WO1987000928A2

WO1987000928A2 - Monitor for a cyclically varying parameter

Info

Publication number: WO1987000928A2
Application number: PCT/GB1986/000469
Authority: WO
Inventors: Roger William Dawson
Original assignee: Eme (Electro Medical Equipment) Limited
Priority date: 1985-08-07
Filing date: 1986-08-05
Publication date: 1987-02-12
Also published as: GB8519819D0

Abstract

A medical monitor for monitoring a cyclically varying parameter, such as the pressure in an airway of a ventilator used to assist patient breathing, comprises a transducer (1) for measuring the pressure as an electronic signal, means (18) for timing the electronic signal at two defined points in the cycle, storage means (22) for storing electronic set-up information relating to the time taken for the value of the electronic signal to depart from one of the points and arrive at the other point, means (24) for comparing current information relating to this time with the stored, set-up information, and means (28) for generating an alarm signal when the current information varies from the stored information by more than a predetermined amount. A base level of Bt and a peak level of Pt are stored as well as a standard pressure rise time of RTsu. If the rise time for the value to rise from Bt to Pt exceeds RTsu by a predetermined amount, such as the rise time RThc, this indicates a high compliance condition which causes the alarm signal to be generated. In a similar way, if the rise time is much shorter, such as RTob, this indicates an obstructed condition which likewise causes generation of the alarm signal.

Description

onitor for a cyclically varying parameter

Background of the Invention

The present invention relates to a monitor, which is particularly though not exclusively suitable for medical use, for instance as an airway pressure monitor.

It is important to monitor a ventilator, that is a source of pressurized gas, typically air, supplied to a patient who has difficulty in breathing on his own. A ventilator is normally set up by a doctor or nurse who will have other duties to attend to. The ventilator may then not be looked at for an hour or more. During this time, various problems may develop. For instance water may accumulate in the tubing as a result of condensation from the water vapour added to the ventilator air. Alterna¬ tively the ventilator may become disconnected or again a problem may develop with the patient's lung causing the lung's compliance to alter.

By compliance is meant increase in volume per unit of applied pressure. The result of an increase in a lung's compliance is that the lung takes longer to be inflated by the ventilator. An extreme example of a change in com¬ pliance is the formation of a hole in- the wall of a lung with the result that air passes into the pleural cavity between the lung and the pleura - a membrane surrounding the lung. When the pleural cavity is thus connected to the ventilator, the latter will take longer to inflate the lung to the pressure to be applied to the lung.

Normally a ventilator will supply air to a lung at a constant pressure for an inspiration period during the initial part of which the lung becomes pressurised and during the rest of which period transfer of oxygen and carbon dioxide with the blood occurs, the lung being full of air. During a succeeding expiration period, the lung deflates and rests prior to the next inspiration. Monitors for ventilators do exist. Such known monitors alarm, that is generate an alarm signal, either when the maximum pressure during an inspiration varies or when the mean pressure during inspiration varies. For instance, should the ventilator become disconnected the maximum pressure will not be reached. Alternatively, should a fault slowly develop the mean pressure may not be reached. However, such monitors are not sensitive for instance to a lung ^"hole condition with which the maximum pressure will still be developed and the mean pressure will be little different from normal, taking account of the normal tolerances expected.

The Invention The object of the present invention is to provide an improved monitor.

According to the invention there is provided a monitor for monitoring a cyclically varying parameter, comprising a transducer for measuring the parameter in terms of an electronic signal; means for timing the electronic signal within one cycle thereof at at least two defined points in the cycle; means for storing initial/set-up information relating to a time characteristic of the value of the electronic signal between the two defined points in the cycle; means for comparing current information relating to the time characteristic with the stored, set-up information relating to the time characteristic; and means for generating an alarm signal when the current information varies from the stored, set-up information by more than a predetermined amount, in accordance with the current time characteristic within the cycle between the two defined points in the cycle varying by a corresponding predetermined amount from the equivalent time characteristic at setting-up of the monitor. It should be noted that, whilst the invention is particularly useful where the cyclically varying parameter -3-

is a medical parameter, for instance airway pressure and possibly other parameters measured with medical equipment, for instance blood pressure or electroencephalograph output, the invention may be useful where a parameter is to be monitored in other areas of technology. For instance in the auto-motive field the monitor of the invention may be used for monitoring internal-combustion-engine cylinder pressure.

