Anatomic, airway, or tracheal, dead space is the part of the tidal volume that does not participate in gas exchange. Knowledge of the size of the dead space is important for proper mechanical ventilation, especially if small tidal volumes are used. Respiratory and medical textbooks state that anatomic dead space can be estimated from the patient’s body weight. Specifically, these references suggest dead space can be predicted using a relationship of one milliliter per pound of body weight. Using a volumetric capnography monitor that incorporates on-airway flow and CO2 monitoring (NICO2, Respironics, Wallingford CT), anatomic dead space can be automatically and directly measured using Fowler’s method in which dead space equals the exhaled volume up to the point when CO2 rises above a threshold [4]. We retrospectively analyzed data collected in 58 (43 male, 15 female) patients to assess the accuracy of weight-based estimation of anatomic dead space. It appears that the average anatomic dead space roughly corresponds to the average body weight for the overall population; however, the poor correlation between individual patient weight and dead space contradicts the suggestion that dead space can be estimated from body weight
