Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
PATIENT ~ONITORING EQUIPMENT
This invention relates to patient monitoring
equipment, It has application in monitoring the depth
05 oi anaesthesia o~ patients to whom anaesthetic or
sedatlve drugs are administered. The term anaesthesia
is used herein in its broadest sense and is intended
to include not only anaesthesia for surgery, but also
~he lighter levels o~ anaesthesia or sedation used in
~ritically ill patients receiving intensive care. The
term anaesthetic is to be understood accordingly.
This application is a division of our copending Cana-
dian patent application Serial No. 388,712 filed October 26,
1981.
The response of individual patients to drugs is
highly variable~ Especially in the case of anaes-
thetic drugs an anaesthetlst is required to employ a
considerable degree of clinical judgement in order to
obtain an optimum effect~ Clinical anaesthesia is not
an "on-off" state but a state of unconsciousness and
variable reflex suppression produced by one or more
drugs. It is traditional to describe the degree of
refle~ suppression as the depth of anaesthesia. At
present the depth of anaesthesia is judged by the
ch~nge in various clinical slgns produced ln response
to surglc~l stimulus. It would be o~ great assistance
1~ some obJectlve ln~ormatlon were av~ ble
l~dlcatin~ the depth o~ annesthesia. Attempts have
b~en made to us~ indirect me~surement~ o~ ~ patient's
~,,
vltal physiologlcal functions such as he~rt rate,
blood pressure and electroencephalogram (EEG)
wavaforms to lndicate depth of anaesthesia. No one of
these measurements alone has proved to be a
05 su~lclently reliable lndex of anaesthesiaO
An artlcle by P. Suppan in the Bri~ish Journal of
Anaesthesia, (1972~ 44, p.l263 describes the use of
pulse rate as an indicator of depth of anaesthesia,
and describes furthermore the use of a feed-back
system to automatically control anaesthetic
administration. Tbe article also describes the
possibility of using blood pressure as an indicator of
the depth of anaest-hesia, but there is no suggestion
of the combined use of the parameters, or any
suggestlon that combining two or more measurements to
produce a "score" can provide a more reliable
indication of depth of anaesthesia.
~ . Dubuis, D.E. Scott, and T.M. Save~e, in an
article in Annals Anaesthesia, France (1979) 3, p215
describe the use of EEG as an indicator of the after
ffects of anaesthesia.
Electronically processed EEG signals have been
employed to monitor the level of electrical activity
in the braln during anaesthesia. A review of this and
otber applications o~ EEG monitoring is given in
Nonltorlng Cerebral Function ~author P~F. Prior,
X
published by Elsevoir (North-Holland Biomedical Press,
1979, Amsterdam).
Finally J.S. Stewart in The Lancet (1969) 1,
pl305 describes a monitoring system for drawing the
05 attention o~ a clinician to a deteriorating condition
o~ a patient, using a combination of various
parameters, such as heart rate, blood pressure, and
o~ygen tension. There is, however, no suggestion in
the Stewart article of the use of a similar system to
measure depth of anaesthesia.
We have dlscovered that the muscular activity in
the oesophagus is related to the depth o~ anaesthesia.
During light anaesthesia there is a great deal of
smooth muscle activity in the form of periodic
contractions. During deep anaesthesia there is little
oesopha~eal smooth muscle activity. We h~ve consis-
tently observed this relationship between oesophageal
activity and depth of anaesthesia with most common
anaesthetic agents. Changes in oesophageal muscle
activity cause corresponding changes in intra-lumenal
oesophageal pressure. Thus, by insertion o~ a
balloon-type cathetar, or some other suita~le pressure
probe, into the oesophagus, and me~suring the internal
pres~ure ln the oesophagus, it ls possible to obtaln
an indication oi the depth of anaesthesia.
The pressure changes produced by oesophageal
contraction generally last 2 - 4 seconds and occur at
frequencies of up to 4 or 5 per minute during light
anaesthesia. Occasionally there are short periods of
05 rapid contractions at rates of up to 15 per minute
accompanied by high resting pressures between contrac-
tions.
Oesophageal activity has in the past been
observed for a variety of purposes, for example N.E.
Leatherman in an article in Critical Care Medicine
(19?8) Vol 6, No 3 pl89 describes the use of an
oesophageal balloon for measuring intra-pleural
pressure in the monitoring of acutely ill patients.
However, we are not aware of any proposals for using
15 -measurements of oesophageal activity to monitor depth
of anaesthesia.
We have also discovered that, whether or not
oesophageal contractions are used as a measure of the
degree of anaesthesia, increased reliabilitY in the
quantification by the anaesthetist of depth of
anaesthesia can be obtained if a plurality of
di~ferent bodily ~unctions are observed, and a score
value assigned to each in accordance ~ith certain
parameters. The scors values may then be summed to
produce a total score indicative o~ the degree of
anaesthesia oi' the patlent.
%
-- 5
In accordance with one aspec-t of -the method of moni-
toring anaesthesia defined in the parent appllcation,
signals are produced indicative of the contractions in
the oesophagus of a patient, and an output is derived
from the signal indicative of the degree of anaesthesia
of the patien-t.
