A blood gas is the fastest test on a surgical ward and the one candidates most often read badly under pressure. The panel wants a method spoken in order: how well oxygenated is the patient for the oxygen they are on, is the pH acidaemic or alkalaemic, is the primary disturbance respiratory or metabolic, is there compensation, and what do the lactate, glucose, potassium and haemoglobin add. In a surgical patient the single most important number is the lactate, because a metabolic acidosis with a raised lactate means tissue hypoperfusion until you have proven otherwise.

How do I assess this patient?

The gas is a stage-two investigation in the CCrISP structure, so the patient comes first. Do the immediate ABCDE management, give oxygen, secure access and take the gas while the fluid is running. The BTS oxygen guideline says which patients need an arterial rather than a venous sample: anyone at risk of hypercapnic respiratory failure, anyone critically ill, and anyone whose saturations do not respond as expected. For a lactate, pH and bicarbonate, a venous gas is usually enough and NG253 asks for a venous gas with lactate in every patient with suspected sepsis.

What are the key investigations and findings?

A five-step read

  • Oxygenation: the PaO2 must be judged against the FiO2. A PaO2 that would be normal on air is abnormal on a reservoir mask
  • pH: acidaemia or alkalaemia, and how severe
  • Primary process: a high PaCO2 with acidaemia is respiratory; a low bicarbonate with acidaemia is metabolic. Look at which one moves in the direction of the pH
  • Compensation: the other system moves in the opposite direction. Full compensation does not return the pH to normal; a normal pH with both values abnormal is a mixed picture
  • The rest: lactate, glucose, potassium, sodium, haemoglobin and base excess. These are often where the diagnosis is

Patterns the panel expects you to recognise

  • Metabolic acidosis with raised lactate: shock, sepsis, ischaemic bowel, a leak. Treat the cause and re-measure
  • Metabolic acidosis with normal lactate: diarrhoea, high-output stoma or fistula, renal failure, or ketoacidosis in a diabetic patient
  • Metabolic alkalosis: vomiting or high nasogastric losses, with hypokalaemia and hypochloraemia
  • Respiratory acidosis: opioid sedation, splinting from pain, a chest wall injury, or an exhausted patient
  • Respiratory alkalosis: pain, anxiety, early sepsis or pulmonary embolism

What is the management?

Treat the patient, not the gas. Give oxygen to the BTS target range of 94 to 98%, or 88 to 92% if the patient has COPD or another risk of hypercapnia, and recheck the gas after 30 to 60 minutes as the guideline advises when you have changed the oxygen. A rising PaCO2 in a sleepy patient means reducing sedation, treating pain properly so they can breathe, and calling for help about ventilatory support. A metabolic acidosis with lactate means fluid resuscitation with a 500 ml crystalloid bolus over less than 15 minutes, a search for the source, and a repeat lactate to prove the trend is the right way.

When do I escalate and to whom?

Escalate to your registrar for any lactate that is raised and not falling with treatment, any respiratory acidosis with a falling conscious level, any pH that is severely deranged, and any gas that does not fit the story. Involve critical care early when the patient may need ventilatory or vasopressor support: a rising PaCO2 despite treatment, refractory hypotension, or a lactate that is climbing. Say that you would take the gas to the person you are calling rather than describe it over the phone.

What is the interviewer listening for?

  • A method said in the same order every time, so the panel can hear it is a habit
  • Oxygenation judged against inspired oxygen, not in isolation
  • Lactate treated as a marker of perfusion that must be re-measured, not a one-off result
  • The link from the gas back to the patient: which surgical cause this pattern suggests
  • Knowing when a venous gas is enough and when arterial sampling is needed

What might they ask next?

  • The lactate is 5 and the patient is day 4 after an anterior resection. What are you thinking?
  • The pH is normal but both the PaCO2 and bicarbonate are abnormal. What does that mean?
  • The patient is on a reservoir mask with a PaO2 of 12. Is that reassuring?
  • When would you give bicarbonate?

The Core Surgery Interview question bank has a scored scenario built around a blood gas result: /Question-Bank/Sample-Questions.