CA1291796C - Hand-held self-contained electronic tonometer - Google Patents

Abstract

S P E C I F I C A T I O N

HAND-HELD SELF-CONTAINED ELECTRONIC TONOMETER

ABSTRACT OF THE DISCLOSURE

Disclosed is a battery powered, hand-held self-contained electronic tonometer with a digital readout for displaying pressure in millimeters of mercury. The tonometer includes a transducer which is a solid state pressure sensitive element and which produces a voltage proportional to intraocular pressure.
An electrical waveform is produced by gently bringing the transducer in contact with the cornea. The waveform is converted to a digital signal and processed by a single chip micro-processor. The baseline of a reference signal is nulled by equalizing the charges on two capacitors on the inputs of two differential amplifiers. Equalizing the differential inputs of the two amplifier stages results in a gain of zero and removes any carrier signal. Microprocessor software detects the baseline condition established by equalizing the differential inputs. The microprocessor then looks for a series of valid measurements and calculates the average intraocular pressure along with an estimate of reliability. This average value and the reliability indicator are displayed on a liquid crystal display.

Description

~291796 173~92 ~ACKG~OUND OF THE INVENTION
The measurement of intraocular pressure is an important function performed by ophthalmologists and other eye care pro-fessionals. Pressure measurements are performed (a) as a routine part of the complete eye examination to identify patients with or at risk for developing glaucoma, (b) to monitor progress and response to treatment in patients with glaucoma and ocular hypertension.

The first tonometer was developed in 1926 and is called the Schiotz tonometer. This simple instrument employs a weighted plunger which is lowered onto an anesthetized eye. The amount of deflection of an indicator is proportional to intraocular pressure; however it is also sensitive to scleral rigidity which could lead to an inaccurate measurement. The intraocular pressure is obtained indirectly using a supplementary table.
This somewhat difficult-to-use and inaccurate instrument is still popular today among older eye physicians and in general medical practice.

The Goldman applanation tonometer was developed in 1957 to measure intraocular pressure using an applanation method. The anesthetized cornea is flattened against a glass plate of known diameter, pr~ducing a meniscus of tear film between the head of the instrument and the cornea. This technique is less sensitive to scleral rigidity. However, the Goldman tonometer must be attached to a slit-lamp microscope so that the manual measurement can be made accurately.

3~

173~92 ~:9~`796 There is a portable version of the Goldman tonometer known as the Perkins tonometer which is a hand held device employing similar applanation technology. However, this instrument is quite difficult to use as the examiner's eye must be literally within inches of the patient's eye and stabilization of the instrument is difficult. Therefore, except for examina-tions under anesthesia, the Perkins tonometer is rarely used.

The McKay/Marg tonometer, introduced in 1959, exploits different technology. This instrument incorporates a small electrical strain gauge in the tip of the hand-held probe which is attached to a large carrying case containing an amplifier, strip chart recorder, and transformer. This is a contact device and therefore requires the use of topical anesthesia. The instrument works by relating a change in voltage to intraocular pressure. The user interprets the strip chart output signal, usually interpolating over several subjectively "acceptable"
signals.

The Pneumotonometer was introduced in 1975. It works by bringing a small air burst toward the cornea. The back pressure is sensed, and is found to be proportional to intraocular pressure. This instrument seems to have inaccuracies, especially at the low range.

Another instrument by A. O. Reichert utilizes an air applanation technique, which does not require tou~hing the instrument to the eye. An air puff of a given force and diameter is used to flatten the cornea. The amount of flattening is sensed by the machine and is proportional to pressure. This is the most popular unit in the optometric community because it does not require topical anesthesia.
Harold Rose and Bruce Sand developed an applanation tonometer which utilizes a digital read-out and is described in United States Letters Patent number 3r724,263.
Although some of the above instruments provide reliable estimates of intraocular pressure, they lack portability, reliability, accuracy, or acceptance in the marketplace, and none of the instruments are ideal in providing some key features. Therefore, eye care professionals and the general medical community is still seeking a precise hand-held portable tonometer to assist them in the diagnosis and management of glaucoma.
The present invention is related to the apparatus di~clo~ed in U.S. Patent No. 4,817,432 i5~ued Aprll 4, 1989.

