Aerosols in the Health Care Field

	Section Contents
	Aerosol Drug Delivery System (Overview)
		Advantages and Disadvantages
		» Factors that affect Deposition
		Loss during Delivery

Three physical mechanisms usually are considered for aerosol particle deposition in the human lungs: inertial impaction, gravitational settling (sedimentation), and diffusion. The detail is described in the Respiratory Deposition program.

However, there are other factors beyond these mechanisms that affect the deposition. Therapeutic aerosol particles may be produced at relatively dry ambient conditions. When they are taken into the airway, temperature and humidity rapidly increase to saturation at 37 ^oC.  Inhaled aerosol drugs are heterodisperse in size and they are also hygroscopic. For example, between ambient and BTPS (body temperature, ambient pressure, saturated with water vapor) conditions, cromolyn sodium powder particles from an MDI increased from 2.31 to 3.02 µm MMAD (Mass Median Aerodynamic Diameter), as shown in studies by Smith G et al.(1980) and Hiller FC et al.(1980). 

Either evaporation or hygroscopic water absorption can change the size of aerosol particles size. The relative rate of particle size change is inversely proportional to the size of a particle. Hence, it takes less time for a small particle to grow or shrink than a large partilce. When exposed to relatively dry gas, small water-based particles shrink. On the other hand, aerosols of water-soluble materials, especially salts, tend to be hygroscopic, absorbing water and growing when introduced into a high-humidity environment.

Particle size is a critical parameter in administering therapeutic aerosols. However, it is not the only determinant of deposition. Inspiratory flow rate, flow pattern, respiratory rate, inhaled volume, ratio of inspiratory time to expiratory time (I:E ratio), and breath-holding all are important parameters that influence the physical mechanisms mentioned earlier. Hence, they all affect where a particle of any specific size is deposited. The presence of airway obstruction is one of the greatest factors influencing aerosol deposition.

How does high humidity in the respiratory system affect the deposition pattern of a hygroscopic particle? If a carrier gas (e.g. O2) is dry, how will that affect the deposition pattern of a mist aerosol? In administering pentamidine, you advise the patient to hold breath. What's the reason? How will you advise a patient to control his/her inspiratory flow rate when using mucokinetic drugs?

Reference

Smith G and others: Aerodynamic size distribution of cromolyn sodium at ambient and airway humidity, Am Rev Respir Dis 121: 513, 1980.

Hiller FC and others: Physical properties, hygroscopicity and estimated pulmonary retention of various therapeutic aerosols, Chest 77(supplement): 318, 1980.