The authors aimed to define the optimum ETCO2 to conclusively exclude a pulmonary embolic event. The ratio of physiologic dead space to tidal volume is usually about 1/3. End-tidal CO2 (ETCO2) can represent dead space ventilation. ![]() 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. They have been attributed to an increase in alveolar dead space, or more directly to right-to- left shunting and V/Q mismatch. Macro level dead space imagine that a patient suddenly develops a segmental pulmonary embolism. ![]() For any given tidal volume, there is less effective CO2 clearance. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Excess dead space can arise at a micro level or a macro level: Micro level dead space scarred alveoli due to ARDS become inefficient at CO2 clearance. In the others a shunt (3 to 17 percent of cardiac output) was recorded along with radiographic evidence of atelectasis or infiltrates and accounted for most of. Dead space is the volume of a breath that does not participate in gas exchange.
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