PHS 3342 Lecture Notes - Lecture 15: Methemoglobin, Fraction Of Inspired Oxygen, Atelectasis
1 May 2018
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March 23, 2018
Respiratory Review
10 Causes of Hypoxia
Hypoxia: inadequate oxygen delivery to the tissues
1. O2 deficit in inspired air (high altitude or low FiO2)
•Accompanied by hypoxemia (reduced paO2), pAO2 is low in this case
•Oxygen isn’t getting to the alveoli because of low availability in the air
2. Alveolar hypoventilation
•Accompanied by hypoxemia (reduced paO2), pAO2 is low in this case
•Oxygen isn’t getting to the alveoli because of poor ventilation
a) Won’t breathe (defective neuronal stimulus)!
Ex. Head trauma, drugs
b) Can’t breathe (defective equipment)!
Ex. Muscular incapacity, restrictive disease, airway resistance (asthmatic crisis)
3. V/Q mismatch (atelectasis)
•Accompanied by hypoxemia (reduced paO2), pAO2 is normal in this case
•Alveoli are under ventilated relative to the volume of pulmonary blood
4. Problems with diffusion
•Ex. Pulmonary fibrosis, emphysema, hypersensitivity, pneumonitis
•Oxygen is not diffusing into the blood because of damage to the alveolar membranes or increased diffusion
distances from alveolus to blood
5. Right —> left shunt (cardiovascular congenital defects)
•Accompanied by hypoxemia (reduced paO2), pAO2 is normal in this case
•Oxygenated blood leaving the lungs is being mixed with deoxygenated blood in the left atrium
6. Hypoxia of transport (anemia, CO poisoning, Met-Hb)
•No hypoxemia (normal paO2)
•Oxygen is diffusing successfully to the blood but there is lack of available hemoglobin (anemia) to carry O2 or
presence of increased levels of dyshemoglobins (Hb that can’t carry oxygen, such as carboxylmoglobin (HB
found to CO) or methemoglobin (Hb with oxidized Fe3+ in the heme group))
7. Ischemic hypoxia
•No hypoxemia (normal paO2)
•Blood is oxygenated properly but the heart can’t circulate the blood effectively
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