The physiological dead space is decreased by:
|B||Positive pressure ventilation|
Physiological dead space (total dead space) = Anatomical dead space + alveolar dead space. In upright position ventilation IS proportionately more in apex (increased V/Q mismatch) thereby increasing alveolar dead space.
Emphysema also increases V/Q mismatch.
Positive pressure ventilation can significantly increase the alveolar dead ventilation (alveoli are over ventilated) and perfusion is decreased (positive intrathoracic pres- I sure decreases venous return).
Positive pressure ventilation also increases the anatomical dead space. Neck flexion by decreasing anatomical dead space decreases the total i.e., physiological dead space.