Arterial Blood Gas OSCE Station Exam

Clinical Chemistry
by Medical Educator Clinical Chemistry

Arterial blood gases, also known as ABGs, are laboratory tests used to evaluate a patient’s blood oxygen and carbon dioxide levels, as well as their acid-base balance.

These tests provide important information about the function of a patient’s respiratory and circulatory systems and are crucial in diagnosing and managing respiratory disorders such as asthma, emphysema, and chronic obstructive pulmonary disease (COPD).

If you want to gain a deeper understanding of ABGs and how they are used in clinical practice, then try our exam paper below. It is focused on one patient scenario and will test your knowledge of this topic, helping you expand your understanding.

Table of Contents

Question 1

A 18 year old man presents to A&E after failing his A level results

He complains of light headedness and dizzyness and the following ABG is taken.

pH7.497.35-7.45
Pa O222>10.6
Pa CO22.44.7-6
HCO32-2720-28
Base excess-1.6+/- 2

What is the abnormality seen on the arterial blood gas?

  • Metabolic acidosis
  • Metabolic Alkalosis
  • Respiratory Acidosis
  • Respiratory Alkalosis
  • Compensated respiratory acidosis
  • Compensated metabolic acidosis
  • Partially compensated respiratory acidosis
  • Partially compensated metabolic acidosis

Respiratory Alkalosis

Whats the pH: alkalotic therefore it must be an alkalosis.

Whats the CO2? Low.

Is the CO2 in keeping with the pH: yes so it’s the primary cause not compensation.

I.e. it’s a respiratory alkalosis

Question 2

Continued from previous question. What is the most likely diagnosis?

  • None of the listed diagnoses
  • Alcohol intoxicatoin
  • Diabetic ketoacidosis
  • Deliberate Overdose of Aspirin
  • Illicit Drug use
  • Vomiting
  • Acute renal failure
  • Sepsis
  • Diarrhoea
  • Neuromuscular failure
  • Acute Asthma

None of the listed diagnoses

The most likely cause is hyperventilation. There is no mention of wheeze to suggest asthma. You would not expect such a high PO2 in an acute asthma attack. The treatment here: breathe into brown paper bag.

Question 3

A 22 year old man presents to A&E after failing his driving test.

He feels non specifically unwell. An ABG is taken.

pH7.367.35-7.45
Pa O217>10.6
Pa CO22.54.7-6
HCO32-1620-28
Base excess-7+/- 2

What is the most likely diagnosis?

  • Metabolic acidosis
  • Metabolic Alkalosis
  • Respiratory Acidosis
  • Respiratory Alkalosis
  • Compensated respiratory acidosis
  • Compensated metabolic acidosis
  • Partially compensated respiratory acidosis
  • Partially compensated metabolic acidosis

Compensated metabolic acidosis.

The PH is normal. So it is not an acidosis, an alkalosis or a partially compensated problem. So its either: a compensated problem, a mixed respiratory acidosis and metabolic alkalosis (2 separate disease processes) or a nomal ABG!

Whats the CO2? Low: its therefore respiratory compensation

Whats the HCO3? Low: Implying an metabolic acidosis

Diagnosis? Compensated metabolic acidosis.

Question 4

Continued from the previous question.

A 22 year old man presents to A&E after failing his driving test.

He tells the staff he feels “just unwell”. An ABG is taken]His ABG shows a compensated metabolic acidosis.

His mother is now in the ED and tells the admitting doctor that he was “fine” first thing this morning.

What is the most likely diagnosis?

  • None of the listed diagnoses
  • Alcohol intoxicatoin
  • Diabetic ketoacidosis
  • Deliberate Overdose of Aspirin
  • Illicit Drug use
  • Vomiting
  • Acute renal failure
  • Sepsis
  • Diarrhoea
  • Neuromuscular failure
  • Acute Asthma

None of the listed diagnoses, Deliberate Overdose of Aspirin

A metabolic acidosis in a fit and well man who’s just failed a driving test with non specific symptoms?

Aspirin overdose is the most likely: Salicylic acid will drop the pH nicely.

But careful when answering any other answer: He has no symptoms of any of the other problems: think metabolic.

The only other plausible one would be illicit drug use: but it wouldn’t specifically give you a metabolic acidosis in this situation i.e. the answer does not fit as well.

REMEMBER IT’S ALWAYS THE “BEST FIT” THAT S THE BEST ANSWER!

Question 5

A 17 year old female presents to A&E feeling non specifically unwell.

Observations.

Weight 52 Kg

Height 1.78m

BM 6.5

BP 102/64

Sats 97% Air

ECG normal

An ABG is taken (shown below)

pH7.487.35-7.45
Pa O212>10.6
Pa CO25.54.7-6
HCO32-3420-28
Base excess+1+/- 2

What is the abnormality seen on the arterial blood gas?

  • Metabolic acidosis
  • Metabolic Alkalosis
  • Respiratory Acidosis
  • Respiratory Alkalosis
  • Compensated respiratory acidosis
  • Compensated metabolic acidosis
  • Partially compensated respiratory acidosis
  • Partially compensated metabolic acidosis

Metabolic Alkalosis

Question 6

Continued from the previous question.

A 17 year old female presents to A&E feeling non specifically unwell.

Observations.

Weight 52 Kg

Height 1.78m

BM 6.5

BP 102/64

Sats 97% Air

ECG normal

An ABG is taken (shown below)

pH7.487.35-7.45
Pa O212>10.6
Pa CO25.54.7-6
HCO32-3420-28
Base excess+1+/- 2

What is the most likely cause for the above change?

  • None of the listed diagnoses
  • Alcohol intoxicatoin
  • Diabetic ketoacidosis
  • Deliberate Overdose of Aspirin
  • Illicit Drug use
  • Vomiting
  • Acute renal failure
  • Sepsis
  • Diarrhoea
  • Neuromuscular failure
  • Acute Asthma

None of the listed diagnoses, Vomiting

The only things for exam purposes that do this are:

Vomiting
Burns
Ingestion of alkali
Some drug treatments.

Her BMI is low (look at the observations). This may support a diagnosis of an eating disorder. However you haven’t got much history this could equally be because of hyperemisis gravidarum etc.