Anatomic dead space is age dependent and is > 3 ml/kg in early infancy. The following relationship between total anatomic dead space (DStotal in ml/kg) and age (in yr) is derived: DStotal = 3.28 - 0.56, with r = 0.95 and P = 0.0001. Mean intrathoracic anatomic dead space was 1.03 ml/kg and was not related to age. Extrathoracic dead space per kilogram decreased exponentially with increasing age, ranging from 2.3 ml/kg in early infancy to 0.8 ml/kg in children older than 6 yr. Intrathoracic dead space was measured by continuous analysis of end-tidal and mixed-expired PCO2 and minute ventilation in 10 patients, aged 18 days to 14.7 yr. Extrathoracic dead space was measured by a "water displacement" technique in 40 patients aged 7 days to 14.2 yr who were intubated with cuffed endotracheal tubes. Because of the relatively large head size of infants and children, we hypothesized that extrathoracic and, therefore, total dead space would be relatively larger in pediatric subjects. physiologic dead space the sum of the anatomic and alveolar dead spaces its volume (V D) is determined by measuring the partial pressure of carbon dioxide in a sample of exhaled gas (PECO2) and in the arterial blood (PaCO2) and (with tidal volume of V T) using the formula V D /V T (PaCO2PECO2)/PaCO2. In adults, anatomic dead space is 2.2 ml/kg.
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