According to a first aspect of the invention defined
in the parent application, there is provided patient
monitoring apparatus, comprising a sensor for providing
signals indicative of smooth muscle contractions in the
oesopnagus of a patient, and means for deriving from the
said signals an output indicative of the depth of anaes-
thesia of the patient.
Preferabl~, the output is clerived from the rate of
generation of signals produced by oesophageal contraction.
Accordingly, in one embodiment of this aspect of the
invention of the parent application, there is provided
patient monitoring apparatus comprising a sensor for
producing signals indicative of smooth muscle contractions
in the oesophagus of a patient, and means for producing
an output indicative of the rate of occurrence of such
contractions. Alternatively, an output may be derived
from the amplitude of such signals.
The changes in pressure that occur and which are
picked up by the sensor will be not only pressure changes
due to oesophageal contraction but also pressure changes
resulting from movement of the heart adjacent to the
oesophagus and changes in intrathoracic pressure due to
ventilation of the lungs. The pressure changes produced
by ventilatlon and movement o-E the heart are rela-tively
small and are usually less than 20 mm Hg, typically abou-t
10 mm Hg, when measured with a balloon--type catheter of
the type hereinafter described. In contrast oesophageal
contractions produce changes in intra-lumenal pressure
which may be as high as 100 mm Hg ~nd are usually in
e~cess of 20 mm Hg. In order therefore to eliminate
signals arising out oi activity not due ~o oesophageal
contractions it is desirable to set a threshold value
of pressure below which no signals are utilised for
monitoring purposes. This can be done by providing
the apparatus with discriminating means for rejecting
signals of less than a desired threshold magnitude.
The threshold magnitude is preferably substantially
greater than, for exampl~- twice as great as the
magnitude of signals produced by forced ventilation of
the lungs of the patient.
In a ~urther embodiment of this aspec-t of the invention
of the parent application, there is therefore provided
patien-t monitoriny appara-tus, comprising a sensor for
producing signals indicative of smooth muscle contractions
in the oesophagus of a patient, and discriminating means
for accepting only signals of greater than a predetermined
magnitude.
Furt~ermore in order to ~inimise spurious signals
arising ~rom irregularities in the pressure waveform,
an inhibition period may be provided immediately
following each contraction in excess of the threshold.
-- 7
In the inhibition period no con~ractions are
recognised. A convenient value for chreshold pressure
is 20 - 25 mm Hg and for the inhibition period a time
of from 5 to 10 seconds may be suitable.
The rate of generation of contraction signals can
be derived from a measurement of the ti~e intervals
between successive contractions. The contractions may
not occur at regular intervals and a rate derived
directl~ in this way would fluctuate frequently. A
measurement of such time interval could be stored and
averaged to overcome this instability. A more stable
and representative rate is ob-tained by providing means
for counting the number of oesopha~eal contractions
over a period of time and indicating the mean or
a~erage rate. --_
In carrying out the invention of the parent applica-
tion, therefore, it is convenient to make the period of time
over which the contractions are averaged a moving and adjus-
table time "window". A convenient value for such a time
"window" is up to nine minutes, pre-ferably from 3 to 9
minutes.
It ls possible to provoke oesophageal contrac-
tio~s. Thes~ provoked contractions are similar to
sponta~eous contractions but can ~e provoked at a
25 depth oi anaesthesia su~lcient to suppress
spontaneous contractions. As anaesthesia is deepened
the oesophageal response to provocation diminishes~
Thus the provision of means for provoking oesophageal
1/~
contractions allows deeper levels of anaesthesia to be moni-
tored.
According to yet a further embodiment of this aspect of
the invention of the parent application, there is there-fore
provided patient monitoring apparatus, comprising means for
stimulating smooth muscle contractions in the oesophagus of
a patient and a sensor for producing signals indicative of
contractions in the oesophagus of the patient.
The amplitude of the provoked oesophageal
response is, in part, related to the depth of anaes-
thesia. Thus in addition to the rate of oesophageal
contractions, the amplitude of the provoked response
ma~ be used as a guide to the depth o~ anaesthesia.
Oesophageal co~tractions may be provoked by the
15 appllcatlon o~ a mechani~l or electrical stimulus to
the oesophagus or contiguous structures for e~ample
th~ pharyn~, lary~x or trachea. A convenlent me~ns of
provo~lng oesophageal contractions is an air or llquid
filled in~latable balloon inserted into the trachea or
more preferably, the oesophagus.
Since the equipment described above provides an
indication of the depth of anaesthesia it is possible
to use the indicatlon obtained to control a drug
delivery system to achieve a desired depth of anaes-
thesia,
>~
Anaes-thetic control equipment may comprise a sensor for
providing signals indicative o-f contractions in the oesopha-
gus of a patient, for example a probe adapted to be inserted
into the oesophagus of the patient and means for controlling
the delivery of anaesthetic drugs to the patient in accor-
dance with a parameter of the signals so ob-tained to achieve
a desired depth of anaesthesia.
A suitable parameter is the rate of generation of
such signals, preferably averaged over a period of
time, However, an amplitude measurement of such
signals may be used, par~icularly where provoked
conkractions are being measured~
It will be appreciated that the anaesthetic
control equipment set out above operates as an
lS automatic closed-loop control system.
