Apparatus for the determination of the respiratory passageway impedance

In the illustrated embodiment, applied high frequency flow is divided into a first flow component traversing a known flow impedance and a second flow component directed toward the mouthpiece of a breathing tube. During calibration, the mouthpiece is sealed, and phase adjustments are made such that during measurement, applied flow and first flow component representing signals are essentially in phase and can be combined in an a.c. subtracter to provide a measure of the second flow component. A divider provides the quotient of rectified average pressure and second flow component signals as a measure of the magnitude (Z) of respiratory passageway impedance. A phase difference measurement circuit measures the phase difference between pressure and second flow components and can be switched over during calibration for use in the phase adjustment procedure. The real and imaginary components of respiratory passage impedance can be obtained using sample and hold circuitry responsive to the phase of the second flow component.

BACKGROUND OF THE INVENTION 
The invention relates to an apparatus for determining the respiratory 
passageway impedance comprising a breathing tube which exhibits a pulse 
generator for subjecting the breath flow with flow pulsations of a higher 
frequency as compared with the respiratory frequency, and which exhibits a 
selective pressure measuring apparatus for the correspondingly 
higher-frequency pressure fluctuations, and whose tube-end not facing the 
mouthpiece is terminated with a flow resistance having a known resistance 
value; for example, a hose, as the comparison resistance, such that a 
first partial current of the higher frequency total current flows over the 
flow resistance, whereas a second partial current flows via the mouthpiece 
into the respiratory tract of an examination subject. 
An apparatus of the type initially cited is known from the U.S. Pat. No. 
4,051,843. With an apparatus of this type, the respiratory passageway 
resistance can be indicated rapidly and in a problem-free manner as a 
function of the impressed flow, the complex comparison flow impedance 
having a known impedance value, and the measured pressure valve. In a 
further development of an apparatus of this type, means were provided in 
the measuring apparatus for the simultaneous detection of the phase angle 
of the pressure measured value and means for representing, or converting, 
respectively, the measured value with the object of determining the 
magnitude and phase of the complex respiratory passageway impedance. 
However, the equations derived for this purpose show that the connection 
between the measured values and the magnitudes to be determined is 
relatively complicated. In said letters patent, the complex respiratory 
passageway impedance must first be determined from diagrams with curved 
coordinates, or by means of programmable computers, respectively. 
SUMMARY OF THE INVENTION 
It is an object of the invention to further develop an apparatus of the 
type initially cited such that the respiratory passageway impedance can be 
indicated (or displayed) directly ("on line") and in linear fashion 
according to magnitude and phase, or that it can also be indicated as a 
real and an imaginary component. The cumbersome evaluation of diagrams and 
the programming of computers, respectively, is to be entirely eliminated 
here. 
In accordance with the invention, the object is achieved by virtue of the 
fact that, in order to determine that particular second partial current of 
the superimposed higher-frequency total current which flows via the 
mouthpiece into the respiratory tract of the subject under examination, 
the first partial current which flows via the comparison impedance and 
whose magnitude is established by the quotient of the higher frequency 
pressure signal and the known comparison impedance, is subtracted by means 
of a subtracter element from the likewise known total current, whereby, in 
order to determine the magnitude and phase, or the real (and imaginary) 
component of the respiratory passageway impedance, a phase tuning device 
is series connected specifically to the subtracter element, said phase 
tuning device bringing the total current and the first partial current 
into phase in a calibration operation while the breathing tube is sealed 
(or closed) off at the mouthpiece. In order to determine the phase of the 
respiratory passageway impedance, a phase measuring device with a 
calibrated indicator apparatus is connected in the signal path for the 
second partial current behind the subtracter element, on the one hand, and 
in the pressure signal path in front of the phase tuning device, on the 
other hand, whereby, in the calibration operation, carried out with the 
breathing tube closed (or sealed) off at the mouthpiece, the phase 
measuring device connected to the signal path for the total current and 
the first partial current is calibrated to zero (or null) value by means 
of the phase tuning device. Thus, with the apparatus according to the 
invention, an "on line" display (or indication) of the magnitude and 
phase, or the real- and imaginary-component of the respiratory passageway 
impedance, is achieved. The technical outlay involved remains within 
reasonable limits. 