~,l, ', "~ ,$
.J., ,s `' SUMMARY OF THE INVENTION
The electronic tonometer is comprised of a precision strain gauge, a three stage high gain amplifier, and a micro-processor. The microprocessor is highly interactive with the amplifier circuitry to insure accurate data acquisition and control. The differential output of the strain gauge is fed into a first stage amplifier where it is converted to a single-ended non-differential output. A modulated carrier signal is successively amplified by the second and third stages of the amplifier circuitry and processed by an internal analog-to-A digital (A/D) converter in a Hitachi 6305 microprocessor. ~hilethe microprocessor is in the data analysis mode, it enters as many as ten states of logical processing to acquire and process the carrier signal. The microprocessor only requires the differ-ential levels of this signal for accurate processing and does not require absolute voltage reference levels. It is necessary that the second and third stages of the amplifiers be nulled before the measurement and analysis process can begin. This involves finding a stable amplifier baseline to reference and calculate the relative amplitude of the pressure waveform. To accomplish this, the microprocessor applies an active high capacitor dis-charge signal for a period of 60 mS. This nulls both the second and third stages of the amplifier circuitry by equalizing the charge on the inputs to the second and third stage amplifiers.
This neutralizing effect equalizes both differential inputs for each amplifier stage, resulting in a gain of zero and removing any carrler signal. This process allows the microprocessor to reset the baseline when needed while dynamically processing the pressure waveform. The amplifiers are effe~tively dc coupled (since there is virtually an infinite time constant) which gives r k _5_ ~917~ 60724-17Ql the microprocessor a dc level signal to process. In a conventional ac coupled amplifier circuit, nulling would have to be accomplished either by a complex precision auto/nulling hardware circuit or by operator manual calibration before instrument use.
Therefore it is an object of the present invention to provide the eye care professionals and general medical community with a precise self-contained hand-held portable tonometer that is reliable and accurate so as to assist them in the diagnosis and management of ocular hypertension and glaucoma.
According to a broad aspect of the invention there is provided a baseline nulling system in which inputs to ampli~ier stage~ are dynamically equalized prlor to and during ~ignal analy 1~ compri~ing amplifier means with a plurallty of ampllfier stages wherein a flrst stage is capacitively coupled to a second stage and said second stage ls capacitively coupled to a third stage, analysis means connected to said third stage for receiving electronic signals from sald thlrd stage, corresponding to pressure measurements, analog switch means comprising a first analog switch and a second analog switch, said flrst analog switch being connected to said analysis means and to said second stage, and said second analog switch being connected to said analysls means and said third stage, and wherein said analysis means causes said first and second switches to open and close for a pre-determined perlod of time between the open state and closed state prlor to said analysis means recelvlng e~ectronic signals from said third stage and whereln said analog swltch means cause~ the gain of ~ald second ~0 and third stages to have a pre-determined galn when ~aid first .~

~ 60724-1701 and second analog switches are open.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective drawing of the tonometer.
Figure 2 is a front view of the liquid crystal display.
Figure 3 is a top view of the tonometer showing the placement of the various components inside the instrument.
Figure 4 is a section view of the tonometer showing component placement.
Figure 5 is a system diagram.
Figure 6 is a perspective drawing of the lnstrument as used to make a pressure readlng on a patient's eye.
Figure 7 is a circult diagram of the three-stage ampllfier wlth its assoclated baseline reference nulling circult.

-6a-~ ?Jg ~t7~ ~ 173/~92 Figure 8 is a waveform resulting from a pressure measurement.

Figure 9A is a section view of the pressure transducer and Figure 9B is a top view of strain gauges on a plate.

