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Dead space is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused. It means that not all the air in each
breath Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cellular ...
is available for the exchange of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
.
Mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s breathe in and out of their lungs, wasting that part of the inhalation which remains in the conducting airways where no gas exchange can occur.


Components

''Total dead space'' (also known as physiological dead space) is the sum of the anatomical dead space and the alveolar dead space. Benefits do accrue to a seemingly wasteful design for ventilation that includes dead space. #Carbon dioxide is retained, making a bicarbonate-buffered blood and interstitium possible. #Inspired air is brought to body temperature, increasing the affinity of
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
for
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, improving O2 uptake. #Particulate matter is trapped on the mucus that lines the conducting airways, allowing its removal by mucociliary transport. #Inspired air is humidified, improving the quality of airway mucus. In humans, 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. Dead space can be increased (and better envisioned) by breathing through a long tube, such as a snorkel. Although one end of the snorkel is open to the air, when the wearer breathes in, they inhale a significant quantity of air that remained in the snorkel from the previous exhalation. Therefore, a snorkel increases the person's dead space by adding even more airway that does not participate in gas exchange.


Anatomical dead space

Anatomical dead space is the volume of the conducting airways (from the
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...
,
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
and
trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...
to the terminal bronchioles). These conduct gas to the
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * ...
but no gas exchange occurs here. In healthy lungs where the alveolar dead space is small,
Fowler's method Nitrogen washout (or Fowler's method) is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways. Procedure A nitrogen washout can be performed with a single n ...
accurately measures the anatomic dead space using a single breath
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
washout technique. The normal value for dead space volume (in mL) is approximately the lean mass of the body (in pounds), and averages about a third of the resting
tidal volume Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass. Mechanical vent ...
(450-500 mL). In Fowler's original study, the anatomic dead space was 156 ± 28 mL (n=45 males) or 26% of their tidal volume. Despite the flexibility of the trachea and smaller conducting airways, their overall volume (i.e. the anatomic dead space) changes little with bronchoconstriction or when breathing hard during exercise. As birds have a longer and wider trachea than mammals the same size, they have a disproportionately large anatomic dead space, reducing the airway resistance. This adaptation does not impact gas exchange because birds flow air through their lungs - they do not breathe in and out like mammals.


Alveolar dead space

Alveolar dead space is defined as the difference between the physiologic dead space and the anatomic dead space. It is contributed to by all the terminal respiratory units that are over-ventilated relative to their perfusion. Therefore it includes, firstly those units that are ventilated but not perfused, and secondly those units which have a ventilation-perfusion ratio greater than one. Alveolar dead space is negligible in healthy individuals, but it can increase dramatically in some
lung disease The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s due to
ventilation-perfusion mismatch In respiratory physiology, the ventilation/perfusion ratio (V/Q ratio) is a ratio used to assess the efficiency and adequacy of the matching of two variables: * V – ventilation – the air that reaches the alveoli * Q – perfusion – the blood ...
.


Calculating

Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution, it is possible to measure physiological dead space, employing the concept of
mass balance In physics, a mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have b ...
, as expressed by
Bohr equation The Bohr equation, named after Danish physician Christian Bohr (1855–1911), describes the amount of physiological dead space in a person's lungs. This is given as a ratio of dead space to tidal volume Tidal volume (symbol VT or TV) is ...
. Bohr, C. (1891). Über die Lungenathmung. Skand. Arch. Physiol. 2: 236-268. article
/ref> ::\frac = \frac :where V_ is the dead space volume and V_ is the tidal volume; ::P_ is the partial pressure of carbon dioxide in the arterial blood, and ::P_ is the partial pressure of carbon dioxide in the mixed expired (exhaled) air.


