Aerosols are deposited in the respiratory tract primarily as a result of inertial impaction and sedimentation. Inertial impaction, which occurs primarily in the upper airway, larynx, and at bifurcations of the first few bronchial divisions, is favored by high inspiratory flow rates and narrowed bronchospastic airways. Sedimentation occurs in regions of reduced airflow in more peripheral airways and is the most important mechanism for the deposition of therapeutic aerosols, Clinical factors, such as high inspiratory flow rates and rapid shallow breathing, can occur in patients with acute airflow obstruction who are breathing in an uncontrolled manner via a HHN. This would favor the deposition of aerosol in more proximal airways.

Severe bronchospasm may result in greater turbulent airflow and further loss of aerosol due to inertial impaction in upper airways. The slow deep inhalation followed by a breath-hold, as occurs with the use of MDI-spacer combination, would favor the sedimentation of bronchodilator aerosol in more peripheral airways. An increase in dosage of bronchodilator could compensate for the unfavorable deposition of the delivery technique. The “standard” dose of metaproterenol delivered by HHN is more than ten times the “standard” dose of MDI metaproterenol (1.3 mg for two inhalations MDI as compared to 15 mg in the HHN).