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Increased dead space may precipitate ventilatory failure in these patients.

Patients with obstructive lung disease have impaired ventilatory capacity and may be unable to accommodate the increased demand for minute ventilation created by conditions that produce dead space. The minute ventilation used in mechanically ventilated critically ill patients is frequently around 150 mL/min per kg predicted bodyweight or above, in contrast to surgical patients undergoing mechanical ventilation in the operating room (in whom minute ventilation is around 100 mL/min per kg predicted bodyweight). A physiological consequence of increased dead space as the increased work of breathing required to maintain a normal PaCO2. This increased minute ventilation does not mean alveolar ventilation increases because much of the minute ventilation is directed toward dead space units. In conditions producing dead space, the body tries to maintain a normal PaCO2 by increasing the minute ventilation. VD/VT increases as a larger percentage of the VT becomes dead space. Pulmonary embolism and shock are conditions that cause increased alveolar dead space. Increased alveolar dead space decreases alveolar ventilation if minute ventilation stays the same. Any factor that decreases perfusion increases alveolar dead space. Dead space is the term used to describe alveoli that have normal ventilation but no blood flow through their capillaries. A normal PaCO2 associated with high minute ventilation indicates that much of this patient's ventilation is not in contact with blood flow.