Figure 10A through 10C are flow diagrams of a program.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The tonometer consists of a housing that is contoured such that it is easily grasped, in a manner of a writing pen.
The tip of the instrument is the solid state pressured transducer element. The other functioning components of the instrument include an activation button, located on the anterior dorsel sur-face in close approximation to the index fingertip of the user, a liquid crystal display, a reset button, and a removable battery cover.

The measurement transducer is a ~olid phase pres~ure sensitive element which produces a change in voltage with a change in intraocular pressure. The electrical waveform produced by gently bringing the transducer in contact with the cornea is converted to a digital signal and processed by a microprocessor.
The microprocessor is highly interactive with the amplifier circuitry, insuring accurate data acquisition and control. The microprocessor uses multiple criteria such as slope and configuration of the waveform for accepting a reading as valid and then calculates the average intraocular pressure along with an estimate of its reliability. An average presCure value and the reliability are then read out on a liquid crystal display.

~796 173/l92 Figure 1 shows a perspective view of the tonometer 20.
The tonometer 20 has a housing 22 which is formed so that a user can grasp the instrument and have his or her index finger over activation switch 24. The transducer housing 26, contains a strain gauge that is used to convert the pressure indications from the cornea to electrical impulses. The contact head 28 of the transducer housing 26 has a thin rubber membrane which covers a central post 34 (Figure 4) attached to the strain gauge 36.
After repeated measures are obtained by intermittent contact with the cornea, the pressure is then read out on the liquid crystal display 30 shown in Figure 2.

~ igure 3 is a top view of the tonometer 20 with its various components. The batteries 40 are located toward the rear of the tonometer. Adjacent to the batteries 40 is the display 30 and adjacent to the display 30 is a microprocessor 42. A three-stage high-gain amplifier 46 and its baseline reference nulling circuit are located forward of the microprocessor on a printed circuit board 44 (Figure 4). The activation button 24 is located on the top forward portion of the instrument for easy operation by the user.

In Figure 9A, the transducer comprises a contact head 28 and an internal assembly 29. The contact head 28 contains two components, a base 48 and a central post 34. The central po~t 34 is flush with the base 48, but may vary up to .5 microns from the base 48 without affecting the measurement. The central post 34 is welded to two flexures 33 and 35 which are .002 of an inch thick and one half inch in diameter. Multiple cutouts 37 are shown which serve to decrease mass while preserving strength of ~ 7~ 173/192 the elements. The anterior flexure 33 is passive, serving primarily to align the posts. The posterior flexure 35 is active in the measurement in intraocular pressure. Mounted on the ~ 5 C ~ ) r~
flexure are four miniature impedance-matched solid state 39, two of which are configured to be altered by stress and two by strain. The circuitry is configured as a Wheatstone bridge.
Balancing resistors and thermal correction resistors are added to the circuit, as required. A voltage of two to six volts is utilized to activate the bridge when pressure is applied to the central post 34. This force causes a change in the flexure state-which is proportional to an output voltage. A stop 41 is placed posterior to the active flexure in order to protect against accidental long excursions of the post.

The central post has a mass which produces a measurable orce when the transducer is moved from a "tip down" to a "tip tr~ o d c up" position. The calibration ~e is initiated by two presses of the activation button 24 in rapid succession. ~here is an automatic ca~ib~ation of the electrical output of the trans~ucer to an inte~val va~ue ~eplesenti~g the folce supp~ied by g~avity on the mass of the central post 34. If the discrepancy between stored and calibrated values differ by 10%, the instrument cannot be put into the measurement mode. Recalibration, however, can be attempted.

