Dead space

Overview
In physiology, dead space is air that is inhaled by the body in breathing, but does not partake in gas exchange.

In adults, it is usually in the range of 150 mL.

Understanding dead space
Not all the air we breathe in is able to be used for the exchange of oxygen and carbon dioxide. About a third of every resting breath is exhaled exactly as it came into the body.

Because of dead space, taking deep breaths more slowly (e.g. ten 500 mL breaths per minute) is more effective than taking shallow breaths quickly (e.g. twenty 250 mL breaths per minute). Although the amount of gas per minute is the same (5 L/min), a large proportion of the shallow breaths is dead space, and does not allow oxygen to get into the blood.

Dead space can be enlarged (and better envisaged) by breathing into a long tube. Even though one end of the tube is open to the air, when one inhales, it is mostly the carbon dioxide from expiration. Using a snorkel increases a diver's dead space in the airways.

Components
Dead space can be divided into two components: "anatomic" and "physiologic".

Anatomical dead space
Anatomical dead space is the gas in the conducting areas of the respiratory system, such as the mouth and trachea, where the air doesn't come to the alveoli of the lungs.

It is normally equal in milliliters to your body weight in pounds. A 150 lb (68 kg) male would have an anatomical dead space of about 150 mL. 1 mL per lb or 2.2 mL per kilogram of body weight. This is the same conversion of kilograms to pounds, except the final unit is in mL. This is about a third of the resting tidal volume (450-500 mL).

Anatomic dead space is the volume of the conducting airways. It may be measured by Fowler's Method, a nitrogen washout technique. It increases with an increase in tidal volume and is dependent on posture.

Physiological dead space
The physiological dead space is equal to the anatomical dead space plus the alveolar dead space.

Alveolar dead space is the area in the alveoli that does get air to be exchanged, but there is not enough blood flowing through the capillaries for exchange to be effective. It is normally very small (less than 5 mL) in healthy individuals. It can increase dramatically in some lung diseases.

Physiologic dead space can be measured by Bohr's method.

An equation and example are provided below:


 * $$ V_\mathrm{D} = V_\mathrm{T} \,\frac{P_\mathrm{a\,CO_2}-P_\mathrm{E\,CO_2}}{P_\mathrm{a\,CO_2}} $$


 * $$ V_\mathrm{D} = 0.5\,\frac{0.056-0.040}{0.056} = 0.143 \ \mathrm{L} $$


 * VD = dead space
 * VT = tidal volume
 * ''PaCO 2 = partial pressure of carbon dioxide in arteries
 * ''PECO 2 = partial pressure of carbon dioxide in exhaled air