Physiological dead space equals anatomical plus alveolar dead space
Mnemonic
“Anatomical plus alveolar equals physiological”:
- Anatomical dead space - the conducting airways, about 2 ml/kg, so roughly 150 ml in an adult. Ventilated but never involved in gas exchange
- Alveolar dead space - alveoli that are ventilated but not perfused. Close to zero in health
- Physiological dead space - the sum of the two, measured by the Bohr equation
Alveolar dead space is the abnormal one, and it rises in pulmonary embolism, hypotension, positive pressure ventilation and emphysema, which is the basis of a raised end tidal to arterial carbon dioxide gradient in embolism.
Anatomical dead space is increased by neck extension, jaw protrusion and bronchodilators, and decreased by intubation or tracheostomy, which bypass roughly half of it.
West zone 1 is alveolar dead space by definition, since alveolar pressure exceeds arterial pressure there.
Expansion
- Anatomical dead space - gas in the conducting airways, which never reaches alveoli. About 150 ml, or 2 ml per kilogram
- Alveolar dead space - alveoli that are ventilated but not perfused. Negligible in health
- Physiological dead space - the sum of the two; the total volume not taking part in gas exchange
In health, physiological and anatomical dead space are almost equal, about 30 per cent of tidal volume.
Alveolar dead space rises with pulmonary embolism, low cardiac output, hypotension, emphysema and excessive positive pressure ventilation, all of which reduce perfusion of ventilated alveoli.
Anatomical dead space is increased by neck extension and jaw protrusion, and reduced by tracheostomy, which bypasses the upper airway and removes roughly half of it.
Because dead space is a fixed volume, rapid shallow breathing wastes a greater proportion of each breath and reduces alveolar ventilation even when minute ventilation is preserved.