Note: Descriptions are shown in the official language in which they were submitted.
CA 02466105 2011-10-24
Sensor and method for measuring physiological parameters
The invention relates to a sensor for the measurement of physiological pa-
rameters according to the preamble of claim 1. The invention relates fur-
ther to a method for the measurement of physiological parameters accord-
ing to the preamble of claim 25.
The measurement of one or more physiological parameters of the human
body is increasingly gaining in significance. Thus, a combination sensor
for the combined measurement of the oxygen saturation of the haemoglo-
bin in arterial blood and also of the arterial partial pressure of carbon di-
oxide is known from the document EP 0 267 978 Al. For the measure-
ment of the oxygen saturation (SP02) a non-invasive, optical, and gener-
ally known method is used which is termed pulse oximetry. A pulse
oximeter system of this kind comprises a sensor which is applied to a
location of the human body with a good blood supply, a pulse oximeter,
and also a connection cable which connects the sensor to the pulse
oximeter. For the measurement of the CO2 concentration in the blood, the
transcutaneous carbon dioxide partial pressure (tcpCO2) is determined
with the aid of an electrochemical measuring apparatus. Detailed
information concerning these generally known measurement methods are,
for example, to be found in the following review article: "Noninvasive
Assessment of Blood Gases, State of the Art" by J.S. Clark et al., Am. Rev.
Resp. Dis., Vol. 145, 1992, pages 220-232. Details of the pulse oximetry
measurement method are for example to be found in the document WO
00/42911.
CA 02466105 2004-05-05
2
Disadvantages of the sensor disclosed in the document EP 0 267 978 Al
are the facts
- that disturbing signals which arise falsify the measurement,
- that the sensor has to be calibrated frequently,
- that the sensor has a relatively thick cable,
- that the sensor is relatively heavy and has a large diameter,
- that the sensor together with the evaluation device is relatively expen-
sive,
- and that the sensor only permits relatively simple measurements to be
carried out.
It is the object of the present invention to propose a more advantageous
sensor. This object is satisfied by a sensor having the features of claim 1.
The subordinate claims 2 to 24 relate to further advantageously designed
sensors. The object is further satisfied with a method having the features
of claim 25. The subordinate claims 26 to 35 relate to further advanta-
geous method steps.
The object is in particular satisfied by a sensor for the measurement of
physiological parameters such as oxygen or carbon dioxide in the blood
comprising at least one measuring apparatus and also a digital sensor sig-
nal processor arranged in the sensor which is connected in signal-
conducting manner to the measuring apparatus and which makes the
measured values available in digital form for the further processing, and
further comprising a signal transmitting communication means for trans-
mitting the output signal, wherein the communication means is formed as
an at least two pole cable.
CA 02466105 2004-05-05
3
The sensor of the invention has a digital sensor signal processor which
digitises the signal measured by the measuring apparatus, so that this
signal is available in the sensor for further processing in digital form. A
digital sensor signal processor of this kind is also referred to in English as
DSSP (Digital Sensor Signal Processor) or as Single Chip MCU (MiroCom-
puterUnit). A sensor signal processor of this kind comprises not only a mi-
crocontroller with memory, microprocessor and interfaces on a single chip,
but rather also an analog-to-digital converter and also a digital-to-analog
converter. The digital sensor signal processor enables, amongst other
things, the values measured by the measuring apparatuses arranged in
the sensor to be converted into digital values within the sensor.
From this, the following advantages arise in particular:
- The measured, analog signal is converted within the sensor into a digital
signal which is insensitive to disturbance. This also permits weak analog
signals to be measured cleanly.
- The signal is digitally transmitted between the sensor and a subsequent
evaluation device, which extensively precludes signal falsification due to
stray radiation. This further enables a bidirectional communication i.e. an
exchange of data transfer between the sensor and the subsequent evalua-
tion apparatus.
- The evaluation device can be arranged within the sensor or spaced apart
from the sensor. In an advantageous embodiment, the evaluation of the
signals is executed in the sensor signal processor.
- Two conductive wires i.e. a two pole cable are sufficient for the transmis-
sion of the digital data. The connection cable between the sensor and the
evaluation device, consisting of two conductive wires only, can thus be
CA 02466105 2004-05-05
4
made very thin and flexible. Further the sensor can be supplied with en-
ergy through this two conductive wires.
- A digital signal processing is essentially carried out in the evaluation de-
vice. This enables a favourably priced evaluation device obtainable as a
standard product, with a processor and matched software corresponding
to the sensor.
In a further advantageous embodiment the sensor is designed such that
its digital output signals have fixed normalised values. In a preferred em-
bodiment a reference curve, for example a calibration curve of the measur-
ing apparatus arranged in the sensor is stored in the sensor. Such sen-
sors have the advantage that no time-consuming or costly calibration is
required on changing the sensor, since all sensors have a specified output
signal.
In a further advantageous embodiment the sensor has a circuit board
which is equipped with the essential or all required electronic compo-
nents. A sensor of this kind can be manufactured at extremely favourite
cost.
In a further advantageous embodiment the sensor has an inner space
which is electrically screened against the outside, which yields the advan-
tage that disturbing signals cannot or can hardly be superimposed on the
measured signals.
