Process and apparatus for the detection of catheter-tissue...

Surgery – Diagnostic testing – Measuring electrical impedance or conductance of body portion

Reexamination Certificate

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C600S443000, C600S523000

Reexamination Certificate

active

06304776

ABSTRACT:

The invention relates to a process and an apparatus for the detection of contact with tissue by a catheter which is arranged in a patient's vessel, particularly in a patient's blood stream, and furthermore to a process and an apparatus for the detection of the interaction of high frequency energy with the patient's tissue.
In many medical applications or treatments it is of the greatest importance for the doctor to detect, to follow, and in many cases also to record, the contact of an instrument with the patient's body, with high precision, since the result of the treatment frequently depends on this to a considerable degree.
In particular, in catheter ablation, the desired treatment effect is as a rule only obtained when a contact between the ablation catheter and the patient's tissue to be treated can be insured during the whole period during which ablation power is delivered.
Resistance measurements between at least two catheter electrodes, or the cathode electrodes and a neutral electrode arranged on the patient's body, were heretofore carried out for the detection of catheter-tissue contact. However, this manner of proceeding is disadvantageous, particularly in high frequency catheter ablation, since the resistance measurement is connected in principle with a flow or current through the region to be measured. However, additional currents are thereby introduced during the ablation, and can cause severe disturbances, for example in the detection of EKG signals. If direct current resistance measurement methods are used, in order to avoid high frequency signals which affect the detection of EKG signals, undesired electrolytic effects are produced, and lead to a further chemical stress for the patient. Furthermore, resistance measurements are difficult to carry out and subject to disturbance, where corrosive effects and surface contamination hinder the flow of current or falsify the measurement result. Such a situation is however already present in the field of medical applications, since here the blood or body fluids of the patient have salts or coagulating substances which can undesirably interact with the surface of electrodes or contacts. Furthermore, the tissue contact during the ablation can be judged by means of impedance measurements, only with difficulty, since many factors can affect the impedance value.
Consequently, the invention has as its object to avoid the abovementioned disadvantage and to contribute to an improved detection of contact between the catheter and the patient's tissue. This object is attained in a most surprising manner by a process according to claim
1
and an apparatus according to claim
11
.
The inventor has found in a surprising manner that in many cases, when electrodes are placed on a patient's tissue, in particular in the case of tissues bathed in blood, voltages arise between the electrodes and in particular during HF ablation. The preferably metallic electrode of an ablation catheter for example produces the voltage signal shown in
FIG. 1
and obtained according to the invention in the measuring arrangement shown in FIG.
2
.
The inventor has found in a most surprising manner that a catheter arranged in the blood stream produces at first only a very weak voltage signal, denoted here by U
0
, in contrast to which there then follows an abrupt rise in voltage when the catheter electrode comes into contact with the tissue. The instantaneous value or amplitude of the voltage signal derived in this manner represents a measure of the quality of the cathetertissue contact and can be detected without the use of additional external currents. Consequently in the manner according to the invention no electrolytic processes arise, and as a result of this, further measurements such as the recording of EKG signals, for example, are not adversely affected.
It has furthermore been found that the voltage measurement according to the invention is clearly superior to the conventional resistance measurement in regard to the measurement data obtained. Firstly, the voltage is an extremely accurate measure for the contact of the catheter electrode with the tissue, while an impedance measurement during the ablation permits only conditional conclusions about the tissue contact of the catheter, and secondly this signal appears nearly without any time delay, which makes this suitable for real time measurements.
It has further been found in a particularly surprising manner that the height of the measured signal is very precisely correlated with the temperature of the tissue, particularly that of a tissue which is heated during ablation. It could be shown by parallel measurement processes that with the invention, temperature values of the tissue in contact with the catheter could already be measured with an accuracy of ±1° C. Furthermore, accuracies of ±2° C. could be measured with simple means, i.e., simple, high ohmic voltage measurement devices. Here the temperature is in regular, linear proportion to the measured voltage, which made it possible to allocate standardized voltage measurement values to the same ablation catheter or to a group of catheters of like construction.
Since voltages can in principle also be measured without current, for example by impressing a counter-voltage of equal height, or by measuring with very high-ohmic instrumentation amplifiers, transition resistances of contacts within the measurement region play a far smaller part than in all the conventional measurement processes. Consequently, the apparatus according to the invention or the process according to the invention is exposed to fewer disturbances and can be used more conveniently and reliably. Furthermore, thermal (transit) resistances, and also thermal capacitances, of the catheter have far smaller effects on the accuracy and also speed of the measurement of the temperature.
It has been found in a further most surprising manner that in catheter ablation itself, its results can be considerably improved. If, for example, the catheter temperature is taken as the control value, as in the conventional catheter temperature control, and thus as a measure of the energy delivery, it can happen that with a relatively cold catheter, for example, at a temperature of only about 40° C., a marked lesion in the tissue is already produced, when the catheter is located in very close contact with the tissue and hence a large portion of the ablation energy could be delivered into it.
The conventional, pure temperature control with thermal sensors would not have detected the lesion in this case. Furthermore, only small lesion effect or ablation effect take place even at a higher catheter temperature when the catheter-tissue contact is very poor. In both cases, an inexact or false opinion of the result of the treatment could be derived from the conventional processes and apparatuses.
The starting point according to the invention, however, is that it is always insured by the voltage signals which arise and are detected that the energy delivery takes place substantially into the tissue, and consequently in the invention the power delivered by the catheter is allocated more directly and exactly to a treatment effect. Furthermore, after the abrupt voltage rise has detected the catheter/tissue contact, the absolute height of the voltage signal can be used for very accurate temperature measurement.
Furthermore, it is possible to use the arising voltage signal for the control or monitoring of the ablation process itself. If, for example, during high frequency ablation, the voltage signals are determined during the delivery of high frequency power, and if switching off or at least a reduction of the delivery of high frequency power is effected on exceeding or falling below the value, it is always insured that the high frequency power was substantially delivered into the patient's tissue, and this leads to a treatment with overall cooler catheter electrodes and higher action.
It is furthermore advantageous to integrate the voltage signal and thus to at

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