Respiratory Failure: First Assessment

• Type I (hypoxemic): weigh V/Q mismatch against shunt, hypoventilation, and diffusion; check the A-a gradient.
• Type II (hypercapnic) failure splits into 'won't breathe' (central, NM weakness) versus 'can't breathe' (obstructive, chest wall).
• Escalation follows the ABG and SpO₂/FiO₂ trend: HFNO → NIV (COPD, APE) → intubation.
• Treat the cause and dose oxygen like a drug: target 92–96% (88–92% in COPD retainers).

Etiologies of Hypercapnic Respiratory Failure by Respiratory Pump Component

• Hypercapnic respiratory failure = failure of ≥1 respiratory pump component
• CNS
• Medullary stroke · drug effect (narcotics, sedatives) · central apnea/hypoventilation syndromes · metabolic alkalosis · hypothyroidism · idiopathic (Ondine's curse)
• Anterior horn cell
• Poliomyelitis · ALS/motor neuron disease · cervical spine injury
• Motor nerve
• Critical illness polyneuropathy · Guillain–Barré syndrome · fish toxins, tick paralysis, diphtheria toxin
• Neuromuscular junction
• Botulism · myasthenia gravis · Eaton–Lambert myasthenic syndrome · organophosphate poisoning
• Muscle
• Muscular dystrophy · myopathy (drugs, steroids, infectious, critical illness, hypothyroidism) · polymyositis/dermatomyositis · diaphragmatic dysfunction
• Airways and alveoli
• Pulmonary fibrosis · COPD, asthma, cystic fibrosis · pulmonary edema
• Work of breathing excessive
• Upper airway obstruction · obesity · chest wall disorders, scoliosis · tense ascites, abdominal compartment syndrome · sepsis, metabolic acidosis

General Strategy for Hypoxemic Respiratory Failure

• Hypoxemic respiratory failure — compute the alveolar–arterial gradient
• Alveolar–arterial gradient increased?
• Does PaO₂ rise with supplemental O₂?
• V/Q mismatch
• Airways disease: COPD, asthma, CF, BOS · interstitial lung disease: IPF, sarcoid, NSIP, DIP · pulmonary vascular disease: thromboembolism, fat embolism · alveolar filling (see shunt list)
• Shunt
• Alveolar filling: pulmonary edema, left heart failure, mitral valve disease, ALI/ARDS of any cause, pneumonia, trauma/contusion, alveolar hemorrhage, alveolar proteinosis, drugs (heroin, paraquat), TRALI, acute interstitial pneumonitis, acute eosinophilic pneumonia, BOOP/COP, aspiration, upper airway obstruction, near-drowning · atelectasis: postoperative, immobility · intracardiac shunt: PFO, ASD, VSD · intrapulmonary vascular shunt: pulmonary AVM, hepatopulmonary syndrome
• PaCO₂ elevated?
• Hypoventilation — evaluate respiratory pump failure (→ Pathway 7.1)
• High altitude — inspired PO₂ low