It is of central importance that the sensor of the invention also allows
weak signals to be measured unambiguously and that the measured sig-
nals can be supplied free of disturbance to a signal evaluation device. This
CA 02466105 2004-05-05
has the consequence that no complicated method is required for the signal
evaluation in order to unambiguously and reproducibly evaluate an oth-
erwise normally noisy signal.
A preferred embodiment of the sensor according to the invention is a com-
bination sensor for the combined measurement of the oxygen saturation of
the haemoglobin in arterial blood and of the arterial partial pressure of
carbon dioxide. To get a measurement signal of stable quality, the combi-
nation sensor known from the document EP 0 267 978 Al requires a rela-
tively large and heavy reference electrode made of silver. The combination
sensor according to the invention requires for the measurement of the ar-
terial partial pressure of carbon dioxide only a reference electrode with a
small diameter i.e. a small contact area, because the digital sensor signal
processor is able to process also relatively a weak electrical signal, and be-
cause the sensor operates with digital signal which is insensitive to dis-
turbance. As a consequence, the use of a small reverence electrode allows
to build the entire sensor relatively small and in particular very light-
weight. In an advantageous embodiment, the entire sensor has a weight of
less than 10 grams, preferably even a weight of less than 5 grams.
It has proved to be particularly advantageous to use the sensor of the in-
vention on the ear, in particular on the earlobe. In fact the earlobe is a dif-
ficult place to measure, because the available measurement surface is
small, and because higher pressure forces exerted onto the earlobe disturb
the blood flow. The sensor according to the invention can be built with a
sensor surface adapted to lay on the earlobe and having a small diameter.
The sensor according to the invention is very lightweight, and therefore
only a small squeezing force is required to attach the sensor to the ear.
CA 02466105 2004-05-05
6
The connection cable of the sensor can be built very thin and flexible, so
that movements of the head scarcely influence the measurement of the
sensor. Moreover, it is particularly advantageous to heat the sensor with a
heating device in order to thereby keep the earlobe at a reproducibly con-
stant temperature. The earlobe proves to be a particularly advantageous
measurement position, because the ear is located relatively close to the
heart with respect to the blood circulation, substantially closer than the
finger cap, which is, for example, also suitable for the measurement of
oxygen or carbon dioxide in the blood. Moreover, as a result of the heating,
hardly any vascular construction or hardly any vessel narrowing arises at
the ear by reason of the heating. The sensor of the invention thus enables
measurement to be made at the earlobe with a very low Signal to Noise
(S/N) ratio, so that a measurement signal of excellent quality is available.
This high signal quality now in turn enables further physiological parame-
ters to be determined from the measured values, such as, for example, the
blood pressure, which is measured by means of the so-called CNIBP
method, which signifies in English "Continuous Noninvasive Blood Pres-
sure". In this connection the systolic blood pressure is, for example, de-
termined by means of the pulse oximetric measurement method which can
be measured at the earlobe, as is for example described in detail in the fol-
lowing document: "Can Pulse Oximetry Be Used to Measure Systolic Blood
Pressure?, R. Chawla et al., Anesth Ana1g, 1992; 74:196-200". The sensor
of the invention permits the pulse curve shape, for example the so-called
plethysmogram to be measured in that the oxygen content of the blood is
measured pulse oximetrically for example 50 or 100 times per second and
the blood pressure is determined from the resulting shape of the curve.
CA 02466105 2004-05-05
7
The high signal quality enables the hemotacrit, abbreviated internationally
with "HCT" to be determined as a further physiological parameter. The de-
termination of this parameter is, for example, disclosed in detail in the
document US 5,803,908.
The sensor in accordance with the invention is also suitable for the meas-
urement of the composition of the respiratory air.
In a further advantageous embodiment a data memory is arranged in the
sensor in which measured values or patient data can for example be
stored due to the fact, that there is a bidirectional data communication
between the sensor and a subsequent signal evaluation apparatus. In an
advantageous embodiment this data memory is made sufficiently large
that data measured over a longer period of time can be stored in the sen-
sor, for example in a nonvolatile memory, also termed an EEPROM.
In an advantageous method, the measured values are supplied to a signal
evaluation device arranged after the sensor and the evaluated data are
supplied at least partly to the sensor again and stored in its nonvolatile
memory. The sensor can also be connected to different signal evaluation
devices temporally one after the other, which respectively store the evalu-
ated data in the memory of the sensor. A signal evaluation device can, if
required, also access the data stored in the memory, for example the pa-
tient data.
The sensor in accordance with the invention is, for example, also suitable
for the longer term monitoring of patients, for example also in emergency
situations, in which the sensor is, for example, secured to the patient's ear
CA 02466105 2004-05-05
8
and the sensor remains continually with the patient and the data of the
signal evaluation devices, to which the sensor is respectively connected
one after the other, is at least partly stored. Thus, it is possible with a
sin-
gle sensor to continuously monitor a patient substantially without gaps,
starting for example at the site of an accident, and subsequently in the
ambulance, in the operating theatre through to the wake-up station.
