Respiratory failure
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Here is a comprehensive overview of respiratory failure drawn from authoritative pulmonary and critical care textbooks.
Respiratory Failure
Definition
Respiratory failure is characterized by inadequate blood oxygenation or carbon dioxide removal — specifically the failure of the respiratory system to meet tissue requirements for oxygen uptake and CO₂ elimination. Clinically, it is defined by arterial blood gas values. — Fishman's Pulmonary Diseases and Disorders
Classification
Respiratory failure is classified along two axes:
By Gas Exchange Abnormality
| Type | Definition | Key ABG Finding |
|---|---|---|
| Hypoxemic (Type 1) | Failure to oxygenate | PaO₂ < 60 mm Hg on room air |
| Hypercapnic (Type 2) | Failure to ventilate | PaCO₂ > 45–50 mm Hg |
| Postoperative (Type 3) | Atelectasis-driven; subset of Type 1/2 | Occurs post-surgery from pain/sedation |
| Shock-related (Type 4) | Metabolic demand exceeds respiratory capacity | e.g., sepsis, fever |
The two types frequently coexist — disorders initially causing hypoxemia may progress to pump failure and hypercapnia. — Washington Manual of Medical Therapeutics
By Time Course
| Acute | Chronic | |
|---|---|---|
| Hypercapnic | PaCO₂ >45 mmHg + acidemia (pH <7.30); develops in minutes to hours | Long-standing CO₂ retention with renal HCO₃⁻ compensation |
| Hypoxemic | Abrupt change in mental status; develops rapidly | Polycythemia, cor pulmonale as clues |
Pathophysiology
Respiratory failure can arise from any component of the respiratory pump or the alveoli:
Central Nervous System → Peripheral Nerves → Respiratory Muscles/Chest Wall → Airways → Alveoli
Defects in the first four (the "respiratory pump") tend to cause combined hypercapnia and hypoxemia; alveolar disorders initially cause predominantly hypoxemia. — Fishman's Pulmonary Diseases and Disorders
Mechanisms of Hypoxemia (Type 1)
| Mechanism | Examples | A-a Gradient | O₂ Response |
|---|---|---|---|
| V/Q Mismatch | COPD, pneumonia, PE, pulmonary edema | Elevated | Responds to O₂ |
| Shunt | Atelectasis, ARDS, ASD/VSD, pneumonia | Elevated | Minimal/no response |
| Diffusion limitation | Pulmonary fibrosis, pulmonary hypertension | Elevated | Responds to O₂ |
| Hypoventilation | CNS depression, neuromuscular disease | Normal | Responds to O₂ |
| Low inspired O₂ | High altitude | Normal | Responds to O₂ |
Causes of Hypercapnia (Type 2)
Hypercapnia follows the equation: PaCO₂ ∝ VCO₂ / VA — any decrease in alveolar ventilation (VA) or increase in dead space raises PaCO₂.
CNS ("won't breathe"): Opiate overdose, CNS infection, metabolic alkalosis, obesity-hypoventilation syndrome, meningoencephalitis, stroke
Neuromuscular ("can't breathe"): Guillain-Barré syndrome, myasthenia gravis, ALS, muscular dystrophies, spinal cord injury, pharmacologic paralysis
Chest wall/thoracic: Kyphoscoliosis, flail chest, morbid obesity, massive ascites, abdominal distension
Airway obstruction: COPD (most common), acute asthma, cystic fibrosis, epiglottitis, foreign body, tracheal tumor, bronchiolitis obliterans
Alveolar/parenchymal: Severe ARDS, extensive pneumonia, pulmonary edema — through increased dead space and ventilatory demand exceeding supply
Hypermetabolic states (increased CO₂ production): Sepsis, fever, thyrotoxicosis, seizures, serotonin syndrome
Severity Classification (in COPD Exacerbation)
| Severity | RR | Work of Breathing | Mental Status | Hypoxemia | PaCO₂ / pH |
|---|---|---|---|---|---|
| No failure | 20–30/min | Normal | Baseline | Mild, responds to NC | Normal |
| Acute RF | >30/min | Increased, accessory muscles | Baseline | Responds to <35% FiO₂ | 50–60 mmHg, pH >7.25 |
| Severe RF | — | — | Altered | Requires >35% FiO₂ | PaCO₂ >60 mmHg or pH ≤7.25 |
— Rosen's Emergency Medicine
Management Overview
Non-invasive Oxygen Therapy
| Device | FiO₂ Range | Notes |
|---|---|---|
| Nasal cannula | ~24–44% | +4% per L/min; max 6 L/min |
| Simple face mask | 35–55% | Min 5 L/min to flush CO₂ |
| Venturi mask | 24–50% | Precise FiO₂ control |
| Non-rebreather mask | Up to 80% | Reservoir bag with one-way valve |
| HFNC | Up to 100%, up to 60 L/min | Flushes dead space, small PEEP effect; reduces intubation need in hypoxemic RF |
Non-invasive Positive Pressure Ventilation (NPPV)
- CPAP (5–15 cm H₂O): Prevents alveolar collapse; used in pulmonary edema, OSA
- BiPAP: Reduces work of breathing; first-line for COPD exacerbation with acute hypercapnic RF (if cooperative and hemodynamically stable)
- BiPAP starting settings: IPAP 5–10 cm H₂O above EPAP; EPAP 5 cm H₂O
Invasive Mechanical Ventilation
Indicated when:
- Non-invasive methods fail
- Patient cannot protect airway
- Altered mental status
- Hemodynamic instability
- Acute hypercapnic RF with pH ≤7.25
ECMO
- VV ECMO: Severe hypoxemic RF (e.g., refractory ARDS) with intact cardiac function
- VA ECMO: Combined cardiac and respiratory failure
- Reserved for cases where conventional ventilation fails
— Washington Manual of Medical Therapeutics; Sabiston Textbook of Surgery; Murray & Nadel's Textbook of Respiratory Medicine
Key Distinguishing Points
- Shunt is the one cause of hypoxemia that does not respond to supplemental oxygen — a critical clinical discriminator
- Acute hypercapnia raises PaCO₂ abruptly with acidosis; chronic hypercapnia has a compensated (higher) bicarbonate with near-normal pH
- HFNC has shown mortality benefit over standard O₂ in hypoxemic non-hypercapnic RF (90-day mortality improvement)
- Obesity hypoventilation syndrome causes hypercapnia on a central basis — distinct from obstructive sleep apnea alone
ECMO detailed review
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