Notes on Four Causes of Hypoxemia

  >   Rahul's Noteblog   >   Notes on Pulmonology   >   Notes on Four Causes of Hypoxemia

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

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Random Pages:

What is Time? Pictures of old Kuwaiti Dinars
Video of me playing Unknown Easy Blues Piano Notes on Female Reproductive System
Notes on Osteology of Pectoral Region Notes on Chlamydia
Notes on Energy Metabolism Notes on Hypothesis Testing
Significance of Glycosylated Hemoblogin (HbA1c) Notes on Functions of the Liver
Notes on Basic Gastrointestinal Physiology One Powerful Prayer written by an Anonymous Author
Pictures of old Kuwaiti Dinars What is an ELEK`s Test?
Why did I decide to become a doctor? Medical School Admissions Essay Video: Titanic Piano Theme: The Portrait
Corporate Failure: The Enron Case My Experience during the Iraqi Invasion of Kuwait
USMLE Blood Lab Values Regulation of Heart Rate by Autonomic Nervous System
Images of Antibodies Video of me playing Hagood Hardy`s "The Homecoming"
Notes on Renal/Urinary System Differentiation and Anatomy of a Blastocyst
Notes on Cell Components Notes on Nervous Tissue
Voices from Hell: My Experience in Mussoorie, India Video of Cardiology Examination in a Clinical Setting

Please Do Not Reproduce This Page

This page is written by Rahul Gladwin. Please do not duplicate the contents of this page in whole or part, in any form, without prior written permission.