Notes on Four Causes of Hypoxemia

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Four causes of hypoxemia are:

• Hypoventilation.

• Diffusion impairment.

• Ventilation-perfusion defect.

• Shunt.


• Underventilated / overperfused region; high blood flow, smaller alveoli.


• 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.


• 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
6. Systemic vs Pulmonary Circulation FAQ
7. Principles of Gas Exchange in Lungs
8. Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve
9. FAQ on Mechanics of Breathing
10. FAQ on Control of Breathing
11. Criteria for Transudate Pleural Effusion
12. Light's Criteria for Exudate Pleural Effusion
13. Notes on Lung Sounds
14. Patient with Acute Respiratory Distress Syndrome (ARDS)
15. Management of Acute Deep Venous Thrombosis
16. Notes on Asthma Treatment

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

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