Etiology

• Hypoxemia is caused by one of the following mechanisms:
  • Hypoventilation.
  • Diffusion impairment.
  • Ventilation/perfusion mismatch Ventilation-perfusion mismatching.
  • Shunt.
  • Decreased fraction of inspired oxygen.
  • Hemoglobin abnormalities/deficiencies.

Pathophysiology

• Hypoventilation:
  • Alveolar oxygen pressure (PAO2) decreases, while PACO2 rises.
  • May be caused by drugs (such as narcotics), CNS disease, neuromuscular disease, cervical spinal cord disease, large airway obstruction and respiratory muscle fatigue.
  • Supplemental oxygen should be effective at making up for the reduced flow of inspired gas.
• Diffusion impairment:
  • Thickened alveolar barrier prevents oxygen from reaching red blood cells (RBCs).
  • Ficks law states that the rate of diffusion of a gas is inversely proportional to the thickness of the tissue.
  • May be caused by diseases resulting in severe pulmonary parenchymal pathology such as pneumonia Lung: aspiration pneumonia , inflammatory lung disease, neoplasia Lung: pulmonary neoplasia , pulmonary edema Lung: pulmonary edema , pulmonary fibrosis and intra-alveolar bleeding.
  • Supplemental oxygen should improve arterial oxygen-hemoglobin saturation.
• Ventilation-perfusion mismatch:
  • The most common cause of hypoxemia.
  • A small degree of ventilation-perfusion mismatch Ventilation-perfusion mismatching is present in the normal animal, with blood flow being minimal in the dorsal lung fields in the standing animal.
  • May be caused by any severe pulmonary parenchymal disease, with a classic example being pulmonary thromboembolism Lung: pulmonary thromboembolism.
  • Supplemental oxygen should improve arterial oxygen saturation.
• Shunt:
  • Refers to blood that enters the systemic arterial system without going through ventilated areas of lung.
  • A small degree is present in the normal animal from the bronchial and coronary arterial blood.
  • Most commonly caused by cardiac disorders with right to left shunting such as tetralogy of Fallot Tetralogy of fallot or reverse PDA Patent ductus arteriosus. Some degree of shunting will occur when large areas of the lung are not ventilated due to conditions such as atelectasis or consolidation Lung: atelectasis (such as seen with pneumonia or neoplasia).
  • Shunts typically do not respond to supplemental oxygen therapy, but it may provide mild benefit if the shunt fraction is small.
• Decreased fraction of inspired oxygen:
  • Most commonly occurs at high altitudes or during anesthetic accidents.
  • Responds very well to supplemental oxygen.
• Hemoglobin abnormalities Hematology: hemoglobin concentration :
  • Methemoglobinemia:
    • Normal hemoglobin molecule with ferrous ion oxidized to the ferric form.
    • Unable to transport oxygen and causes shift of the hemoglobin-oxygen dissociation curve to the left (making it more difficult to oxygenate tissue).
    • Clinical signs occur at >20% methemoglobin levels.
    • May be caused by oxidative stress resulting from toxins such as acetaminophen and benzocain or by methemoglobin reductase deficiency.
  • Carboxyhemoglobin:
    • Normal hemoglobin with carbon monoxide bound in place of oxygen.
    • Carbon monoxide binds to hemoglobin with much greater affinity and prevents the carriage of oxygen. It also shifts the hemoglobin-oxygen dissociation curve to the left.
  • Supplemental oxygen will be of no help until the hemoglobin abnormality is corrected.