Respiratory · Easy
This case is fictional and exists for study. It describes no real patient.
67 years · male
A 67-year-old male with a 20-year history of chronic obstructive pulmonary disease (COPD) presents to the emergency department with a 2-day history of worsening shortness of breath, increased cough, and thick green sputum. He relies on a tiotropium inhaler and a combination salmeterol/fluticasone inhaler daily. On assessment, he is tachypnoeic and speaking in short sentences. Chest examination reveals bilateral widespread polyphonic expiratory wheezes and prolonged expiration. An arterial blood gas (ABG) is performed on room air.
Vital signs
- HR
- 104 bpm
- BP
- 134/82 mmHg
- RR
- 26/min
- Temp
- 37.8 °C
- SpO₂
- 88%
Examination findings
- Widespread polyphonic expiratory wheezing bilaterally
- Prolonged expiratory phase
- Use of accessory respiratory muscles
Investigations
| pH (ABG) | 7.31 pH units | reference: 7.35-7.45 |
| PaCO2 (ABG) | 6.6 kPa | reference: 4.7-6.0 |
| PaO2 (ABG) | 7.1 kPa | reference: 11.0-14.4 |
| HCO3- (ABG) | 27 mmol/L | reference: 22-26 |
Question 1 of 3
What is the most likely diagnosis?
The patient exhibits classic features of an acute COPD exacerbation: worsening dyspnoea, increased sputum volume, and sputum purulence (Anthonisen criteria) in a patient with established COPD. Asthma typically presents with a history of atopy and variable airflow obstruction without chronic purulent sputum. Pneumonia usually features focal crackles or consolidation rather than diffuse polyphonic wheezing. Pulmonary embolism causes sudden dyspnoea and chest pain without increased purulent sputum.
Teaching point: An acute exacerbation of COPD is clinically characterized by a sustained worsening of the patient's baseline dyspnoea, cough, or sputum production.
Question 2 of 3
Which of the following best describes the arterial blood gas (ABG) findings?
The pH is reduced (< 7.35), indicating acidosis. The PaCO2 is elevated (> 6.0 kPa), confirming a respiratory cause for the acidosis. The PaO2 is significantly reduced (< 11.0 kPa, severe hypoxia at < 8.0 kPa), indicating type 2 respiratory failure. Two edits, both required. EDIT 1 — vignette.labs, the "PaCO2 (ABG)" entry (line 32): change "value" from "6.6" to "7.4". { "name": "PaCO2 (ABG)", "value": "7.4", "unit": "kPa", "reference": "4.7-6.0" }, EDIT 2 — acuity.drivers[2] (line 56), which re-quotes the old figure and must move with it: "Acute hypercapnic respiratory acidosis (pH 7.31, PaCO2 7.4 kPa)" Nothing else changes. With PaCO2 7.4 kPa the printed pH of 7.31 is exact (6.1 + log10(27 / (0.225 x 7.4)) = 7.310), HCO3- 27 now sits between the acute-expected 25.6 and the fully-chronic-expected 29.4 so "minor chronic retention, but not enough to normalise the pH" becomes true as written, and the questions[1] explanation's other clauses ("PaCO2 is elevated (> 6.0 kPa)", "severe hypoxia at < 8.0 kPa") remain correct. OPTIONAL, separate register fix for this PLAB case (British English), not required for the arithmetic: line 84 "normalize" -> "normalise"; line 72 "characterized" -> "characterised".
Teaching point: A low pH with an elevated PaCO2 on ABG indicates decompensated hypercapnic (type 2) respiratory acidosis.
Question 3 of 3
What is the most appropriate target oxygen saturation range for this patient during initial management?
In patients with exacerbation of COPD at risk of hypercapnia (or with confirmed hypercapnia on ABG), controlled oxygen therapy targeting an SpO2 of 88-92% (often using a Venturi mask) is recommended to prevent worsening hypercapnia from loss of hypoxic drive, ventilation-perfusion mismatching, and the Haldane effect. Targets of 94-98% or higher risk worsening CO2 retention. 80-84% is dangerously low.
Teaching point: Target oxygen saturation in patients with acute COPD exacerbation at risk of hypercapnia is 88-92%.
Key learning points
- An acute exacerbation of COPD is defined by increased breathlessness, cough, and sputum purulence or volume.
- Target oxygen saturation in COPD exacerbations at risk of hypercapnic respiratory failure is 88-92%.
- Arterial blood gases in acute COPD exacerbations typically show decompensated or partially compensated respiratory acidosis with hypoxaemia (Type 2 respiratory failure).
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