A detailed analysis of the pneumatic conditions in the known apparatus for 
determining the respiratory passageway impedance (according to FIG. 1) 
served as the basis of the invention. From the parallel-connection of the 
respiratory passageway impedance Z and the comparison impedance Z.sub.o, 
in analogy with laws of electrical engineering, there result the equations 
for the currents Q.sub.i and the impedances Z.sub.i : 
##EQU1## 
The symbols here denote the following (complex) magnitudes: 
______________________________________ 
-Z = Z .multidot. e.sup.j.phi. 
(Respiratory passageway impedance) 
-Z.sub.o = Z.sub.o .multidot. e.sup.j.phi..sbsp.o 
(Comparison impedance) 
-Q.sub.o = Q.sub.o .multidot. e.sup.j.omega.t 
(impressed pulsation) 
-Q.sub.1 = Q.sub.1 .multidot. e.sup.j(.omega.t+ .phi..sbsp.1.sup.) 
(first partial current flowing over the 
comparison impedance) 
-Q.sub.2 = Q.sub.2 .multidot. e.sup.j(.omega.t+ .phi..sbsp.2.sup.) 
(second partial current flowing over the 
respiratory passageway impedance of the 
test subject) 
-P = P .multidot. e.sup.j(.omega.t+ .psi.) 
(mouth pressure) 
______________________________________ 
If equation (2) is solved according to Z, the respiratory passageway 
impedance is basically determinable as a function of the specified (or 
predetermined), and measurable magnitudes. In the case of known apparatus, 
the second partial current Q.sub.2 is particularly also measured with a 
pneumotachograph in the respiratory current passageway, whereby, as a 
consequence, however, due to the unavoidable errors during the measurement 
of small alternating currents of high frequency, the value, according to 
magnitude and phase, of the respiratory passageway resistance becomes 
imprecise. 
However, in accordance with the present invention, Q.sub.2 is 
electronically determined. The circuit proceeds from the 
definition-equation for a flow impedance as a quotient of alternating 
pressure and alternating current; specifically for the magnitude and phase 
of the respiratory passageway impedance in an apparatus of the type 
initially cited the following relations are then valid: 
EQU Z=P/Q.sub.2 and .phi.=.psi.-.phi..sub.2 with Q.sub.2 =Q.sub.o -Q.sub.1 
EQU and Q.sub.1 =P/Z.sub.o, .phi..sub.1 =.psi.-.phi..sub.o 
If the second partial current Q.sub.2 is known according to magnitude and 
phase, then the magnitude of the respiratory passageway impedance Z can be 
determined by means of quotient formation in a divider member, and the 
phase of the respiratory passageway impedance can be determined by means 
of direct phase measurement. For the real- and imaginary-component of the 
respiratory passageway impedance Z, as is known, the following relations 
are valid: 
EQU Re(Z)=Z cos .phi. 
EQU Im(Z)=Z sin .phi. 
However, the invention preferably proceeds from the fact that the pressure 
divided by the magnitude of the second partial current, at the time of 
maximum flow corresponds to the real component, and that this quotient 
corresponds to the imaginary component of the respiratory passageway 
impedance during zero (or null) instantaneous value of the flow. In an 
advantageous further development, the corresponding voltages are retained 
(or held) with sample- and hold-elements, whereby the sample pulses are 
supplied by means of extreme or peak value detectors- or zero (or null) 
value detectors, respectively, in the signal line for the second partial 
current. 
Further advantages and details of the invention shall be apparent from the 
following description of a sample embodiment on the basis of the 
accompanying sheets of drawings; other objects, features and advantages 
will be apparent from this detailed disclosure and from the appended 
claims.

