Computer Calculations of Exercise Dead Space: The Role of Laminar Flow, and Development of a Clinical Prediction Formula

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Abstract
The behaviour of the anatomical dead space cannot be described by passage of a ‘square’ wave front through the conducting airways, with subsequent diffusion of alveolar gas back up the bronchial tree. A computer model based on these assumptions leads to overestimation of the dead space during both rest and exercise. It is suggested that these discrepancies arise from laminar (axial) flow in the smaller airways. A simple description of the anatomical dead space is given by the equation VD = Vf+ (Ve-200)e-kt where VD is the volume of the anatomical dead space, Vf is the volume of the conducting airway to the fifteenth order bronchioles, Ve is the volume from the sixteenth order bronchioles to the second or third order alveolar ducts, 200 is an empirical correction for laminar gas flow, k is a diffusion constant (1.0 for nitrogen, 0.8 for carbon dioxide) and t is the half length of the respiratory cycle, measured in seconds.

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