Method and apparatus for monitoring respiration

A method and an apparatus are provided for monitoring waveforms representing the respiration of a patient and for detecting waveforms having characteristics indicative of apnea. The method comprises processing a first electrical signal having a waveform representing the respiration of the patient and a second electrical signal representing the electrocardiogram of the patient. In order to increase the reliability of apnea alarms and the accuracy of measuring apnea duration, the method comprises: comparing the amplitude of the first signal, or the amplitude of a signal obtained by transformation of the first signal, at the time of incidence of each QRS complex with a predetermined threshold value, resetting to zero the first signal or said signal obtained by transformation of the first signal in synchronism with the incidence of each QRS complex on the second signal, and triggering an alarm if the amplitudes of the first signal, or of said signal obtained by transformation thereof, at the said times do not exceed the threshold value over a predetermined interval of time, said alarm being indicative of apnea.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus and a method of monitoring waveforms 
representing the respiration of a patient, and of detecting waveforms 
having characteristics indicative of apnea, this method comprising 
processing a first electrical signal having a waveform representing the 
respiration of the patient and a second electrical signal representing the 
electrocardiogram of the patient. 
In the method of monitoring waveforms representing a patient's respiration, 
the reliability of apnea alarms (indicating cessation of respiration), 
accuracy of measurement of the duration of each apnea, and the sensitivity 
of the measuring system for detecting apnea, are greatly restricted by the 
presence of artefacts, which are synchronous with cardiac activity, on the 
wave of the signal representing the patient's respiration, this signal 
being produced, for example, by measuring the variation in the 
transthoracic impedance. In this case, the artefacts, known as 
cardiovascular artefacts, are due to the fact that the movement of the 
heart and the variation in blood flow produce a variation in the impedance 
of the rib cage. 
If the respiratory signal wave did not contain the above artefacts, apnea 
could be detected, at least in principle, by monitoring the amplitude of 
the respiratory signal wave and by detecting the intervals in which said 
amplitude is below a predetermined threshold value. In actual fact, a 
method of this kind is not reliable, because the amplitude of the 
cardiovascular artefacts, which are present even in the case of apnea (an 
apnea should be indicated by a shallow respiration wave of practically 
zero amplitude), may very often exceed the selected detection threshold 
and may therefore erroneously be thought to indicate the presence of 
normal respiration. If a low detection threshold is selected, in fact, 
there is a risk of apnea occurring unnoticed. If, on the other hand, a 
high detection threshold is chosen, there is a risk of false apnea alarms 
being triggered when the respiratory signal is of low amplitude but not 
yet low enough to be an apnea. Since the cardiovascular artefacts may 
often have an amplitude equal to or even greater than that of the 
respiratory signal if the latter is shallow, the user of such a method 
finds it impossible to choose a suitable detection threshold to enable him 
to obtain reliable apnea indication and to measure the duration of such 
apnea accurately. 
Fluctuations in the baseline of the respiratory signal and other 
interference signals frequently superimposed thereon also make reliable 
detection of apnea difficult. 
The following methods have already been proposed to improve the reliability 
of apnea detection despite the presence of cardiovascular artefacts: 
In a first known method (French patent application published under the No. 
2,267,734), the interval of time corresponding to the period of the 
respiratory signal is compared with the interval between successive QRS 
complexes of the signal representing the patient's electrocardiogram. and 
if the difference between these intervals remains below a predetermined 
threshold value over a predetermined interval of time it is assumed that 
the apparently existing respiratory signal is in fact a cardiovascular 
artefact. 
In a second known method (French patent application published under the 
number 2 192 790) the phase difference is measured between the respiratory 
signal and the signal representing the patient's electrocardiogram, and if 
this difference remains below a predetermined threshold value over a 
predetermined interval of time it is assumed that the apparently existing 
respiratory signal is in fact a cardiovascular artefact. 
The above two known methods have the following disadvantages: 
Since cardiac artefacts are often at the limit of detection, their random 
detection eludes the frequency or phase comparison means and results in 
respiration being thought to be normal, with no apnea detection. 
