Traditionally, dead space is expressed as a fraction of the tidal volume (V T), ie, physiologic V D/V T, but it may also be expressed as anatomical V D/V T or alveolar V D/V T. The sum of anatomical V D and alveolar V D is referred to as the physiologic dead space. 1 Dead space is divided into the normal anatomic dead space-this exists in the large and small airways, which normally do not participate in gas exchange-and the alveolar dead space when there is reduced or no blood flow to a given area of lung that is still receiving ventilation. The concept of dead space was originally described by Bohr. Dead space may be manifested as low V̇/Q̇ or absolute, with no blood flow, that is V̇/Q̇ = 0.Īlthough much of the management of patients in respiratory failure on artificial ventilation has focused primarily on imaging and respiratory mechanics, dead-space measurements have been shown to provide very useful information in patients with severe respiratory failure both for management and for prognosis. A shunt exists in a pulmonary segment when where there is pulmonary arterial blood flow and no ventilation, that is V̇ = 0/Q̇. Traditionally, V̇/Q̇ in the lungs is described in the 3-compartment model: normal V̇/Q̇, shunt, and dead space (V D). Respiratory failure is manifested by a derangement of arterial blood gases, and the relationship of arterial blood gases to the matching of ventilation (V̇) to perfusion (Q̇) in the lungs is critical to understanding the mechanisms of respiratory failure.
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