Respiratory failure and arterial blood gas interpretation

After this session, you will be able to define respiratory failure, classify its types, identify common causes, and understand the basic pathophysiology. Always check drug doses and local protocols against the Kenya MoH guidelines and your hospital formulary.

Definition

Respiratory failure is a clinical state where the respiratory system fails to maintain adequate arterial oxygenation, with or without carbon dioxide elimination. This leads to impaired gas exchange, resulting in hypoxemia (PaO2 < 8 kPa or 60 mmHg) and often hypercapnia (PaCO2 > 6.0 kPa or 45 mmHg) at sea level, breathing ambient air.

Anatomy and physiology you need

Effective gas exchange relies on:

1. Patent airways: From nose/mouth to alveoli.
2. Intact respiratory muscles: Diaphragm, intercostals, accessory muscles for ventilation.
3. Functional central respiratory drive: Brainstem control of breathing.
4. Healthy lung parenchyma: Alveoli and surrounding capillaries for diffusion.
5. Adequate pulmonary blood flow: Perfusion of the alveolar-capillary membrane.
6. Intact pleura: For lung expansion.

Oxygen (O2) moves from alveoli to blood down a partial pressure gradient. Carbon dioxide (CO2) moves from blood to alveoli down its partial pressure gradient. Ventilation (V) is the air reaching the alveoli, and perfusion (Q) is the blood flow through pulmonary capillaries. Optimal gas exchange requires a matched V/Q ratio.

Causes and risk factors

Pathophysiology

Type 1 Respiratory Failure (Hypoxemic) V/Q mismatch → Areas of lung are perfused but not ventilated (shunt) OR ventilated but not perfused (dead space) → Inefficient gas exchange → PaO2 falls. Diffusion limitation → Thickening of alveolar-capillary membrane (e.g., fibrosis) → Slower O2 transfer → PaO2 falls. Right-to-left shunt (e.g., intracardiac shunt) → Deoxygenated blood bypasses lungs → PaO2 falls. Hypoventilation (if severe enough) → Reduced alveolar O2 → PaO2 falls (and PaCO2 rises).

Type 2 Respiratory Failure (Hypercapnic) Decreased ventilatory drive (e.g., opioids, brainstem injury) → Reduced respiratory rate/tidal volume → Alveolar hypoventilation → CO2 retention → PaCO2 rises. Respiratory muscle weakness/fatigue (e.g., GBS, myasthenia gravis) → Ineffective breathing → Alveolar hypoventilation → CO2 retention → PaCO2 rises. Increased airway resistance (e.g., severe asthma, COPD) → Increased work of breathing → Muscle fatigue → Alveolar hypoventilation → CO2 retention → PaCO2 rises. Chest wall abnormalities (e.g., kyphoscoliosis) → Restrict lung expansion → Alveolar hypoventilation → CO2 retention → PaCO2 rises.

Clinical features

History questions that matter:

• Onset and duration of symptoms: Acute (minutes to hours) vs. chronic (days to weeks) helps differentiate acute exacerbation from chronic disease, and acute respiratory failure from chronic.
• Dyspnoea: Severity (e.g., at rest, on exertion), character (e.g., gasping, shallow), associated factors (e.g., position, activity). This is the cardinal symptom of respiratory failure.
• Cough and sputum: Productive vs. dry, colour, quantity, consistency. Suggests underlying infection or inflammatory process.
• Chest pain: Pleuritic (sharp, worse on inspiration) suggests pleural involvement (e.g., pneumonia, PE), central (heavy, crushing) suggests cardiac ischaemia.
• Wheezing/stridor: Wheezing suggests lower airway obstruction (e.g., asthma, COPD), stridor suggests upper airway obstruction (e.g., foreign body, epiglottitis).
• Fever/chills: Suggests infection (e.g., pneumonia, sepsis).
• Fatigue/weakness: General malaise often accompanies severe illness.
• Altered mental status: Confusion, drowsiness, agitation. Key sign of hypercapnia (CO2 narcosis) or severe hypoxemia.
• Past medical history: Pre-existing lung disease (COPD, asthma, ILD), cardiac disease (HF), neurological conditions (myasthenia gravis, GBS), renal disease, diabetes. These predispose to respiratory failure.
• Medications: Current medications, recent changes, adherence. Some drugs can cause respiratory depression or lung injury.
• Smoking history: Quantify pack-years. Major risk factor for COPD, lung cancer.
• Occupational/environmental exposures: Dusts, chemicals. Can cause occupational lung diseases.
• Travel history: Exposure to endemic infections.

