Type 1 Respiratory Failure (Hypoxemic Respiratory Failure) 

Definition 
Type 1 respiratory failure is characterized by low arterial oxygen tension (PaO₂ < 8KPa or 60 mmHg) with normal or low carbon dioxide (PaCO₂ ≤ 6KPa or 45 mmHg). This is also called hypoxemic respiratory failure. 

Key point: Type I RF is hypoxemia without hypercapnia. 

Pathophysiology 

Type 1 respiratory failure usually occurs when oxygen transfer from the alveoli to the blood is impaired. This can be due to: 

Ventilation-perfusion (V/Q) mismatch 
Most common cause. 
Areas of the lung are perfused but not ventilated adequately. 
Examples: pneumonia, pulmonary oedema, pulmonary embolism. 

Shunt 
Blood bypasses ventilated alveoli. 
Examples: severe pneumonia, ARDS, congenital heart disease with right-to-left shunt. 

Diffusion impairment 
Rare; occurs when oxygen cannot cross the alveolar-capillary membrane efficiently. 
Examples: interstitial lung disease, pulmonary fibrosis. 

Low inspired oxygen (hypoxemia due to low FiO₂) 
High altitude or hypoventilation in certain scenarios. 

Why CO₂ is normal or low: CO₂ diffuses ~20 times more easily than O₂, so unless there is severe ventilation failure, CO₂ may remain normal or even decrease due to compensatory hyperventilation. 

Causes 
Some common causes include: 

Pulmonary causes: 
Pneumonia 
Pulmonary oedema (cardiogenic or non-cardiogenic, e.g., ARDS) 
Pulmonary embolism 
Interstitial lung disease 
Pneumothorax 

Extra-pulmonary causes: 
High altitude 
Severe anaemia (rare, mainly affects O₂ content/carrying capacity) 

Clinical Features 

Patients may present with: 
Dyspnoea (shortness of breath) 
Tachypnoea (fast respiratory rate) 
Cyanosis (late sign) 
Restlessness, confusion, or agitation (due to hypoxia) 
Use of accessory muscles 
Important: Patients may not appear hypercapnic because PaCO₂ is usually normal or low. 

Investigations 

Arterial Blood Gas (ABG) 
PaO₂ < 8KPa or 60 mmHg 
PaCO₂ ≤ 6KPa or 45 mmHg 
pH usually normal or slightly alkalotic (from hyperventilation) 

Chest X-ray / CT scan 
To identify underlying cause: consolidation, pulmonary oedema, embolism. 

Pulse oximetry 
Low oxygen saturation (SpO₂ < 90%) 

Other labs 
FBC, electrolytes, renal function, markers of infection (if relevant) 

Management 

Goal: Correct hypoxemia and treat the underlying cause. 

Oxygen therapy 
Nasal cannula, face mask, or high-flow oxygen. 
Target SpO₂: 92–96% (avoid hyperoxia in certain cases like COPD) but remember hypoxia kills before hypercarbia…. 

Treat underlying cause 

Antibiotics for pneumonia 
Diuretics for pulmonary oedema 
Anticoagulation for pulmonary embolism 

Mechanical ventilation 

If hypoxemia is severe or not responsive to oxygen. 
Usually non-invasive ventilation (NIV) or invasive ventilation depending on cause. 
 
Summary Table 


 
Key Points: 

Type 1 RF = Hypoxemia without hypercapnia. 
Caused by V/Q mismatch, shunt, or diffusion defects. 
Hyperventilation often keeps PaCO₂ normal or low. 
Treatment: Oxygen first, then underlying cause.