In a third known method (UK patent application No. 2 060 892), a signal is 
formed which represents the first derivative of the respiratory signal 
with respect to time and the slope of this signal is examined at intervals 
of time corresponding to the intervals of time between successive QRS 
complexes of the signal representing the patient's electrocardiogram, and 
if this slope assumes a negative value in the interval during which it is 
examined, and if this occurs a plurality of consecutive times, it is 
assumed that the apparently existing respiratory signal is in fact a 
cardiovascular artefact. 
This third known method has the following disadvantages: 
it is sensitive to the polarity of the signal and does not operate if the 
electrodes are not correctly placed, 
it is sensitive to noise of any kind and, in particular, a very noisy 
random signal may be classed as apnea because it has every likelihood of 
having negative slopes in the examination interval, 
it takes into account the form of the artefact at the time of incidence of 
the QRS complex, such form is however not identical from one patient to 
another and depends on the positioning of the electrodes. 
This method may therefore be inoperatve on certain patients. 
SUMMARY OF THE INVENTION 
The primary underlying object of the invention described hereinafter is to 
provide a method of and apparatus for performing the same whereby the 
disadvantages of the above-described known methods and apparatus can be at 
least partially eliminated so as to give reliable apnea alarms and enable 
the duration of each apnea to be measured with greater accuracy and 
sensitivity despite the presence of cardiovascular artefacts on the 
respiratory signal wave. Another object is to provide a method and 
apparatus of this kind at the lowest possible price and in the smallest 
possible size so that their incorporation in a respiration monitoring 
system will not excessively increase its cost. 
DESCRIPTION OF THE INVENTION 
According to the present invention, the above objectives are realized with 
a method involving processing a first electrical signal having a waveform 
representing the respiration of a patient and a second electrical signal 
representing the electrocardiogram of the patient, which method comprises 
(a) comparing the amplitude of the first signal or of the amplitude of a 
signal obtained by transformation of the first signal, at the time of the 
incidence of each QRS complex with a predetermined threshold value, 
(b) resetting to zero the first signal or said signal obtained by 
transformation thereof in synchronism with the incidence of each QRS 
complex on the second signal, and 
(c) triggering an alarm if the amplitudes of the first signal or of said 
signal obtained by transformation thereof at the said times do not exceed 
the threshold value over a predetermined interval of time, this alarm 
being indicative of apnea. 
In the preferred embodiment, the method according to the invention 
comprises 
(a) forming a third signal by transformation of the first signal, 
(b) forming a fourth signal by transformation of the third signal, 
(c) comparing the amplitude of the fourth signal at the time of the 
incidence of each QRS complex with a predetermined threshold value, 
(d) resetting the fourth signal to zero in synchronism with the incidence 
of each QRS complex on the second signal, and 
(e) triggering an alarm if the amplitudes of the fourth signal at the said 
times do not exceed the threshold value over a predetermined interval of 
time, said alarm being indicative of apnea. 
A further preferred embodiment of this method comprises 
(a) forming a third signal which represents the first derivative of the 
first signal with respect to time, 
(b) forming a fourth signal which represents the integral of the third 
signal over first integration intervals, each of these intervals 
corresponding to the interval between two successive QRS complexes of the 
second signal, 
(c) comparing the amplitude of the fourth signal at the time of the 
incidence of each QRS complex which marks the end of one of the said 
integration intervals with a predetermined threshold value, 
(d) resetting to zero the fourth signal in synchronism with the incidence 
of each QRS complex on the second signal, and 
(e) triggering an alarm if the amplitudes of the fourth signal at the said 
times do not exceed the threshold value over a predetermined interval of 
time, said alarm being indicative of apnea. 
The invention also relates to apparatus for monitoring the respiration of a 
patient, comprising means for forming a first electrical signal having a 
waveform representing the respiration of the patient and a second 
electrical signal representing the electrocardiogram of the patient, and 
means for detecting waveforms of the first signal having characteristics 
indicative of apnea. According to the invention, this apparatus comprises: 
(a) means for deriving a third signal by conversion of the first signal, 
(b) means for forming a fourth signal by conversion of the third signal, 
(c) means for comparing the amplitude of the fourth signal at the time of 
the incidence of each QRS complex with a predetermined threshold value, 
(d) means for resetting the fourth signal to zero in synchronism with the 
incidence of each QRS complex on the second signal, and 
(e) means for triggering an alarm if the amplitudes of the fourth signal at 
the said times do not exceed the threshold value over a predetermined 
interval of time, said alarm being indicative of apnea. 
