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.

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)

Recently Added Pages:

Hormonal Signaling Pathways Notes on Heart Muscle Mechanics
Notes on Electrical Activity of the Heart Notes on Heart Sounds and Murmurs
Notes on Diagnostic and Statistical Manual IV (DSM IV) Major Depressive Disorder vs Dysthymic Disorder
How to Stop Feedback Form & Guestbook Spam Pictures of old Kuwaiti Dinars
Body-Mass-Index, Waist-to-Height Ratio, Body Fat, Basal Metabolic Rate Calculator Digestion FAQ, Defecation reflex, etc.
Notes on Basic Gastrointestinal Physiology Inferential Statistics
Notes on Osteogenesis 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 What is Steady State Concentration?
Generalized vs Specialized Transduction 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.