U.S. patent number 3,741,208 [Application Number 05/117,889] was granted by the patent office on 1973-06-26 for lung ventilator.
Invention is credited to Sven Ingelstedt, Bjorn Jonsson, Sven Gunnar Olsson.
United States Patent |
3,741,208 |
Jonsson , et al. |
June 26, 1973 |
LUNG VENTILATOR
Abstract
A lung ventilator is provided with means for controlling the
magnitude of a variable of the flow of respiratory gas in the
inspiration line or in the expiration line or in both of them and
means for automatically modulating said control means in response
to the instantaneous magnitude of said variable of said flow
thereby maintaining a desired pattern of said magnitude of the
flow.
Inventors: |
Jonsson; Bjorn (Lomma,
SW), Ingelstedt; Sven (Lund, SW), Olsson;
Sven Gunnar (Sollentuna, SW) |
Family
ID: |
22375363 |
Appl.
No.: |
05/117,889 |
Filed: |
February 23, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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749205 |
Jul 31, 1968 |
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Current U.S.
Class: |
128/204.21;
128/204.26; 128/202.22; 137/82; 137/102 |
Current CPC
Class: |
A61M
16/205 (20140204); A61M 16/20 (20130101); A61M
16/024 (20170801); A61M 16/204 (20140204); A61M
16/0051 (20130101); A61M 2016/0042 (20130101); A61M
2016/0039 (20130101); A61M 2016/0021 (20130101); A61M
16/0009 (20140204); Y10T 137/2544 (20150401); Y10T
137/2278 (20150401) |
Current International
Class: |
A61M
16/00 (20060101); A61M 16/20 (20060101); A61m
016/00 () |
Field of
Search: |
;128/145,145.5,145.6,145.8,147,142,142.2,188 ;137/102,87,88
;73/198,274,279 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Dunne; G. F.
Parent Case Text
This application is a continuation-in-part of copending U.S. Pat.
application Ser. No. 749,205, filed July 31, 1968 now abandoned.
Claims
We claim:
1. A lung ventilator comprising means for supplying respiratory
gas, inspiration means including an inspiration line connectible to
the patient, expiration means including an expiration line
connectible to the patient, first means in at least one of said
lines for measuring the instantaneous magnitude of at least one gas
flow parameter from the group consisting of rate-of-flow and
pressure, second means providing a signal proportional to said
instantaneous magnitude of said flow parameter, means for providing
a reference signal defining a desired flow pattern for said
respiratory gas, third means for comparing the signal from said
second means with the signal from said reference signal means for
generating a signal proportional to the magnitude and sense of the
difference therebetween, and fourth means actuated by said
difference signal for automatically modulating said flow of
respiratory gas, thereby to maintain a desired pattern of flow of
the respiratory gas independently of the pressure of the
respiratory gas furnished by said supply means and independently of
the variable flow resistance and variable patient resistance.
2. A lung ventilator as defined in claim 1 wherein said first means
comprises a flow meter unit, said third means comprises an error
signal calculator receiving signals from said reference signal
means and from said flow meter unit for generating an error signal
defining the difference between the signal from said reference
signal means and said flow meter unit, and said fourth means
comprises a flow regulator unit for adjusting the flow of
respiratory gas in response to said error signal.
3. A lung ventilator as defined in claim 2 comprising means for
zeroing said flow meter unit, means in said flow regulator unit for
indicating a closed position of said unit, and means for energizing
said means for zeroing said flow meter unit in response to an
indication from said indicator unit that said flow regulator unit
is closed and that the flow consequently is zero.
4. A lung ventilator as defined in claim 3 wherein said zeroing
means comprises an electrical capacitor, a field effect switch and
said indicating means of said flow regulator unit.
5. A lung ventilator as defined in claim 2 wherein said flow meter
unit comprising means defining two parallel passages therein, a
fine-mesh netting disposed in one of said passages, a disc disposed
in the other passage carried at one end by a rigid thread, a
pressure sensitive member contacting said rigid thread, said
pressure sensitive member being constituted by a thin silicon
plate, a resistor mounted at each side of said silicon plate, said
resistors being connected in a bridge connection for generating a
signal in response to deformation of said silicon plate which is
deformed by said rigid thread in response to the force exerted on
said disc by the flow of respiration gas through said passages of
said flow meter unit.
6. A lung ventilator as defined in claim 1 wherein said first means
comprises a flow meter unit and which further includes an
integrator, means applying said reference signal to said
integrator, means applying the signal from said flow meter unit to
said integrator, means in said integrator for integrating the
deviation between the said signals applied thereto to produce a
deviation signal, and means applying said deviation signal to said
third means which produces said difference signal in such sense as
to reduce said difference signal to zero.
