Page 158 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF RESPIRATORY DEVICES 137

4.7.3 Safety
Most respiratory devices are noninvasive, which reduces the safety burden somewhat because
there is no direct route for introduction of microbes into the interior of the body as with other
types of devices. One of the main causes for safety concern is due to saliva: this fluid could be the
pathway for stray electrical currents to enter the body. Even more important is the transmission of
respiratory diseases.
Saliva contains a mixture of ionic substances and can conduct electricity. For this reason, there can
be no direct electrical pathway from the ac supply to the patient. Thus use of isolation transformers
should be seriously considered, especially when young children are to be measured. An alternative is
the use of battery-powered units, which are especially attractive because there cannot be a stray path
to ac electrical ground as long as the unit is not hooked to an ac electrical outlet. Although most hospitals
have grounded ac outlets and ground fault interrupters, respiratory medical devices are being used in
homes, clinics, and schools where such safety precautions may not be installed.
Saliva can carry disease organisms. To minimize the transmission of disease, disposable card-
board 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 filters may also help prevent microbe transmission.
Some ventilators and pulmonary function equipment connect to cylinders of gas, for example, to
supplemental oxygen, carbon monoxide, helium, and others. Connectors for gas cylinders of differ-
ent gases are different to prevent accidentally connecting to the wrong kind of gas. If a mistake is
made, asphyxiation and death may be the result. There have been hospital deaths attributed to this
simple mistake. Be sure to use the correct connector.

4.7.4. Costs
The range of costs for pulmonary function equipment is from less than $300 for simple spirometers
to $50,000 for hospital body plethysmographs. The cost must be appropriate for the use. In general,
home-use devices are much less costly than hospital devices. Home devices are usually much sim-
pler and may not be as accurate or reliable as hospital devices. The design engineer must know the
market for a new device before investing inordinate amounts of time or before including too many
options that lead to inappropriate purchase prices.
Much of the cost of a new medical device is incurred due to governmental regulatory require-
ments. There is no practical solution to this because approval to manufacture requires amounts of
device details and human trials to assure that the device is safe and effective.

4.7.5 Materials
Many of the materials incorporated in respiratory medical devices do not directly contact the person
using the device. Therefore, materials are not subject to the same biocompatibility constraints as are
implantable or surface contact devices. There is a need, however, to be sure that materials are rugged
enough to stand up to repeated use under sometimes frantic circumstances. An endotracheal tube, for
instance, must not fail during insertion into the trachea. In addition, materials in the air passage can-
not give off toxic gases or undesirable odors or flavors. Finally, parts that are to be disinfected peri-
odically must be able to withstand the disinfection process. This is especially important, given the
prevalence of chemical disinfection with fairly corrosive agents.

4.7.6 Legal Liability

All medical devices are possible sources for legal action. Misuse or malfunctioning of the device can
bring tort claims against the hospital, the owner of rented equipment, and the company of manufacture.