What are impaction, sedimentation, and diffusion in aerosol therapy for a mechanically ventilated patient with Chronic Obstructive Pulmonary Disease (COPD) or asthma?

Aerosol Deposition Mechanisms in Mechanically Ventilated Patients

Impaction, sedimentation, and diffusion are the three distinct physical mechanisms by which aerosol particles deposit in the respiratory tract of mechanically ventilated COPD or asthma patients. 1

Impaction

Impaction occurs when larger aerosol particles (>5 μm) collide with and deposit on airway surfaces due to their inertia, particularly at airway bifurcations and areas of turbulent flow. 1

• This mechanism is enhanced by turbulent flow conditions, which explains why increasing flow rates in high-flow nasal cannula systems cause greater impactive aerosol losses within the tubing rather than reaching the lungs. 2, 1

• Higher inspiratory flow rates and rapid breathing patterns increase impaction in the upper airways and artificial airways (endotracheal tubes), reducing therapeutic drug delivery to the lower respiratory tract. 1

• Slow, controlled inspiratory flow (≤30 L/min or 3-5 seconds) minimizes impactive losses and optimizes aerosol delivery to the lungs. 1

• The endotracheal tube and ventilator circuit create multiple sites for impactive losses, which is a major reason why only 1.5-6% of aerosol reaches the lungs in ventilated patients versus 10-15% in spontaneously breathing patients. 1, 3

Sedimentation

Sedimentation is the gravitational settling of medium-sized particles (1-5 μm) that occurs when airflow velocity decreases, particularly during breath-holding or prolonged inspiratory pauses. 1

• This mechanism is time-dependent, which explains why a 10-second breath hold after inhalation is recommended to maximize drug deposition in the lower respiratory tract. 1

• Particles in the optimal size range for sedimentation (1-5 μm) are most likely to reach and deposit in the smaller airways and alveoli, making this the primary deposition mechanism for therapeutic aerosols. 1

• Vibrating mesh nebulizers produce particles with mass median diameters of 4.6-5.1 μm, which is optimal for avoiding excessive impaction while still allowing effective sedimentation in the lower airways. 1

• Actuating the MDI at the precise onset of the ventilator's inspiratory cycle maximizes the time available for sedimentation in the lungs rather than the circuit. 1, 3

Diffusion

Diffusion is the random Brownian motion of submicron particles (<0.5 μm) that causes them to collide with and deposit on airway surfaces, predominantly in the peripheral airways and alveoli where airflow velocity is lowest. 1

• Diffusion is enhanced in areas of low flow velocity and long residence time, which is why particles that reach the alveolar regions through other mechanisms may ultimately deposit via diffusion. 1

• This mechanism becomes increasingly important for very small particles that have escaped impaction and sedimentation in the upper and central airways. 1

Clinical Implications for Mechanically Ventilated Patients

Understanding these three mechanisms is essential for optimizing aerosol therapy in ventilated patients, as the artificial airway and ventilator circuit dramatically alter normal deposition patterns. 1, 4

• The delivery of inhaled drugs to mechanically ventilated patients is complicated by deposition of aerosol particles in the ventilator circuit and endotracheal tube, where impaction and sedimentation cause significant losses before the drug reaches the lungs. 4

• Meticulous adherence to proper technique—including timing actuation with the inspiratory cycle, using appropriate flow rates, and positioning devices correctly in the circuit—is necessary for successful aerosol therapy in mechanically ventilated patients. 4, 5