What is the initial diagnostic investigation for a patient with a known history of Chronic Obstructive Pulmonary Disease (COPD) presenting with shortness of breath (SOB), drowsiness, respiratory acidosis (pH 7.25), and hypoxemia (oxygen saturation 82%)?

Initial Diagnostic Investigation for COPD Patient with Acute Respiratory Failure

The initial diagnostic investigation is a chest x-ray (Option B), which should be performed urgently alongside arterial blood gas measurement to assess for precipitating causes of acute decompensation such as pneumonia, pneumothorax, or pulmonary edema. 1

Rationale for Chest X-Ray as Initial Investigation

This patient presents with acute-on-chronic hypercapnic respiratory failure, evidenced by:

• Drowsiness indicating CO2 narcosis 1
• Severe respiratory acidosis (pH 7.25) 1
• Hypoxemia (oxygen saturation 82%) 1

Urgent investigations on admission for COPD exacerbation must always include arterial blood gas measurement (which has already been obtained) and a chest radiograph. 1 The chest x-ray serves multiple critical purposes in this acute setting:

• Identifies treatable precipitants such as pneumonia, pneumothorax, pleural effusion, or pulmonary edema that require specific interventions 1
• Guides immediate management decisions regarding antibiotics, chest tube placement, or diuresis 1
• Establishes baseline for monitoring response to treatment 1

Why Other Options Are Not the Initial Priority

Chest CT (Option A)

• Not indicated as the initial investigation in acute COPD exacerbation 1
• Reserved for specific scenarios such as suspected pulmonary embolism when clinical suspicion is high, or when chest x-ray findings are inconclusive 2
• Delays immediate management and requires patient transport, which is inappropriate in a drowsy, acidotic patient 1

Sputum Culture (Option C)

• Should be sent within the first 24 hours if sputum appears purulent, but is not the initial diagnostic priority 1
• Results take 48-72 hours and do not guide immediate management 1
• Empiric antibiotic therapy should be started based on clinical presentation, not delayed pending culture results 1

CBC (Option D)

• Should be performed within the first 24 hours as part of comprehensive assessment 1
• Does not identify the precipitating cause of acute decompensation 1
• Less urgent than imaging to exclude life-threatening complications 1

Critical Management Context

This patient requires immediate intervention beyond diagnostic testing:

• pH 7.25 indicates severe acidosis requiring urgent consideration for non-invasive ventilation (NIV) 1, 3
• Drowsiness suggests impending respiratory failure and potential need for intubation if NIV fails 1, 3
• Oxygen saturation of 82% requires controlled oxygen therapy targeting 88-92% to avoid worsening hypercapnia 1, 3

The chest x-ray must be obtained urgently but should not delay initiation of NIV, controlled oxygen therapy, bronchodilators, and corticosteroids. 1 A pH below 7.26 is predictive of poor outcome and indicates the need for intensive monitoring, preferably in an ICU or high-dependency unit where intubation is readily available. 1, 3

Common Pitfalls to Avoid

• Delaying chest x-ray to obtain CT scan wastes critical time and may miss immediately treatable conditions 1
• Waiting for culture results before starting antibiotics in a patient with purulent sputum and severe acidosis increases mortality 1
• Over-oxygenation while awaiting investigations can worsen hypercapnia and precipitate further respiratory failure 1, 3