One preferred embodiment of the apparatus according to the invention 
comprises 
(a) means for forming a third signal representing the first derivative of 
the first signal with respect to time, 
(b) means for forming a fourth signal representing the integral of the 
third signal over first integration intervals, each of said intervals 
corresponding to the interval between two successive QRS complexes of the 
second signal, 
(c) means for comparing the amplitude of the fourth signal at the time of 
incidence of each QRS complex which marks the end of one of said 
integration intervals with a predetermined threshold value, 
(d) means for resetting the fourth signal to zero in synchronism with the 
incidence of each QRS complex on the second signal, and 
(e) means for triggering an alarm if the amplitudes of the fourth signal at 
said times do not exceed the threshold value over a predetermined interval 
of time, said alarm being indicative of apnea. 
The advantages obtained by performing the method according to the invention 
and by using apparatus according to the invention are as follows: 
considerable improvement of apnea alarm reliability, 
increased sensitivity of the measuring system for apnea detection, 
a considerable increase in the accuracy of measurement of the duration of 
each apnea, and 
the simplicity and low cost of the means required for performing the method 
and for constructing the apparatus according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows the positioning of electrodes 111 and 112 to produce a signal 
representing a patient's respiration and a signal representing a patient's 
electrocardiogram, the patient, for example, being a newborn baby. In the 
example shown in FIG. 1, these two signals are produced by the same set of 
electrodes. Within the scope of this invention each of these signals may, 
however, be produced independently of one another by separate means. 
An optional third electrode 113 can be used as a potential reference. 
The electrodes shown in FIG. 1 enable the respiratory signal to be produced 
by measuring the transthoracic impedance. This signal may also be produced 
by other means, e.g., by a thoracic strain gauge, or a displacement or 
acceleration pick-up, an acoustic pick-up, or a pneumatic pick-up for 
detecting the variation in the pressure of a pneumatic mattress, etc. 
FIG. 2 shows typical waveforms of the following signals, which are obtained 
simultaneously from a patient: 
a wave 21 representing the electrocardiogram of the patient 11, 
a wave 22 representing the patient's respiration in the ideal case in which 
said wave has no cardiovascular artefacts, and 
a waveform 23 representing the respiratory signal when the same contains 
cardiovascular artefacts. 
In the interval of time 27 in FIG. 2, the wave 22 of the pure respiratory 
signal has a practically zero amplitude, and this corresponds to apnea. 
However, the wave 23 has an amplitude which exceeds a detection threshold 
26 because it contains a cardiovascular artefact represented by the 
portion 25 of the wave 23. This artefact therefore prevents detection of 
the apnea by means of the detection threshold 26 and leads to the 
interpretation of normal respiration being present in the interval 27. The 
following difficulties are met in the attempt to detect apnea by comparing 
the amplitude of the respiratory signal 23 with a detection threshold 26: 
if the threshold chosen is low there is a risk of apnea not being detected 
(this is the case in the interval 27 in FIG. 2), 
if, on the other hand, the threshold selected is high, there is a risk of 
false apnea alarms being triggered in the event of a normal respiratory 
signal of low amplitude. 
In every case, i.e., irrespective of the selected threshold 26, the 
presence of cardiovascular artefacts interferes with the accuracy of 
measurement of the real time of respiratory pauses (apnea) and makes it 
difficult for the user to select a suitable detection threshold 26 (by 
adjusting the sensitivity), because the respiratory signal amplitude 
varies not only from one patient to another but also with time in the case 
of one and the same patient. These difficulties are obviated by the use of 
the methods according to the invention as described below. 
In the case of normal respiration without apnea, a first embodiment of the 
method according to the invention is illustrated by the waveforms shown in 
FIG. 3, in which the time axes are denoted by broken horizontal lines. 
