Dead space (physiology)

Blood gas, acid-base, & gas exchange terms

PaO2	Arterial oxygen tension, or partial pressure
PAO2	Alveolar oxygen tension, or partial pressure
PACO2	Arterial carbon dioxide tension, or partial pressure
PaCO2	Alveolar carbon dioxide tension, or partial pressure
PvO2	Oxygen tension of mixed venous blood
P(A-a)O2	Alveolar-arterial oxygen tension difference. The term formerly used (A-a DO2) is discouraged.
P(a/A)O2	Alveolar-arterial tension ratio; PaO2:PAO2 The term oxygen exchange index describes this ratio.
C(a-v)O2	Arteriovenous oxygen content difference
SaO2	Oxygen saturation of the hemoglobin of arterial blood
SpO2	Oxygen saturation as measured by pulse oximetry
CaO2	Oxygen content of arterial blood
pH	Symbol relating the hydrogen ion concentration or activity of a solution to that of a standard solution; approximately equal to the negative logarithm of the hydrogen ion concentration. pH is an indicator of the relative acidity or alkalinity of a solution
v ·		It has been suggested that Deadspace (in breathing apparatus) be merged into this article or section. (Discuss) Proposed since November 2010.

In physiology, dead space is air that is inhaled by the body in breathing, but does not take part in gas exchange. Not all the air in each breath is able to be used for the exchange of oxygen and carbon dioxide. About a third of every resting breath has no change in O₂ and CO₂ levels. In adults, it is usually in the range of 150 mL.^[1]

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

Total dead space (also known as "physiological" dead space) can be divided into anatomical dead space and alveolar dead space.

Anatomical dead space

Anatomical dead space is the gas in the conducting areas of the respiratory system, such as the mouth and trachea, where air does not come into contact with the alveoli of the lungs. Birds, which, relatively, have a far longer and wider trachea than mammals, have a higher proportion of dead space.

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.^[2][3][4]

Alveolar dead space

Alveolar dead space is caused by air contacting alveoli without bloodflow in their adjacent pulmonary capillaries, i.e. ventilation without perfusion. As a result, no gas exchange can occur.^[5] Alveolar dead space is negligible in healthy individuals, but can increase dramatically in some lung diseases.

Calculating the dead space

Overview

Anatomical and alveolar dead space can both be measured using the Bohr equation.^[6][7] Formally, Bohr's method is used to calculate the former. In practice, it is more commonly used to calculate the latter.

The Bohr equation states that the dead space (V_d) is calculated as follows:

$\frac{\mathbf{V}_{\mathrm{d}}}{\mathbf{V}_{\mathrm{t}}} = \frac{{\mathrm{P}_{\mathrm{a}}{\mathrm{CO}}_{\mathrm{2}}} - {\mathrm{P}_{\mathrm{e}}{\mathrm{CO}}_{\mathrm{2}}}}{\mathrm{P}_{\mathrm{a}}{\mathrm{CO}}_{\mathrm{2}}},$

where V_d is dead space volume, V_t is tidal volume, P_aCO₂ is the partial pressure of carbon dioxide in the arterial blood, and P_eCO₂ is the partial pressure of carbon dioxide in the expired air.

Depending on how the expired CO₂ is measured, this equation gives the physiological or alveolar dead space. When calculating the:

• physiological dead space, a large plastic bag (that is, a Douglas bag) is used to collect all of the patients expired gas, and the total expired CO₂ is measured as a fraction of total expired gas. This gives P_eCO₂, which is then substituted into the Bohr equation.
• alveolar dead space, the end-tidal CO₂ in capnography is used as a surrogate for the expired CO₂. P_etCO₂ is then used instead of P_eCO₂ in the Bohr equation.

Example

If a patient's tidal volume is 500 mL, their arterial carbon dioxide (PaCO₂) is 42 mmHg (5.6 kPa), and their end-expired carbon dioxide (ETCO₂) on capnography is 40 mmHg (5.3 kPa), the alveolar dead space can be calculated as follows:

$\frac{\mathbf{V}_{\mathrm{d}}}{\mathbf{V}_{\mathrm{t}}} = \frac{{\mathrm{P}_{\mathrm{a}}{\mathrm{CO}}_{\mathrm{2}}} - {\mathrm{P}_{\mathrm{e}}{\mathrm{CO}}_{\mathrm{2}}}}{\mathrm{P}_{\mathrm{a}}{\mathrm{CO}}_{\mathrm{2}}},$

$\mathbf{V}_{\mathrm{d}} = \frac{ 500ml (42mmHg - 40mmHg) }{ 42mmHg } = 24 ml$

See also

• Bohr equation
• Christian Bohr
• Respiratory physiology
• Ventilation (physiology)

References

1. ^ Wasted Ventilation
2. ^ http://www.med.mun.ca/surgery/presentations/LungVentilation/LungVentilation.htm#AnatomicDeadSpace
3. ^ Physiology at MCG 4/4ch3/s4ch3_17
4. ^ Heller H, Könen-Bergmann M, Schuster K (1999). "An algebraic solution to dead space determination according to Fowler's graphical method". Comput Biomed Res 32 (2): 161–7. doi:10.1006/cbmr.1998.1504. PMID 10337497. 
5. ^ Physiology at MCG 4/4ch3/s4ch3_20
6. ^ Physiology at MCG 4/4ch3/s4ch3_18
7. ^ Klocke R (2006). "Dead space: simplicity to complexity". J Appl Physiol 100 (1): 1–2. doi:10.1152/classicessays.00037.2005. PMID 16357075.  article

External links

• Overview at jhmi.edu
• Overview at ccmtutorials.com