Physiological dead space

The Bohr equation is used to measure physiological dead space. Unfortunately, the concentration of carbon dioxide (CO2) in alveoli is required to use the equation but this is not a single value as the ventilation-perfusion ratio is different in different lung units both in health and in disease. In practice, the arterial partial pressure of CO2 is used as an estimate of the average alveolar partial pressure of CO2, a modification introduced by Henrik Enghoff in 1938 (Enghoff H. Volumen inefficax. Bemerkungen zur Frage des schadlichen Raumes. Upsala Läkarefören Forhandl., 44:191-218, 1938). In effect, the single arterial pCO2 value averages out the different pCO2 values in the different alveoli, and so makes the Bohr equation useable. The quantity of CO2 exhaled from the healthy alveoli is diluted by the air in the conducting airways (anatomic dead space) and by gas from alveoli that are over-ventilated in relation to their perfusion. This dilution factor can be calculated once the mixed expired pCO2 in the exhaled breath is determined (either by electronically monitoring the exhaled breath or by collecting the exhaled breath in a gas impermeant bag (a Douglas bag) and then measuring the pCO2 of the mixed expired gas in the collection bag). Algebraically, this dilution factor will give us the physiological dead space as calculated by the Bohr equation: :\frac = \frac


Alveolar dead space

The alveolar dead space is determined as the difference between the physiological dead space (measured using the Enghoff modification of the Bohr equation) and the anatomic dead space (measured using Fowler's single breath technique). A clinical index of the size of the alveolar dead space is the difference between the arterial partial pressure of CO2 and the end-tidal partial pressure of CO2.


Anatomic dead space

A different maneuver is employed in measuring anatomic dead space: the test subject breathes all the way out, inhales deeply from a 0% nitrogen gas mixture (usually 100% oxygen) and then breathes out into equipment that measures nitrogen and gas volume. This final exhalation occurs in three phases. The first phase (phase 1) has no nitrogen as that is gas that is 100% oxygen in the anatomic dead space. The nitrogen concentration then rapidly increases during the brief second phase (phase 2) and finally reaches a plateau in the third phase (phase 3). The anatomic dead space is equal to the volume exhaled during the first phase plus the volume up to the mid-point of the transition from phase 1 to phase 3.


Ventilated patient

The depth and frequency of our breathing is determined by chemoreceptors and the brainstem, as modified by a number of subjective sensations. When mechanically ventilated using a mandatory mode, the patient breathes at a rate and tidal volume that is dictated by the machine. 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.


Mechanical dead space

Mechanical dead space is dead space in an apparatus in which the
breathing gas A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed h ...
must flow in both directions as the user breathes in and out, increasing the necessary respiratory effort to get the same amount of usable air or breathing gas, and risking accumulation of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
from shallow breaths. It is in effect an external extension of the physiological dead space. It can be reduced by: * Using separate intake and exhaust passages with one-way valves placed in the mouthpiece. This limits the dead space to between the non return valves and the user's mouth or nose. The additional dead space can be minimized by keeping the volume of this external dead space as small as possible, but this should not unduly increase work of breathing. * With a
full face mask A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece, demand valve or constant flow gas supply that provides the diver with breathing gas. The full face mask h ...
or demand diving helmet: ** Keeping the inside volume small ** Having a small internal
orinasal mask An orinasal mask, oro-nasal mask or oral-nasal mask is a Breathing mask (disambiguation), breathing mask that covers the mouth and the nose only. It may be a complete independent item, as an oxygen mask, or on some anaesthetic apparatuses, or it ...
inside the main mask, which separates the external respiratory passage from the rest of the mask interior. **In a few models of full face mask a mouthpiece like those used on diving regulators is fitted, which has the same function as an orinasal mask, but can further reduce the volume of the external dead space, at the cost of forcing mouth-breathing (and acting like a gag, preventing clear talking).


See also

* * * *


References


Further reading

*Arend Bouhuys. 1964. "Respiratory dead space." in ''Handbook of Physiology. Section 3: Respiration.'' Vol 1. Wallace O. Fenn and Hermann Rahn (eds). Washington: American Physiological Society. *John B. West. 2011. ''Respiratory Physiology: The Essentials.'' Lippincott Williams & Wilkins; Ninth edition. .


External links


The ''Dead space'' page on Johns Hopkins School of Medicine ''Interactive Respiratory Physiology'' website.
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