7. A lung ventilator as defined in claim 1 wherein the parameter of
the respiratory gas measured is its rate-of-flow, and which further
includes a servo valve, said servo valve comprising a step motor, a
cam mounted on the rotatable shaft of said step motor, a cam
follower, and means including a lever actuated by said cam follower
for engaging and varying the gas flow rate through a flexible hose
constituting at least a section of said inspiration and expiration
lines in accordance with the operation of said step motor.
8. A lung ventilator as defined in claim 1 wherein the parameter of
the respiratory gas measured is its rate-of-flow and wherein said
means providing a reference signal defining a desired flow pattern
for said respiratory gas comprises a clock pulse generator wherein
a predetermined number of the pulses produced thereby is definitive
of one breathing cycle, a first counter receiving clock pulses from
said generator for determining the duration of the inspiration
phase of the breathing cycle in response to a preset number of said
clock pulses, means for generating a signal from said first counter
in response to the duration of said inspiration phase, a second
counter receiving clock pulses from said generator for determining
the duration of the pause phase of said breathing cycle by counting
a preset number of clock pulses corresponding to the duration of
said pause time, and a logic circuit for generating an expiration
signal when sensing that there is neither inspiration phase nor
pause phase.
9. A lung ventilator as defined in claim 8 wherein said means
providing a reference signal defining a desired signal flow pattern
for said respiratory gas further includes a signal generator
receiving signals from said clock pulse generator and from said
signal generating means of said first counter, said signal
generator comprising means for generating a voltage inversely
proportional to the length of the inspiration phase of a
respiratory cycle, means in said signal generator for integrating
the clock pulses such that the area of the reference signal is
independent of the relation between the length of the inspiration
phase and the length of the entire respiratory cycle and that the
area of the reference signal during the inspiration phase will vary
with the clock pulse frequency so that the area of all reference
signals will be constant during a predetermined period of time.
10. A lung ventilator as defined in claim 9 and which further
includes means for generating a desired output signal form.
11. A lung ventilator as defined in claim 9 and which comprises a
further counter connected to said clock pulse generator for
counting the frequency cycles of said generator, each frequency
cycle corresponding to one breathing cycle, and means for
decreasing the pulse frequency of said clock pulse generator after
counting a preselected number of breathing cycles during the
inspiration phase of the following breathing cycle thereby to
simulate a sigh function.
12. A lung ventilator comprising means for supplying respiratory
gas, inspiration means including an inspiration line connectible to
the patient, expiration means including an expiration line
connectible to the patient, first means in at least one of said
lines for measuring the instantaneous magnitude of the rate-of-flow
of said gas, second means providing a signal proportional to said
measured gas flow rate, means generating a reference signal
defining a desired flow pattern for the respiratory gas, third
means for comparing the signal from said second means with the
signal from said reference signal means for generating a signal
proportional to the magnitude and sense of the difference
therebetween, fourth means actuated by said difference signal for
automatically modulating the gas flow rate, a monitoring unit for
monitoring the magnitude of at least one other variable parameter
of the gas in addition to its rate-of-flow for generating a signal
proportional to said other monitored variable, means for indicating
said last signal, and means for applying said last signal to said
reference signal means to modify the signal produced therefrom.
13. A lung ventilator as defined in claim 12 wherein said
monitoring unit comprises a mean value flow rate integrator, means
applying the signal produced by said second means indicative of the
instantaneous gas flow rate in said expiration line to said
integrator, means applying clock pulses from a clock pulse
generator of said reference signal generating means to said
integrator, means for prolonging said clock pulses, means for
energizing said integrator during the duration of the prolonged
clock pulses and means for continuously generating an output signal
from said integrator proportional to the integrated value of said
signal produced by said second means during said prolonged clock
pulses.
14. A lung ventilator comprising means for supplying respiratory
gas, inspiration means including an inspiration line connectible to
the patient, expiration means including an expiration line
connectible to the patient, first means in at least one of said
lines for measuring the instantaneous magnitude of the product of
the gas pressure and of the gas flow rate, second means providing a
signal proportional to said product, means providing a reference
signal defining a desired flow pattern for said respiratory gas,
third means for comparing the signal from said second means with
the signal from said reference signal means for generating a signal
proportional to the magnitude and sense of the difference
therebetween, and fourth means actuated by said difference signal
for modulating said flow of respiratory gas, thereby to maintain a
desired pattern of flow of the respiratory gas independently of the
pressure of the respiratory gas furnished by said supply means and
independently of the variable flow resistance and variable patient
resistance.
Description
The present invention relates to lung ventilators.
