The Effect of Nonuniform Ventilation on the Dispersion of Inspired Aerosol Boluses—A Modeling Study

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Abstract
To investigate the potential use of aerosol dispersion (AD) and the skewness (SK) of expired aerosol boluses in detecting and characterizing lung injury, AD and SK were computed in a model of inhomogeneous ventilation. The lung was represented as two parallel compartments with volumes V1 and V2 and ventilatory time constants τ1 and τ2. Within a compartment, inspired aerosol was assumed to disperse as a Gaussian function of inspired volume. Expired aerosol concentration was computed by the complete mixing of expirate from the compartments. AD was defined as the square root of the difference of the variances of inspired and expired boluses with the coefficient of dispersion (CD) = AD/VP. For V1 = V2 and τ2 > τ1, CD and SK increased with (τ2 − τ1) and flow rate and decreased with VP. For V2 < V1, SK was maximum with V2 = 0.33V1 but was still increased up to 59% over baseline (τ1 = τ2) values with V2 = 0.01V1. The results suggest that: 1) CD and SK are markers of parallel inhomogeneity in lung ventilation, 2) CD and SK may be more sensitive to lung injury when measured at higher flow, and 3) SK may be particularly sensitive to a small region of injured lung.