Normally the transducer will be incorporated with the other components of the monitor as a self contained unit. However, it is anticipated the transducer may be provided remote from the unit and be electrically connected thereto.

A plot of ventilator pressure against time normally shows a modified square wave with the rising and falling edges being at a steep gradient tapering into the final pressure. This curve shape provides a number of possibi¬ lities for analysing its wave form. In particular, the rise time, the dwell time at high pressure, the fall time and the dwell time at low pressure may be measured. In the case of the dwell times, these may be measured in terms of time from when the electrical signal achieves and departs again from a certain band of pressure. In the case of the rise time and the fall time these may. be measured in terms of time from when the pressure departs from one band of pressure and arrives at another band of pressure. The bands of pressure are determined by the initial peak/inspi¬ ration pressure and an associated tolerance and by the initial base/expiration pressure and an associated tolerance. These bands of pressure are stored. In the case of the dwell times when the measured value is within the band concerned, the changes are a change of pressure into the band concerned and out of the same band; whereas in the case of the rise time and fall time the change is a change of pressure out of one band and into .another band. Preferably, the rise time and the dwell time at high pressure are monitored and of these the former alone, if one only is to be monitored.

It is envisaged that in place of measuring rise time in terms of time between two trigger levels, it may be measured in terms of rate of change of signal between the two trigger levels by feeding the signal to a differentiator during the rise time between the trigger levels and comparing the output of the differentiator with a set-up output thereof. It is believed that if the rise time is monitored and remains within tolerance, then the operator can be confident that the lungs are being adequately inflated and remaining inflated for a sufficient length of time, providing the cycle time of the ventilator remains constant..

Monitoring of the rise time has particular advantage as described below in the case of a hole developing in the lungs as explained in more detail below.

The monitor may further include means for generating an alarm signal if the electrical signal during the dwell times departs from a stored peak level and a stored base level ^"by more than a certain tolerance, or if the mean value of the signal departs from a stored mean value by more than a certain tolerance.

Preferably the monitor incorporates a visual display unit for displaying a plot of the monitored parameter against time. Conveniently the monitor incorporates means for automatically altering the gain at which the parameter is displayed whereby the plot occupies a substantial pro¬ portion of the height of the screen. Further, the monitor also conveniently incorporates means for automatically altering the time base of the plot whereby not less than 1- and not more than 6 cycles of monitored parameter are displayed at any one time.

To help understanding of the invention, two specific embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which:- The Drawings

Figure 1 is a block diagram of an analogue/digital monitor according to the invention;

Figure 2 is a graph of typical pressure plots of ventilator pressure to be monitored;

Figure 3 is a block diagram of a microprocessor controlled monitor according to the invention;

Figure 4 is a software flow chart for the monitor of Figure 3; Figure 5 is a front view of a typical display of the monitor of Figure 3;

Figure 6 is a perspective view of the monitor of Figure

3 connected to a ventilator; and

Figure .7 is a graph of a particular pressure plot of ventilator pressure where a hole in a lung is present.