Prior art lung ventilators of the type providing automatic positive
over-pressure ventilation of the lungs operate according to
different modes. In pressure controlled lung ventilators the
pressure of the respiratory gas is varied in a predetermined manner
in a tube or hose, connected to the respiratory passages of the
patient. The gas volume supplied to the patient at each breath in
this type of lung ventilator will vary in dependence of the
pressure variations as well as the condition of the respiratory
passages and the lungs of the patient. Therefore, the ventilation
cannot be directly preset in the lung ventilator, and this is a
substantial disadvantage extensively limiting the utility of this
type of lung ventilator in advanced respiratory treatment.
Volume controlled lung ventilators operate to supply a
predetermined adjustable volume of respiratory gas to the patient
at each breath the ventilation thereby being determined
substantially by the adjustment of the lung ventilator.
Prior art volume controlled lung ventilators are of two different
types. In one type bellows or other container is filled during the
expiration phase to predetermined degree with respiratory gas such
gas being supplied to the air passages and lungs of the patient
during the inspiration phase. These lung ventilators are bulky,
noisy, mechanically complicated and generally difficult to clean
and sterilize. Moreover, they generally do not allow patient
responsive control, i.e., that the patient starts the supply of
respiratory gas from the lung ventilator by an effort to perform an
inspiration, since the respiratory cycle is fixed by the mechanical
components of the lung ventilator. For the same reason it is not
possible to control, without difficulty, the ratio of the periods
for inspiration and expiration. Further, the lung ventilators of
this type provide large compressible volumes, which means that when
the pressure in the air passages of the patient rises part of the
gas supplied by the bellows or other container will return to
increase the pressure in the compressible volumes. This involves
difficulties to determine and to maintain the gas volume supplied
to the patient. Finally, volume controlled lung ventilators of the
type described take air from the surrounding space to be used as
respiratory gas, dust and bacteria entrained in the air being
supplied to the respiratory passages of the patient which involves
risk for infections since filters provided in the lung ventilator
generally do not completely or sufficiently clean the air supplied
as respiratory gas.
The second type of volume controlled lung ventilator operates
according to the high impedance mode wherein the respiratory gas is
exposed to a very high constant pressure and a large constant flow
resistance is imposed to the gas when allowed to flow to the
patient during the inspiration phase. Since the pressure variations
in the respiratory passages of the patient are small compared to
the high primary pressure the gas flow is substantially constant
during the inspiration phase. Thus, the gas volume during each
breath will be governed by the rate of flow and by the period of
each inspiration. A disadvantage of this type of lung ventilator is
the difficulty to maintain the high primary pressure at a constant
level when large variations of the output flow rate occur. This
necessitates a very high capacity of the high pressure apparatus.
Moreover, this type of lung ventilator cannot be used when
supplying certain anaesthetic agents which may cause explosions at
the high pressure involved. Further, such pressure necessitates a
heavy and complicated control equipment to be used.
In the above lung ventilators the respiration pattern is defined by
the structure of the lung ventilator in question without any
possibility of changing the respiration pattern. As patients with
obstructive or restrictive pulmonary diseases and with circulatory
disturbance have different optimal insufflation patterns, said lung
ventilators are insufficient for the ventilation of several
patients.
In order to detect a leakage in the patient circuit the prior lung
ventilators have required the use of a gas meter by means of which
the gas volume expired during one minute has intermittently been
measured. This method is laborious and often delays the detection
of such a leakage.
The object of the invention is to provide a lung ventilator in
which said disadvantages have been obviated.
In accordance with the invention there is provided a lung
ventilator comprising a respiratory circuit having an inspiration
connection line and an expiration connection line, means for
generating a flow of respiratory gas to said respiratory circuit
through said inspiration connection line and means for discharging
a flow of respiratory gas from said respiratory circuit through
said expiration connection line, means for controlling the
magnitude of a variable of said flow of respiratory gas through at
least one of said inspiration and expiration lines, and means for
automatically modulating said control means in response to the
instantaneous magnitude of said variable of said flow thereby
maintaining a desired pattern of said magnitude of said flow.
Two different embodiments of the lung ventilator according to the
invention will be described with reference to the accompanying
drawings in which
FIG. 1 is a block diagram of a first embodiment of a lung
ventilator.
FIG. 2 is a block diagram of an inspiration servo unit of the lung
ventilator of FIG. 1.
FIG. 3 is a block diagram of an expiration servo unit of the lung
ventilator of FIG. 1.
FIG. 4 is a block diagram of a reference voltage generator of the
lung ventilator of FIG. 1.
FIG. 5 is a block diagram of a monitoring unit of the lung
ventilator of FIG. 1.
FIG. 6 is a perspective view of a flow meter of the lung ventilator
of FIG. 1.
FIG. 7 is a sectional view of a servo valve of the lung ventilator
of FIG. 1.
FIG. 8 is a schematic diagram of a zero-shift compensating device
of the lung ventilator of claim 1.