As mentioned above ~e have also discovered that,
whether or not oesophageal contractions are used as a
measure of the degree of anaesthesia, increased
reliability in the quantification by the an~esthetist
o~ depth of anaesthesia can be obtained if a plural1ty
o~ di~erent bodily functions are observed, and a
score value assigned to each in accordance with
certain parameters, the score values then being be
summed to produce a total score indicative of the
degree of anaesthesia of the patient.
According to a fur-ther aspect of the inven-tion defined
in the parent application, there is provided a method of
monitoring the degree of anaesthesia or sedation of a
X
- 10 --
pa~ient, which method comprises assigni~g a score value
to each of a plurality of diferent bodily functions, the
said ~core values bein~ indicative o~ a depth of anaes-
thesia or sedation, and summing the score values to
obtain a total score indlca~ive of the degree of anaes-
thesia or sedation o~ the patlent.
The invention of the pare~t application also provides
a patient monitoring appara-tus, which comprises means for
assigning a score value to each of a plurality of differ-
ent bodily ~unctions of a patient, the said score valuesbeing indicative of a depth of anaesthesia or sedation,
means for summing the score values, and means ~or produ-
cing an output ~rom the summed score values indicative of
the amount of anaesthetic to be given to the patien~.
Means may be provided for measuring one or more of
the said bodily functions and for automatically genera-
ting a score value from the measured value. This is
particularly suitable for func~ions such as heart rate
and blood pressure. Alternatively the means ~or assigning
a score value to a bodily function may take the form of
a keypaA, for entering a score value in accordance
with a clinical assessment of a selected bodily
function. The apparatus of the invention preferably
includes means for displaying the resultant score.
E~amples of bodily functions that may be measured
are EEG activity, cardiac outpu and o~ygen consump~
tion. Specific cardlac function indices that may be
measured include blood pressure (normally sys-tolic
blood pressure, although diastolic or mean blood
pressure may alternatlvely be used) and heart beat
rate. ~unctions in which clinical assessments may be
made and a score value assigned via ~he keypad
are the degree o~ sweating and the formation of tears.
Values of bodily functlons which are readily measured
by lnstruments, such as systollc blood pressure and
heart bsat rate may of course be measured by
conventional methods, and score values assigned using
the keypad-
Alternatively, or additionally to theabove-listed functions, other bodily functions may be
measured or assessed.
We have ~ound that because the summed score value
relates to a plurality o~ different bodily functions
the score is a much more reliable indication of the
depth of anaesthesia than measurement of any one
individual function by itself.
Means may be included ~or recording the total
score and if desired the individual score values. It
may be convenient to record other associated informa-
tion, ~or e~ample the time at which the measurements
are takenO
l'he displayed score can be compared wi-th a
desired score in a comparator to derive a score error
and the rate of administration of appropriate drugs
may then be determined in accordance with the score
error to achieve a desired depth of an~esthesia. By
-- I 2
providlng means for entering ~n initial rate of
administration into the e~uipment and then modif~lng
this rate by the score error a requlred rate of
administratlon of a drug may be obtained, dlsplayed
and automatlcally co~trolled.
As indicated above the score o~tained ma~ be
used to automatically control the delivery of drugs to
a patient.
It is desirable to provide a clock which provides
control signals to the measuring means to update the
score values at regular intervals and where on~ or
mor~ score value asse~sments are included lt may be
deslrable ~o include means for prompting the clinician
or anaesthetist to enter his curren~ assessments.
In accordance with-o~e aspect of the invention of this
application, there is provided an elongate probe for measur-
ing oesophageal contractions, having a distal end adapted to
pass downwardly through the oesophagus of the patient, and a
proximal end adapted in use to lie externally of the mouth,
the probe including stimulating means adapted in use to lie
within the oesophagus of a patient and operable by means
external to the patient for stimulating the oesophagus, to
cause contractions therein, pressure sensing means adapted
in use to lie within the oesophagus of the patient for
enabling the measurement of pressure wi-thin the oesophagus,
thereby t,o sense contractions of the oesophagus.
In accordance with a fur-ther aspect of the invention of
- 13 -
this application, there is provided a probe for measuring
oesophageal contractions, comprising an elongate body por-
tion for insertion into the oesophagus and having at least
first and second lumens sealed with respect to each other,
the first lumen being in communication at its distal end
with a sensing balloon -for sensing oesophageal pressure, and
being provided at its proximal end with means for connection
to a pressure measuring device, and -the second lumen being
adapted to receive a fluid under pressure to sti~ulate the
oesophagus to cause contractions therein.
A number of particularly preferred embodiments of
the invention will now be described with reference to
the accompanying drawings in which:-
~ igure 1 illustrates a probe adapted to be
inserted into the oesophagus together with associatedequipment;
Figure 2 illu~trates monitoring and control
equipment suitable for use with the probe of Figure 1;
Figures 3 to 5b illustrate alternative construc-
tions o~ oesophageal probes which may be used with the
apparatus of Figure ~;
Flgure 6 shows a typical trace obtained with
apparatufi a~ illustrated ln Figures 1 ~nd 2 sho~ing
pro~oked oesophageal contractions;
Figure 7 is a block diagram of apparatus in
accordance with the second aspect of the invention of the
parent application;
- 14 -
Figure 8 illustrates yet a further alternative
construction of an oesophageal probe, and
Figure 9 is a block schematlc diagram o~ an
elec~ronic circuit suitable for use with the probe of
05 Flgure 8.
Referring now to Figure 1 there is shown therein
a probe which is adapted to be inserted into an
oesophagus. The probe comprises a hollow flexible
stem l which contains two passageways 2 and 3. At the
end of the probe there is provided a balloon 4 which
ls connected to passageway 2 and adjacent to balloon 4
ls a ~urther balloon 5 coupled to passageway 3.