This method is carried out with two signals produced simultaneously from 
one patient: the signal 21 representing the electrocardiogram and the 
respiratory signal 31. The signal 31 contains cardiovascular artefacts but 
they have no appreciable effect on the result of the method in the case 
shown in FIG. 3, and in order to simplify the representation the 
representation of the signal 31 in FIG. 3 does not show such artefacts. 
In the method according to the invention, a signal 41 is formed whose 
waveform represents the first derivative of the waveform of the 
respiratory signal 31 with respect to time, and a signal 42 is formed 
whose waveform represents the integral of the signal 41 over integration 
intervals, each of said intervals being defined by the interval between 
two successive QRS complexes of the signal 21, with resetting to zero on 
the incidence of each QRS complex. In FIGS. 3 and 4 the incidence of these 
complexes denoted by broken vertical lines. 
The amplitude of the signal 42 at the time of incidence of each QRS complex 
marking the end of one of the integration intervals is compared with a 
threshold value 44 for the amplitudes with a positive polarity and with a 
threshold value 46 for the amplitudes of negative polarity. If the 
amplitudes of the signal 42 at the said times do not exceed the threshold 
values 44 and 46 respectively over a predetermined interval of time, an 
alarm indicative of apnea is triggered. 
The above-described method has the following advantages: 
the use of the first derivative with respect to time (formation of signal 
41), on the one hand, and resetting to zero at the end of each integration 
interval (on formation of signal 42), on the other hand, make the 
amplitude of the signal 42 insensitive to the fluctuations (drift) of the 
base line of the respiratory signal 31. 
the integration operation applied to the signal 41 also comprises a 
low-pass filtration which reduces the influence of other artefacts (other 
than cardiovascular artefacts) and reproducibility errors. 
The above-described method, although quite usable in general, becomes 
inoperative in the specific case shown in FIG. 4 in which the frequency of 
the respiratory signal 34 is exactly half the frequency of the ECG signal 
21 and, in addition, signal 34 has an unfavourable phase ratio with 
respect to signal 21. In this specific case, the amplitude of signal 42 
always assumes the value zero at the end of each integration interval and 
leads to the interpretation that apnea is present. To obviate this 
difficulty, in a preferred version of the method described above, a signal 
43 is additionally formed, the waveform of which represents the integral 
of the signal 41 over integration intervals 47, 48, 49, etc., each of 
which is offset by an interval 33, e.g., 200 milliseconds (ms), with 
respect to one of the integration intervals used for forming the signal 
42, and the amplitude of the signal 43 at the time marking the end of each 
of these integration intervals 47, 48, 49, etc., is compared with the 
predetermined threshold values 44 and 46 respectively. In this preferred 
version of the first embodiment of the method according to the invention 
the apnea alarm is triggered only if the amplitudes of the signal 42 and 
the amplitudes of the signal 43 at the end of the respective integration 
intervals do not exceed the threshold value over a predetermined interval 
of time, e.g., 3 seconds. 
In the extremely unfavourable case shown in FIG. 4, the amplitudes of the 
signal 43 at the end of the integration intervals exceed the threshold 
value 46 and reliably prevent a false apnea alarm from being triggered. 
In the more general and frequent case illustrated in FIG. 3, the values of 
the amplitudes of the signals 42 and 43 at the end of the respective 
integration intervals exceed the threshold values 44 and 46, respectively, 
and reliably prevent a false apnea alarm from being triggered. 
In the case of an apnea in the presence of cardiovascular artefacts, the 
first embodiment of the method according to the invention is illustrated 
by the waveforms shown in FIG. 5, in which the time axes are denoted by 
broken horizontal lines. As before, this method is performed by means of 
two signals produced simultaneously from one patient: signal 21 
representing the ECG and a signal 32 obtained with the same means used to 
produce the respiratory signal 31 in FIG. 3. Since the signal 32 
represents an apnea (absence of respiration), its waveform represents 
practically only cardiovascular artefacts. 
In the method according to this invention, a signal 51 is formed whose 
waveform represents the first derivative of the waveform of the signal 32 
with respect to time, and a signal 52 is formed whose waveform represents 
the integral of the signal 41 over integration intervals, each of said 
intervals being defined by the interval between two successive QRS 
complexes of the signal 21, with resetting to zero on the incidence of 
each QRS complex. In FIG. 5 the incidence of each of these complexes is 
denoted by a vertical broken line. 