Examination findings and why each occurs:

• General appearance:
  • Distress: Obvious struggle to breathe, use of accessory muscles. Indicates increased work of breathing.
  • Cyanosis: Bluish discoloration of lips, tongue, nail beds. Sign of severe hypoxemia (deoxyhaemoglobin >5g/dL).
  • Altered mental status: Confusion, agitation, somnolence. Due to hypoxemia or hypercapnia (CO2 narcosis).
  • Diaphoresis: Sweating. Sign of sympathetic activation due to stress or hypercapnia.
• Respiratory system:
  • Respiratory rate: Tachypnoea (>20 breaths/min). Early sign of respiratory distress, body's attempt to compensate for hypoxemia or acidosis. Bradypnoea (<12 breaths/min). May indicate severe respiratory muscle fatigue or CNS depression (e.g., CO2 narcosis).
  • Work of breathing:
    • Accessory muscle use: Sternocleidomastoid, scalene, intercostal retractions. Indicates increased effort to breathe.
    • Nasal flaring: Widening of nostrils during inspiration. Common in children, indicates respiratory distress.
    • Pursed-lip breathing: Exhaling through pursed lips. Increases positive end-expiratory pressure, preventing airway collapse in obstructive disease.
    • Paradoxical abdominal breathing: Abdomen moves inward during inspiration. Sign of diaphragmatic fatigue.
  • Auscultation:
    • Wheezes: High-pitched whistling sounds. Airway narrowing (e.g., asthma, COPD).
    • Crackles (rales): Discontinuous popping sounds. Fluid in alveoli (e.g., pulmonary oedema, pneumonia) or interstitial fibrosis.
    • Rhonchi: Low-pitched, coarse sounds. Secretions in large airways.
    • Diminished breath sounds: Airflow obstruction, pleural effusion, pneumothorax.
  • Percussion:
    • Dullness: Consolidation (pneumonia), pleural effusion.
    • Hyper-resonance: Pneumothorax, severe emphysema.
• Cardiovascular system:
  • Heart rate: Tachycardia. Sympathetic response to hypoxemia, hypercapnia, or stress.
  • Blood pressure: Hypertension (early) or hypotension (late). Early: sympathetic response. Late: severe hypoxemia, acidosis, or shock.
  • Jugular venous pressure (JVP) elevation: Right heart strain, fluid overload (e.g., cor pulmonale from chronic lung disease, heart failure).
• Neurological system:
  • Asterixis (flapping tremor): Seen in severe hypercapnia.
  • Papilloedema: Rare, but can occur with chronic severe hypercapnia due to cerebral vasodilation and increased intracranial pressure.

Differential diagnosis

Investigations

Diagnosis and severity

The diagnosis of respiratory failure is primarily made by Arterial Blood Gas (ABG) analysis, supported by clinical features and other investigations.

Diagnosis:

• Type 1 Respiratory Failure (Hypoxemic): PaO2 < 8 kPa (60 mmHg) with normal or low PaCO2.
• Type 2 Respiratory Failure (Hypercapnic): PaCO2 > 6.0 kPa (45 mmHg) with or without hypoxemia (PaO2 < 8 kPa).

Severity:

Severity is assessed by the degree of hypoxemia and hypercapnia, the patient's clinical presentation, and the presence of organ dysfunction.

• Mild: Hypoxemia responsive to low-flow oxygen, no significant hypercapnia, patient alert and able to protect airway.
• Moderate: Significant hypoxemia requiring higher oxygen concentrations, mild to moderate hypercapnia, increased work of breathing, mild altered mental status.
• Severe/Life-threatening: Profound hypoxemia despite high-flow oxygen, severe hypercapnia, severe acidosis (pH < 7.25), marked increased work of breathing, exhaustion, significant altered mental status (e.g., somnolence, coma), haemodynamic instability.

Clinical signs of severity include:

• Increasing respiratory rate (>30/min)
• Accessory muscle use, paradoxical breathing
• Inability to speak in full sentences
• Tachycardia (>120/min)
• Hypotension
• Cyanosis
• Altered mental status (confusion, agitation, drowsiness, coma)
• Exhaustion or impending respiratory arrest

Management

Immediate Stabilisation (ABCs):

