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

Aerosol Deposition Mechanisms in Mechanically Ventilated Patients

In mechanically ventilated COPD or asthma patients, aerosol particles deposit in the respiratory tract through three distinct physical mechanisms: impaction (large particles collide with airway walls at bifurcations and bends), sedimentation (medium-sized particles settle by gravity during breath-holding), and diffusion (submicron particles undergo random Brownian motion and deposit in alveoli). 1

Impaction

Impaction occurs when larger aerosol particles (typically >5 μm) traveling with high velocity cannot follow the airstream around curves and bifurcations, causing them to collide with and deposit on airway walls. 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. 1

• In mechanically ventilated patients, the endotracheal tube itself serves as a major site for impactive losses, functioning as both a deposition surface and a barrier that reduces overall lung delivery to only 1.5-6% of the actuated dose compared to 10-15% in spontaneously breathing patients. 2, 3

• Higher inspiratory flow rates and rapid breathing patterns increase impaction in the upper airways and artificial airways, which is why slow, controlled inspiratory flow (30 L/min or 3-5 seconds) is recommended for optimal aerosol delivery. 4, 3

• The geometry of the ventilator circuit creates additional impaction sites—this is why positioning the spacer within 30 cm of the Y-piece is critical, as greater distances increase particle losses through impaction in the tubing. 2

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. 4

• In mechanically ventilated patients, aerosol delivery shows a linear correlation with both inspiratory time and duty cycle (r > 0.91), because longer inspiratory times allow more opportunity for gravitational sedimentation to occur. 3

• 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, 5

• The design of some jet nebulizers allows gravitational flow of patient secretions into the medication reservoir through this same sedimentation principle, which can lead to contamination. 1

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

• This mechanism becomes increasingly important for very small particles that remain suspended in air for extended periods and can penetrate deep into the respiratory tract. 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

• The smallest particles depositing by diffusion are also those most likely to escape into the environment as fugitive aerosols, contributing to environmental contamination during nebulization. 1

Clinical Implications for Mechanically Ventilated Patients

Understanding these mechanisms explains why only 1.5-6% of aerosol reaches the lungs in ventilated patients versus 10-15% in spontaneously breathing patients—the endotracheal tube and ventilator circuit create multiple sites for impactive and sedimentary losses. 2, 3

• Humidification dramatically reduces delivery by approximately 50% because water vapor causes particle growth, increasing impaction and sedimentation in the circuit rather than the lungs. 2, 3

• Actuating the MDI at the precise onset of the ventilator's inspiratory cycle maximizes the time available for sedimentation and diffusion in the lungs rather than the circuit. 2

• 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. 6, 5