The amplitude of the signal 52 at the time of incidence of each complex QRS 
marking the end of one of the integration intervals is compared with the 
threshold value 44 for the positive polarity amplitudes and with the 
threshold value 46 for the negative polarity amplitudes. 
FIG. 5 shows that at the above-defined times in an apnea interval the 
amplitudes of the signal 52 are less than the threshold values 44 and 46. 
According to the first embodiment of the method according to the 
invention, an alarm is triggered which indicates an apnea if said 
amplitudes remain below said thresholds over a predetermined interval of 
time. 
In order to improve the reliability of the method according to the 
invention, in the preferred version thereof described above, there is also 
formed a signal 53, whose waveform represents the integral of the signal 
51 over integration intervals 47, 48, 49, etc., each of which is offset by 
the interval 33 with respect to one of the integration intervals used for 
the formation of the signal 52, and the amplitude of the signal 53 at the 
time marking the end of each of these integration intervals 47, 48, 49, 
etc., is compared with the threshold values 44 and 46, respectively. FIG. 
5 shows that in an apnea interval the amplitudes of the signals 52 and 53 
at the end of the respective integration intervals are less than the 
threshold values 44 and 46 respectively. According to the preferred 
version of the first embodiment of the method according to the invention, 
an alarm indicative of an apnea is triggered if said amplitudes remain 
below said threshold over a predetermined interval of time. 
The efficiancy of the methods according to the invention is based on 
certain properties of the cardiovascular artefacts. As shown in FIG. 6, 
the waveform 12 of a signal representing cardiovascular artefacts may have 
any arbitrary form but is generally fairly periodic, in synchronism with 
the signal 21 representing the ECG and repetitive in its form. As shown in 
FIG. 6, the waveform 12 of the signal representing a cardiovascular 
artefact resumes the same amplitude value for each time of its period. 
Thus the amplitude of the point 13 of the wave 12 on the incidence of a 
QRS complex of signal 21 at time 35 is the same as that of point 15 at 
time 36 on the incidence of the next QRS complex. The points 14 and 16 
prior to the points 13 and 15, respectively, of the wave 12 and which are 
separated from these points by intervals of time 17, also have the same 
amplitude. The latter characteristic of the waveform of cardiovascular 
artefacts is in fact an adequate condition for the methods according to 
the invention to allow reliable apnea detection and accurate measurement 
of apnea duration during monitoring of a patient's respiration. 
FIG. 7 shows a first embodiment of an electronic system for performing a 
method according to the invention. This system comprises the series 
connection of the following circuits: a differentiating circuit 72, an 
integrating circuit 73, a threshold detector 74 and a bistable type D 
flip-flop 79. 
The respiratory signal is applied to the input 71 of the differentiating 
circuit 72. At its output the latter delivers a signal representing the 
first derivative of the respiratory signal with respect to time. This 
signal is applied to the input of the integrator 73. At its output the 
latter delivers a signal representing the integral of the signal applied 
to its input over integration intervals each corresponding to the interval 
between two successive QRS complexes of the patient's ECG. The integrator 
73 is reset to zero on the incidence of each QRS complex by the closing of 
a switch 69 for a short time. This closing operation is controlled by a 
signal representing the respective QRS complex, said signal being derived 
from the signal representing the ECG by means of a suitable detector 
circuit. In FIG. 7, the control for closing switch 69 is represented by a 
broken line 68. The signal delivered at the output of integrator 73 is 
applied to a first input 75 of the threshold detector 74. A voltage 
corresponding to a predetermined threshold value is applied to a second 
input 76 of the threshold detector 74. According to the result of the 
comparison of the signals applied to these inputs, the threshold detector 
74 outputs a signal corresponding to a logic 1 or 0. This signal is 
applied to the input D of the flip-flop circuit 79 via line 78. On the 
incidence of each QRS complex a signal corresponding thereto is applied 
via a line 77 to a second input of the flip-flop circuit 79. This trigger 
input is the one receiving the clock pulses when the trigger circuit is 
used in digital circuits. The output signal of the trigger circuit 79 is 
delivered over line 81. This output signal is applied to an analyzer 
circuit (not shown in FIG. 7) which triggers an apnea alarm if the output 
signal from flip-flop 79 indicates that the amplitudes of the output 
signal of the integrator do not exceed the threshold value over a 
predetermined interval of time. 