1. Airway: Ensure patent. Suction secretions if needed. Consider airway adjuncts (oropharyngeal/nasopharyngeal airway).
2. Breathing:
   • Oxygen Therapy: Crucial for Type 1 and Type 2 RF.
     • Targets:
       • General: SpO2 94-98%.
       • COPD/Risk of Type 2 RF: SpO2 88-92%. Start low flow (e.g., nasal prongs 1-2 L/min) and titrate up carefully.
     • Delivery devices: Nasal prongs (1-6 L/min), simple face mask (5-10 L/min), non-rebreather mask (10-15 L/min), Venturi mask (precise FiO2).
   • Ventilatory Support:
     • Non-Invasive Ventilation (NIV): CPAP (for Type 1, e.g., cardiogenic pulmonary oedema) or BiPAP (for Type 2, e.g., COPD exacerbation, asthma, chest wall deformities).
       • Indications: Acute exacerbation of COPD with respiratory acidosis (pH < 7.35, PaCO2 > 6 kPa), cardiogenic pulmonary oedema, asthma, immunocompromised patients with RF, post-extubation failure.
       • Contraindications: Cardiac arrest, severe encephalopathy/coma, inability to protect airway, severe facial trauma/burns, upper GI bleed, haemodynamic instability, pneumothorax (untreated).
     • Invasive Mechanical Ventilation (IMV): Endotracheal intubation and mechanical ventilation.
       • Indications: Failure of NIV, severe hypoxemia despite maximal oxygen, severe respiratory acidosis (pH < 7.25), apnoea, altered mental status with inability to protect airway, haemodynamic instability, severe work of breathing with impending exhaustion.
3. Circulation: Assess and support haemodynamics. IV access, fluid resuscitation if indicated, vasopressors if hypotensive.

Definitive Treatment: Address the underlying cause (e.g., bronchodilators for asthma/COPD, antibiotics for pneumonia, diuretics for pulmonary oedema, thrombolysis for PE).

Monitoring: Continuous SpO2, heart rate, respiratory rate, blood pressure. Serial ABGs. Level of consciousness.

Contraindications and Safety:

• Avoid excessive oxygen in COPD to prevent worsening hypercapnia (loss of hypoxic drive).
• NIV requires patient cooperation and an intact airway reflex.
• Careful monitoring for complications of IMV (e.g., ventilator-associated pneumonia, barotrauma).

Complications and Prognosis

Complications:

• Acute: Cardiac arrhythmias, multi-organ failure (renal, hepatic), ventilator-associated pneumonia, barotrauma (pneumothorax), deep vein thrombosis, pulmonary embolism, critical illness polyneuropathy/myopathy.
• Chronic: Persistent disability, reduced quality of life, chronic respiratory failure requiring long-term oxygen or ventilatory support.

Prognosis: Varies widely depending on the underlying cause, severity of respiratory failure, presence of comorbidities, and timely initiation of appropriate treatment. Acute severe respiratory failure carries a high mortality rate.

High-yield Exam Points

• Oxygen in COPD: Target SpO2 88-92%. Higher oxygen can suppress hypoxic drive, leading to worsening hypercapnia and acidosis.
• Compensation vs. Mixed Disorders:
  • Compensation: Body's physiological response to restore pH. In respiratory acidosis, kidneys retain HCO3-. In respiratory alkalosis, kidneys excrete HCO3-. In metabolic acidosis, lungs hyperventilate (↓PaCO2). In metabolic alkalosis, lungs hypoventilate (↑PaCO2).
  • Mixed Disorders: When two or more primary acid-base disturbances coexist (e.g., metabolic acidosis and respiratory acidosis). Look for unexpected pH, PaCO2, or HCO3- values given the primary disorder.
• Lactate: Elevated in Type A lactic acidosis (tissue hypoperfusion/hypoxia) and Type B (e.g., sepsis, liver failure, certain drugs). Important marker of tissue oxygenation and severity of illness.
• When to Intubate: Failure of NIV, severe hypoxemia despite maximal oxygen, severe acidosis (pH < 7.25), apnoea, GCS < 8, inability to protect airway, haemodynamic instability, severe work of breathing with exhaustion.

Clinical Case

A 68-year-old male with a history of COPD presents to the ER with increasing shortness of breath, productive cough, and confusion for 2 days. On examination, he is drowsy, tachypnoeic (RR 30/min), using accessory muscles, and has widespread expiratory wheezes. SpO2 is 85% on room air.

Questions:

1. What type of respiratory failure is this patient likely experiencing?
2. What is the initial oxygen target for this patient, and why?
3. Assuming an ABG shows pH 7.28, PaCO2 8.5 kPa, PaO2 6.0 kPa, HCO3- 28 mmol/L, what is the most appropriate next step in ventilatory support?

