Respiratory acidosis

see also hypercapnia

Background

• Acidemia = pH < 7.35
• Respiratory acidosis = PaCO2 > 42
• Acute respiratory acidosis: Change in pH = 0.008 X (40 - PaCO2)
  • For every 10 mmHg increase in PaCO2, HCO3- should increase by 1 mEq/L
• Chronic respiratory acidosis: Change in pH = 0.003 X (40 - PaCO2)
  • For every 10 mmHg increase in PaCO2, HCO3- should increase by 4 mEq/L
• Determine if another primary acid/base disturbance is occurring
  • Calculate AG
  • if HCO3- < 24 + (pCO2-40)/10 x 3(+/-1) then there is a superimposed primary metabolic acidosis
    • for every 10mm increase in pCO2 >40, HCO3expected increases by 2-4mEq (2 if acute/limited time for metabolic compensation, 4 if chronic i.e. COPD)
  • if HCO3- > 24 + (pCO2-40)/10 x 3(+/-1) then suspect primary metabolic alkalosis

Etiology

• Hypoventilation - acute vs chronic

Differential Diagnosis

• Respiratory disorders
  • COPD
  • Pneumonia
  • ARDS
  • Pulmonary edema
  • Pulmonary fibrosis
  • Trauma
• Central respiratory depression
  • Central sleep apnea
  • Drug overdose (opiates, benzodiazepines)
  • Trauma
  • Stroke
  • Status epilepticus
• Airway obstruction
  • Obstruction sleep apnea
  • Foreign body aspiration
  • Tumor
  • Bronchospasm
• Neuromuscular dysfunction
  • Guillain-Barre syndrome
  • Myasthenia gravis
  • Brainstem or spinal cord injury

Management

Improve alveolar ventilation

1. Bronchodilators
2. CPAP
3. Intubation (esp of pH < 7.25)
   • Do not reduce pH too quickly (>5Hg/h)
     • Can lead to abrupt Hypocalcemia/Hypokalemia

See Also

• Acid-base disorders