The electronic system described above allows the performance of the first 
embodiment of the method of the invention described above with reference 
to FIG. 3. FIG. 8 shows a more complete version of this system. This 
version allows performance of the preferred embodiment of the method 
according to the invention described above with reference to FIGS. 3 and 
4. 
The electronic system shown in FIG. 8 comprises all the elements shown in 
FIG. 7 and additionally the following elements: 
a monostable multivibrator 62, to which there is fed via line 61 as an 
input signal pulses corresponding to respective QRS complexes. These same 
pulses control the closing of the switch 69 in the integrator 73 (via a 
control shown by broken line 68) and are also applied to the input 77 of 
the flip-flop 79. 
an integrator 83 having the same configuration and in principle the same 
operation as the integrator 73. Resetting of integrator 83 to zero is 
effected by a switch 67 being closed for a short time. This switch is 
controlled (via a control denoted by a broken line 66) by the pulses 
delivered at the output of the monostable multivibrator 62. These pulses 
are delayed a certain amount with respect to the pulses representing the 
QRS complexes, the delay being, for example, 200 ms. 
a threshold detector 84 having an input connected to the output of the 
integrator 83 by a line 85 and an input receiving a voltage corresponding 
to a threshold value via a line 86. The output signal of this detector is 
delivered over line 88. The operation of the threshold detector 84 is the 
same as that of the threshold detector 74. 
A bistable type D flip-flop 89, the input signal of which is, on the one 
hand, the threshold detector output signal fed via line 88, and, on the 
other hand, the output signal of the monostable multivibrator 62 fed on 
line 87. The output signal of flip-flop 89 is fed via line 91 to an 
analyzer (not shown in FIG. 8). This analyzer triggers an apnea alarm only 
if the output signals of the flip-flops 79 and 89 indicate that the 
amplitudes of the output signals of the integrators 73 and 83 at the end 
of the respective integration intervals do not exceed the threshold value 
over a predetermined interval of time. 
FIG. 9 shows a second embodiment of the systems according to FIGS. 7 and 8. 
In the system shown in FIG. 9, the differentiating circuit 72 is retained 
but the other elements shown in FIGS. 7 and 8 are replaced by a 
microprocessor 93, which also carries out the function of the analyzer 
mentioned (but not shown) with reference to FIGS. 7 and 8. An 
analog-digital converter 118 is provided between the output of the 
differentiating circuit 72 and an input 119 of the microprocessor 93. 
Pulses corresponding to the QRS complexes and arriving via line 92 are fed 
to the "interrupt" input of the microprocessor 93. The latter performs 
integration operations by cumulating the values of the output signal of 
the differentiating circuit 72 every 1 ms in the integration intervals, 
with resetting to zero at the end of each of these intervals. The 
microprocessor 93 also compares the results of these integration 
operations with detection thresholds and, where applicable, triggers an 
alarm which is made audible by means of a loudspeaker 82 actuated by an 
output signal of the microprocessor 93 fed via line 94. 
FIG. 10 illustrates an entirely digital version of the system shown in FIG. 
9. In this version, a microprocessor 103 also performs the function of the 
differentiating circuit 72. The respiratory signal delivered via line 71 
is fed to the input of an analog-digital converter 118, the output of 
which is fed to the input 119 of the microprocessor. Pulses corresponding 
to the QRS complexes and arriving via line 102 are fed to the "interrupt" 
input of the microprocessor. As in the version of FIG. 9, a loudspeaker 82 
is connected to the output 104 of the microprocessor and where applicable 
enables the apnea alarms triggered by the microprocessor to be made 
audible. 
In the microprocessor 93 (FIG. 9) and 103 (FIG. 10), the value of a signal 
representing the first derivative of the respiratory signal is calculated 
every 1 ms by calculating the difference between two successive values of 
the respiratory signal. 
