Dead space of the respiratory system


 Dead space is the volume of inspired air that takes no part in gas exchange.

Divided into 2 parts:

anatomical dead space: mouth, nose, pharynx and large airways not lined with respiratory epithelium. 

alveolar dead spaceunperfused alveoli. Ventilated lung normally contributing to gas exchange, but not doing so because of impaired perfusion. Thus represents one extreme of V̇/Q̇ mismatch.



Physiological dead space equals anatomical plus alveolar dead space. It is calculated using the Bohr equation. Assessment may be useful in monitoring V̇/Q̇ mismatch in patients with extensive respiratory disease, especially when combined with estimation of shunt fraction. Normally equals 2–3 ml/kg; i.e., 30% of normal tidal volume. In rapid shallow breathing, alveolar ventilation is reduced despite a normal minute ventilation, because a greater proportion of tidal volume is dead space.


Increased by:

◗ increased lung volumes.

◗ bronchodilatation.

◗ neck extension.

◗ PE/gas embolism.

◗ old age.

◗ hypotension.

◗ haemorrhage.

◗ pulmonary disease.

◗ general anaesthesia and IPPV.

◗ atropine and hyoscine.

◗ apparatus (see later).

Decreased by:

◗ tracheal intubation and tracheostomy.

◗ supine position.

Apparatus dead space represents ‘wasted’ fresh gas within anaesthetic equipment. Minimal lengths of tubing should lie between the fresh gas inlet of a T-piece and the patient, especially in children, whose tidal volumes are small.

Facemasks and their connections may considerably increase dead space.


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