PULMONARY PHYSIOLOGY-HYPOXEMIA CAUSES

SUMMARY

1. Ventilation/Perfusion (V/Q) mismatch: due to inadequate perfusion of ventilated areas or inadequate ventilation of perfumed areas; asthma, COPD, pneumonia, ILD, pulmonary vascular diseases

2. Right-to-left shunting: due to alveolar collapse or intra-alveolar filling; ARDS, atelectasis, intra-cardiac shunt, vascular shunt

3. Hypoventilation: due to decreased tidal volumes, low RR, apnoea; the A-a gradient is normal

4. Impaired O₂ diffusion: decreased DLCO; low DLCO occurs with ILD, emphysema, pulmonary vascular disease & anemia; increased DLCO occurs with alveolar haemorrhage, polycythemia

5. Reduced O₂ tension: as occurs at high altitudes, leading to low P_b

6. Low mixed venous O₂: can induce hypoxic vasoconstriction, increasing right-to-left baseline anatomical shunting; caused by low CO or increased tissue extraction of O₂

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].