Diagnosis

Respiratory acidosis

Etiology

Causes: [oan][wiy]

• Brain damage: Encephalitis, stroke, tumor, and spinal cord injury

• CNS depressants: Opiate, alcohol, benzodiazepines

• Pulmonary conditions (Dead space ventilation): Acute respiratory distress syndrome (ARDS), bronchitis, bronchiectasis, emphysema, COPD, pulmonary embolism, Interstitial lung disease, atelectasis, pneumonia, congestive heart failure, pulmonary edema

• Chest wall and pleural disorders: Flail chest, kyphoscoliosis, hyperinflation, large pleural effusions, obesity-hypoventilation syndrome (Pick-wick syndrome), and thoracoplasty

• Foreign body

• Neuromuscular transmission: Amyotrophic lateral sclerosis, botulism, Guillain-Barre syndrome, myasthenia graves, organophosphate poisoning, poliomyelitis, spinal cord injury, tetanus, and transverse myelitis

• Muscle abnormalities: Diaphragmatic paralysis, diffuse atrophy, muscular dystrophy, and ruptured diaphragm

Pathophysiology [oan][wiy]

1. The respiratory centers in the pons and medulla control alveolar ventilation

2. PCO2, PO2, and pH regulate the ventilation

3. The respiratory system provides O2 and removes CO2 in this equilibrium:

   CO2 + H2O <--> H2CO3- <--> HCO3- + H+

4. Type 1 respiratory failure: Inadequate supply of oxygen --> hypoxemia

   • Alveolar hypoventilation, low atmospheric pressure/fraction of inspired oxygen, diffusion defect, ventilation/perfusion mismatch, and right-to-left shunt

5. Type 2 respiratory failure: inadequate removal of carbon dioxide --> hypercapnia

   • Respiratory pump failure (reduced respiratory frequency and intensity) and increased carbon dioxide production --> accumulation of CO2 --> decreased bicarbonate (HCO3-) --> lowering of pH (acidosis)

   • Metabolic compensation: kidneys excrete more acid (hydrogen and ammonium) and reabsorb bicarbonate

Epidemiology

Incidence per 100.000 [oan][nym][fzy]

Approach

• Blood test

• Blood gas: elevated PCO2, elevated HCO3- (if chronic condition), and decreased pH

• Capnometry

• Pulse oximetry

• Chest x-ray

• Lung ultrasound

• Urine stix

• CT thorax

• CT caput

Treatment

1. Airway

2. Breathing: Arterial oxygen tension (PaO2) > 60 mmHg or SaO2 > 90% [oan]

   • O2 supplementation if hypoxemia: Avoid O2 toxicity -> CO2 narcosis

   • Ventilatory support if hypercapnia (respiratory acidosis)

   • Indications for Mechanical Ventilation:

     • Apnea with respiratory arrest

     • Respiratory rate >30 breaths per minute

     • Disturbed conscious level or coma

     • Respiratory muscle fatigue

     • Hemodynamic instability

     • Failure of supplemental oxygen to increase PaO2 to 55-60 mmHg

     • Hypercapnia with arterial pH < 7.25

3. Circulation

4. Treat the underlying cause: Bronchodilators, naloxone

Differential diagnoses

Acute respiratory distress syndrome, Asthma, Atelectasis, Botulism, Brain stem injury, Bronchitis, Central sleep apnea, COPD, Cor pulmonale, Diaphragmatic paralysis, Drug side effects, Emphysema, Fat embolism syndrome, Myocardial infarction, Neuromuscular disease, Obesity-hypoventilation syndrome, Obstructive sleep apnea, Pleural effusion, Pneumoconiosis, Pneumonia, Pneumonitis, Pneumothorax, Pulmonary edema, Pulmonary embolism, Pulmonary fibrosis, Pulmonary hypertension, Respiratory failure, Scoliosis, Shock, Spinal cord injury