Dead space (physiology)

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.[1]

1. Carbon dioxide is retained, making a bicarbonate-buffered blood and interstitium possible.
2. Inspired air is brought to body temperature, increasing the affinity of hemoglobin for oxygen, improving O₂ uptake.[2]
3. Particulate matter is trapped on the mucus that lines the conducting airways, allowing its removal by mucociliary transport.
4. Inspired air is humidified, improving the quality of airway mucus.[2]

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. 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.

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, as expressed by the Bohr equation.[8][9]

${\displaystyle {\frac {V_{d}}{V_{t}}}={\frac {P_{a\,{\ce {CO2}}}-P_{e\,{\ce {CO2}}}}{P_{a\,{\ce {CO2}}}}}}$

where ${\displaystyle V_{d}}$ is the dead space volume and ${\displaystyle V_{t}}$ is the tidal volume;

${\displaystyle P_{a\,{\ce {CO2}}}}$ is the partial pressure of carbon dioxide in the arterial blood, and

${\displaystyle P_{e\,{\ce {CO2}}}}$ is the partial pressure of carbon dioxide in the mixed expired (exhaled) air.

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:

• 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 the work of breathing.
• With a full face mask or demand diving helmet:
  • Keeping the inside volume small
  • Having a small internal orinasal mask 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).

• Bohr equation – Equation describing the amount of physiological dead space in a person's lungs
• Christian Bohr – Danish physician and professor of physiology
• Multiple inert gas elimination technique – Medical technique
• Respiratory system – Biological system in animals and plants for gas exchange

• 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. ISBN 978-1609136406.

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