacidosis, which commonly causes abdominal pain. Treat with fluids, fixed-rate insulin and potassium, and re-examine: the pain usually settles as the ketosis resolves. Pain that persists needs a surgical cause looked for.
After you answerThe teaching pointShow

Always check the glucose and ketones in an acute abdomen. Taking a patient in DKA to theatre without resuscitation is dangerous.

Case 7 — Pre-operative assessment

The patient

A 70-year-old man with severe COPD is assessed before an elective inguinal hernia repair. He is well and at his usual baseline.

Arterial blood gas
pH7.36
PaCO27.6 kPa (57 mmHg)
HCO331 mmol/L
PaO28.1 kPa on air
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
The pH is normal but on the acid side, with a high PaCO2 and a high bicarbonate: a compensated chronic respiratory acidosis. The PaCO2 is 17 mmHg above 40; chronic compensation predicts an HCO3 of about 30, so 31 fits.
Question 2What matters for his surgery?Show the model answer
He retains CO2. Target oxygen saturations of 88–92%, as high-flow oxygen can worsen his hypercapnia. Consider local or regional anaesthesia, and plan physiotherapy and early mobilisation.
After you answerThe teaching pointShow

In a chronic respiratory acidosis the raised bicarbonate is compensation, not a separate disorder. Do not try to ‘correct’ it.

Case 8 — The normal-looking gas

The patient

A 69-year-old woman is septic from cholangitis. She has also vomited repeatedly for two days. Her blood pressure is 85/50.

Arterial blood gas
pH7.40
PaCO25.3 kPa (40 mmHg)
HCO324 mmol/L
Na140
Cl90
Lactate6.1 mmol/L
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
pH, PaCO2 and HCO3 are all normal — but the anion gap is 140 − (90 + 24) = 26, and the lactate is 6.1. There is a high anion gap (lactic) metabolic acidosis hidden by a metabolic alkalosis from vomiting. The bicarbonate has not fallen at all, so the delta ratio is very high.
Question 2What is the lesson?Show the model answer
She is in septic shock with a lactic acidosis, despite a ‘normal’ gas. Resuscitate, give antibiotics and arrange biliary drainage. Always calculate the anion gap.
After you answerThe teaching pointShow

Two opposite disorders can cancel out in the pH. The anion gap and the lactate reveal what the pH hides.

Case 9 — After a trauma resuscitation

The patient

A 28-year-old man was given 6 litres of 0.9% saline during resuscitation after a road traffic collision. He is now stable and warm, with a normal lactate.

Arterial blood gas
pH7.34
PaCO24.1 kPa (31 mmHg)
HCO316 mmol/L
Na145
Cl121
Lactate1.4 mmol/L
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Acidaemia with a low bicarbonate: metabolic acidosis. Winter’s: 1.5 × 16 + 8 = 32 mmHg (30–34): compensation is appropriate. Anion gap 145 − (121 + 16) = 8: normal anion gap, hyperchloraemic acidosis.
Question 2What caused it?Show the model answer
The large volume of 0.9% saline, which contains 154 mmol/L of chloride. Switching to a balanced crystalloid such as Hartmann’s or Plasma-Lyte reduces the risk.
After you answerThe teaching pointShow

A normal lactate and a normal anion gap point away from ongoing shock. The acidosis is iatrogenic — but check the lactate again, rather than assuming.

Case 10 — Two problems at once

The patient

On day two after an emergency laparotomy for a perforated duodenal ulcer, a 79-year-old man is breathless and confused. He is on 40% oxygen and his abdomen is distended.

Arterial blood gas
pH7.11
PaCO27.5 kPa (56 mmHg)
HCO317 mmol/L
PaO27.2 kPa on 40%
Lactate5.0 mmol/L
Question 1Interpret the gas: primary disorder, compensation and anything else.Show the model answer
Severe acidaemia with both a high PaCO2 and a low bicarbonate: a mixed respiratory and metabolic acidosis. The PaO2 is 7.2 kPa on 40% oxygen, when about 30 kPa would be expected: type 2 respiratory failure with a severe gas-exchange problem. The lactate shows tissue hypoperfusion.
Question 2What do you do?Show the model answer
This is a peri-arrest gas. Call the critical care team now. He needs respiratory support, resuscitation, and a search for the cause — pneumonia, aspiration, abdominal sepsis or a leak.
After you answerThe teaching pointShow

When both components push the pH the same way, the pH falls very fast. A mixed acidosis in a surgical patient means critical care, not a repeat gas in an hour.

Related pages

GuideABG interpretation, step by stepThe five steps and the formulas.CalculatorABG calculatorCheck any gas step by step.OSCEDry stations27 OSCE stations with model answers.PracticeClerk a patientNine simulated surgical histories.

The patients are written for teaching and are not real.

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