The present invention relates to a sensor for measuring conductance of material and in particular a sensor that may be used, among other applications, for measuring periodic respiratory and cardiac motion by detecting changes the in conductance of patient tissue.
The conductance of a material, that is, how well it conducts electricity, may be determined by applying a voltage to the material with a pair of electrodes and measuring the corresponding current flow.
This approach, while straightforward, has several disadvantages. First, it is sensitive to localized high resistances at the interface between the electrodes and the material. Second, the electrodes used to apply the voltage must contact the material raising issues of contamination of the materials and corrosion of the electrodes.
For this reason it is known to make non-contacting measurement of conductivity using a pair of electrical coils immersed in the material being measured. The premise behind such systems is that electrical coupling of an alternating current signal between the two coils is functionally related to the conductivity of the material between the coils. Such systems have the disadvantage of requiring multiple coils.
A single coil resistivity measuring system (resistivity being the inverse of conductivity) is described in U.S. Pat. No. 4,536,713. In this system, eddy currents are induced in a drilling fluid for an oil well or the like by a magnetic field applied via a ferrite core. The measured eddy currents, as reflected on the driving voltage of the coil, provide a coarse indication of drilling liquid resistivity.
Electronic monitoring of respiration may be performed for critically ill or comatose patients, small children at risk of sudden infant death (xe2x80x9cSIDxe2x80x9d), or patients with sleep apneas (partial suspension of breathing sometimes associated with insomnia).
There are two types of sleep apnea. Obstructive apnea, is caused by blockage of the windpipe or mouth and nose, and is characterized by a change in the phase relationship between respiratory movement of the chest and abdomen. In central apnea, the patient ceases all signs of breathing.
Distinguishing central apnea from obstructive apnea requires independent detection of abdominal and thoracic respiratory motion.
A number of different types of respiration sensors are known in the art, including mechanical sensors employing air flow measuring turbines or the like, that require the use of a face mask or mouthpiece, and inflatable cuffs which measure chest or abdomen extension manifest as a change in cuff pressure. Such mechanical systems can suffer from problems of reliability.
Respiratory motion may be also detected electronically by conductive coils wrapped around the patient. In one prior art device, multiple inductive coils are positioned on the patient to move with respect to each other as the patient breathes. The mutual inductance between the coils changes with their relative motion and is measured to detect respiratory motion.
In a somewhat different design, respiratory deformation of a single coil causes a change of inductance in that coil which may be measured by the detuning of an oscillator using the coil""s inductance as a tuning element. The change in frequency of the oscillator is used as a measure of respiratory motion.
With these electrical sensors, the sensing coil must be snugly fit to the patient so as to move with the patient and for this reason may create an unpleasant confining sensation or pressure on the patient""s trunk and stomach. Such constraining pressure is particularly problematic with small infants. Such electronic sensors are unsuitable for measuring cardiac motion which does not result in significant distention of the patient""s chest or abdomen.
The present invention provides a single-coil non-contacting conductance sensor that provides improved accuracy in conductance measurement. By driving the single coil at its resonant frequency (as coupled to the measured material) capacitive and inductive effects are minimized making the eddy current measurement more robust against the influence of the geometry of the measurement and qualities of the material other than its conductance.
Specifically, the sensor includes a conductive coil driven by an auto-tuning oscillator. The auto-tuning oscillator seeks a resonant frequency of the conductance coil as coupled to a measured material and its environment to induce eddy currents in the measured material. An impedance measuring circuit is connected to the conductive coil providing a measure of the effective resistance of the coil at the resonant frequency as reflects the magnitude of the induced eddy currents within the sample.
Thus, it is another object of the invention to provide a simplified conductance measuring sensor that is resistant to inductive and capacitive effects. By driving the coil at its natural resonant frequency, the resistive effects to be measured dominate.
The auto-tuning oscillator may include a gain controlled amplifier having a gain control input signal reflecting the amplitude of the signal driving the coil and the impedance measuring circuit may derive resistance from the gain control signal.
Thus it is another object of the invention to provide a conductance sensor that is resistant to amplitude dependent effects in the conductance measuring process.
The gain control signal may be provided by a synchronous rectifier, rectifying an oscillator signal from the oscillating current synchronously with the oscillating current. The synchronous rectifier may be a multiplier multiplying the oscillator signal by itself.
Thus it is another object of the invention to provide a conductance sensor that is resistant to asynchronous noise sources. The synchronous rectifier is not switched by larger amplitude out of phase or different frequency signals which will then be averaged out in a subsequent filtering stage.
One application of the conductance sensor is electronic monitoring of respiratory (or cardiac) motion which relies on the measurement of changes in conductivity in the patient""s tissue within a limited cross-sectional area. Because the measurement does not rely on changes in the coil""s dimensions, the sensor may be less constraining to the patient and more robust against deformation of the coil in use.
Specifically, the conductive coil is sized to fit about the body of a person. The auto-tuning oscillator, connected to the conductive coil, drives the coil with an oscillating current at a resonant frequency to induce eddy currents in the person. Attached to the conductive coil is the impedance measuring circuit which provides a measure of the effective resistance of the coil at the first frequency as reflects the magnitude of the induced eddy currents within the person. An electronic filter receiving this impedance signal extracts a periodic plethysmographic signal.
Thus, it is one object of the invention to provide a method of measuring periodic physiological motion, such as cardiac or respiratory motion, by detecting changes in areal conductance of patient tissue within a coil without the need to deform the coil in time with the physiological motion. Because the invention does not require outward distention of the patient, it is equally effective to measure periodic internal conductance changes, for example those caused by the heartbeat, as with those conductance changes which are reflected in outward movement of the patient, such as breathing.
The patient monitor may include two coils sized to fit about the body of the person, one in the abdominal area of the person and one in the thoracic area of the patient. Here, the impedance measuring circuit provides measures of the effective resistance of the two coils separately. An alarm may compare these separate resistance measurements to determine when distress of the person is indicated.
Thus, it is another object of the invention to provide an electronic respiratory detector that can provide separate measures of two different regions of the body in proximity to each other. Because only a single coil is needed to measure conductivity, multiple measurements may be made by adjacent multiple coils.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims herein for interpreting the scope of the invention.