What blood gas finding suggests an acute exacerbation of chronic obstructive pulmonary disease (COPD) in a patient with a history of COPD and prior partial pressure of carbon dioxide (PCO2) of 50 mm Hg with a normal pH?

Blood Gas Findings Suggesting Acute COPD Exacerbation

The blood gas showing pH 7.24, PCO2 60 mm Hg, bicarbonate 24 mEq/L indicates an acute exacerbation of COPD in this patient.

Understanding the Clinical Context

This patient has chronic hypercapnia (baseline PCO2 of 50 mm Hg with normal pH), indicating compensated respiratory acidosis where the kidneys have retained bicarbonate over time to maintain a normal pH despite chronically elevated CO2 1. This is the typical baseline state for patients with severe stable COPD 1.

Why the First Option Indicates Acute Exacerbation

The blood gas with pH 7.24, PCO2 60 mm Hg, and bicarbonate 24 mEq/L demonstrates:

• Acute respiratory acidosis (pH < 7.35) 1
• Elevated PCO2 (60 mm Hg, increased from baseline of 50 mm Hg) 1
• Normal bicarbonate (24 mEq/L), which has not had time to compensate for the acute rise in CO2 1

This represents acute-on-chronic respiratory acidosis 1. The bicarbonate level was equilibrated with the previous CO2 level of 50 mm Hg but is insufficient to buffer the sudden further increase to 60 mm Hg that occurred during the acute exacerbation 1.

A pH below 7.26 is predictive of poor outcome and requires aggressive management 1.

Why the Other Options Are Incorrect

Option 2: pH 7.28, PCO2 60 mm Hg, bicarbonate 30 mEq/L

• This shows partial compensation with elevated bicarbonate (30 mEq/L) 1
• While still acidotic, the elevated bicarbonate suggests this has been present long enough for renal compensation to begin 1
• This could represent either a subacute process or chronic stable state with mild decompensation

Option 3: pH 7.38, PCO2 40 mm Hg, bicarbonate 24 mEq/L

• This is completely normal 1
• Does not reflect the patient's known chronic hypercapnia
• Suggests resolution or improvement, not acute exacerbation

Option 4: pH 7.49, PCO2 25 mm Hg, bicarbonate 23 mEq/L

• This shows respiratory alkalosis (pH > 7.45 with low PCO2) 1
• Inconsistent with COPD exacerbation, which causes CO2 retention, not elimination
• Could suggest hyperventilation from another cause (anxiety, pain, pulmonary embolism)

Clinical Implications

Immediate management priorities for pH 7.24 with PCO2 60 mm Hg include:

• Controlled oxygen therapy targeting saturation 88-92% using 28% Venturi mask or 2 L/min nasal cannulae 1, 2, 3
• Recheck blood gases within 60 minutes to ensure oxygen therapy is not worsening acidosis 1
• Nebulized bronchodilators (salbutamol 2.5-5 mg and ipratropium 0.25-0.5 mg) immediately 2, 3
• Systemic corticosteroids (prednisolone 30 mg daily or hydrocortisone 100 mg IV) 2, 3
• Consider non-invasive ventilation if pH remains below 7.26 despite initial medical management 3

Common Pitfall

Do not confuse chronic compensated hypercapnia (high PCO2 with normal pH and elevated bicarbonate) with acute decompensation 1. The key distinguishing feature is the normal bicarbonate level in acute exacerbation, which indicates insufficient time for renal compensation 1.