Thanks to the fact that all the measurement data is available at every sta-
tion and, moreover, that even further patient data is eventually available,
reliable data is also available at any time in emergency situations, which
enables an ideal care of a patient.
The invention will be described in the following with reference to embodi-
ments in which are shown:
Fig. 1 a longitudinal section through a sensor in accordance with the
invention;
Fig. 2 a plan view of a sensor;
Fig. 3 schematically, a block diagram of the sensor;
Fig. 4 schematically, a further block diagram of a sensor;
Fig. 5 a detail aspect of the sensor signal processor;
Fig. 6 a plurality of sensors connected to a signal evaluation device;
Fig. 7 a correction curve for a measurement apparatus;
CA 02466105 2004-05-05
9
Fig. 8 a longitudinal section through a further sensor in accordance
with the invention which contacts an earlobe;
Fig. 9 a longitudinal section through a further sensor in accordance
with the invention;
Fig. 10 a cross-section through a micro-pH-electrode;
Fig. 11 a longitudinal section through a respiratory gas measuring
apparatus;
Fig. 12 a longitudinal section through a further respiratory gas meas-
uring apparatus;
In the following the same reference numerals are used for the same items.
The sensor shown in Fig. 1 permits a combined measurement of the arte-
rial oxygen saturation (SpO2) and of the transcutaneous CO2 partial pres-
sure (tcpCO2) = For the measurement of the oxygen saturation the sensor 1
has a pulse oximetric measurement system 17 which includes a two-
colour light emitting diode 2 (LED) and also a photodetector 3. The two-
colour light emitting diode 2 comprises two light emitting diodes 2a, 2b
arranged alongside one another in a common housing, with the one light
emitting diode 2a having a wavelength of approximately 660 nm (red) and
the other light emitting diode 2b having a wavelength of 890 nm (infrared).
The sensor 1 has a surface lb over which a membrane 50 is arranged in
the illustrated embodiment with a thin layer electrolyte 51 therebetween.
CA 02466105 2004-05-05
This membrane 50 is applied to the skin at a point of the human body well
supplied with blood, for example at a finger or at the earlobe. The light
emitted from the two light emitting diodes 2a, 2b shines through the elec-
trolyte 51 located between the light emitting diodes 2a, 2b and also the
membrane 50 and is directed into the non-illustrated body part which is
well supplied with blood and is scattered there and partly absorbed. The
light reflected by the body part is measured by the photodetector 3. The
signal measured by the photodetector 3 is supplied to a digital sensor sig-
nal processor 13 designated in abbreviated from also as DSSP.
The illustrated sensor 1 moreover comprises an electrochemical measur-
ing apparatus 19 for the measurement of the transcutaneous carbon diox-
ide partial pressure tcpCO2 measurement, with this measuring apparatus
19 including a micro-pH-electrode 4 and also an Ag/AgC1 reference elec-
trode 5. The transcutaneous carbon dioxide partial pressure is measured
potentiometrically in that the pH value of the thin layer of the electrolyte
solution 51 is measured, which communicates with the skin via the hy-
drophobic membrane 50 which is readily permeable to gas. A change of
the pCO2 value at the skin surface causes a pH change of the electrolyte
solution which behaves proportional to the logarithm of the pCO2 change.
The pH value is measured in that the potential between the miniature pH
electrode 4 and the Ag/AgCI reference electrode 5 is measured. The micro-
pH-electrode 4 is connected in signal conducting manner to the digital
sensor signal processor 13 via the inner electrical conductor 4a.
The illustrated sensor 1 includes moreover a heating system 18 compris-
ing a heating-up device formed as an electrical resistance and also a tem-
perature sensor 7 for the temperature regulation. The heating system 18
CA 02466105 2004-05-05
11
is advantageously used in combination with the electrochemical measur-
ing apparatus 19 in order to heat up the underlying skin via the sensor
surface 1b. For the transcutaneous measurement of the carbon dioxide
partial pressure pCO2 or of the oxygen partial pressure pO2, the sensor
surface lb is for example heated up to a temperature of about 40 C to
45 C.
The sensor 1 includes a multilayer rigid circuit board 10 which is
equipped with electronic components 2, 3, 6, 7, 12, 13 and which 'has a
plurality of non-illustrated electrically conductive tracks in order to con-
nect the electronic components such as the light emitting diode 2, the
photodetector 3, the resistor 6, the temperature sensor 7, a second tem-
perature sensor 7a or further electronic components, such as amplifiers
12, 12a in a signal conducting manner, in particular to the digital sensor
signal processor 13. All electronic components are formed using SMD
technology (Surface Mounted Device) which results in the advantage that
the circuit board 10 can be automatically equipped and can thus be
manufactured at very favourable cost. At the centre of the circular circuit
board 10 an opening 10c is cut out in the form of a round hole in which
the electrochemical measuring apparatus 19 comprising the micro-pH-
electrode 4 and also the Ag/AgCl reference electrode 5 is arranged. The
micro-pH-electrode 4 is formed as a glass electrode and has an inner elec-
trolyte 4c which is surrounded by a shaft glass 4b and also a membrane
glass 4d. The potential arising in the inner electrolyte 4c is supplied via
the inner conductor 4a to an amplifier 12. An insulator 45 is arranged by
between the micro-pH-electrode 4 and the reference electrode 5.