DETAILED DESCRIPTION 
In FIG. 1, 1 designates a breathing tube, at the one end of which a 
mouthpiece 2 is located for the purpose of connecting the test subject, 
whose respiratory tract is represented by the complex respiratory 
passageway impedance Z. At the other end of the breathing tube leading 
into free space, there is arranged a complex comparison impedance 3 with 
the value Z.sub.o. There is disposed on the breathing tube 1 a connecting 
branch 4 for the purpose of connecting an alternating flow pump 5 with an 
impressed, known flow pulsation Q.sub.o, whose frequency lies above the 
breathing frequency. In accordance with the rule of sums, the total 
current Q.sub.o branches at the juncture of connecting branch 4 with tube 
1 into the two partial currents Q.sub.1 and Q.sub.2. The schematically 
illustrated alternating flow pump 5 is connected to a drive unit 6. There 
is mounted onto the drive unit 6 a semicircular plate (or disk) 7 which, 
when drive unit 6 is in operation, periodically interrupts the path of 
rays of a light barrier constructed from an optical transmitter 8 and 
optical receiver 9. A square wave signal for Q.sub.o is thereby produced 
synchronously with the operation of the alternating flow pump 5, said 
signal being conveyed via signal line 10 to the evaluation unit. In 
addition, there is disposed on breathing tube 1 a tapping (or measuring) 
connection piece 11 for measuring pressure. Via a microphone 12 operating 
as a sensing transducer, pressure signal P is likewise conveyed in the 
form of an electrical signal via signal line 13 to the evaluation unit. 
In FIG. 2, light barrier 8, 9 with signal line 10, as well as pressure 
transducer 12 with signal line 13 from FIG. 1 is schematically 
illustrated. The square wave signal of light barrier 8, 9, is first 
delivered to a selective filter 14 for the alternating frequency, which 
filters out (transmits) the fundamental wave of the square wave signal; 
i.e., a continuous sinusoidal waveform signal as the fundamental wave, 
said fundamental sine wave signal being synchronous with the alternating 
flow pump 5. A phase-synchronous signal for Q.sub.o is thus available. 
Instead of light barrier 8, 9, it is also possible to employ as the signal 
generator a permanent magnet rotating with the pump drive 6, said 
permanent magnet inducing in a fixed coil an alternating current 
synchronous with the impressed alternating flow Q.sub.o. The output signal 
of measuring transducer 12 is likewise initially conveyed via line 13 to a 
filter 15 which is selective for the alternating frequency, which 
separates (transmits) the higher frequency pressure component 
corresponding in frequency to the flow pulsation Q.sub.o from the 
respiratory pressure component. Via an amplifier 16 with adjustable 
amplification, the pressure signal is conveyed to a phase shifter 17 with 
which the known phase angle .phi..sub.o of the comparison impedance 
Z.sub.o is compensated. In the case of a real comparison impedance 
(.phi..sub.o =0), the phase shifter 17 can be eliminated. Thus, the output 
signal of the phase shifter member 17 represents a signal which is 
phase-synchronous with the first partial current Q.sub.1. The fundamental 
sine wave output signal, which is phase-synchronous with Q.sub.o, of 
filter 14 is delivered via an adjustable phase shifter 18, together with 
the output signal--which is phase synchronous with Q.sub.1 --of phase 
shifter 17 to a subtracter unit 19. With the adjustable phase shifter 
member 18, the phase relationship between Q.sub.o and Q.sub.1 is initially 
equalized (or balanced) in the calibration operation while the breathing 
tube 1 is closed (or sealed) off at the mouthpiece 2. During the 
measurement operation, the difference between Q.sub.o and Q.sub.1 is then 
formed by means of subtracter unit 19, whereby a phase-synchronous signal 
for Q.sub.2 results. In order to determine the magnitude Z of the 
respiratory passageway impedance Z, the output signals of the phase 
shifter 17 and the subtracter unit 19 are then delivered, for the purpose 
of full wave rectification in each instance, to amplitude members 20 and 
21 with following low-pass filters 22 and 23, for the purpose of mean 
value formation of the rectified signals, and subsequently the quotient of 
the averaged magnitudes is determined in a divider element 24. The output 
signal of output divider element 24 then still merely needs to be weighted 
(or evaluated) in a calibration potentiometer 25 with the factor Z.sub.o, 
and it can then be directly indicated on a calibratable indicator 26 as 
the magnitude of the respiratory passageway resistance Z. The indicator 
apparatus 26 can be switched over by means of switch 27 from a measurement 
position M to a calibration position E. In the calibration position E, the 
absolute value of Q.sub.2 is delivered directly to display apparatus 26. 