BQ1100n 4 is liquid-filled and passageway ~ is coupled
to a pressure transducer so that the external pressure
applied to balloon 4 can be monitored. Balloon 5 is
connected to passageway 3. A suitable length for stem
1 is about 100 cm while the total length of the two
balloons 4 and 5 can be about 10 cms. The two
balloons each have a ma~imum diameter of appro~imately
2 cms.
Passageway 3 is connected to an air supply line
9. An air-pump 6 supplles a reservoir 7 ~hlcb ~eeds
supply line ~ t~rough a valve 8. A pressure gauge 10
i~ coupled to reservolr 7. In an alternative and
pre~erred embodiment, gauge 10 is coupled to line 9 so
a3 ko measure directly the provoking pressure.
L~ ~
- 15 -
Reservoir 7 also has a regulating valve 11 which can
be ad~usted to prevent excessive rise in ths pressure
in reservoir 7. Valve 8 is controlled by a timing
u~it 12.
05 In use of the equipment shown in ~igure 1 probe 1
is inserted into the oesophagus so that the balloon 4
is in the lower part of the oesophagus, in an adult,
typically 35 cm from the incisor teeth, and the timing
unit 12 functions to control valve 8 to connect
reservoir 7 to intermittently inflate balloon 5. At
the end o~ each inflation period valve 8 connects line
~ to a vent 13 to allow balloon 5 to deflate. With
reservoir 7 having a volume of approximately 200 cc
and being held at a pressure of around 200 mm Hg and
with balloon 5 having an inflated volume of about 5 cc
the opening of valve 8 will cause full and rapid
e~pansion of balloon 5 to its maximum capacity. It is
convenient to hold balloon 5 inflated for periods of 5
seconds or ~here- abouts with the intervals between
inflations being in the range of 1 - 10 mins.
In order to safeguard against deleterious ef,t'ects
on ~ patient arlslng out of rupture o~ halloon 5 it
may be desirable to provide means for limi-ting the
volume or rate o~ air~low out of reservoir 7.
2S second valve may be provided, operated by the timing
unit 12, between the air pump 6 and the reservoir 7
- 16 -
and open when valve 8 is closed, to restore pressure
in the reservoir 7 only when ~he reservoir is not
directly connected to balloon 5.
Any contraction of the oesophagus which is either
05 spontaneous or else is trlggered by inflation of
balloon 5 is monitored by liquid-filled balloon 4 and
the pressure signal therefrom fed through passageway 2
to a pressure transducer (not shown).
The monitor balloon 4 described above is liquid
filled, preferably with water. It is however possible
to use an air or gas filled balloon for monitoring
pur.poses al~hough in such cases there ~ay be a loss of
fidelity in the recording of pressure amplitude.
Alternatively in place of a balloon a catheter tip
15 ~trarsducer can be inserted into the oesophagus in a
so~t balloon sleeve, in the position of balloon 4
While a separate monitoring balloon 4 and
provoking balloon 5 have been described with reference
to Flgure 1 it is possible to replace the two balloons
by a single balloon which fulfils both functions. In
such a case a switching valve is required which
normally co~nects the balloon to the pressure
tr~nsducer but ~hich is switched to valve 8 whenever a
provoking stimulus is called for by the timing unit
~-
-- 17 -
Timing unit 12 is optionally provided with an
inhibit input along a line 14~ When an inhibit pulse
is received timing unit 12 does not function for a set
period thereafter so that inflation of provoking
05 balloon 5 is inhibited during that period. The
inhibit pulses are derived from the pressure
transducer 21 to which balloon 4 is coupled as shown
in Figure 2. Use of the inhibit pulses ensures that
when there are spontaneous contractions operation of
the provoking balloon, which is unnecessary, is
prevented. A further option provides for manual
triggering o~ timing unit 12 to operate valve 8.
Figure 2 shows monitoring and control equipment
in block diagrammatic form. The pressure signal
obtained from the balloon 4 shown in Figure 1 is fed
to a transducer 21 which provides an output si~nal of
magnitude proportional to the amplitude o~ the
pressure signal fed to it. Transducer 21 may include
of~set and gain controls.
The output signal from transducer 21 includes not
only ma~or signals derived from oesophageal contrac-
tions but also other signal~ which arise from
heartbeats and lung ventilation as well as other
background sigllals lncluding noise. The output from
transducer 21 ls applied to a filter 22 to remove low
level background signals and thence to an adjustable
_ 18 -
threshold circuit 23 to block all remaining signals
below a set level. Since the oesophageal con-
tractions result in pressure signals of ~uch higher
level of amplitude than signals from other sources the
05 setting of an appropriate threshold level in circuit
23 ensures that the output therefrom comprises signals
due to the oesophageal contractions only.
The signals from circuit 23 are utilised to
provide a count o~ events in a moving time "window".
