Biomedical Engineering Reference
In-Depth Information
FIGURE 2-56
Fiber optic pressure
sensor from FISO
Technologies
employs a
MEMS-based
sensing catheter for
in vivo physiological
measurements.
(Courtesy
http://www.
fiso.com/.)
For measuring blood pressure, the chamber containing the diaphragm is coupled via
a thin plastic catheter to an artery. The catheter is filled with saline solution so that the
arterial blood pressure is coupled directly to the diaphragm outside the patient. However,
in this configuration it is important to realize that both ends of the catheter must be at
the same height to avoid hydrostatic effects and that the tube must be sufficiently stiff to
minimize compliance effects on the frequency response of the sensor. Air bubbles in the
catheter and obstructions due to clotted blood or other materials can introduce distortions
into the measured waveform due to resonance and damping effects.
It is now possible to obtain miniature pressure sensors that are located at the tip of
the catheter and to measure pressure within the blood vessel (Allen, 2002; Cromwell,
Weibell et al., 1973). As illustrated in Figure 2-56, these devices are sufficiently small
to be inserted into an artery or a vein through the needle of a hypodermic syringe, and
because of their small size they have an insignificant effect on the pressure or flow at the
measurement site.
Long-term stability of pressure monitors is a problem, and drift, particularly when
measuring low pressures such as venous blood or cerebrospinal fluid, can result in signif-
icant errors. Pressure transducers need regular recalibration, which can pose significant
problems, particularly if the sensor is implanted.
Table 2-8 shows a comparison of the properties of some pressure sensing technologies.
Measurement of respiratory processes is generally achieved by measuring expired
volume using a spirometer or flow rate using a pneumotachograph, as discussed in Chap-
ter 10. However, air pressure is measured as part of the operation of negative and positive
pressure respiratory devices. Sensors for measuring air pressure in these devices need not
be miniaturized but must be capable of measuring pressures from less than 2 cm of H
2
O
up to 40 cm H
2
O to assist or provide full ventilation.
