Bacterial and Viral Filters to the Expiratory Circuit

Abstract

Air filters in the expiratory circuit are medical devices used in breathing circuits with the aim to protect patients, equipment, and the environment from cross infection due to pathogens able to spread by air. Filters have variable composition and form that determine mechanical ability to withhold microorganisms and particles, allowing to identify efficiency particulate air (EPA), high-efficiency particulate air (HEPA), and ultra-low penetration air filters. Heat moisture and exchangers (HMEs), instead, can retain heat and humidity from exhaled air and return them to the patient during the following inspiration. HMEs also provide a filtration function that can be either electrostatic or mechanical, and they can be classified as either EPA or HEPA based on their filtration efficiency. Connecting a bacterial--viral filter to the breathing circuit modifies its mechanical characteristics because it could increase the compliance of the circuit, increase dead space if placed at the airway interface, and it could add a resistance that causes a pressure drop between the inlet and the outlet of the filter. In this chapter, we will analyze the types of filters, the mechanisms underlying filtration, the positions of the filters in the breathing circuits, and their advantages/disadvantages.