The transducer must be aligned perpendicularly to the corneal surface as shown in Figure 6 wherein the tonometer 20 is shown aligned with the visual axis 23 of the eye 21. Incomplete or off-axis contact results in ~low and/or inade~uate excursion of the post. When a correct applanation of the tonometer onto P ~ 173/~92 the eye is made, a sharply rising edge of the electrical waveform is elicited from the transducer shown as edge 90 in Figure 8.
Continuing pressure beyond that necessary to contact the cornea results in an artificial elevation of intraocular pressure by the instrument itself, shown in Figure 8 as peak 93. At the point of optimal contact, there is a minimal indentation of the cornea by the base of the transducer tip. This results in a sm~ll transient depression 92 of voltage which best correlates with intraocular pressure as determined by manometric techniques.

As the transducer is removed from the cornea, a mirror image of the applanation waveform is produced such that the case of minimal corneal indentation is again achieved with a second corresponding minimum 94 in an otherwise stable voltage, follow-ing which the voltage rapidly returns to baseline as shown by Ealling edge 96. This waveform is shown in Figure 8. Thus, voltage depression 92 and voltage depression 94 are substantially equivalent and best correlate with intraocular pressure.

The output voltage of the transducer 36 is ac coupled.
To prevent a wandering baseline between measurements, a capacitor is shorted just prior to activating the transducer. The analog electrical signal i5 then digitized by the microprocessor 42 at a sampling rate of 200 to 300 Hz. Up to 32 sequential values are stored in random access memory in the microprocessor 42 and analyzed.

Analysis begins upon momentary application of the contact head 28 to the eye and consists of determining criteria for the baseline, for perpendicular application and release of 173~92 the transducer to/from the cornea, and for optimal indentation.
With each readable momentary application of the contact head 28 to the eye, a brief click is heard by the user, supplied by the microspeaker 50, and which is elicited by a train of electrical signals delivered from the microprocessor 42 to the microspeaker 50 mounted on the printed circuit board. All readable measures of intraocular pressure are averaged after six measurements are obtained. The range is then computed. A "beep" is given by means of a medium frequency output from the microprocessor 42 to the microspeaker 50, signaling that a reading has been obtained. The mean intraocular pressure in millimeters of mercury (Hg) is then shown on the liquid crystal display 30. One or more of four annunciator bars 34A, 34B, 34C and 34D may be illuminated denoting a correlation variance which is plus or minus 5~ of the mean, shown by annunciator 34A, plus or minus 10%
of the mean, shown by annunciator 34B, plus or minu~ 20~ of the mean, shown by annunciator 34C, and greater than plus or minus 20% of the mean, shown by annunciator 34D. If ten applications of the transducer are made without achieving six readable measurements, no numeric value is displayed and the "beep" is given. The annunciator bars are shown in Figure 2 in relationship to the display 30. At any new touch of the activa-tion button, the microprocessor allows new measurements of intraocular pressure to be obtained. Figure 6 shows the tono-meter 20 being used to make a pressure measurement on a patient's eye 21 along the visual axis as shown by dotted line 23.

Figure 5 shows a detailed system block diagram which consists of a three-stage high-gain amplifier 46 with its associated baseline reference nulling circuit, a microprocessor ~9~ 173/l92 42 and a display 30. The strain gauge 36 (Figure 4) is used to convert the intraocular pressure of the eye to an electrical impulse. When the contact head 28 of the transducer housing 26 is put in contact with the surface of the eye, then the central post 34 of the strain gauge 36 is caused to move which in turn causes the plate 35 on which the strain gauges are mounted to bend slightly. That in turn causes the resistance of the strain gauges 39 to increase/decrease. The strain gauge forms two of the resistive elements of a Wheatstone bridge. The output of the Wheatstone bridge is connected to the three-stage high-gain amplifier 46 where the signal is amplified for input to the microprocessor 42. The microprocessor then follows the sequence chown in the flow chart of Figures lOA through lOC to perform the analysis on the waveform from the three-stage high-gain amplifier 46. In that analysis, shown in the flow chart of Figures lOA
through lOC, the dc component offset or baseline must be subtracted from the pressure waveform, shown in Figure 8, to determine the relative differential signal of interest.

