Notes on Four Causes of Hypoxemia
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Notes on Pulmonology
Notes on Four Causes of Hypoxemia
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.
Pulmonology Equations:
There are four variables we need to understand: PaO2, PaCO2, PAO2, and PACO2.
A-a gradient = PAO2 - PaO2.
A = alveolar; a = arterial.
PAO2 = [FiO2 x (PB - PH2O)] - (PaCO2/R).
FiO2 = Fraction of inspired air that is O2 (normal = 0.21).
R = Ratio of CO2 production to O2 consumption.
PB = Barometric pressure (normal = 760 mm Hg at sea level).
PH2O = Water vapor pressure (normal = 47 mm Hg at sea level).
The above equation can be simplified as:
PAO2 = 150 - (PaCO2/0.8).
Additional Reading:
Basic Pulmonology
1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia
5. Control of Respiration
Pulmonology Videos
1. Video of Pulmonology Examination in a Clinical Setting
Related Topics
1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders
3. Pulmonary Examination for Internal Medicine
Medical Images
Useful Medical Images & Diagrams (link opens in a new window)
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