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 space: unperfused 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.