![]() ![]() The ratio of physiologic dead space to tidal volume is usually about 1/3. In other words, dead space occurs when plenty of air reaches the alveoli, but there is a lack of. It’s often referred to as wasted ventilation, as the inhaled air is not involved in the exchange of oxygen and carbon dioxide. Alveolar dead space is the volume of gas within unperfused alveoli (and thus not participating in gas exchange either) it is usually negligible in the healthy, awake patient. Dead space ventilation is defined as the volume of ventilated air that does not participate in gas exchange. The equation is, V D /V T (F A CO 2 - F E CO 2 ) / F A CO 2. The Bohr equation can be used to determine physiological dead space from the difference between the exhaled CO 2 and alveolar CO 2, but the latter is hard to measure. Anatomic dead space is the volume of gas within the conducting zone (as opposed to the transitional and respiratory zones) and includes the trachea, bronchus, bronchioles, and terminal bronchioles it is approximately 2 mL/kg in the upright position. Physiological dead space can be measured using the Bohr-Enghoff method. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Dead space is the volume of a breath that does not participate in gas exchange.
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