Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-04-19
2001-11-20
Schaetzle, Kennedy (Department: 3762)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S309000, C600S364000, C600S347000, C607S022000, C204S406000, C204S416000
Reexamination Certificate
active
06321101
ABSTRACT:
The present invention relates to a method for electro-chemical determination of the concentration of at least one dissolved chemical entity in a liquid medium by means of a working electrode and a counter electrode, an apparatus for performing the method, an active implant containing such an apparatus and the use of such a method in an active implant such as a pacemaker or the like.
DESCRIPTION OF THE PRIOR ART AND RELATED SUBJECT MATTER
In the description the following expressions are intended to have the following meaning:
Measuring Potential: the applied potential, as related to a reference potential, during the measurement. in the description denoted E. corresponding to a
Measuring Voltage: used in algorithms. which are part of the present invention in the description denoted U.
Floating Potential: the potential, as related to a reference potential, an electrode will acquire when placed in an electrolyte and no current is allowed to pass through an outer circuit, i.e. not passing through the electrolyte, in the description denoted E
0
.
Chemical Entity: a chemical entity is defined for the purpose of this application as either a gas or a chemical substance or compound dissolved in a liquid medium.
Such entities can be subjected to analysis by electrolytic reduction/oxidation reactions and the corresponding reduction/oxidation potential, or rather the electric current evoked by said potentials for characterizing the amount and substance reduced/oxidized.
Working Electrode: herein and below relates to the electrode at the surface of which the reduction of the chemical entity takes place, e.g. the working electrode can be used as an oxygen-sensor.
Sensor Rate: A calculated rate increase to be added to a basic pacing rate when the sensor is used in an active implant. The sensor herein and below being an oxygen sensor comprising a working electrode and a counter electrode and optionally a reference electrode.
A chemical entity is defined for the purpose of this application as either a gas, a chemical compound or substance dissolved in a liquid medium, said entities to be reduced or oxidized under the influence of an electric potential applied between electrodes.
Such entities can be subjected to analysis by electrolytic reduction/oxidation reactions and the corresponding reduction/oxidation potential, or rather the evoked electric current for characterizing the amount and substance reduced/oxidized.
In modern medicine, implantable heart pacemakers are used increasingly for the therapy of heart arrhythmia. It is a well-know fact that physical demands on the body requires levels ofoxygen adapted to the degree of physical activity to be delivered to the body. Thus the body activity lowers the venous oxygen concentration in the blood as a function of the degree of activity. The oxygen concentration in the blood and the changes in the concentration may be used as an indicator for a change in the stimulation pulse rate in order to regulate the rate of the stimulation pulses emitted by the pacemaker. See Pace, Vol. 17, p1939-1943, Johan Carlsten et al.
It is to be noted that oxygen in the blood exists in an equilibrium, The greater part of the oxygen is attached to the hemoglobin molecules while the some part is dissolved in the blood plasma and transported thus through the vascular system including the heart. The amount of oxygen combining with the hemoglobin is dependent on the oxygen's partial pressure in the blood, measurements of either one will give indication as to the amount of oxygen present. There are also other factors, which govern the ability of hemoglobin to combine with the oxygen, such as temperature and pH.
The oxygen saturation of the blood, which is a measure of the amount of oxygen bound by the hemoglobin, may be measured by different methods, e.g. transmission photometry and reflection photometry in the venous blood or measured indirectly by electrochemical methods, see above.
The photometric measurements do not show a linear dependency on the oxygen saturation and the measurement values have to be compensated in various ways, which is not the case in the electrochemical methods for measuring the oxygen partial pressure. In these later methods there is a linear dependency in the measurements of the measured current/measured voltage as a function of the oxygen partial pressure.
The measurements using electrochemical methods make use of the fact that the oxygen molecules dissolved in the blood are chemically reduced on the surface of the working electrode when the potential during a measurement pulse is forced to negative potential (about 1 volt) relative to a reference electrode/potential. The counter electrode is below assumed, at least partly, to have a surface made from carbon. In the reduction process, hydroxide ions are produced and the amount of these ions are dependent on the concentration of dissolved oxygen according to the reactions:
at the working electrode 2 H
2
O+O
2
+4e
−
→4 OH
−
at the counter electrode 4 OH
−
+C→CO
2
+2 H
2
O+4e
−
The above equations show a simplified picture of the process in the liquid (electrolyte), but for the purpose of this description they are sufficient.
The electrical current flowing through the liquid medium to the working electrode WE during the measurement pulses is carried by the hydroxide ions. This current called the oxygen current (I
pO2
), is proportional to the amount of hydroxide ions formed on the working electrode surface. During the measurement pulse the carbon coating of the counter electrode is oxidized to minute amounts of carbon dioxide (CO
2
), which is removed by the blood stream.
United States patents disclosing different aspects of electrochemical measurement teachings are e.g. U.S. Pat. Nos. 4,602,637 and 4,611,604 and 4,779,618 and 4,853,091. A factor influencing these measurements is the drift of the reference electrode.
One of the more stable reference electrodes that may be used is the Ag/AgCl-electrode. However, the bio-compatibility of the reference electrode is important as the reference electrode is to be implanted. Other types of reference electrodes with better bio-compatibility are not as stable as the Ag/AgCl-electrode. The Ag/AgCl-electrode also must be protected by e.g. a membrane when used in vivo because of the above reason.
During testing of electrodes it has been observed that the measured oxygen current (the current depending on the voltage imposed on the working electrode) increases or decreases by time.
SUMMARY OF THE INVENTION
It is therefore a principal object of the present invention to improve the certainty in the measurements and to eliminate the uncertainties emanating from the possible drift over time of the potential of the reference electrodes used in the prior art.
It is also a purpose of the present invention to improve the sensitivity of the oxygen/working electrode by diminishing the influences of the drift of the reference electrode by making it possible to perform the measurements without a reference electrode.
Another object is the possibility of using a measuring current that will be in a range most preferred for the reaction of the oxygen at the working electrode, i.e. where the sensitivity of the measurements is optimal.
Yet another object is to reduce as much as possible the amount of energy used for the measuring pulses and still make certain that a relevant value is attained.
The above objects are achieved in accordance with the principles of the present invention in a method and in a device for electrochemical determination of a concentration of at least one dissolved chemical entity in a liquid medium, employing a working electrode and a counter electrode with a measurement voltage being impressed on the working electrode relative to the counter electrode, the dissolved chemical entity reacting by oxidation or reduction at the working electrode with the reaction producing a measurement evoked current, and wherein the measurement evoked current is compared with a predetermined value and the meas
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Schaetzle Kennedy
Schiff & Hardin & Waite
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