CA 02466105 2004-05-05
12
Within the circuit board 10 a substantially full area, electrically conduc-
tive layer 10b is arranged which serves for the screening of electrical
fields. Many non-illustrated conductor tracks extend in insulated manner
and transversely through the electrically conductive layer 10b in order to
connect electronic components arranged on the circuit board 10 above
and below it. The layer 10b is of full area design, with the exception of the
apertures caused by the transversely extending conductive tracks.
The heat sensitive electrode 4 is adhered by an electrically conductive ep-
oxy resin to the circuit board 10, with this adhesive being arranged such
that an electrically conductive connection is formed between the layer 10b
opening at the aperture 10c and the Ag/AgCl reference electrode 5. The
reference electrode 5 is thus connected to the and forms moreover a
screen against electrical fields.
The circuit board 10 is contained in a housing 9. The housing 9 consists
of a plastic body provided with an electrically conductive surface, such as
a metal layer. The electronic components arranged above the circuit board
are surrounded by a heat conducting cover 8 which is preferably de-
signed as a heat conducting potting. The space between the surface of the
circuit board 10 and the sensor surface lb is preferably potted with a heat
conducting, electrically insulating epoxy resin. The sensor surface lb is
processed such that the components light emitting diode 2, photodetector
3 and the electrochemical measuring apparatus 19 and the heat conduct-
ing potting 8 which open at the surface form a planar surface. The heat
conducting potting 8 has the advantage that the heat produced by the re-
sistor 6 can be transferred with low loss and uniformly distributed to the
CA 02466105 2004-05-05
13
sensor surface lb, so that the skin which contacts the membrane 50 dur-
ing the measurement can be uniformly warmed.
The electronic components 12, 13 arranged beneath the circuit board 10
are surrounded by an electrically insulating cover 14, 15. For this cover
14, 15 an epoxy resin, and preferably an electrically highly insulating ep-
oxy resin is used which is cast in place. The inner conductor 4a extending
from the electrode 4 to the circuit board 10 is embedded in the highly in-
sulating epoxy resin in order to largely suppress disturbing electrical in-
fluences.
The cover 14, 15, the housing 9, and also parts of the heat conducting
cover 8 are surrounded by a metallic cover 16 in order to protect the inte-
rior of the sensor 1 from disturbing electrical and electromagnetic influ-
ences. The metallic layer 10b providing blanket coverage within the circuit
board 10 or arranged at its surface is not absolutely essential, but has,
amongst other things, the advantage that it prevents the propagation of
disturbing electromagnetic influences produced within the sensor 1, since
the electronic components 2, 3, 6, 7 arranged above the circuit board 10
are screened from the electronic components 12, 13 arranged beneath the
circuit board. The electronic components 2, 3, 6, 7, 12, 13 are moreover
arranged distributed in the sensor 1 such that they exert as far as possi-
ble no mutually disturbing influences. As can be seen from the section of
Fig. 1 the high ohmic, and thus disturbance sensitive, inner conductor 4a
is led extending to the left to an amplifier 12, whereas the digital sensor
signal processor 13 is arranged at the right-hand side. These components
are thus spatially separated, with the grounded reference electrode 5 lo-
cated between them for additional screening. The digitally operating digital
CA 02466105 2004-05-05
14
sensor signal processor 13 is thus screened from the remaining electronic
components 2, 3, 6, 7, 12 which operate in analog manner. The sensor 1
with components and screens arranged in this way enables a signal proc-
essing with an extremely low interference signal component.
The electrode 4 is of elongate design and has a longitudinal direction L.
The circuit board 10 extends perpendicular to the longitudinal direction L
and is arranged within the length of the electrode 4. This arrangement of
the circuit board 10 with the respective electrode 4 has the advantage that
the light emitting diode 2, the photodetector 3 and the heating device 6
come to lie close to the sensor surface lb and that the electrical connec-
tion lines are very short. This arrangement has, amongst other things, the
advantage of a low interference signal component.
The sensor 1 moreover includes electrical connections 11 which are con-
nected in signal conducting manner to the sensor signal processor 13 to
which a non-illustrated cable can be connected which supplies the electri-
cal signals to a subsequent signal evaluation device 37. Fig. 1 shows es-
sentially a section along the line A-A of Fig. 2. The sensor 1 disclosed in
Fig. 1 has preferably a total weight of less than 10 grams. In an advanta-
geous embodiment, the diameter of the sensor 1, which is disclosed in Fig.
2, is chosen so that the sensor surface covers the ear lobe of a person
completely or in most parts.
A possible arrangement of the components described in Fig. 1 is shown in
the view of the sensor surface lb shown in Fig. 2. An insulator 45 is ar-
ranged between the micro-pH-electrode 4 and the reference electrode S.
