Biomedical Engineering Reference
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compared with the glass electrodes. Their results suggested that the electrochemical
limitations of antimony catheters could be overcome by careful calibration and thermal
compensation.
Wise et al. [165] reviewed the potential drift of antimony pH electrodes in 100 con-
secutive 24 hour ambulatory esophageal pH studies. Their fi ndings suggested that drift
in pH during 24 hour pH studies using antimony electrodes was common, but large
degrees of drift are less frequent. Drifts of at least 0.1 pH occurred in 88% of studies
while drifts of 0.4 pH or higher occurred in only 9% of studies.
Although they are effective, catheter-based testing systems may be limited by
patient discomfort and interference with normal diet and activities. The probe can
cause a great deal of discomfort and sometimes patients do not tolerate it. Wireless pH
measurement devices have been used in studies to reduce these limitations and to pro-
vide prolonged, continuous recordings of esophageal pH for up to 48 hours [166, 167].
The wireless pH capsule (Medtronic Inc.) is oblong in shape and contains an anti-
mony pH electrode, a reference electrode at its distal tip, a battery, and a RF trans-
mitter. The whole device is encapsulated in epoxy. The capsule is introduced into the
esophagus on a catheter through the nose or mouth and is attached to the lining of
the esophagus with a clip. The probe monitors the pH in the esophagus and transmits
the information via RF telemetry at a rate of 6 per second (0.17 Hz) to a pager-sized
receiver that is worn by the patient on a belt. Prior to implantation, the capsule is cali-
brated with its receiver in pH buffer solutions of pH 1.07 and pH 7.01 [168].
Kahrilas and Pandolfi no [160] have reviewed certain clinical performances, such
as safety, tolerability, and diagnostic accuracy of the Bravo wireless pH system in
ambulatory esophageal pH measurements. Des Varannes et al. [168] analyzed the cor-
relation between wireless pH systems and catheter-based systems. They reported that
strong correlations between esophageal acid exposures recorded with the two devices
were established during simultaneous recordings of esophageal acid exposure. They
observed that the wireless pH system reported a lower pH and signifi cantly underre-
corded acid exposure (especially for brief refl ux events 12 s or less) compared to the
catheter-based pH device. The discrepancy in lower pH readings was attributable to
the pH sensor calibration. The discrepancy in recorded acid exposure was explained
by the different response characteristics of the two electrodes. The sampling frequency
of the wireless Bravo electrode was lower (0.17 vs 0.25Hz), and the response time
was longer than that of the catheter electrode. In addition, poor reproducibility of the
catheter system due to the movement of the pH sensor tip may also contribute to such
detection differences [160, 169].
Studies to determine tolerance of such implanted devices have been carried out
[167, 170]. Generally speaking, the devices are well tolerated by the patients, although
some mild symptoms are reported. The symptoms range from foreign body sensation
to chest discomfort or pain. It is reported that such symptoms are more obvious in
women and younger patients [171]. Although more studies are still needed to examine
the correlation between refl ux events and symptoms, the Bravo pH capsule represents
a signifi cant advancement in the development of wireless, implantable, and solid-state
pH measurement systems.
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