It appears that the anatomic dead space estimate methods were sufficient when used (as originally intended) together with other assumptions to identify a starting point in a ventilation algorithm, but the poor agreement between an individual patient's measured and estimated anatomic dead space contradicts the assumption that dead space can be predicted from actual or ideal weight alone.Dead space represents the volume of ventilated air that does not participate in gas exchange.

The mean +/- SD error between the body weight estimate and the measured dead space was 60 +/- 54 mL. The coefficient of determination (r2) between the anatomic dead space estimate based on body weight and the measured anatomic dead space was r2 = 0.0002. Anatomic dead space was measured during the first 10 min of monitoring and compared to the estimates. We analyzed data from 58 patients (43 male, 15 female) to assess the accuracy of 5 anatomic dead space estimation methods. With a volumetric capnography monitor that measures on-airway flow and CO2, the anatomic dead space can be automatically and directly measured with the Fowler method, in which dead space equals the exhaled volume up to the point when CO2 rises above a threshold. Many respiratory and medical textbooks state that anatomic dead space can be estimated from the patient's weight by assuming there is approximately 1 mL of dead space for every pound of body weight. With smaller tidal volumes, the percentage of each delivered breath that is wasted in the anatomic dead space is greater than it is with larger tidal volumes. Some contemporary ventilation protocols, such as the Acute Respiratory Distress Syndrome Network protocol, call for smaller tidal volumes than were traditionally delivered. Anatomic dead space (also called airway or tracheal dead space) is the part of the tidal volume that does not participate in gas exchange.

Anatomical dead space