First Embodiment

Referring first to Figure 1, there is shown a monitor operating with electrical signals whose analogue value is indicative of pressure measured by a transducer 1 whose output signal is fed to an amplifier 2. The output signal from the amplifier 2 is passed on line 3 to a reset switch 4 and on line 5 to two comparators 6,7.- When the reset switch

4 is made, the output signal is passed to a peak level measurement circuit 8 and a base level measurement circuit 9. Each of circuits 8,9 gives a constant output signal indicative of respectively the peak level of the signal from the amplifier 2 and the base level from the amplifier 2, these outputs being on lines 10,11. Trigger level adjust- ment potentiometers 12,13 are connected to both of lines 10 and 11 and feed their output to lines 14,15. Thus according to their settings the potentiometers 12,13 on their tappings connected to lines 14,15 will give signals a certain percentage below the peak level towards the base level and a certain percentage above base level towards peak level. The lines 14,15 are connected to the comparators 6,7 whose output is connected to respective Schmitt triggers 16,17. The arrangement is such that the trigger 16 is-ON when the comparator 6 identifies that the output signal from the amplifier 2 is equal to or above the trigger level set in the potentiometer 13. Similarly the trigger 17 is ON when the comparator 7 identifies that the signal is equal to or above the trigger level set in the potentiometer 12.

Schmitt triggers 16,17 act as interfaces between the comparators 6,7 and a counter 18 for counting clock pulses from a clock 19. When the Schmitt trigger 16 switches ON the counter.18 is enabled to count until the Schmitt trigger 17 is switched ON, whereupon after a short delay caused by a delay circuit 20, the counter is cleared. The output of the counter is fed to a latch 21 which is enabled as soon as the Schmitt trigger 17 switches ON, i.e. immediately before the counter is cleared.

The output from the latch 21 is fed to a further latch 22 which is enabled via a second reset switch 23 ganged to the reset switch 4. The output from both latches is fed to a subtractor 24. The time stored in the latch 22 is the rise time for the monitored pressure during setting up of the monitor, whilst the time stored in the latch 21 is the rise time of the presently monitored parameter. Thus the output of the subtractor 24 represents any change in rise time. This is fed to a digital-to-analogue converter 25 whose analogue output is fed to a comparator 26. Also fed to the comparator 26 is an adjustable reference voltage from a potentiometer 27. The comparator 26 controls an audio and/or visual alarm 28, switching it on when the absolute value of the D/A converter exceeds the reference voltage.

In order for the counter to be enabled only when the output signal from the amplifier 2 is positive going, the signal is passed to differentiating circuit 29 which controls a switch 30. The latter earths the enable contact 31 of the counter when the signal from the amplifier 2 is negative going.

Referring now to Figure 2, there are shown plots of airway pressure, on the ordinate, induced by a ventilator against time, on the abscissa. In full lines is shown the plot corresponding to setting up of the ventilator, whilst in a dashed line is shown a plot corresponding to the ventilator tube being obstructed and in chain dotted lines 0 is a plot corresponding to the compliance of the lung/ ventilator system increasing substantially. The plot identifies a base level B of pressure and a peak level P. Corresponding trigger levels B and P. , set by the potentio¬ meters 13 and 12. are shown. The rise time at set-up RTsu 5 is shown, as is the rise time for the high compliances condition RT.he and the rise time for the obstructed con- dition RT , . It will be noted that the high compliance rise time RT. is substantially longer than the set-up rise time

RTsu,' whilst the obstruction rise time RTo.b is substantially 0 shorter than the set-up ^c rise time RT su

The manner in which the monitor detects these serious deviations from the set-up conditions will now be described. On set-up the normal ly open set-up/reset switches 4 , 23 are closed to pass the voltage signal embodying the voltages 5 equivalent to peak and base pressures P , B to the level measurement circuits 8 , 9. The potentiometers 12 , 13 facili¬ tate the setting of the trigger levels P , B in terms of voltages on lines 14 , 15.

Stil l during set-up, the counter 18 counts the clock

30 pulses from clock 19 during the set-up rise time RT , because - once a first cycle is completed the trigger voltage level s are set - the counter is started once the s ignal voltage reaches that corresponding to B and is s topped once the voltage reaches the P. voltage . The set-up

