Page 159 - Biomedical Engineering and Design Handbook Volume 2, Applications

138 MEDICAL DEVICE DESIGN

The designer of respiratory medical devices must design the device to minimize accidental misuse
by the user by making the device as foolproof as possible, especially during emergency situations
where the attention is on the patient and not the device. If an injury can be attributed to a device,
either because of malfunction or faulty design, the manufacturer can face severe penalties.

4.7.7 Optional Functions
There is a tendency on the part of a designer to incorporate many additional functions to give the
device additional capabilities. There are cases where this is counterproductive, especially if the com-
plexity of device use increases beyond the point where mistakes can be made. Increasing the num-
ber of options often shifts the burden for a measurement from the device to the nurse or technician.
Especially if these options are rarely used, they may not be useful even if they are appealing to the
designer and marketing specialists. If options are to be included, make them hierarchical: the most
important functions should be obtained with little effort by the user. Additional functions can be
accessed with extra effort. For instance, the display on a computerized pulmonary function device
should not show all possible measurements made with the device; only the one to three most impor-
tant measurements should be displayed. Additional values, tables, or graphs can be displayed with
additional switches, knobs, or touching the screen. Keep it simple.

4.7.8 Calibration

All hospital equipment requires periodic calibration to assure accurate measurements. Automatic
calibration features allow for quick calibration at the point of use. If the instrument can self-calibrate,
even for just the most important points, then it will be much more useful than if it must be moved to
a shop for calibration. Some instruments undergo self-calibration when they are turned on. Other
instruments are normally always powered, and can be calibrated either by pressing a button or by a
timer (although this may interfere with a measurement). More thorough calibrations still need to be
completed in a biomedical maintenance laboratory.
If the device has a linear input-output response, then there are normally only two calibration
points, one at the device zero (null input), and the other at the span value (maximum input). It is not
uncommon that significant drift occurs in the zero value alone; it is not so common that the span
value drifts independent of the zero value. That is fortunate, because a null input is easier to calibrate
than a span input. The instrument can be made to measure the output for null input and correct all
readings accordingly.
Calibration of the device should be an important consideration when the product is being
designed.

4.7.9 Human Factor Issues

Human factor considerations can be easily overlooked in the initial design of a medical instrument,
which can result in the need for costly redesigns and delays in testing and approval. In these days
when concurrent engineering practices are being applied to designs of everything from light bulbs to
automobiles, it is important for the biomedical engineer to understand the medical environment in
which the device is to be used. Especially important is to understand the various expectations for the
device from personnel involved in its use.

The Patient. There is not one stereotypical patient, but several. One of these is a normal, healthy
individual who is undergoing a routine physical examination. This examination might be for school,
for personal reasons, or for work. There may be some apprehension exhibited by the patient when
confronted by medical surroundings. Especially with a new medical device or test, the patient will
wonder what it will do to him or her. The patient will likely exhibit slight hyperventilation, her/his