Effect of Humidity on Size Distributions of MDI Particles Exiting a Mechanical Ventilation Holding Chamber

The efficacy of drug delivery from metered-dose inhalers is dramatically reduced in the confined, humid setting typical during mechanical ventilation. Although it is widely accepted that increased particle sizes in the presence of high concentrations of ambient water vapor lead to greater inertial impaction in the ventilator circuit, the primary mechanism through which particle sizes increase has been debated. The present study proposes a modification to the popular assumption of particle growth by condensation, in which water is allowed to evaporate back into the ambient airflow subsequent to an initial transient nucleated condensation. The hypothesis is supported by particle size data from two commercial metered-dose inhalers, measured downstream from a mechanical ventilation holding chamber.