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. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals 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.

Benefits do accrue to a seemingly wasteful design for ventilation that includes dead space.^[1]

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.^[3]

Dead space can be increased (and better envisioned) by breathing through a long tube, such as a snorkel. Even though 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. Thus, a snorkel increases the person's dead space by adding even more "airway" that doesn't participate in gas exchange.

The total dead space (also known as physiological dead space) is the sum of the anatomical dead space plus the alveolar dead space.

Anatomical dead space is that portion of the airways (such as the mouth and trachea to the bronchioles) which conducts gas to the alveoli. No gas exchange is possible in these spaces. In healthy lungs where the alveolar dead space is small, Fowler's method accurately measures the anatomic dead space by a nitrogen washout technique. ^[4][5]

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 (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.^[4] 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.^[4][6]

Birds have a disproportionately large anatomic dead space (they have a longer and wider trachea than mammals the same size), 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.^[7]

Alveolar dead space is sum of the volumes of those alveoli which have little or no blood flowing through their adjacent pulmonary capillaries, i.e., alveoli that are ventilated but not perfused, and where, as a result, no gas exchange can occur.^[1] Alveolar dead space is negligible in healthy individuals, but can increase dramatically in some lung diseases due to ventilation-perfusion mismatch.

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 anatomical and alveolar dead space, employing the concept of mass balance, as expressed by Bohr equation.^[8][9]

The concentration of carbon dioxide (CO2) in healthy alveoli is known. It is equal to its concentration in arterial blood since CO2 rapidly equilibrates across the alveolar–capillary membrane. The quantity of CO2 exhaled from the healthy alveoli will be diluted by the air in the conducting airways and by air from alveoli that are poorly perfused. This dilution factor can be calculated once the CO2 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 mixed gas in the collection bag). Algebraically, this dilution factor will give us the physiological dead space as calculated by the Bohr equation:

When the poorly perfused alveoli empty at the same rate as the normal alveoli, it is possible to measure the alveolar dead space. In this case, the end-tidal sample of gas (measured by capnography) contains CO2 at a concentration that is less than that found in the normal alveoli (i.e. in the blood):^[10]

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 has no nitrogen, and is the air that entered the lung only as far as the conducting airways. The nitrogen concentration then rapidly increases during the brief second phase and finally reaches a plateau, the third phase. The anatomic dead space is equal to the volume exhaled during the first phase plus half that exhaled during the second phase. (The Bohr equation is used to justify the inclusion of half the second phase in this calculation.)^[4]

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 is dead space in an apparatus in which the breathing gas 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 from shallow breaths. It is in effect an external extension of the physiological dead space.

It can be reduced by: