This invention relates to a process for measuring a conducting volume and a device for using this process, this volume being placed in an homogeneous conducting body which is itself placed in an environment which has large dimensions and resistivity compared with the dimensions and resistivity of the volume to be measured.
The rest of the description will refer to an example consisting of a device for measuring the heart rate making use of implanted sensors.
This type of device is used for the detection of irregularities in the heart rate and for the control of stimulators in a number of pathologies, or to determine the activity state of a patient.
Many parameters related to heart activity are measured, and in particular the partial pressure of oxygen in the blood, the mechanical measurement of contraction, measurement of the blood flow, direct measurement of electric activity (ECG) by electrodes, measurement of the blood pressure, measurement by the Doppler effect, measurement of the acceleration, etc.
The publications described below, the references of which are given at the end of the description, describe various embodiments according to prior art.
Reference [1] describes a system for monitoring the movement quantity or speed of cardiac masses by the use of an implanted sensor, this sensor being fixed to the internal muscular wall of the heart. Accelerometers, for example piezo-electric accelerometers, may be used as sensors.
Reference [2] describes a stimulation device including an accelerometric sensor and an electrode.
Reference [3] uses mechanical deformation sensors based on propagation lines, the impedance of which varies as a function of their geometry.
Reference [4] describes a system for measuring the velocity using a catheter comprising at least two staged electrodes. These electrodes form a polarized galvanic cell sensitive to the blood flow.
Reference [5] presents a system based on measuring the partial pressure of oxygen by electrodes.
Methods of measuring variations of the heart rate activity according to prior art do not use information about the global volume of a heart cavity. This magnitude is an extremely interesting marker, since the ejection volume may vary from 20 to 60% each time the heart beats. Furthermore, arrhythmia problems and other pathologies appear to be directly related to monitoring of this magnitude.
The purpose of this invention is a process for measuring this type of conducting volume.
This invention relates to a process for measuring at least one dimension of a conducting volume, characterized in that a measuring sensor is placed inside an existing cavity inside an homogeneous conducting or non-conducting body, the dimensions and resistivity of this cavity being large compared with the dimensions and resistivity of the volume to be measured, the sensor having characteristics that vary as a function of the medium immediately surrounding it. For example, the measurement sensor may include a self inductance mutually coupled to the medium, which may be a medium with electromagnetic losses.
The volume to be measured may vary with time.
In the first example embodiment, the process according to the invention is used for measuring the volume of a heart cavity which varies with variations in the heart cycle. Advantageously, the sensor may be placed at the end of a catheter which may consist of a coaxial transmission line.
In this example embodiment, the process according to the invention compares the normal cycle and the difference from the normal in order to detect any more or less serious dysfunctions in real time and in situ.
In a second embodiment, the process according to the invention is used to measure the diameter of a pipe with cylindrical geometry, for example such as a blood vessel.
The process according to the invention may then be coupled to a velocity measurement to measure the blood flow inside this vessel, this flow being an essential magnitude for evaluating cardiovascular pathologies.
More generally, the invention may be applied to any volume or diameter measurement of a pipe, provided that there is a contrast between the resistivity of the volume to be measured or the pipe and the resistivity of the surrounding medium.
The invention also concerns a device for embodiment of this process in which the measurement sensor comprises a self-inductance wound with adjacent or non-adjacent turns, and readout electronics comprising the following in sequence:
a 3 dB coupler receiving sensor data as input;
an amplifier;
a detection circuit, an amplifier being connected to this circuit and to the coupler;
a low-pass filter;
a shaping circuit connected to a signal output.