Applied Resp – Gases

Master answer

Hypoxaemia

• Hypoxaemia = Acute ↓ PaO₂ = (low partial pressure of oxygen in arterial blood)
• Normal PaO₂ = 75-100mmHg
• Hypoxaemia can lead to tissue hypoxia and subsequent adverse effects

Causes of early post-operative hypoxaemia:

• Low inspired oxygen concentration
  • PAO₂ = PiO₂ – PCO₂/R + F, where PiO₂ = (P_ATM-47mmHg) x FiO₂
  • FiO2 may be insufficient to meet the metabolic demands of the patient
    • may been ↑due to recent surgery
• Hypoventilation
  • VA = (TV – dead space) x RR
  • may be due to:
    • pain
    • respiratory depression 2° drugs (opioids, barbiturates)
    • paralysis of respiratory muscles
    • damage to chest wall
    • high resistance to breathing
  • results in a ↓PaO₂ and a ↑PaCO₂
  • easily reversed by increasing FiO₂
  • along with V/Q scatter are the major causes of post-operative hypoxaemia
• V/Q Ratio Differences
  • V/Q scatter refers to areas of low ventilation compared with perfusion
    • atelectasis
  • V/Q mismatch refers to areas of low perfusion compared with ventilation
    • severe hypotension
    • PE
• Shunt
  • V/Q = 0
  • refers to blood that enters the arterial system without going through ventilated areas of lung
  • cannot be abolished by an ↑FiO₂
  • may be anatomical or physiological
• Increasing oxygen consumption
  • may be due to:
    • pyrexia (fever)
    • shivering
    • hypermetabolic states (thyrotoxicosis, malignant hyperthermia)
• Diffusion barrier
  • very minor role in healthy patients (likely to be present pre-op though)
  • PaO₂ usually very close to PAO2 but never quite equal
  • PaO₂ will decrease with blood-gas barrier thickening as it ↑ the diffusion distance between the alveolar gas and pulmonary capillary blood
    • Fick’s Law of Diffusion
• Increasing Age:
  • the PaO2 ↓ with ↑age from about 20 years of age
  • this is due to closing capacity being greater than FRC
    blood draining from these alveoli has a lower PO2

JC 2020

Answer by JC