Notes on Four Causes of Hypoxemia

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Four causes of hypoxemia are:
• Hypoventilation.
• Diffusion impairment.
• Ventilation-perfusion defect.
• Shunt.

Base = underventilated / overperfused region; high blood flow, smaller alveoli.
Apex = overventilated / underperfused region; low blood flow, larger alveoli.

Alveolar ventilation : blood flow ratio:
• Less than 0.8 (base) = underventilated due to airway blocking; 0 = pulmonary shunt; less gas for more blood.
• Greater than 0.8 (apex) = overventilated due to thrombus blocking pulmonary artery; more gas for less blood; no blood flow; infinity = dead space; increases in exercise.

Normal state: PaCO2 = 40 mm Hg; PaO2 = 100 mm Hg.

Hypoventilation: PCO2 = 80 mm Hg; PaO2 = 60 mm Hg.

Diffusion impairment: A-a exists in defective gas exchange between alveolar and systemic arterial blood; pulmonary capillary PO2 is less than alveolar PO2 due to diffusion impairment.

Pulmonary shunt:
• No gas exchange; eg., atelectasis; leads to hypoxemia.

Shunting of blood in heart:
• ASD: PO2 appears in right atrium.
• VSD: PO2 appears in right ventcricle.
• PDA: PO2 appears in pulmonary artery.

Additional Reading:

Basic Pulmonology

1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia

Related Topics

1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders

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