The count is made in a timer/counter circuit 24 into
which the desired lengths or duration of the time
"window" is entered. Conveniently circuit 24 has a
plurality of registers each of which records the
inputs received from circuit 23 in a ii~ed time period
(e.g. one minute) in succe~sion. The number o~ the
most rece~tly ~illed registers that contribute their
contents to the summation is determined by the length
of the time "window". Thus for one minute registers
and a time "window" o~ four minutes the four
last-filled registers are summed and their sum is
outputted. The above e~ample of a one minute time
perlod and a time ~indow of four minutes is purely to
illu~trate the manner of operatlon of circuit 24 and a
dl~erent time period can be provided in the circuit
and the length of the time "windo~l' may be adjustable
~or greater or smaller durations than the fi~ure
-- 19 --
quoted. The summation of the register contents may be
ad~usted to bias the sum in favour of some part of the
window~.
It can occur that contractions of the oesophagus
05 may be stimulated, for e~ample by mo~ement of the
patient during an operation. The inclusion of signals
due to such stimulation may give rise to misleading
estimations of degree of anaesthesia. For this
reason, it is preferable to provide as a function of
the apparatus means for selectively rejecting any
particular signal at the discretion of the operator,
such that the particular signal does not contribute to
the degree of anaes~hesia indicated.
The output from circult 24 is applied to an
invert0r 25 to glve a rate of contraction and this
rate is displayed in an analogue display 261 for
e~ample as a bar of varying length. Alternatively or
in addition it ls displayed in digital form in a
digital display 270
It may be desired to have a visual record of
oesophageal activity. A temporary record may be shown
on a cathode ray tube or similar display, A permanent
~ecord may be obtained with a chart recorder 28 which
is ~ed with the signal from transducer 21. Recorder
28 can also c~rr~ indications of the input pulses to
timer/counter circuit 24 by ~eeding signals ~rom
- 20 -
circuit 23 to an event marker 29 to provide a suitable
lndication on the chart of the occurrence of each
supra-threshold signal. Additionally the operatlon of
valve ~ may be recorded by a second event marker 34
05 triggered from a line 3S from timing unit 12~ Thus,
as well-as provldin~ a record of oesophageal activity,
the chart recorder may also produce a separate record
of those sensed pulses greater than the preset
threshold value, and the inflation of the provoking
10 balloon~
A typical trace obtained from apparatus as
illustrated in Figllre 1 is shown in Figure 6, in which
the lower trace 41 represents the amplitude of
provoking pulses applied to balloon 5, and the upper
trace 42, represents the amplitude of pressure-
waveform sensed by the liquid-filled balloon 4. Upper
trace 42 can be seen to consist of a regular
background pattern of pressure-waves 43 approximately
10 mm Hg~ in amplitude due to forced ventilation of
the lungs of the patient. On the top of this re~ular
trace 43 are superimposed waveforms 45 of much greater
amplitude, typically 30 to 50 mm Hg., each following a
provoking pulse 44 bg a matter of some 5 to 10
seconds. The occurrence or non-occurrence of these
provoked pressure waves 45, above a preset threshold
magnltude, for e~ample 25 mm Hg. can be used as a
simple "on-o~f" indicatlon of whether the depth of
X
~ 21 -
anaesthesia of the patient is greater or less than A
deslred level.
The equipment thus far described operates to
~onitor depth o~ anaesthesia. However the signal from
05 inverter 25 can be used directly to control the
delivery o~ anaesthetic drugs to a patient. Drugs may
be delivered to a patient by two routes depending on
the type of drug usedO Certain drugs may be given as
a solution administered by controlled intravenous
in~usion for example by means of a syringe pump or
drip controller. Where the anaesthetic drug ls a gas
or volatile liquid it may be administered by
inhalation of a gas or vapour mi~ture produced by
controllable gas mixing valves and vapourising
systems. Where drug delivery is by means of a syringe
pump loaded with the appropriate drug, the plunger of
the syringe may be driven by a stepper motor. The
rate of drive pulses supplied to the stepper motor
determLnes the rate of delivery of the drug.
Comparison o~ the rate signal from inverter 25 with a
~alue set by th~ anaesthetist enables a control signal
to be obtained which elther speeds up or slows do~n
the drlve pul~e rate of a syringe pump stepper motor
i~ ~ccordance with whether the rate signal that is
monitored ls abo~e or below the set value.
.X
The equipment required for this control function
comprises a co~para~or 31 whicn is fed with the output
from inverter 25. Comparator 31 also has fed to it a
desired rate which is set by the anaesthetist. Comp-
05 arator 31 compares the actual rate of contraction ofthe oesophagus with the desired rate and provides ~n
error signal giving the magnitude and sign of the
difference between the actual rate and the desired
rate set by the anaesthetistO The error signal is
~0 applied to a pulse generator 32 to modify the rate of
pulse generation therefrom. The pulse output from
generator 32 is in the form of drive pulses to a
stepper motor 33 which drives a syringe pump.
An alarm 30 is preferably included to give an
audible or visual alarm or both. Trig~ering of a
alarm 30 may be caused when the signal from inverter
25 falls below or rises above set safety levels.
Additional alarm trigger signals can be obtained from
various parts of the circuit, for example from pulse
generator 32 if the pulses similarly are above or
belo~ set safety levels.
