Respiratory Failure Diagnosis and Management

ABG Interpretation Criteria

The British Thoracic Society recommends that Type 1 respiratory failure is defined by PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂, while Type 2 respiratory failure is defined by PaO₂ <8 kPa AND PaCO₂ >6.0 kPa (45 mmHg) 1, 2
The BTS/ICS guidelines specify that PaO₂ <8 kPa (60 mmHg) is equivalent to SpO₂ approximately 90% 1, 3
Normal PaCO₂ levels are considered to be between 4.6-6.1 kPa 2, 3
Type 1 respiratory failure results from failure of oxygenation despite adequate ventilation 1
Type 2 respiratory failure is characterized by PaCO₂ >6.0 kPa (45 mmHg), which is the critical distinguishing feature 2, 4
The BTS/ICS guidelines recommend considering NIV when PaCO₂ is between 6.0-6.5 kPa, but starting NIV when PaCO₂ ≥6.5 kPa with pH <7.35 after optimal medical therapy 4

Acute hypercapnic respiratory failure is defined by pH <7.35 with elevated PaCO₂, indicating insufficient renal compensation 1, 2
Chronic compensated hypercapnic respiratory failure is characterized by normal or near-normal pH despite elevated PaCO₂, due to renal bicarbonate retention over hours to days 2
Acute-on-chronic hypercapnic respiratory failure is defined by elevated PaCO₂ with pH <7.35 despite baseline bicarbonate elevation 2
pH <7.35 (H⁺ >45 nmol/L) defines respiratory acidosis and triggers consideration for NIV 1, 4

The British Thoracic Society recommends targeting SpO₂ 94-98% in most patients with Type 1 failure 5
High-flow nasal oxygen may reduce intubation rates in Type 1 failure, according to the Thorax journal 6
Type 2 failure typically requires targeting SpO₂ 88-92% to avoid worsening hypercapnia 5, 6
Repeat ABG after 1 hour of optimal medical therapy to determine if NIV is needed in Type 2 failure 1, 4
Start NIV when pH <7.35 and PaCO₂ ≥6.5 kPa persist after initial treatment in Type 2 failure 4
Repeat ABG 1-2 hours after starting NIV, then again at 4-6 hours to assess response in Type 2 failure 1, 6