CA 02466105 2004-05-05
Moreover, a further electrochemical measuring apparatus 19 for the
measurement of the transcutaneous oxygen partial pressure tcpO2 could
be arranged in the sensor 1. As Fig. 2 indicates the photodetector 3 could
be replaced by an electrode 20 in accordance with Clark, which has a
platinum wire 20a arranged at its centre. The electrode 20 forms, together
with the Ag/AgCI reference electrode 5, the further electrochemical meas-
uring apparatus 19 for the measurement of the oxygen partial pressure
P02. In this measurement method, the light emitting diode 2 could be dis-
pensed with. The platinum wire 20a could also be arranged extending
within the photodetector 3 or the light emitting diode 2, with its end face
opening at the surface. The platinum wire 20a could also be arranged, as
can be seen in the section through the mirco-pH-electrode 4 shown in Fig.
10, between an outer shaft glass 4e and an inner shaft glass 4b, with the
end of the platinum wire 20a opening to the surface of the membrane
glass 4d.
Only a single measuring apparatus 17, 19 could also be arranged in the
sensor 1. Measuring apparatuses for the measurement of the most diverse
physiological parameters can also be arranged in the sensor 1. For exam-
ple, a measuring apparatus known per se for the measurement of the core
temperature Tk of the human body which permits the core temperature Tk
to be determined by means of infrared radiation, i.e. with an infrared
transmitter and infrared receiver, with the aid of a measurement at the
timpanic membrane of the ear could be arranged in the sensor 1. Further
measuring apparatuses known per se could also be arranged in the sensor
1, for example for the measurement of the hematocrit (HCT) of the blood
pressure (CNIBP). One or also more such measuring apparatuses can in
each case be arranged in the sensor 1.
CA 02466105 2004-05-05
16
Fig. 3 shows schematically, by way of a block diagram, the electronic
components arranged in the sensor 1. Of particular importance is the digi-
tal sensor signal processor 13 arranged in the sensor 1 which enables an
extensive digital signal processing in the sensor 1. A sensor signal proces-
sor of this kind, also termed in English "Digital Sensor Signal Processor"
(DSSP) normally comprises:
- at least one analog input and output 13a (AFE) with at least one ana-
log/digital converter 35 and at least one digital/ analog converter 36,
- a digital input and output 13b (DIO),
- a nonvolatile memory 13c (FLSH),
- a central processor 13d (CPU),
- a working memory 13e (RAM).
The software for the control of the central processor 13d is stored in the
nonvolatile memory 13c. This software determines how the individual
components of the sensor signal processor and the measuring appara-
tuses 18, 19 are controlled and how the digital data are exchanged with a
higher level signal evaluation device 37.
The sensor signal processor can include further components, such as for
example an oscillator 13g (OSC) or a status monitoring device 13f (SUP),
which resets the sensor signal processor 13 into a defined starting state,
for example with a sudden failure.
The sensor signal processor 13 is connected via analog signal lines 21
with at least one of the measuring apparatuses 17, 18, 19. The optical
measuring apparatus 17 comprises the light emitting diode 2 and the
CA 02466105 2004-05-05
17
photodetector 3. The heating system 18 comprises the electrical resistor 6
and the temperature sensor signal processor 7. The electrochemical
measuring apparatus 18 comprises the reference electrode 5 and the mi-
cro-pH-electrode 4 for the tcpCO2 measurement in accordance with Stow-
Severinghaus and/or the electrode 20 for the tcpO2 measurement in ac-
cordance with Clark. The sensor 1 preferably comprises two temperature
sensors, a digital temperature sensor 7 and also an analog temperature
sensor 7a. In comparison to a sensor which operates in a purely analog
manner, the substantially digital sensor of the invention has a substan-
tially higher redundancy. Moreover, the state of the sensor can be moni-
tored in a substantially more differentiated manner. Thus, it can, for ex-
ample, be determined whether a digital signal is present or not. Conclu-
sions can be drawn concerning the heating up of the sensor 1 from the
total current consumed by it and, if necessary, the current supply or the
heating power reduced. Through the use of two temperature sensors 7,
their temperature can be compared at regular intervals and, if a greater
deviation is present, it can be concluded that one of the temperatures sen-
sors 7 is defective. The sensor 1 of the invention thus has the advantage
that it operates substantially more reliably, that possible disturbances can
be recognised early on and that temperatures in the sensor 1 which are
too high, which could, for example, damage tissue such as the earlobe, are
precluded.
The sensor signal processor 13 is connected via a digital input and output
13b and via a digital signal line 22 to a serial interface 24 also termed
UART (Universal Asynchronous Receiver Transmitter) which enables via
two serial conductors 26a, 26b a bidirectional exchange of data via a con-
nection means 26 to the higher level signal evaluation device 37. In the
CA 02466105 2004-05-05
18
illustrated embodiment the serial interface 24 is designed in accordance
with the standard RS-232. An electrostatic dissipation device 25 (ESD)
protects the serial interface 24 from over-voltages.
The sensor signal processor 13 can, moreover, be connected via digital sig-
nal line 22 to an additional memory 23, with the additional memory 23
preferably being formed as an EEPROM, which is referred to in English as
"Electrical Erasable Programmable Read Only Memory". The individual
components of the sensor 1 are supplied with electrical current via an en-
ergy supply 28. A feed voltage 27a (VCC) and also an ground 27b (GND)
are supplied to the energy supply 28 via two conductors.
