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. 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. The equation states VD is equal to VT multiplied by the partial pressure of arterial carbon dioxide (PaCO2) minus partial pressure of expired carbon dioxide (PeCO2) divided by PaCO2. Understanding the equation will simplify the concept of dead space greatly. Any factor that decreases perfusion increases alveolar dead space. The Bohr equation can be used to calculate the amount of dead space in a lung. 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.
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