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Respiratory Failure

  • Respiratory failure – when pulmonary gas exchange fails to maintain normal arterial O2 + CO2 levels

Pathophysiology

Type I Respiratory Failure – V/Q mismatch

  • When disease impairs ventilation of part of a lung – e.g. pneumonia/asthma
    • Perfusion of that region results in hypoxic and CO2-laden blood entering the pulmonary veins
  • Increased ventilation of neighbouring regions of normal lung can ↑CO2 excretion, but cannot augment oxygen uptake because the Hb is already fully saturated
  • Admixture of blood from the underventilated and normal region results in hypoxia with normocapnia
  • Common causes – pneumonia, acute lung injury, cardiogenic pulmonary edema, lung fibrosis

Type II Respiratory Failure – alveolar hypoventilation ± V/Q mismatch (mechanical failure)

  • Caused by conditions that cause alveolar hypoventilation which is insufficient to remove CO2
  • This leads to arterial hypoxia with hypercapnia
  • Common causes – COPD, Chest wall deformities , respiratory muscle depression ( GBS ), drug overdose

Chronic Respiratory Failure

  • COPD is MCC
  • Unless there is an acute illness, it remains compensated
    • Despite ↑PaCO2, there is no acidosis as kidneys retain bicarbonate
  • An acute illness (e.g. exacerbation of COPD) can precipitate acute on chronic respiratory failure
    • With acidosis + respiratory disease – can progress to coma
  • Patients with chronic respiratory failure have lost their sensitivity to ↑PaCO2
    • So they depend on hypoxia for respiratory drive
    • These pts are at risk of respiratory depression if given ↑[O2]

Clinical features

Symptoms of hypoxia

  • Dyspnea, restlessness, confusion, central cyanosis
  • Chronic hypoxia – polycythemia, pulmonary HTN, cor pulmonale

Symptoms of hypercapnia

  • Headache, tachycardia, tremor, Papilloedema , confusion, coma

Assessment

  • Accessory muscle use, tachycardia, Paradoxical respiration , sweating
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