Biomedical Engineering Reference
In-Depth Information
tip that is 1 mm in diameter and 10 mm in depth. When the electrode was fully inserted
perpendicularly into myocardial tissue, a 10 mm depth was reached. The electrode that
was previously developed in conjunction with Vascular Technology Inc. (VTI) was
discontinued in 2001. A new system has been under development for the myocardial
tissue pH measurement in conjunction with Terumo Cardiovascular Systems (http://
www.terumo-cvs.com).
In a study published by Skrobik and Filep [154], two such electrodes were used.
One electrode was surgically implanted in the right ventricular myocardium (epicar-
dial) and the other electrode was implanted in the jugular vein to the level of the endo-
cardium of the right ventricle in pigs, to monitor endomyocardial and epicardial tissue
pH, respectively. Khuri
et al.
[155] have reported a study using these glass electrodes
in a large number of patients to monitor the intramyocardial pH to examine the rela-
tionship between intraoperative regional myocardial acidosis and long-term survival of
patients undergoing cardiac surgery with cardiopulmonary bypass and aortic clamping.
They reported the fi ndings based on data from 496 patients who were followed up for
an average of 10 years postoperatively. They concluded that regional myocardial aci-
dosis, as measured with tissue pH electrodes during cardiac surgery, was predictive of
long-term survival of these patients, and reducing acidosis during operations improved
long-term patient survival rate.
Simultaneous and continuous measurements of extracellular pH, potassium K
, and
lactate in an ischemic heart were carried out to study lactic acid production, intracel-
lular acidifi cation, and cellular K
loss and their quantitative relationships [6, 7]. The
pH sensor was fabricated on a fl exible kapton substrate and the pH sensitive iridium
oxide layer was electrodeposited on a planar platinum electrode. Antimony-based pH
electrodes have also been used for the measurement of myocardial pH in addition to
their application in esophageal acid refl ux detection.
Rosenfeldt
et al
. [156] evaluated a miniature antimony electrode for the measure-
ment of myocardial pH. The electrode was in a coated wire format with about 0.7 mm
in diameter and a layer of polycrystalline antimony was coated on a copper wire. The
antimony electrodes had low impedance at sub M
range, which is much lower than
a glass electrode. The electrode response was calibrated in a 100 mM phosphate buffer
of pH 6.2-7.8 and the linear response slopes were 45 to 52 mV/pH at 25ºC. The elec-
trode properties were compared to those of a conventional glass electrode
in vitro
and
an optical pH sensor
in vivo
(Paratrend 7, Biomedical Sensors Ltd, UK). They found
that antimony electrodes were more sensitive to temperature changes than glass elec-
trodes. In fact, the temperature effect was observed to be about three times higher in
the antimony electrode than that in the glass electrode, within the temperature range of
10ºC to 40ºC. Their results indicated that such temperature effects were fairly repro-
ducible on all the antimony electrodes tested and suggested the possibility of thermal
compensation to overcome such limitation.
The antimony electrode used by Rosenfeldt
et al.
[156] did respond proportionally to
the myocardial pH changes in dogs produced by infusion of sodium bicarbonate or inha-
lation of carbon dioxide. However, pH measured by the antimony electrodes was con-
sistently about 0.26 units higher than that which was measured with the Paratrend optical
Ω
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