In order to determine the phase angle .phi. of the respiratory passageway 
impedance Z, the sinusoidal output signals of amplifier 16 and subtracter 
unit 19 (i.e., the signal lines for P and Q.sub.2) are delivered to 
measurement inputs M of a phase measurement element 28 with a following 
indicator unit 29 which determines the phase difference of the input 
signals in a conventional manner. The phase measurement element 28 
additionally has two calibration inputs E to which the signal lines 
carrying Q.sub.1 and Q.sub.o can be connected for the purpose of 
calibration. A switch 30 is here coupled in front of the phase measurement 
element 28 with switch 27 such that both indicator units 26 and 29 can 
each be simultaneously brought into calibration position. 
For zero (or null) balance of the circuit according to FIG. 2, the 
breathing tube according to FIG. 1 is tightly closed (or sealed) off at 
the mouthpiece 2, and switches 27 and 30, respectively, are set to the 
calibration position E. Since, on account of Z.fwdarw..infin., the 
relations Q.sub.2 =0, and Q.sub.1 =Q.sub.o are pneumatically valid, the 
signals for Q.sub.1 and Q.sub.o must be made identical in phase and 
amplitude in the calibration position. Accordingly, with phase shifter 
unit 18, the phase of Q.sub.o is adjusted such that the value of zero is 
indicated on indicator apparatus 29. The amplitude of Q.sub.1 is then 
adjusted at the adjustable amplifier 16 such that a minimum value is 
present on the indicator apparatus 26. The limit indication of zero (or 
null) can be only achieved if the signals for Q.sub.o and Q.sub.1 are 
completely free of harmonic components. Subsequent to the zero (or null) 
balance, in the measurement position M of switch 27, with the aid of a 
known calibration resistance which is connected to the mouthpiece 2 
according to FIG. 1 instead of the test subject, the amplitude deflection 
of the indicator apparatus 26 is adjusted to a calibration mark by means 
of potentiometer 25. The indicator apparatus 26 is thus calibrated for the 
measurements. 
The circuit in FIG. 3 proceeds from the circuit means illustrated in FIG. 2 
for determining the second partial current Q.sub.2 (at the output of 
subtracter 19) and the pressure signal value P (at the output of 
adjustable amplifier 16). In order to directly detect the real component 
and the imaginary component of the respiratory passageway impedance, the 
quotient P/Q.sub.2 is retained (held) at specific times in each instance. 
The circuit is based on the fact that the pressure value P, divided by the 
magnitude of Q.sub.2 corresponds at the time of the maximum and the 
minimum (i.e. at the time of the peak positive and peak negative 
instantaneous values) of the second partial current Q.sub.2 (i.e. 
.phi..sub.2 =90.degree., 270.degree.) to the real component, and that at 
the time of zero (or null) instantaneous values of the second partial 
current Q.sub.2 (i.e. .phi..sub.2 =0.degree., 180.degree.), it corresponds 
to the imaginary component of the respiratory passageway impedance Z. The 
corresponding voltages in the signal lines are retained or held with 
sample- and hold-elements. The sample pulses are obtained with zero (or 
null) value detectors. Prior to the quotient formation, the P- and Q.sub.2 
signal again undergo a full-wave rectification at amplitude elements 31 
and 32. There are supplied to the divider element 33 and the following 
calibration potentiometer 34 (analogously to FIG. 2) the rectified 
pressure signal and the magnitude signal of the second partial current 
Q.sub.2. 