Many of the functions of the monitoring and
control equipment oi Figure 2 can be incorporated in
an appropriately programmed microprocessor. The
Z5 inputs to the microprocessor include the signal from
~iltPr 22 as well as the various set values such as
- 23 ~
pressure threshold, window length and the desired rate
of contractions. The output from the microprocessor
includes display information and signals to control
pulse generator 32~ Conditions for operating alarm 30
05 can also be incorporated.
The pattern of oesophageal contractions, e.g. the
amplltude, rate, degree of repetition of any
particular sequence of contractions, may be
characteristic for a particular patient. This allows
the possibility of providing for the apparatus to be
adaptive, in the sense that the microprocessor can be
programmed to generate a response "norm" for a
particular patient, based on observation over a
period, and to produce an output corresponding to
deviation from the norm, whether in amplitude, rate,
or any other parameter of the contractions, which will
indicate the degree of anaesthesia.
8efore being processed, signals from the
transducer may be converted from analogue to digital
forrll, and the digitised form of -the input may be
connected directly to the microprocessor circuitry.
Using this technique it is possible for the micro-
processor to compare the output signal directly with
pre-programmed "norm", as regards for e~ample
amplitude, duratlon, and profile or "shape". If the
~ignal fits the pre-set norm the ~ave~orm is
X
_ 24 -
recognised as a significant contraction. Clearly it
may be useful to employ a threshold discriminator in
combination with the above technique.
The amplitude of tbe contractions of the
05 oesophagus has been ~ound to vary along ~he length of
the oesophagus, and to be greatest approximately 5 to
10 cm from the entry to ths stomach. It is therefore
preferable to provide means for retaining the sensor
in the appropriate position in the oesophagus, for
e~ample a balloon adapted to pass into the stomach
where lt is inflated to locate the sensor in the
desired position.
Figures 3 to 5b show ~lternative embodiments of
the oesophageal probe, the same re~erence numerals
being used as in Figure 1 to deno-te corresponding
parts .
Figures 3a and 3b show respectively a schematic
side and end view of an oesophageal probe, in which A
provoking balloon 5 and a measuring balloon 4 are
arranged in a parallel, rather than a series
configuration. In use, the lumens 2 and 3 are
connected as in Figure lo
In the arrangement shown in Figure 4, two
provokirlg ball~ons, 5a and 5b are used. Thus, the
oesophagus may be provoked in two positions
6imultaneou~1y. By provlding communicating holes o~
.X
- ~5 -
dif~erellt sizes from the balloons shown ln 5a and 5b
to the outer lumen 3, the rate of expansion of the
balloons 5a and 5b may be made to differ from each
other, so as to give a peristaltic-like efiect.
05 Multiple balloons of this kind may be ut~lised with
any o~ the probe designs proposed herein.
Figures 5a and 5b show side and end schematic
views respectively of a probe in which a provoking
balloon 5 is provided concentric with a sensing
balloon 4. The sensing balloon continues to be
sensitive to pressure changes, provided tha~ the
provoking balloon is deflated. During the application
of brief provoking pressure pulses to the provoking
balloon 5, ths sensing balloon ~ is not responsive to
changes in oesophageal activity, but sensitivty
returns to normal, when the provoking pulse has
ceased.
A number of other modifications may be used with
the probes described above. For example, the sensing
balloon may be provided with a foam insert, to
maintain the balloon volume. This is particularly
usei'ul when the sensing balloon is gas filled, since a
co~traction of the oesophagus will ~ive rise to a
greater pressure slgnal.
X
- 26 -
When the sensing balloon is gas filled,
progressive drift in the system pressure is likely to
occur, due to both temperature change of gas within
the balloon, and by the diffusion of anaesthetic
05 gases, for example, nitrous oxide, from the body
tissues through t~e balloon wall. It is therefore
desirable when a gas ~illed sensing balloon is used to
incorporate a small controlled leak into the gas line
connected to the monitor balloon, ~o allow very slow
equalisation of any progressive pressure drifto The
leak is not, of course, so large as to interfere
significantly with the transmission of transient high
pressure signals. Instead of a fluid-filled balloon
for pressure sensing, a pressure transducer may be
inserted into a soft sleeve in place of balloon 4.
This may be combined ~ith means for provoking the
oesophagus ~o contraction, for example, a pressure
balloon of the kind described above.
As an alternative to a balloon ~or provoking the
oesophagus to contraction, a pair of spaced stimu-
lating electrodes may be provided, and these may be
spaced along the length of a balloon type catheter
used ~or sensing oesophageal pressure.
In yet a ~urther alternatlve embodiment,
provocation of the oesophagus may be carrled out by a
- 27 -
suitable balloon inserted into the tr~chea of the
patient.
Any of the oesophageal probes discussed above may
have incorporated therein a microphone, a temperature
05 prob~ or electrodes ~or recording an electrocardio-
gram. The microphone mQy be either incorporated ln
the region of $he probe tip, or else connected either
to the provoking lumen 3, or sensing lumen 2, sound
from the oesophagus belng transmit~ed by the inter-
medlate fluid. Thus, a stethoscope function m~y beprovided.
