Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 288

sensors. The authors stated that this difference could be explained by the infl uence
of oxygen. It is clear that the oxygen interference with the antimony electrode would
result in the inaccurate measurement of pH in cardiac surgery, since aortic clamping
and cardioplegia produce dramatic changes in myocardial oxygen levels.
The corrosion of antimony electrodes was also measured using ICP-MS (inductively
coupled plasma mass spectrometry) for dissolved antimony in vivo [156]. After the
electrodes were inserted in the plasma, the antimony concentration showed a linear rise
with time at a rate approximately of 94 g/L/h ( r 2 0.997). Although the projected

antimony concentration is lower than the safe limit, accumulation of dissolved antimony
and localized toxic effects in tissue may prevent the antimony electrode from long-term
implantable applications.

µ

10.5.5 Measurement of pH in the esophagus
The esophagus is the muscular tube that carries food and liquid from the throat to the
stomach. Gastroesophageal refl ux disease (GERD) is a common chronic acid-related
disorder caused by refl ux of gastric contents into the esophagus. This refl ux is respon-
sible for the discomfort of heartburn, a common symptom of GERD. This condition
affects an estimated 35-40% of Western populations, with approximately 7% experi-
encing daily symptoms [157, 158]. An ambulatory 24 hour continuous pH monitoring
in vivo is the method of choice for diagnosis of GERD and determination of the effec-
tiveness of treatments [159, 160].
The pH measurement device consists of a small, thin polymer catheter (about
1.5-2 mm in diameter) with a pH sensor probe at the end of the tubing. The thin tub-
ing is gently inserted through one nostril, down to the end of the esophagus as the
patient swallows. The catheter is connected to a waist worn recorder. The patient is
sent home with the catheter and recorder in place. Over 12 to 24 hours, the pH in the
lower esophagus is recorded and the refl ux or other symptoms the patient experiences
are recorded by pushing buttons on the recorder. A decrease in esophageal pH below
4.0 indicates that acid gastric contents refl ux in the esophagus. The percentage of time
spent below pH 4 over total pH monitoring time is defi ned as the refl ux index (RI) and
a cut-off RI value for “normal” vs “abnormal” is set at 5% [159].
In some studies, a probe with two pH sensors in the proximal and distal esophagus
has been used to monitor pH simultaneously [161]. Such a catheter probe for esopha-
geal pH can also be used for monitoring the hypopharynx for laryngopharyngeal refl ux
(LPR). In this case, one pH sensor is positioned in the hypopharynx and the other in
the esophagus [162]. To further accommodate variable esophageal lengths in different
patients, a triple-pH sensor catheter probe has recently been developed [163].
In addition to glass electrodes [159], the pH sensors incorporated in the catheter
for esophageal pH monitoring were reported to be mostly antimony electrodes such
as the Slimline from Medtronic Inc. [95, 158, 164]. In a recent study, Pandolfi no et al.
[33] compared the accuracy of the Slimline antimony pH monitoring system to that
of a conventional glass electrode catheter pH system during ambulatory conditions in
18 patients. They reported that the antimony electrodes had acceptable performance