35. rise time RT su is stored in the latch 22. During monitoring, the present rise time is subtracted from the stored set-up rise time. If and when the difference reaches the value set by the potentiometer 27, the alarm 28 is triggered. A typical value of peak pressure for a ventilator in use assisting the breathing of neonates is 20cms of water above ambient, with the base pressure being 2cms of water above ambient. The trigger levels are typically at 5% of the base-to-peak range above base pressure for the trigger level B , ie 2.9cms of water, and 5% below peak level for the trigger level P , ie 19.2cms of water. In the preferred embodiments, the monitor alarms when the peak pressure deviates from the set-up peak pressure by +20%. A typical rise time is 100msec. In the event of the ventilator tube becoming blocked with mucuous, the rise time can fall to 50msec. Similarly, in the event of the ventilator tube developing a leak the rise time can rise to 200msec. Accordingly a typical tolerance range on the rise time is +40%, beyond which the monitor alarms. However in the preferred embodiments, the tolerance is adjustable between ±10% and ±90% of the set-up rise time.

Second Embodiment

Turning now to Figure 3, there is shown a second embodiment of a monitor in accordance with the invention. This monitor has a pressure transducer 51 whose output signal is amplified in an amplifier 52 and fed via an analogue-to-digital converter 53 to a processing unit 54. The processing unit is operated under control of software stored in a PROM 55. A RAM 56 is connected to the pro¬ cessing unit 54, as is a clock 57 and "a display 58.

The PROM 55 controls the processing unit 54 to record in the RAM 56 a set-up rise time when a set-up switch 59 is made. Base and peak trigger levels can be adjusted by adjusting controls 60,61 whereby the set-up rise time is the

-9-

time taken for the monitored pressure to rise from a base trigger pressure B to a peak trigger pressure P as shown in Figure 2. When, during monitoring, the presently moni¬ tored rise time from B to P deviates by more than a certain amount, adjustable by control 62, from the set-up rise time RTsu,' the p^rrocessing³ unit 54 causes an audio and/or visual alarm 89 to be activated.

Figure 4 shows a flow chart of the software stored in the PROM 55 for controlling the processing unit 54. As with the embodiment of Figure 1, the monitor of Figure 3 measures the rise time by counting clock pulses produced by the clock 57 during the rise time period. Then, when the pressure signal reaches B counting is initiated and when the pressure signal reaches P the counting ceases. The set-up rise time is stored in the RAM 56 in terms of number of clock pulses counted during it, and during monito¬ ring, it is the number of clock pulses counted in the present rise time which are directly compared with the number stored in the RAM 56 for determining whether the deviation is such that the alarm should be activated.

This monitor incorporates various additional features. A plot of the monitored signal is displayed on the visual display unit 58 -which is a liquid crystal display. Since the monitor is intended to be used both with ventilators being used to assist the breathing of adults and with ventilators being used to assist the breathing of neonates, it is convenient that the ordinate scale of the plot be changeable. Normally, the monitor will be used by medical staff unfamiliar with oscilloscope controls. Accordingly, means is provided for automatically changing the ordinate scale. When the processing unit detects the plot is theore¬ tically occupying more than 100% of the display screen because the amplitude of the signal exceeds a certain threshold, it supplies a control signal on line 63 to the amplifier 52 to decrease gain by a pre-determined step. Similarly, when occupation of less than 30% for example of the height of the screen is detected, the gain is increased by a pre-determined step. Change of gain does not in fact affect the measurement of rise time since gain changes are arranged to occur only during set-up and the set-up rise time is remeasured whenever a gain change has occurred. Similarly, the set-up rise time is remeasured whenever the trigger levels are manually changed.

The abscissa scale is also controlled to provide that for normal rates of breathing between 1% and 3 cycles are displayed on the screen. Should the processing unit detect that the plot is displaying fewer or more cycles, by measure¬ ment of the frequency of the signal, it controls stepwise the time base of the display via its time base 64 to make an appropriate adjustment to return the display to a condition wherein between l and 3 cycles are being displayed.

The processing unit controls the display to show certain information in addition to a plot of the presently monitored pressure. Referring to Figure 5 these are inspi- ration time 65, expiration time 66, breathing rate 67, inspiration time/expiration time ratio 68, rise time 69, duration of pressure above base pressure 70, peak pressure 71, mean pressure 72, base pressure 73 and a percentage by which the monitored rise time must vary from the set-up rise time for the alarm to be activated 74.