Similarly, a temperature probe, for example a
thermistor, may be provided in the probe tip to record
body temperatureO
A particularly preferred embodiment of an
oesophageal probe ~or use in the method of the
invention is illustrated in Figure 8. The probe of
Figure 8 includes two sensing balloons, 84a and 84b
each connected to separate transducers via passage-
ways 82a and 82b respectively. The two sensing
balloons each have a length of appro~imately 3 to 5
cm, and a ma~imum diameter o~ appro~imately 2 cm.
Appro~imately 1 cm ~rom the distal sensing
balloon~ 84a; i3 a single provoking balloon 85. The
provoking balloon 85 is in communication via holes 86
~ith a passageway 83, through which provoking pulses
X
- 28 -
ca~ be applied, in the same way as described above,
The dlstance between the provoking balloon 85 and the
monitoring balloon 84b is approximately 5 cm.
The provision of two transducers linked to
05 passageways 82a and 82b enables oesophageal pressure
to be monitored at two points in the oesphagus spaced
by appro~imately 10 - 15 cm.
The human oesophagus is unusual in that it has a
nervous control mechanism which controls the whole of
the oesophagus, despite the fact that the lower part
is made of involuntary or "smooth" muscle, and the
upper part of voluntary or "skeletal" muscle. The
ability to measure oesopha.geal contractions at two
po~nts spaced in the oesophagus, using a probe as
illustrated in Figure 8, is a useful one, since there
~re certain drugs, for e~ample curare-like muscle
rela~ants, which selectively paralyse "skeletal", but
not "smooth" muscle.
Wben muscle relaxants have been given to a
patient9 oesophageal con~ractions can only be
elicited in the lower part of the oesophagus. As the
mu~cle rela~ant wears off (the half-life of many
rela~nts is around 15 to 20 minutes) an increasing
amount o~ activity can be recorded in the upper
oeæophagus. Thus, the rel~tive smplitude of contrac-
X
3~
- 29 --
tions in t~le upper and lo~er ot~sopha~us provide~ a
means fvr ~ssessing t~le extent of sk~lt-~tal mus~L~
paralysis.
FigUI`e 9 lS a bloclc schematic diagram
05 illuscratLng a clrcuit suitable for use with the probe
of Figure 8 to make use of this effect.
Balloons 84a and ~4b are connected to transducer
a and ~ransducer b respectively, and the amplitude of
the signals is compared in a comparator. The
comparator may ~unction continuously, activated b~
contractions in excess of a threshold value, or by any
of the other identification methods outlined above, as
monitoreti by the distal monitorin~ balloon 84a.
Alternatively, the system may operate only when
enabled by a si~nal from the provoking unit.
When the provoking balloon 85 is inflated,
provoked contractions will be observed in both sensing
balloons 84a and 84b within 5 to 10 seconds. Thus,
the urran~ement will normally provide for the
comparator to be enabled for a period of from 5 to 10
secor.ds. The display provides an output corresponding
to the ratio o~ amplitudes of pressure senst?d hy
oalloons 84b and 84~. \Yhen this r~tio i~ near unity,
there is little muscle paralysi~s. when the ratio is
~5 close to zero, skeletal muscle iS paralysetl fully.
- 30
Thus, according to yet a further embodiment of the
inventlon, there is provided a method for determining
the degrse of skelet~l muscle relaxation of a patient,
which method comprises providing signals indicative of
~5 contractions at two spaced points in the oesophagus of
a patient, and comparing the signals to derive
therefrom an output indicative of the degree of
skeletal muscle rela~ation of the patient. The
invention includes ~ithin its scope apparatus for
carrying out the above method.
A number o~ the probes described above are in
themselves novel, and such probes should be considered
as lying within the scope of ~his aspect of the
inventton.
A preferred embodiment of the second aspect of the
invention discussed above, involving allocating a
"score" for various bodily functions is illustrated in
more detail with reference to ~igure 7, a~d ~he
following table. The table illustrates a sultable
scoring system for the four parameters systolic blood
pressure, heart rate, sweat, and tears.
X
- 31 -
T A B L E
INDEX CONDITION SCORE
SYSTOLIC ~LOOD LESS THAN CONTROL ~ 15 O
PRESSURE LESS THAN CONTROL ~ 30
(mm Hg) ~ORE THAN CONTROL ~ 30 2
HEART RATE LESS THAN CONTROL + 15 O
(beats/min) LESS THAN CONTROL + 30
MORE THAN CONTROL + 30 2
SWEAT NIL O
SKIN MOIST TO TOUCH
VISIBLE BEADS OF SWEAT 2
TEARS OR NO EXCESS TEARS WIT~ EYE-
LACRIMATION LIDS OPEN O
EXCESS TEARS VISIBLE
WITH ~YELIDS OPEN
TEAR OVERFLOW FROM CLOSED
EYELIDS
In the illustrated embodiment each of these
~unctiQns is assigned one of three score values O, 1
or 2. Clearly, a different number of values and a
dl~erent value system can be used if desired. The
total score of the ~our iunctions in the above system
c~ thus vary ~rom O to 8. A lo~ score would be
ch~racteristlc o~ deep anaesthesia and a high ~core of
light anaesthesla. The ~our ~unc~ions that are
- 32 -
selected are systolic blood pressure, heart beat rate,
s~sating and tears. The first two are obtained by
direct measurements of the patient while the last two
functions are assessed by a clinician. In the
05 embodiment illustrated the measured or assessed values
are entered by the clinician via a keypad, ~lthough in
an alternative embodiment, suitable electrical
transducers may be used to measure one or more of the
functions directly, the transducers providing the
appropriate electrical input to the apparatus.