Referring to Figure 7, which is a circuit of the three-stage high-gain amplifier 46 with its associated baseline reference nulling circuit, state 1 of the microprocessor logic involves finding a stable amplifier baseline to reference and calculate the relative amplitude of the pressure waveform shown in Figure 8. To accomplish this, the microprocessor applies an active high (capacitor discharge) signal from lead 78 to control lines 54 and 56 of the analog switches 58 and 60 for a period of 60 MS. This nulls both the second stage 62 and the third stage 64 of the amplifier circuitry by equalizing the charge on both sides of capacitors 66 and 68. The charge of the capacitors are 1~91~796 neutralized because there is a 1.5 volt reference voltage supplied on lead 71 which equals the reference voltage of the second stage amplifier and is applied on the second-stage amplifier side of capacitor 66. Similarly, a 0.4 volt, set by resistors 72 and 74/ is directly applied to the third-stage side of the capacitor 68 which equals this stage's reference voltage.
This neutralizing effect equalizes both differential inputs for each amplifier stage resulting in a gain of zero, and removing any carrier signal. After 60 milliseconds, the capacitor discharge signal on leads 54 and 56 is terminated, which opens both of the-analog switches 58 and 60 controlled by the micro-processor 42. During the 60 millisecond time period the microprocessor is processing data already received.

This design is unique because the microprocessor is able to use the capacitor discharge control to reach the baseline when needed, while dynamically processing the pressure waveform data.
The amplifiers 62 and 64 are effectively dc coupled (since there is virtually an infinite time constant) which gives the micropro-cessor 42 a dc level signal to process. In a conventional ac coupled amplifier circuit, nulling would have to be accomplished either by complex precision auto-nulling hardware circuit or by operator manual calibration before instrument use.

When the activation switch has not been depressed for twenty seconds, the microprocessor 42 and transducer elements are turned off in order to conserve power and preserve the battery life. A small discrete circuit performs this function and also responds to depression of the activation button by activating the electronic elements and the transducer.

~?,9~796 173/l92 All elements of the tonometer instrument are connected to a multilayered circuit board. Mounted off the circuit board are four silver oxide batteries. Mounted on the circuit board 44 are the microprocessor, the microspeaker, and the discrete cir-cuitry related to "wake-up" and transducer signal processing.
Also on the circuit board are connectors to the display 30, the activation button 24, and the RS232 computer interface connector 32.

While the preferred embodiment of the system of the present invention has been illustrated and described, certain modifications and alternatives will be apparent to those skilled in the art and the present disclosure is intended to include such modifications and alternatives within the scope of the appended claims.

Claims (4)

1. A baseline nulling system in which inputs to amplifier stages are dynamically equalized prior to and during signal analysis comprising amplifier means with a plurality of amplifier stages wherein a first stage is capacitively coupled to a second stage and said second stage is capacitively coupled to a third stage, analysis means connected to said third stage for receiving electronic signals from said third stage, corresponding to pressure measurements, analog switch means comprising a first analog switch and a second analog switch, said first analog switch being connected to said analysis means and to said second stage, and said second analog switch being connected to said analysis means and said third stage, and wherein said analysis means causes said first and second switches to open and close for a pre-determined period of time between the open state and closed state prior to said analysis means receiving electronic signals from said third stage and wherein said analog switch means causes the gain of said second and third stages to have a pre-determined gain when said first and second analog switches are open.

2. A system as in claim 1 wherein, said predetermined gain in zero.

3. A system as in claim 1 including, a display means is connected to said analysis means and is adapted to receive electronic signals corresponding to pressure measurements and to digitally display numbers corresponding to said pressure measurements.

4. A system as in claim 3 wherein, an electrical bridge is connected to said amplifier and supplies said amplifier with electronic signals corresponding to pressure measurements, and transducer means connected to said electrical bridge for supplying said bridge with electronic signals corresponding to pressure measurements.