In the time diagram, the resistance signal P/Q.sub.2 is illustrated 
together with the phase shifted Q.sub.2 signal. From this illustration, it 
is apparent that during the zero (or null) instantaneous value of the 
second partial current, the measured value represents the pure imaginary 
component, and during maximum or minimum values of the second partial 
current, on the contrary, the measured value represents the pure real 
component of the respiratory passageway impedance Z. Accordingly, the 
circuit according to FIG. 3 exhibits a differentiating element 35 with a 
following zero (or null) detector 36, to which the second partial current 
Q.sub.2 is delivered, so that, by means of zero (or null) detector 36, 
sample pulses are supplied in the time intervals .phi..sub.2 =90.degree., 
or 270.degree., respectively. The output signal of zero detector 36, on 
the one hand, actuates a sample and hold circuit 37 (for the purpose of 
peak value formation of Q.sub.2. Circuit 37 has its input connected to 
rectifier 32 and its output connected to divider element 33. The output 
signal from zero detector 36, on the other hand, is supplied to an 
additional sample and hold circuit 38, which is connected with the output 
of divider element 33 via a calibration potentiometer 34. Thus, with this 
interconnection, the value of P/Q.sub.2 is determined at the times 
.phi..sub.2 =90.degree., 270.degree., which represents the real component 
of the respiratory passageway impedance Z, and which is indicated as the 
measured value on the display apparatus 39. 
There is further connected to the input Q.sub.2 of FIG. 3 a zero detector 
40 for detecting the zero crossings of the second partial current Q.sub.2. 
The zero detector 40 has two outputs, the first output of which delivering 
sample pulses in the times .phi..sub.2 =0.degree., 180.degree. to a sample 
and hold element 41 in the output line for the quotient P/Q.sub.2, and the 
second output of which, on the contrary, merely delivering a pulse at the 
times corresponding to .phi..sub.2 =0.degree., which actuates a sample and 
hold element 42 in the pressure signal line. The output signal of the 
sample and hold element 41 represents the magnitude of the imaginary 
component of the respiratory passageway impedance Z. However, since the 
imaginary component can be positive or negative; i.e., it can be of an 
inductive or of a capacitive nature, a polarity determination must take 
place before the ascertained value is indicated as the measured value on 
an indicator apparatus 43. The imaginary component of the respiratory 
passageway impedance Z is positive or negative, respectively, when the 
pressure signal P at the time .phi..sub.2 =0.degree. is greater, or 
smaller, respectively, than zero. The pressure signal P is retained (or 
held) at this time interval by means of the sample and hold element 42, 
whose output signal proceeds to the first input of a comparator 44. The 
second input of comparator 44 is connected to ground potential. In the 
case of a positive output voltage of sample and hold element 42, the 
indicator instrument 43 is connected directly via line 46 and the 
comparator 44 to the output of the sample and hold element 41. If, on the 
contrary, the output signal is negative, the display apparatus 43 is 
connected by means of comparator 44 to the output of an inverter 45, which 
is connected to the output of the sample and hold element 41. An 
indication, which is true to the polarity of the imaginary component of 
the respiratory passageway impedance Z is thereby guaranteed on the 
indicator apparatus 43. 
As will be apparent the zero detector 40 may include a zero crossing 
detector circuit for the full sine waveform shown in the lower part of 
FIG. 3A for providing actuating pulses for circuit 41 at the zero 
crossings of the lower waveform of FIG. 3A. To transmit pulses for zero 
crossings only at phase angle 0.degree. and not at 180.degree. for 
actuation of circuit 42, a differentiator circuit of component 40 may 
generate a positive pulse at 0.degree. and a negative pulse at 180.degree. 
from the waveform at the lower part of FIG. 3A, and a diode may transmit 
only the positive pulses from the differentiator; and such transmitted 
positive pulses then may be used to gate out only the zero crossing pulses 
at phase 0.degree. to circuit 42. 
It will be apparent that many modifications and variations will be effected 
without departing from the scope of the novel concepts and teachings of 
the present invention.