![]() ![]() The ratio of physiologic dead space to tidal volume is usually about 1/3. Recent recognition of the prognostic relevance of measurements of physiological dead space for patients with the acute respiratory distress syndrome (ARDS) has brought new attention to a simple gas exchange calculation described over 120 years ago. 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. 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. The Bohr equation is used to quantify the ratio of physiological dead space to the total tidal volume, and gives an indication of the extent of wasted ventilation. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Measurement of the ratio of physiologic dead space (V D) to tidal volume (V T) with mixed expired carbon dioxide and blood gas analysis can be accomplished with various commercially available monitors. Dead space is the volume of a breath that does not participate in gas exchange. ![]()
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