The monitor may also be arranged to activate the alarm, should the peak pressure deviate by more than a determined amount from a set-up peak pressure.

Figure 6 shows the monitor 75 of Figure 3 connected via a tube 76 to the manifold 77 of a ventilator 79 supplying air from a pressure bottle 78.

Figure 7 shows a normal set-up trace 79 (dashed line) in comparison with a trace 80 (solid line) corresponding to a hole in a lung. A hole in .a lung tends to be in the form of a flap of tissue which closes the hole when the pressure -11-

in the lung is low. When the pressure reaches a certain value, the flap moves to open the hole which then causes a drop in pressure 81. Thereafter as the pleural cavity-and the lung is inflated, the pressure rises as normal to the normal peak pressure. The lung remains at peak pressure for substantially the normal time. Accordingly, it will be apparent from Figure 7 that any monitor relying on a de¬ tection of peak or mean pressure change is liable not to detect this condition. However, it will be apparent that the change in rise time due to the delay in reaching the peak trigger pressure is substantial as percentage of the set-up rise time and that a hole in the lung exhibiting this plot of pressure against time may be detected.

Claims

1. A monitor for monitoring a cyclically varying parameter, comprising a transducer for measuring the parameter in terms of an electronic signal; means for timing the electronic signal within one cycle thereof at at least two defined points in the cycle; means for storing initial/set¬ up information relating to a time characteristic of the value of the electronic signal between the two defined points in the cycle; means for comparing current information relating to the time characteristic with the stored, set-up information- relating to the time characteristic; and means for generating an alarm signal when the current information varies from the stored, set-up information by more than a predetermined amount, in accordance with the current time characteristic within the cycle between the two defined points in the cycle varying by a corresponding predetermined amount from the equivalent time characteristic at setting-up of the monitor.

2. A monitor as claimed in claim 1, wherein the timing means is adapted to time the electronic signal between it departing from one threshold value at a first defined point and arriving at another-threshold value at a second defined point, whereby the time characteristic is a rise or fall time.

3. A monitor as claimed in claim 1 or claim 2, wherein the timing means is adapted or further adapted to time the electronic signal between it arriving at one threshold value at a first defined point and departing from the threshold value at a second defined point, whereby the time charac¬ teristic is a dwell time.

4. A monitor as claimed in claim 1, wherein the timing means is a differentiator adapted to give a differential output in accordance with the time at which the electronic signal departs from one threshold value and the time at which the electronic signal arrives at another threshold value, whereby the time characteristic is a rise or fall time.

5. A monitor as claimed in claim 1, claim 2 or claim 3, wherein the timing means is adapted for adjustable setting of the threshold value(s) at determined trigger level (s) an adjustable amount from peak levels.

6. A monitor as claimed in any preceding claim, wherein the means for generating an alarm signal is adapted for adjustment of the predetermined amount by which the present information varies from the set-up information for alarm.

7. A monitor as claimed in any preceding claim, wherein the means for generating an alarm signal is operative to generate the alarm signal when the value of the electronic signal during a period of unchanged value departs from a stored peak value or a stored base value by more than a predetermined amount.

8. A monitor as claimed in any preceding claim, wherein the means for generating an alarm signal is operative to generate the alarm signal when the mean value of the electronic signal departs from a stored mean value by more than a predetermined amount.

9. A monitor as claimed in any. preceding claim, incorporating a visual display unit for displaying a plot of the values of the electronic signals representing the monitored parameter against time.

10. A monitor as claimed in claimed in claim 9, incorporating means for automatically altering the gain at which the values of the electronic signals are displayed, whereby the plot is made to occupy a substantial proportion of the height of the screen of the visual display unit.

11. A monitor as claimed in claim 9 or 10, incorporating means for automatically altering the time base of the plot, whereby not less than 1% and not more than 6 cycles of monitored parameter are displayed at any one time.