FIG. 11 shows a typical form of the amplitude-frequency characteristic 
curve of the differentiating circuit 72 in FIGS. 7-9 and 12. The cut-off 
frequency at 3 hertz (Hz) shown in this figure is only a typical example. 
It is not an essential condition of the method. 
FIG. 12 is a block schematic diagram of an apparatus comprising the system 
shown in FIG. 9 for apnea detection during monitoring of the respiratory 
signal. In addition to the elements of the system described above with 
reference to FIG. 9, the apparatus shown in FIG. 12 comprises the 
following elements: 
a floating amplifier 114 whose inputs are connected to the electrodes 111, 
112 and 113 shown in FIG. 1. One of the outputs of the amplifier 114 
delivers a signal representing the patient's respiration. This signal is 
amplified by an amplifier 115 and the amplified signal is fed to the input 
of the differentiating circuit 72. 
a second output of the floating amplifier 114 delivers a signal 
representing the patient's electrocardiogram. This signal is amplified by 
an amplifier 116 and the amplified signal is fed to the input of a circuit 
117 for the detection of the QRS complexes. The output of the circuit 117 
delivers pulses representing said complexes. The output of circuit 117 is 
connected to the "interrupt" input of the microprocessor 93 via a line 92. 
a keyboard 153 is connected to one input of the microprocessor 93 for 
inputting certain parameters, e.g., the duration of the predetermined 
interval of time after which an apnea alarm is triggered in the method 
acording to the invention. 
A digital display 152, e.g., a liquid crystal display, is connected via a 
line 121 to an output of the microprocessor 93. With this system it is 
possible to display alarm conditions, alarm causes, the contents of 
incident memories, and so on. 
FIG. 12 illustrates an apparatus in which the respiratory signal and the 
signal representing the patient's ECG are produced with the same group of 
electrodes 111, 112, 113. In that case the floating amplifier 114 contains 
the means required for separating the respiratory signal from the ECG 
signal, e.g., a filter 122 for extracting the respiratory signal and the 
series circuit comprising an amplifier 123 and a filter 124 for extracting 
the ECG signal, in order to deliver these signals via separate outputs. 
Each of these signals, however, can be produced independently of the 
other, i.e., by separate means. 
In the exemplified embodiments described above the differentiating circuit 
72 may be replaced by filtering means having a similar transfer function. 
FIG. 13 illustrated a second embodiment of the method according to the 
invention. In this Figure a broken horizontal line represents the baseline 
of each of the signals shown. The incidence of each QRS complex of the 
electrocardiogram is shown by a vertical broken line. In this second 
embodiment, clamping of the respiratory signal is effected whenever a QRS 
complex is detected. A signal 131 is thus formed whose waveform 
corresponds to that of the respiratory signal in the intervals between 
successive QRS complexes, but which assumes the value zero whenever a QRS 
complex is detected. The values of the amplitude of the signal 131 at the 
times of incidence of the QRS complexes are compared with predetermined 
threshold values 132, 133. If the amplitudes of the signal 131 at said 
times do not exceed the threshold values 132 ad 133, respectively, over a 
predetermined interval of time, an alarm indicative of an apnea is 
triggered. In the opposite case illustrated in FIG. 13, in which the 
amplitudes of the signal 131 exceed said threshold values, the presence of 
normal respiration and hence the absence of apnea are confirmed. 
FIG. 14 diagrammatically illustrates a system for performing the method 
illustrating in FIG. 13. This system enables analog clamping of the 
respiratory signal delivered via line 71 to be effected. The system 
comprises a connecting capacitor 141 and a switch 142. To provide 
clamping, the switch is closed for a short time on incidence of a QRS 
complex. Closing of the switch 142 is controlled by a pulse representing a 
QRS complex. This control is denoted by the broken lines 143 in FIG. 14. 
The clamped respiratory signal 131 is delivered via line 144 to an 
analyzer circuit (not show in FIG. 14), which triggers an apnea alarm if 
the amplitudes of the signal 131 do not exceed the threshold values 132, 
133 over a predetermined interval of time. The function of the system 
shown in FIG. 14 may also be carried out by a microprocessor.