Respiratory Acidosis - Complete MD Exam Notes (10 Marks)

Definition

• Primary disturbance: ↑ PaCO2
• Direction of change: pH ↓, PaCO2 ↑, HCO3- ↑ (compensatory)

Pathophysiology

ABG Profile

Causes (ETIOLOGY)

A. Central Nervous System / Respiratory Centre Depression

• Opiates, barbiturates, benzodiazepines, anesthetics
• CNS lesions (stroke, trauma, tumors, encephalitis)
• Central sleep apnea
• Excessive oxygen therapy in COPD (removes hypoxic drive)
• Alcohol intoxication

B. Neuromuscular Disorders

• Guillain-Barré syndrome
• Poliomyelitis, Amyotrophic Lateral Sclerosis (ALS)
• Multiple sclerosis
• Myasthenia gravis
• Muscular dystrophies
• Electrolyte disorders (hypokalemia, hypophosphatemia)

C. Chest Wall / Pleural Disorders

• Kyphoscoliosis (severe)
• Flail chest
• Massive pleural effusion, pneumothorax
• Pain from thoracic/abdominal incisions (splinting)

D. Airway Obstruction

• Aspiration of foreign body
• Obstructive sleep apnea (OSA)
• Laryngospasm, severe bronchospasm (status asthmaticus)

E. Parenchymal Lung Disease

• COPD (most common chronic cause)
• Severe pneumonia
• Pulmonary edema / ARDS
• End-stage interstitial lung disease

F. Miscellaneous

• Obesity-hypoventilation syndrome (Pickwickian syndrome)
• Permissive hypercapnia (intentional strategy in mechanical ventilation)
• Abdominal compartment syndrome, massive ascites (↓ diaphragmatic excursion)
• Improper mechanical ventilator settings

Compensation Mechanisms

1. Immediate: Chemical Buffering

• CO2 diffuses into cells (especially RBCs)
• Within ICF: CO2 → H⁺ + HCO3⁻; H⁺ buffered by intracellular proteins (hemoglobin) and organic phosphates
• This raises serum HCO3⁻ slightly

2. No Respiratory Compensation

• Since the primary problem IS the respiratory system, no respiratory compensation is possible

3. Renal Compensation (Delayed: kicks in after 24-48 hours, complete by 3-5 days)

• Kidneys increase H⁺ excretion (as NH4⁺ and titratable acid)
• Increased HCO3⁻ synthesis and reabsorption in the proximal tubule
• Collecting duct type A intercalated cells upregulate H⁺-ATPase and H⁺/K⁺-ATPase
• Similar mechanism to renal response in metabolic acidosis

Compensatory Formulas (KEY EXAM POINTS)

• Acute: pH falls by ~0.08 per 10 mmHg rise in PaCO2
• Chronic: pH falls by ~0.03 per 10 mmHg rise in PaCO2 (less fall due to renal compensation)

Acute vs. Chronic Respiratory Acidosis

Clinical Features

A. CNS Effects (CO2 narcosis / Hypercapnic encephalopathy)

• Anxiety, dyspnea, confusion, psychosis, hallucinations
• Headache (CO2 is a vasodilator - increases cerebral blood flow)
• Papilledema (mimics raised intracranial pressure)
• Tremor, myoclonic jerks, asterixis
• Somnolence → coma (CO2 narcosis)
• Abnormal reflexes, focal muscle weakness

• Sleep disturbances, loss of memory
• Daytime somnolence, personality changes
• Poor coordination

B. Cardiovascular Effects

• Peripheral vasodilation (CO2-mediated)
• Increased cardiac output initially
• Arrhythmias (especially with severe acidemia)
• Decreased myocardial contractility in severe cases

C. Respiratory Signs

• Dyspnea, tachypnea or bradypnea (depending on cause)
• Cyanosis (if hypoxemia present)
• Use of accessory muscles

D. Other Signs

• Warm, flushed skin (peripheral vasodilation)
• Bounding pulse
• Signs of underlying disease (barrel chest in COPD, obesity in Pickwickian)

Diagnosis

1. Arterial Blood Gas (ABG): pH ↓, PaCO2 ↑, HCO3⁻ ↑
2. Step-by-step approach:
   • Step 1: Check pH → acidemia
   • Step 2: PaCO2 and pH move in opposite directions → primary respiratory disorder (↑ PaCO2 with ↓ pH = respiratory acidosis)
   • Step 3: Check HCO3⁻ - is compensation appropriate?
   • Step 4: If compensation is greater than expected → superimposed metabolic alkalosis; if less → superimposed metabolic acidosis
3. Identify cause:
   • Pulmonary function tests (spirometry, diffusion capacity, lung volumes)
   • CXR, CT chest
   • Drug history
   • Neuromuscular assessment
   • Polysomnography (if OSA suspected)

Treatment

Acute Respiratory Acidosis

1. Treat the underlying cause (most important)
2. Restore adequate alveolar ventilation - this is the definitive treatment
   • Non-invasive ventilation: BiPAP/CPAP (for COPD, OSA, neuromuscular disease)
   • Endotracheal intubation + mechanical ventilation for severe cases
3. Oxygen therapy - use cautiously in chronic COPD with CO2 retention; excessive O2 removes hypoxic drive and worsens hypercapnia
4. Permissive hypercapnia: In ARDS, intentionally allowing PaCO2 to rise to avoid barotrauma from high tidal volumes; bicarbonate infusion may be given if there is also superimposed metabolic acidosis
5. Avoid rapid correction of chronic hypercapnia - may precipitate cardiac arrhythmias, reduced cerebral perfusion, and seizures (same complications as acute respiratory alkalosis)
6. Bicarbonate: Generally NOT given for pure respiratory acidosis; may be considered only in mixed metabolic + respiratory acidosis, with the goal of keeping pH acceptable, not normalizing it

Chronic Respiratory Acidosis

• Treat underlying lung disease (bronchodilators, steroids in COPD)
• Long-term non-invasive ventilation (NIV/BiPAP)
• Pulmonary rehabilitation
• Gradually lower PaCO2 to baseline levels; provide adequate Cl⁻ and K⁺ to allow renal HCO3⁻ excretion
• Avoid diuretics that cause HCO3⁻ retention (e.g., loop diuretics can cause "post-hypercapnic metabolic alkalosis")

Mixed Disorders Involving Respiratory Acidosis

Important Exam Mnemonics

• Sedatives/CNS depressants
• Thoracic cage disorders (kyphoscoliosis, flail chest)
• Obstruction (airway)
• Pulmonary parenchymal disease (COPD, ARDS, pneumonia)
• Brain/brainstem lesions
• Respiratory muscle disorders (NMJ, motor neuron)
• Electrolyte disorders (↓K, ↓Mg, ↓PO4)
• Abdominal (obesity, distension, compartment syndrome)
• Traumatic (head injury)
• Hypoventilation (primary alveolar, Pickwickian)
• Intubation problems (mechanical ventilator malfunction)
• Narcotics/opioids
• Greater O2 delivery in COPD (excessive)

Summary Table for Quick Revision

• Brenner & Rector's The Kidney (p. 707)
• Harrison's Principles of Internal Medicine 22E (p. 419)
• Costanzo Physiology 7th Ed. (p. 339)
• Sabiston Textbook of Surgery
• Schwartz's Principles of Surgery, 11th Ed.
• Murray & Nadel's Textbook of Respiratory Medicine
• Symptom to Diagnosis, 4th Ed.