It could also be used only a two pole cable comprising the two leads 27a,
27b, in that the digital signal is modulated onto this leads 27a, 27b, so
that the bidirectional data communication also takes place through the
leads 27a, 27b, and therefore the two leads 26a, 26b are not necessary.
The block circuit diagram shown in Fig. 5 shows, for the example of an
optical measuring apparatus 17, a detail aspect of the digital sensor signal
processor 13. The analog input and output 13a comprises an analog con-
trollable signal amplifier 34 which is followed by an analog signal line 21
by an analog-digital converter 35 with 16 bit resolution. The so produced
digital signal is supplied via a digital signal line 22 to the central
processor
13d. The photodetector 3 is connected via the analog signal line 21 to the
signal amplifier 34. The amplification factor of the signal amplifier 34 can
be controlled from the central processor 13d via the digital signal line 22.
In order to ideally exploit the resolution capability of the analog-digital
CA 02466105 2004-05-05
19
converter 35, a weak signal of the photodetector 3 is amplified in accor-
dance with a rule preset by the software of the central processor 13d.
The optical measuring apparatus 17 shown in Fig. 5 for the pulse oximet-
ric measurement of the oxygen saturation (Sp02) measures the light
transmitted through the finger 33 in that the two light emitting diodes 2a,
2b are arranged on the one side of the finger 33 and the photodetector 3
on the other side. The light emitting diodes 2a, 2b are connected via ana-
log signal lines 21a, 21b to a switch over device 31 which in each case
supplies one of the light emitting diodes 2a, 2b with power. The central
processor 13d is connected via the digital signal line 22 to the digital-
analog converter 36 (DAC) which generates an analog signal 21 for the
control of the switch over device 31. Important components of the sensor
signal processor 13 are thus the analog-digital converter 35 and also the
digital-analog converter 36, thanks to which the measuring apparatus 17,
which operates in analog manner, can be operated. A substantial advan-
tage of the digital sensor signal processor 13 can be seen in the fact that
the measuring apparatus 17 is controlled by the central processor 13d,
i.e. by its software. A plurality of further measuring apparatuses or regu-
lating systems, for example the electrochemical measuring apparatus 19
or the heating system 18, can be connected as illustrated to the digital
sensor signal processor 13 in place of the optical measuring apparatus 17
shown by way of example in Fig. 5.
It has proved particularly advantageous to use the sensor signal processor
13 in combination with the optical measuring apparatus 17 for the pulse
oximetric measurement. It is known that the sensitivity of the pulse oxi-
metric measurement is restricted in that interference signals and elec-
CA 02466105 2004-05-05
tronic noise are superimposed on the measurement signals. The sensor 1
of the invention converts the analog signal of the photodetector 3 with the
aid of the sensor signal processor 13 within the sensor 1 into a digital sig-
nal. The analog signal line 21 is very short and is moreover screened
against the outside by the metallic screen 16, so that the analog signal of
the photodetector 3 is hardly impaired by interference signals or electronic
noise. The digital signal can be supplied via the digital signal line 22 with-
out loss of quality to the higher level signal evaluation device 37. It is
known that the primary measuring signals, that is to say the measuring
signals measured by the photodetector 3, can be very weak when using
the pulse oximeter measurement. The sensor 1 of the invention enables
interference signals and electronic noise to be largely avoided, so that
measurements with weak primary measurement signals can also still be
evaluated.
The arrangement shown in Fig. 5 is, for example, controlled by the central
processor 13d in such a way that the photodetector 3 controls one after
the other the light of the infrared light emitting diode 2a, the light of the
red light emitting diode 2b and thereafter measures the light caused by
the environment when none of the light emitting diodes 2a, 2b are ener-
gised. These three measured values are for example detected 4 times
within 16 milliseconds and thereafter sent as a data packet comprising 12
values each having 16 bits, via the digital signal line 22 to the higher level
signal evaluation device 37. Further values can also be transmitted in this
data packet, such as the temperature or the measurement values of the
electrochemical measuring apparatus 19.
CA 02466105 2004-05-05
21
In a further advantageous embodiment the signals measured by the
measuring apparatus 17, 18, 19 are converted by the sensor signal proc-
essor 13 into a normalised digital output signal. For example the tempera-
ture measured in the sensor 1 with the temperature sensor 7 is normal-
ised in such a way that the digital value 0 corresponds to the value 0 C
and the digital value 10000 corresponds to the value 100 C.
In order to further improve the accuracy of the measured values, the char-
acteristic values of at least one of the electronic components used in the
sensor 1, for example the characteristic of the photodetector 3, of the light
emitting diodes 2a, 2b, or as shown in Fig. 7 the characteristic 52 of the
temperature sensor 7, are stored in the sensor 1 in a further advanta-
geous embodiment. The values of the characteristic 52 are preferably
stored in the additional memory 23. The characteristic 52 can be stored in
the most diverse manners, for example as a polygonal chain. The charac-
teristic 52 shown in Fig. 7 is essentially a straight line, so that the pa-
rameters of the characteristic 52 are uniquely determined by the storage
of the slope 53 and also of the offset 54. In this way the characteristics 52
of some or all important electronic components of the sensor 1 can be
stored in the additional memory 23.
