What term refers to the amount of drug that cannot reach the lower lung fields?

The term that refers to the amount of drug that cannot reach the lower lung fields is "respirable mass." This concept is crucial in the context of aerosol therapy, where medication is delivered through inhalation. The respirable mass specifically pertains to the portion of an aerosolized drug that is small enough to penetrate deep into the lungs, ideally reaching the alveolar region for effective absorption.

Inhalable fraction refers to the total amount of aerosol that can be breathed into the lungs, but it does not differentiate between particles that can reach the lower lung regions and those that cannot. Aerosol delivery is a broader term that encompasses the method of providing aerosolized drugs to a patient rather than specifying the fraction that penetrates deeply. Particle size distribution refers to the varying sizes of particles in the aerosol but does not specifically address which particles successfully reach the lower lungs. Therefore, "respirable mass" is the most accurate term for defining the amount of drug that fails to reach the lower lung fields due to size limitations or other factors impacting lung deposition.