Dead space (breathing)

The dead space is the space in the breathing system that is not involved in pulmonary gas exchange, but serves to convey the inspired gas mixture. The dead space serves some fundamentally important conditioning processes during inspiration (inhalation). The inhaled air in the dead space is heated to a physiological temperature, while the inspired air is saturated with water so that a relative humidity of 100% is achieved. There are two different types of dead spaces:

Anatomical dead space

The anatomical dead space includes the nose or mouth , throat , trachea and bronchi . About 30% (approx. 0.15 l) of the inspired air remains in them per breath. The anatomical dead space can also be enlarged by opening the airway e.g. B. is extended by a snorkel or tube . In the case of a tracheostoma after laryngectomy, however, the dead space volume is reduced.

Functional dead space (physiological dead space)

Functional or physiological dead space is the sum of the anatomical dead space (see above) and the alveolar dead space (regions of the lungs that are not supplied with blood or are no longer able to exchange oxygen with the blood due to damage - and vice versa from CO ₂ to the exhaled air - to participate). In healthy people, functional and anatomical dead space largely coincide, since normally all areas of the lungs participate in gas exchange. Classically, a pulmonary embolism, for example, leads to an enlargement of the functional dead space because lung sections behind the occlusion (pulmonary artery or branches) are still ventilated, but not perfused.

Size of dead space

The size of the dead space can be calculated using the Bohr formula:

${\displaystyle V_{\mathrm {D} }=V_{\mathrm {T} }\,{\frac {P_{\mathrm {A\,CO_{2}} }-P_{\mathrm {E\,CO_{2}} }}{P_{\mathrm {A\,CO_{2}} }}}}$

Denote therein

• ${\displaystyle P_{\mathrm {A\,CO_{2}} }}$the partial pressure of in the alveoli, ${\displaystyle CO_{2}}$
• ${\displaystyle P_{\mathrm {E\,CO_{2}} }}$the partial pressure of in the exhaled air, ${\displaystyle CO_{2}}$
• ${\displaystyle V_{\mathrm {T} }}$ the tidal volume, i.e. the volume stroke per breath, for an average person this value is around 0.5 l, as well as with
• ${\displaystyle V_{\mathrm {D} }}$ the volume of the dead space (English dead space).

Example calculation:

${\displaystyle V_{\mathrm {D} }=0{,}5\,\mathrm {l} \cdot {\frac {0{,}056-0{,}040}{0{,}056}}=0{,}143\,\mathrm {l} }$

The ratio is called the dead space fraction. ${\displaystyle V_{\mathrm {D} }:V_{\mathrm {T} }}$

Dead space when diving

When diving, the dead space is increased by equipment such as the snorkel or the second stage of the regulator . With rapid and shallow breathing, there is therefore the risk of breathing in the air that has already been exhaled, which is located in this enlarged dead space and has an increased proportion of carbon dioxide. It is therefore recommended to breathe slowly and deeply, as a result of which the dead air proportion of each breath and the energy consumption expended for breathing are reduced.

literature

• Thomas Pasch, S. Krayer, HR Brunner: Definition and measurands of acute respiratory insufficiency: ventilation, gas exchange, breathing mechanics. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , pp. 93-108; here: p. 97.

Individual evidence

1. ^ P. Lotz: Symbols, Terminology, Formulas. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , p. 425 f.
2. ↑ PADI (Ed.): Open Water Diver Manual . 3.00.02 edition. 2015, ISBN 978-1-61381-963-0 , pp. 86 .