Design of Respiratory Devices

Shade, D. and A. T. Johnson, 2003, Design of respiratory devices, in Standard Handbook of Biomedical Engineering Design, M. Kutz, ed., McGraw-Hill, New York. 

The designer of respiratory devices is, in many ways, faced with the same challenges as designers of other medical devices. There are many different aspects to consider, with device function being but one of these. In addition to device performance, there are safety, user interface, legal, biocompatibility, marketing, cost, and adaptability issues to face. 

In Section 3.5.3, dry powder inhalers have been referred to as breath-controlled devices. The efficacy of dry powder inhalation is a function of many factors, influencing the delivered dose of fine particles and the deposition of these particles in the respiratory tract. Figure 3.4 shows that DPI performance is influenced both directly and indirectly by the design of the inhalation system. The powder formulation, the dose (measuring) system and the powder disintegration principle have to be designed correctly for release of sufficient fine drug particles in... 

Metered dose inhaler has been the most popular aerosol delivery device for the treatment of respiratory diseases, which is attributable to its portability and simple operation. Although seemingly easy to use, the MDI is a sophisticated device in design. The drug(s) are suspended or dissolved in a liquefied propellant system, which may also contain excipients such as cosolvents or surfactants. The formulation is kept pressurized in a small canister, sealed with a metering valve. Upon actuation through an actuator, the valve opens and the metered dose is dispensed as an aerosol spray from the expansion and vaporization of the propellant under ambient pressure. The inhalers may be used alone or with spacer devices, the electrostatic issues of which are considered in a later section. The present discussion focuses on the inherent charging of particles produced from MDIs. 

An oxygen enrichment device is needed for people with impaired respiratory systems. To design such a device, it is necessary to compute the work needed to produce a stream that contains 50 mol % of oxygen. from air (21 mol % oxygen) at 300 K and I bar. If the exit streams are at the same temperature and pressure as the inlet air, and half of the oxygen in the air is recovered in the enriched oxygen stream, what is the minimum amount of work required to operate whatever device is developed for this process ... 

Several approaches to the measurement of respiratory exposure are available. The first was developed by Durham and Wolfe ( ) and employs a respirator with the collection pads protected by cones from direct spray. The second common method uses the personal air sampler with a pump carried by the worker and a collection device In the general breathing zone. The third method involves a more careful experimental design. In this case, the worker wears a pesticide respirator for a certain period of time with the respirator removed for an equal amount of time. Twenty-four hour urine samples are collected each day, with any observed Increase in urinary metabolites Indicating the degree of respiratory exposure. 

As shown from measurements in airway models or theories using modifications of the friction coefficients, the vast majority of the pressure drop across the entire lung occurs within the large airways, say 0 n 8. I us clinical evaluation of total airway resistance can miss diseases of the small airways whose diameters are less than 2 mm, the so-called silent zone of the network. Pressure drop measurements and models are also important for the design of ventilators and other respiratory assist or therapeutic devices that interface with lung mechanics. 

Saliva can carry disease organisms. To minimize the transmission of disease, disposable cardboard mouthpieces are used with many respiratory measurement devices. Respiratory devices should, if possible, be designed to include sterilizable parts in the airflow path. This would reduce the possibility that microbes hitchhiking on aerosol particles would be passed from patient to patient. When appropriate, disposable Alters may also help prevent microbe transmission. 

A protective device for the human respiratory system designed to protect the wearer from inhalation of harmful air contaminants. There are two types of respiratory protective devices (a) air purifiers, which remove the contaminants from the air... 

A respiratory protection device that consists of a supply of respirable air, oxygen, or oxygen-generating material worn by the worker. It is designed for entry into and escape from atmospheres Immediately Dangerous to Life or Health (IDLH) or oxygen deficient. It is one of the highest levels of respiratory protection. Wearers carry the air supply on their back. 

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