What lab values on an arterial blood gas (ABG) indicate respiratory acidosis?

Respiratory Acidosis on Arterial Blood Gas (ABG)

Respiratory acidosis on ABG is indicated by a pH < 7.35 with PaCO2 > 6.0 kPa (45 mmHg), reflecting impaired ventilation and carbon dioxide retention. 1, 2

Key ABG Parameters for Respiratory Acidosis

• pH < 7.35: Indicates acidosis (normal range: 7.35-7.45) 1
• PaCO2 > 6.0 kPa (45 mmHg): Elevated carbon dioxide level indicating hypoventilation 1
• Normal or elevated HCO3-: May be normal in acute respiratory acidosis or elevated in chronic/compensated cases 1

Types of Respiratory Acidosis

Acute Respiratory Acidosis

• pH < 7.35 with elevated PaCO2 > 6.0 kPa 1
• Minimal or no bicarbonate compensation 1
• Often accompanied by tachypnea and tachycardia 1

Chronic (Compensated) Respiratory Acidosis

• PaCO2 remains elevated > 6.0 kPa 1
• pH may normalize or approach normal range due to renal bicarbonate retention 1
• Elevated bicarbonate (HCO3-) level reflecting renal compensation 1

Acute-on-Chronic Respiratory Acidosis

• pH < 7.35 despite elevated bicarbonate 1
• Further acute rise in PaCO2 beyond chronic baseline 1
• Insufficient compensation for the additional CO2 retention 1

Clinical Significance and Management

• When pH < 7.35 and PaCO2 > 6.5 kPa with respiratory rate > 23 breaths/min persisting after optimal medical therapy, NIV should be considered 1
• For PaCO2 between 6.0-6.5 kPa, NIV may be considered but is less urgent 1
• Target oxygen saturation of 88-92% is recommended for patients at risk of hypercapnic respiratory failure 1

Common Pitfalls in Interpretation

• Failing to recognize mixed acid-base disorders (e.g., concurrent metabolic acidosis) 3
• Not calculating anion gap when evaluating respiratory acidosis 3
• Overlooking the possibility of compensatory mechanisms 4
• Misinterpreting partially compensated states 5

Systematic Approach to ABG Interpretation

1. Assess pH to determine if acidosis (pH < 7.35) or alkalosis (pH > 7.45) is present 2
2. Evaluate PaCO2 to determine respiratory component 2
3. Check HCO3- to assess metabolic component 2
4. Determine if compensation is occurring 5
5. Assess oxygenation parameters (PaO2, SaO2) 2

Remember that respiratory acidosis reflects impaired ventilation, which can occur in various conditions including COPD exacerbations, neuromuscular disorders, chest wall deformities, and respiratory depression 1.