When PEEP recruits collapsed lung units, resulting in improved oxygenation, alveolar dead space may decrease however, when PEEP induces overdistention, alveolar dead space tends to increase. Alveolar dead space is potentially large in pulmonary embolism, COPD, and all forms of ARDS.

Lung areas that are ventilated but not perfused form part of the dead space. Lung heterogeneity creates regional differences in CO 2 concentration, and sequential emptying raises the alveolar plateau and steepens the expired CO 2 slope in expiratory capnograms.

In steady-state conditions, CO 2 output equals CO 2 elimination, but during non-steady-state conditions, phase issues and impaired tissue CO 2 clearance make CO 2 output less predictable. Alveolar P CO 2 (P ACO 2) depends on the balance between the amount of CO 2 being added by pulmonary blood and the amount being eliminated by alveolar ventilation (V̇ A). The diffusion of gases brings the partial pressures of O 2 and CO 2 in blood and alveolar gas to an equilibrium at the pulmonary blood-gas barrier.

Define dead space ventilation