In the case of blood pressure Qnd heart beat it
is the deviation from control values that is used in
determining a score value and these control values are
obtained from the patient by measurement beforehand.
15 The scoring system shown in the table of Fig 6 may be
modified to include additional or alternative
~unctions. Clearly a different set of score values
can b~ used giving a smaller or greater number of
ranges.
Suitable equipment incorporating the scoring
system shown in the table is shown in Fig f. The
equipment provides ~or the input o~ parameters of ~our
bodlly ~unction~ These are systolic blood pressure
(indlc~ted by BP~, heart beat rate Sindicated by HR),
s~eating (indicated by SW) and tear~ or lacrimination
(indicated by T~. Initlally ~uitabl~ thumbwheel
X
- 33 -
s~i~ches 51 are set to give control values of BP and
HR. These values are fed to a comparator 52 to which
the current measured values of BP and HR are also fed.
These values may be measured by suitable ~ransducers.
05 Comparator 52 carries out the appropriate calculation
~or determining in which of the three ran~es of
measurement set out in Fig 8 the ~easured values lie
and provides outputs of numerical values 0, 1 or 2 as
appropriate. These values are fed to a summing
circuit 53. There are also further inputs to summing
circuit 53. These are the score values of the
functlons SW and TE as assessed by the clinician.
Summing circuit 53 adds all the score values and
displays the total score in a display unit 54 which
may be analogue or digital, or both if desired.
An alarm circuit 55 may be provided to give an
audible or visual alarm or a combination thereof if
the ,,otal score is above or below preset limits. The
maximum and minimum score limits may be entered by
thumb~heel switches 56 and the output of circuit 53
may be compared in a comparator 57 with the score
limits set by switches 56 to determine whether alarm
55 needs to b~ actlvated.
To e~able the requlred rate of administrQtion of
a drug to be displayed a ~urther comparator 58 is
provided whlch has as one input a de5ired score level
- ~4 -
set by a thumbwheel switch 59 and as its other input
the total score output from circuit 53. The output of
comparator 58 is an error function which is a measure
of the difference between the desired or required
05 level of anaesthesia and the level which is obtained
~rom ~easurement and assessment. This error function
is ~ed to a drug rate calculator 60 which has as a
preset input an initial drug rate obtained from a
thumbwheel switch 61 which is set by the clinician.
Calculator 60 modifies the value of the initial drug
rate held in switch 61 in accordance with the error
function obtained from comparator 58 and displays the
required drug rate in a display 62. This calculation
ma~ be carried out at intervals as controlled by a
clock 63.
The equipment thus far described enables the
depth of anaesthesia of a patient to be monitored and
additionally displays information as to the required
rate of administration of drugs to achieve a desired
dPpth of anaesthesia. Since the equipment provides
such in~ormation it can also include means for
automatically delivering drugs at the required rate.
Drugs may be delivered to patients by two routes
depending on the type o~ drug used. Some drugs are
given as a solution adminstered by controlled intra-
venous infusion ~rom for example a syringe pump or
- 35 -
drip controller. Where the anaesthetic drug is a gas
or volatile liquid it may be administered by
inhalation of a gas or vapour mixture produced by
controllable gas mi~ing valves and vaporising systems.
05 ~here drug delivery is by means of a syringe pump
loaded ~ith the appropriate drug, the plunger o~ the
syringe may be driven by a stepper motor. The rate of
drive pulse supplied to the stepper motor de~ermines
the rate of delivery of the drug. Comparison of the
current total score with the desired score set by the
anaesthetist enables a control signal to be obtained
which either speeds up or slows down the drive pulse
rate to the syringe pump stepper motor in accordance
with whether the score value that is monitored is
~ove or below the set value.
An e~ample of additional item~ of equipment to
control the administration of drugs is also shown in
Fig 7. The output from calculator 60 giving the
required drug rate is switched through a switch 64 if
automatic control is desired to a comparator 65.
Comparator 65 has inputs irom thumbwheel switches 66
wbich give the ma~imum and minimum limits oi drug
rates. If the input from drug rate calculator 60 is
; outside the limits set b~ switches ~6 then an alarm
67 ls energi~ed. I~ the output from calculator 60 is
within the set limits then thi~ value ls ~ed to a
X
- 36 -
variable frequency oscillator 68 to control the
frequency thereof in accordance with the value of the
output of calculator 60. Oscillator 68 controls the
speed of operation of a stepper motor 69 which
05 operates a suitable syringe pump or the like for drug
infusion. A drug rate setting circuit 70 may be
interposed in the input path to oscillator 68 to
enable the rate of drug supply to be changed manually.
The total amount of drug supplied is displayed in a
display 71 fed ~rom oscillator 68.
Nhilst the above description relates primarily to
patients undergoing anaesthesia for surgery, the
apparatus described herein can readily be used for
patients to whom sedative, muscle rela~ant and/or
analgesic drugs may be adm:inistered, for e~ample in
intenslve care units, and the terms "anaesthetic" and
"anaesthesia" in the following claims should be
unders~ood accordin~ly.
