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Respiratory Failure - Detailed Clinical Review

Definition

• PaO2 < 60 mmHg (hypoxemia), and/or
• PaCO2 > 45 mmHg (hypercapnia) with pH < 7.35 (respiratory acidosis)

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Classification / Types

Type 1 - Hypoxemic Respiratory Failure

• PaO2 < 60 mmHg, SpO2 < 90%
• Usually associated with tachypnea and hypocapnia early; can progress to hypercapnia
• Results from the lung's reduced ability to deliver oxygen across the alveolocapillary membrane
• Most common mechanism: V/Q mismatch and shunt
• ARDS is the prototypical severe form

Type 2 - Hypercapnic (Ventilatory) Respiratory Failure

• PaCO2 > 45 mmHg with pH < 7.35
• Caused by decreased minute ventilation or increased dead space
• Common in COPD, neuromuscular disease, chest wall disorders, obesity hypoventilation
• Normal or only mildly abnormal A-a gradient (unless combined with lung disease)

Type 3 - Postoperative Respiratory Failure

• Develops from atelectasis due to pain, sedatives, or diaphragmatic dysfunction postoperatively
• Essentially a subset of Type 1 or 2 but classified separately because of its prevalence

Type 4 - Respiratory Failure from Shock

• Occurs when metabolic demands exceed the respiratory system's capacity (e.g., sepsis, fever)
• Patients often require intubation to offload respiratory muscles and decrease oxygen consumption

Mixed

• Most clinical respiratory failure involves overlapping mechanisms leading to both hypercapnia and hypoxemia simultaneously.

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Pathophysiology of Hypoxemia (5 Mechanisms)

PAO2 = FiO2 × (Patm - PH2O) - PaCO2/R

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Causes by System

• Pneumonia, ARDS, pulmonary edema (cardiogenic and non-cardiogenic)
• COPD exacerbation, acute severe asthma
• Pneumothorax, pulmonary embolism, pleural effusion
• Interstitial lung disease, flail chest, pulmonary contusion

• Drug/sedative overdose, CNS lesions (brainstem stroke, trauma)
• Guillain-Barre syndrome, myasthenia gravis, ALS, phrenic nerve injury
• Cervical spinal cord injury

• Foreign body, severe laryngospasm, epiglottitis, tracheal obstruction

• Sepsis (Type 4), severe metabolic alkalosis (causing hypoventilation), morbid obesity, hypothyroidism

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Clinical Features

Symptoms

• Dyspnea (most common), orthopnea, inability to speak full sentences
• Altered mental status (agitation, confusion, somnolence - CO2 narcosis)
• Fatigue, use of accessory muscles

Signs - Grading (Rosen's Emergency Medicine classification)

Physical Exam Findings

• Tachypnea, tachycardia
• Intercostal/supraclavicular/subcostal retractions
• Pursed-lip breathing (especially COPD)
• Paradoxical abdominal movement (respiratory muscle fatigue)
• Cyanosis (central > peripheral)
• Loud wheeze/stridor (obstructive)
• Dullness to percussion + decreased breath sounds (effusion/consolidation)
• Absent breath sounds (pneumothorax)

Dynamic Hyperinflation (in obstructive disease)

• Inhalation begins before complete exhalation (truncated expiratory time)
• Increases end-expiratory lung volume → alveolar overdistension → decreased compliance
• Elevated work of breathing → respiratory muscle fatigue → hypercapnia → respiratory failure

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Investigations

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Management

Step 1 - Identify and Treat Underlying Cause

• Antibiotics for pneumonia, bronchodilators for COPD/asthma, diuretics for pulmonary edema, anticoagulation for PE, chest tube for pneumothorax.

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Step 2 - Oxygen Supplementation

• Reversal of hypoxic pulmonary vasoconstriction (worsens V/Q mismatch)
• Haldane effect (shifts CO2 dissociation curve rightward, increases PaCO2)

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Step 3 - High-Flow Nasal Cannula (HFNC)

• Delivers humidified, heated oxygen at 30-60 L/min
• Creates low-level CPAP effect (approximately 0.35-1 cmH2O per 10 L/min flow)
• Washes out nasopharyngeal dead space, reduces work of breathing
• First-line in hypoxemic (Type 1) respiratory failure - particularly effective in pneumonia and post-extubation
• Less effective in pure hypercapnic failure
• Increasingly used in COPD as well (reduces RR, improves comfort)

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Step 4 - Non-Invasive Ventilation (NIV)

• Respiratory acidosis: PaCO2 ≥45 mmHg AND pH ≤7.35
• Severe dyspnea with accessory muscle use and fatigue
• Persistent hypoxemia despite supplemental O2 therapy