The storage of the characteristics 52 results in the following decisive ad-
vantages:
- As a result of the manufacturing tolerances, each electronic component
has an individual scatter, which results in an individual characteristic 52
for each electronic component. Since this individual characteristic 52 is
stored in the sensor 1, the central processor 13d can access these values
and can convert the measured analog signals in an extremely precise
CA 02466105 2004-05-05
22
manner into normalised digital values. In place of the individual charac-
teristics 52 only one characteristic 52 can, however, also be stored which
has, for example, the same values for a series of components.
- The measurement accuracy of the sensor is increased.
- The sensor 1 need only be calibrated at relatively large time intervals.
Moreover, the calibration is simplified. For example the electrode 4 has a
characteristic with a constant slope dependent on the electrolyte 51. Since
the electrolyte 51 dries out in the course of time, the offset of this charac-
teristic, however, changes so that during calibration only the offset has to
be newly determined, whereas the value of the slope does not have to be
fed in anew.
- An ageing of the electronic components, for example of the luminous
power of the light emitting diodes can be automatically detected by the
digital sensor signal processor 13 with the aid of the photodetector 3 and
the age-dependent changed characteristic 52 can be stored in the addi-
tional memory 23 in place of the original characteristic 52.
- The digital output values of each sensor 1 can be preset in a consistent
manner. Thus, a defective sensor 1 can, for example, be replaced without
problem. A calibration of the sensor, for example an adaptation to the
higher level signal evaluation device 37 is not necessary.
Further data can be stored in the additional memory 23, for example an
individual number for each sensor 1 or a designation for the type of the
sensor 1, so that the higher level signal evaluation device 37 can auto-
matically recognise the characteristics of the sensor 1. Patient data can
also be stored in the additional memory 23, so that this data is immedi-
ately available to the memory of a new evaluation device 37 changing the
sensor 1 by unplugging it. Data evaluated in the additional memory 23
CA 02466105 2004-05-05
23
could also be stored by the evaluation device 37, so that at least a part of
the evaluated data are stored in the sensor 1. On changing the sensor 1
by plugging it into a further evaluation device 37, this evaluation device
has available all the data stored in the sensor 1.
The sensor signal processor 13 can also include an operating hour
counter with which the entire operating time of the sensor is detected. If
the ageing behaviour of an electronic component, for example of the LED 2
is known, then the change of the characteristic which is to be expected
can be corrected.
In a preferred method only the micro-pH-electrode 4 or the electrode 20 is
periodically newly calibrated. Since the electrolyte 51 looses liquid in the
course of time, the micro-pH-electrode 4 must be repeatedly newly cali-
brated. The respective new values determined with the aid, for example, of
a calibrator, can be stored by the sensor signal processor 13 in the addi-
tional memory 23.
Fig. 6 shows schematically a block circuit diagram of a system for the
measurement of physiological parameters. The sensor 1 is connected in
signal conducting manner via the cable 44 with the higher level signal
evaluation device 37. The cable 44 of the sensor could also be connected
with a transmitter/ receiver 44a, to transmit the digital information by a
cableless connection, for example, transmitted by means of electromag-
netic waves, to the second transmitter/ receiver 44a. The one transmit-
ter/ receiver 44a can be arranged together with a battery in a housing 47.
The sensor 1 has in this arrangement to be supplied with electrical energy
from this battery. The signal evaluation device 37 comprises essentially a
CA 02466105 2004-05-05
24
digital sensor interface 38 and subsequently a computer 39 also termed a
multi parameter processor. The computer 39 uses the software corre-
sponding to the respective sensor 1 in order to evaluate the digital data. If
the sensor 1, for example, has the optical measuring apparatus 17 shown
in Fig. 5, then the computer 39 calculates from the measured values the
oxygen saturation SP02 with the aid of algorithms known per se and addi-
tionally, if required, the pulse frequency. An input and output device 48 is
arranged after the computer 39 with a controller 40 which controls the
computer 39, a keyboard 41, a screen 42 and also a sound generator 43.
The signal evaluation device 37 could also be arranged within the sensor
1. By using an appropriate software to drive the sensor signal processor,
the signal evaluation could also take place within the sensor signal proc-
essor, so that the output of the sensor 1 provides evaluated measurement
signals, for example the oxygen saturation of the haemoglobin in the arte-
rial blood and/or the transcutaneous carbon dioxide partial pressure
(tcpC02) and/or the pulse frequency.
In an advantageous embodiment, a commercially available computer is
used as the signal evaluation device 37 and as the input and output de-
vice 48, for example a handheld device called "Palmtop", which has a low
weight, is cheap and allows to display the measured values. To measure
for example the oxygen saturation and the carbon dioxide partial pres-
sure, only a sensor is required, which is connected to a commercially
available "Palmtop". In an advantageous embodiment, the sensor itself
provides evaluated measured values, so the "Palmtop" essentially has only
to display the evaluated values on its display.