Model Answers:

1. This patient is likely experiencing Type 2 respiratory failure (hypercapnic respiratory failure) due to an acute exacerbation of COPD, evidenced by his history, confusion (suggesting CO2 narcosis), and likely hypoxemia.
2. The initial oxygen target for this patient is SpO2 88-92%. This is to prevent worsening hypercapnia by avoiding suppression of the hypoxic drive, which is the primary respiratory stimulus in chronic CO2 retainers.
3. Given the ABG results (respiratory acidosis with significant hypercapnia and hypoxemia) and the patient's clinical presentation (drowsiness, increased work of breathing), the most appropriate next step is to initiate Non-Invasive Ventilation (NIV), specifically BiPAP, after controlled oxygen therapy. This aims to reduce the work of breathing, improve ventilation, and correct the acidosis without the risks of intubation.

Practice Questions

Question 1

A 55-year-old male with no significant past medical history presents with sudden onset severe shortness of breath. His SpO2 is 88% on room air. An ABG shows pH 7.48, PaCO2 4.0 kPa, PaO2 7.0 kPa, HCO3- 24 mmol/L. Which of the following is the most likely primary acid-base disturbance?

A. Metabolic acidosis B. Respiratory alkalosis C. Metabolic alkalosis D. Respiratory acidosis

Answer: B. Respiratory alkalosis Explanation: The pH is elevated (alkalosis) and the PaCO2 is low, indicating a primary respiratory alkalosis. The PaO2 is low, suggesting hypoxemia is driving the hyperventilation.

Question 2

A 70-year-old patient with a history of severe COPD is admitted with an acute exacerbation. His SpO2 is 86% on room air. Which of the following is the most appropriate initial oxygen therapy strategy?

A. High-flow nasal cannula at 15 L/min to achieve SpO2 > 95% B. Nasal prongs at 1-2 L/min, titrating to achieve SpO2 88-92% C. Non-rebreather mask at 10 L/min to rapidly correct hypoxemia D. Venturi mask with 60% FiO2 to ensure adequate oxygen delivery

Answer: B. Nasal prongs at 1-2 L/min, titrating to achieve SpO2 88-92% Explanation: In COPD patients at risk of Type 2 respiratory failure, oxygen should be administered cautiously at low flow rates, targeting an SpO2 of 88-92% to avoid suppressing the hypoxic drive and worsening hypercapnia.

Question 3

Which of the following ABG findings is most consistent with uncompensated Type 2 respiratory failure?

A. pH 7.50, PaCO2 3.0 kPa, PaO2 9.0 kPa, HCO3- 24 mmol/L B. pH 7.25, PaCO2 8.0 kPa, PaO2 6.0 kPa, HCO3- 24 mmol/L C. pH 7.35, PaCO2 7.0 kPa, PaO2 7.5 kPa, HCO3- 32 mmol/L D. pH 7.15, PaCO2 4.0 kPa, PaO2 8.0 kPa, HCO3- 10 mmol/L

Answer: B. pH 7.25, PaCO2 8.0 kPa, PaO2 6.0 kPa, HCO3- 24 mmol/L Explanation: This ABG shows acidosis (low pH), hypercapnia (high PaCO2), hypoxemia (low PaO2), and a normal bicarbonate, indicating an acute, uncompensated respiratory acidosis (Type 2 RF).

Question 4

A patient with severe community-acquired pneumonia develops increasing work of breathing and hypoxemia despite maximal oxygen via a non-rebreather mask. An ABG shows pH 7.20, PaCO2 6.5 kPa, PaO2 5.0 kPa, HCO3- 20 mmol/L. The patient is drowsy and unable to protect their airway. What is the most appropriate next step in management?

A. Initiate Non-Invasive Ventilation (NIV) B. Increase oxygen flow rate to 15 L/min via a Venturi mask C. Prepare for endotracheal intubation and mechanical ventilation D. Administer a bronchodilator nebulizer

Answer: C. Prepare for endotracheal intubation and mechanical ventilation Explanation: Despite the presence of respiratory acidosis and hypoxemia, the patient's inability to protect their airway and severe drowsiness are absolute contraindications to NIV and strong indications for invasive mechanical ventilation.

Question 5

Which of the following conditions is an absolute contraindication to initiating Non-Invasive Ventilation (NIV)?

A. Acute exacerbation of COPD with pH 7.30 B. Cardiogenic pulmonary oedema with SpO2 88% C. Severe facial trauma with active bleeding D. Immunocompromised patient with acute hypoxemic respiratory failure

Answer: C. Severe facial trauma with active bleeding Explanation: Severe facial trauma with active bleeding compromises the seal of the NIV mask and can worsen airway issues, making it an absolute contraindication. The other options are common indications for NIV.

Drafted with AI assistance from standard references and reviewed for structure. Check details against your lecture notes and the Kenya MoH guideline.