• Respiratory/cardiac arrest
• Active vomiting / high aspiration risk
• Facial trauma
• Depressed mental status unrelated to hypercapnia
• Hemodynamic instability
• Inability to protect airway / clear secretions

• IPAP: 12-16 cmH2O (titrate up for CO2 reduction)
• EPAP: 4-6 cmH2O (enough to overcome intrinsic PEEP)
• Backup RR: 14-16 breaths/min
• FiO2: titrate to SpO2 88-92%

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Step 5 - Invasive Mechanical Ventilation (IMV)

• NIV failure or intolerance
• Persistent diminished consciousness
• Respiratory or cardiac arrest
• Hemodynamic instability despite resuscitation
• Life-threatening hypoxemia not corrected by less invasive means
• Inability to protect airway or clear secretions

1. Position patient in "sniffing position" (neck flexed, head extended); ramp for obese patients
2. Preoxygenate with 100% O2 via BVM to SpO2 >95% for 3-5 minutes
3. Administer IV sedation + paralytic (RSI)
4. Insert laryngoscope, visualize cords
5. Advance ETT: 21 cm at teeth for women, 22 cm for men; inflate cuff
6. Confirm with end-tidal CO2 colorimetry + auscultation + CXR

• Tidal volume: 6 mL/kg ideal body weight (ARDSnet protocol)
• Plateau pressure: < 30 cmH2O
• PEEP: Titrated up (5-15+ cmH2O) to recruit collapsed alveoli
• FiO2: Minimize to achieve SpO2 88-95%
• Permissive hypercapnia is acceptable

• Lower RR, longer expiratory time (I:E ratio 1:3 to 1:4)
• Avoid air trapping / auto-PEEP
• PEEP set at ~80% of auto-PEEP to reduce triggering effort

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Step 6 - Rescue Therapies for Severe ARDS

• Prone positioning (>12-16 hours/day) - improves V/Q matching and mortality in severe ARDS (PaO2/FiO2 < 150)
• Neuromuscular blockade (cisatracurium infusion) - reduces ventilator dyssynchrony and patient self-inflicted lung injury in early severe ARDS
• Inhaled NO / prostacyclin - selective pulmonary vasodilators, improve oxygenation transiently
• VV-ECMO (Veno-Venous Extracorporeal Membrane Oxygenation) - for refractory severe ARDS when conventional MV fails; provides gas exchange while allowing "lung rest"
  • Indications: PaO2/FiO2 < 80 despite optimal MV, pH < 7.15 with optimized ventilation
  • Provides oxygenation and CO2 removal independent of lung function

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ARDS Severity (Berlin Definition)

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Weaning from Mechanical Ventilation

• FiO2 ≤ 40%, PEEP ≤ 5-8 cmH2O
• Hemodynamically stable, no vasopressor escalation
• No active sedation/paralysis
• Rapid Shallow Breathing Index (RSBI = RR/TV in L) < 105 predicts successful extubation
• Pass SBT for 30-120 minutes → consider extubation

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Summary Algorithm

Respiratory failure suspected?
        ↓
ABG + SpO2 + CXR + identify cause
        ↓
Mild hypoxemia (SpO2 88-94%)?
   → Supplemental O2 (NC / Venturi)
        ↓
Type 1 (hypoxemic), moderate-severe?
   → HFNC 40-60 L/min
   → If worsening → BiPAP (if no contraindication)
        ↓
Type 2 (hypercapnic) / COPD / NMD?
   → BiPAP (IPAP 12-16, EPAP 4-6)
        ↓
NIV failure / cardiac arrest / altered MS?
   → RSI + Endotracheal Intubation
   → Lung-protective ventilation
        ↓
Severe ARDS (PaO2/FiO2 <150)?
   → Prone positioning + NMB
   → If refractory → ECMO referral

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Key Clinical Pearls

1. Don't give high-flow O2 blindly in COPD - target SpO2 88-92%, not 98-100%, to avoid worsening hypercapnia.
2. Early NIV in COPD exacerbation with pH ≤7.35 + PaCO2 ≥45 mmHg reduces mortality and intubation rates.
3. HFNC before NIV in pure hypoxemic failure (pneumonia/post-extubation) - better tolerated, similar efficacy.
4. Lung-protective ventilation (TV 6 mL/kg IBW) is mandatory in ARDS - reduces mortality.
5. Prone positioning >12 hours/day reduces 28-day mortality in severe ARDS by ~17% (PROSEVA trial).
6. RSBI < 105 is the standard threshold for predicting successful extubation.
7. Dynamic hyperinflation (auto-PEEP) in obstructed patients requires longer expiratory times and lower RR on the ventilator.
8. In end-stage CF or chronic progressive respiratory failure, mechanical ventilation is often not beneficial unless there is an acute reversible precipitant (e.g., viral pneumonia).

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