CA 02466105 2004-05-05
The arrangement in accordance with Fig. 6 has the advantage that a sig-
nal evaluation device 37 suffices in order to evaluate the measurement
signals of different sensors 1. The signal evaluation device 37 must have
different software programs available so that it can access the correspond-
ing evaluation software depending on the sensor 1. The hardware of the
signal evaluation device 37, however, remains identical. Since, in a pre-
ferred embodiment, the signal evaluation device 37 automatically recog-
nises the type of the sensor 1, different sensors 1 can also be connected to
the sensor interface 38 without problem, without additional adjustments
being required. In a further embodiment two or even more sensors 1, 1 a
can also be connected to the same signal evaluation device 37.
Fig. 4 shows in a block circuit diagram a further embodiment of a sensor
1. The analog measurement values of the electrochemical measuring ap-
paratus 19 or of the photodetector 3 are amplified in an impedance buffer
39 and are thereafter passed via the analog signal line 21 to the digital
sensor signal processor 13. The measurement signal of the temperature
sensor 7 is also supplied to it. The sensor signal processor 13 is connected
via digital signal lines 22 to the additional memory 23, to the digital tem-
perature sensor 7a, to the serial RS-232 interface 24 and also, if required,
to a programming device 30. The serial interface 24 is connected via the
serial digital signal line 22a to a cable connection device 32 from which
the leads 26a, 26b, 27a, 27b are led in the form of a common cable to the
signal evaluation device 37. The feed voltage 27a and the ground 27b are
supplied as lines 27 to the amplifiers 31 and also to the sensor signal
processor 13. The sensor signal processor 13 controls the amplifier 31 via
the analog signal lines 21 and thus the two-colour light emitting diode 2
and also the electrical resistor 6.
- ---- - ------
CA 02466105 2004-05-05
26
In the illustrated embodiment the cable includes four conductors 26a,
26b, 27a, 27b. This cable can be made very thin and flexible which gives
the advantage that a movement of the cable has no effect or hardly any
effect on the position of the sensor 1. Bidirectional digital data can also be
exchanged via a single conductor 26a, so that one cable comprising the
conductors 26a, 27a and 27b is sufficient for the operation of the sensor
1. A cable of this kind can be made particularly thin and flexible.
Fig. 8 shows in a longitudinal section a further embodiment of a sensor 1
which is clipped with the aid of a clip 58 with a light reflecting surface 58a
to an earlobe 57. In distinction to the embodiment shown in Fig. 1, the
electrochemical measuring apparatus 19 is formed as a semiconductor
which permits the carbon dioxide to be measured directly and without an
electrolyte 51. The measuring apparatus 19 could also be designed as a
solid state electrode without an internal liquid electrolyte. The measuring
apparatus 19 could also be designed as a semiconductor, for example in
thick film technology, with a pH-sensitive layer. The sensor 1 shown in
Fig. 8 has a housing 9 comprising first and second part housings 9a, 9b.
The measured signals are led away via a connection means 26 formed as a
cable. Fig. 9 shows in a longitudinal section a further embodiment of a
sensor. In distinction to the embodiment shown in Fig. 1, the circuit board
has two flexible sections 10a which is provided with conductive tracks.
A preamplifier 12a is arranged at the end of the one flexible section 1 Oa
and is connected to the inner conductor 4a. This arrangement permits the
high ohmic and thus disturbance-sensitive signal of the inner conductor
4a to be amplified with the preamplifier 12a close to the output point. This
enables a connection between the electrode 4 and the circuit board 10
CA 02466105 2004-05-05
27
which conducts a disturbance-free signal. The flexible section 10a enables
a cost favourable manufacture of the sensor in that the circuit board 10 is
equipped with the electronic components 2, 3, 6, 7, 12, 13, the electro-
chemical measuring apparatus 19 is then bonded to the circuit board 10,
the preamplifier 12a is contacted with the inner conductor 4a and then
the space bounded by the second housing part 9b and the circuit board
is potted with an electrically highly insulating epoxy resin. On the other
flexible section 10a there is arranged a contact point 11 and also a cable
26 leading to the outside. The clamping ring 55 with membrane 50 which
is only indicated in Fig. 1 is moreover shown in Fig. 9 and is releasably
connected to the sensor 1 via a snap device 56.
Fig. 11 shows a longitudinal section through a sensor 1 designed as a res-
piratory gas measuring apparatus 60. In the interior of the tubular hous-
ing 60a there are arranged, in addition to further non-illustrated compo-
nents, a measuring apparatus 19, a temperature sensor 7, a moisture
sensor 62 and also a digital sensor signal processor 13. In the embodi-
ment shown in Fig. 12 the measuring apparatus 19 consists of the two
components light emitting diodes 2 and photodetector 3. This measuring
apparatus 19 could also be designed as semiconductor chip which permits
light of different wavelength to be produced and/or measured in order to
measure a spectrum and determine different gas proportions in the respi-
ratory gas.
In a preferred embodiment, a sensor signal processor with very low energy
consumption is applied in the sensor 1. Such a sensor signal processor is
also called low power processor or ultra low power processor. By using
such a processor, only a low interference power and radiation power is
CA 02466105 2004-05-05
28
produced within the sensor 1. The advantage of this sensor 1 is, that it
causes no or only a very small electromagnetic radiation, which could af-
fect human beings. Further, a very thin cable is sufficient to provide the
sensor with energy.