FIG. 9 is a diagrammatic, partially sectional view of a second
embodiment of the lung ventilator according to the invention having
a flow regulator in the inspiration connection line as well as the
expiration connection line of the respiration circuit; and
FIG. 10 is a block diagram of the components of an electronic unit
and a transducer forming part of the lung ventilator shown in FIG.
9, together with associated solenoids of the flow regulators and a
supplementary system providing a sigh feature in the lung
ventilator.
Referring to FIG. 1 respiration gas is supplied to the respirator
or lung ventilator from some type of pressure source and is
supplied via non-return valves and sterilizing filters to a device
1 which has the purpose of giving the working pressure of the lung
ventilator a constant value. The device 1 has as its principle part
a bellows to which the respiration gas is supplied and which is
exposed to a compressing force which is constant within a
predetermined working range independently of the filling degree of
the bellows. From the pressure source the bellows is supplied with
respiration gas through an on-demand mechanism automatically
maintaining the filling degree of the bellows to about the half
volume thereof. Thus, the bellows acts both as a reservoir and as a
pressure reducting valve. The bellows is provided with several
inlets for allowing the mixing of gases of different kinds before
the gases are supplied to the patient.
The gas flow from the device 1 to the patient is controlled by
means of a servo unit 2 which continuously keeps the flow to the
patient set in response to an electric signal which is supplied to
the servo unit 2 from a reference voltage generator 3. Also the gas
flow from the patient may be controlled by means of a servo unit 4
which in response to an electric signal from the reference voltage
generator 3 determines the gas flow from the patient. Thus, the
inspiration servo unit 2 and the expiration servo unit 4 operate
alternatingly. The lung ventilator also has a monitoring unit 5
which monitors the pressure in the patient circuit, the expired
volume of respiration gas and/or other desired variables. The
monitoring unit 5 generates output signals in dependence on the
sensed variables in order to actuate, if desirable, the reference
voltage generator in such a way that its output signal to the servo
unit 2 and/or the servo unit 4 is corrected in the desired manner,
or in order to give an alarm if the desired ventilation course
cannot be maintained, e.g. due to leakage in the connections to the
patient.
The servo unit 2, which continuously keeps the flow to the patient
set in dependence on an electric signal which is supplied to the
servo unit 2 from the reference voltage generator 3 will be
described in the following with reference to FIG. 2. The servo unit
acts in principle in such a way that a flow-dependent signal
generated by a flow meter unit 6 consisting of a flow meter and an
amplifier is supplied to an error signal calculator 7 in which the
signal is compared with the signal from the reference voltage
generator and which in dependence hereon generates an error signal
which is supplied to a flow control unit 8 in order to adjust said
unit in such a way that the gas flow to the patient desired at any
moment and defined by the reference signal is obtained. The servo
unit 2 further contains a device 9 which is adapted to compensate
for zero shift, if any, in the flow meter unit 6. This is achieved
by the device 9 by zeroizing the flow meter unit 6 in a manner
described in greater detail in the following each time it can be
reliably established that the flow through the flow meter unit is
zero, which is the case when the flow control unit 8 is closed.
Thus, the device 9 zeroizes the flow meter unit 6 every time when
it receives a signal from the control unit which indicates that the
control unit is closed.
The flow meter unit 6 does not give a linear output signal and this
non-linear signal is fed to a linearizing and setting unit 10 where
the signal is linearized in a diode filter in a conventional
manner. The quantity of gas supplied to the patient, the so-called
minute volume, may be controlled by means of the unit 10 by
changing the amplification of the signal which is now linearly
dependent on the flow. The minute ventilation may in that case be
set between 0.5 and 30 liters per minute. The linearly
flow-dependent signal is supplied, as previously mentioned, to the
error signal calculator in order to control the flow control unit
in such a way that the desired flow is maintained at any
moment.
In order to achieve that the average value of the flow becomes
correct in spite of the momentaneous error in the desired flow
which is due to the inherent inertia of the system, the servo
system 2 contains an integrator 11 which is supplied with the
linearly flow-dependent signal from the linearizing and setting
unit 10 and is supplied with the signal from the reference voltage
generator 3. In the integrator 11 the error or deviation between
the desired and the actual flow is integrated by integration of the
difference between the signal received via the linearizing and the
setting unit from the flow meter unit 6 and the said reference
signal from the reference voltage generator, an output signal
generated by the integrator in dependence hereon being supplied to
the error signal calculator 7 in order to actuate the output signal
thereof in such a way that the remaining error will be 0. If the
patient coughs e.g., thus building up such a large counter pressure
against inspiration that the desired flow cannot be maintained
during an inspiration in spite of the valve being open to its
maximum, a signal is generated by the integrator which achieves a
compensation for this during the subsequent breaths. As a result of
this the average flow becomes correct in spite of the fact that a
too small volume of respiration gas during a ventilation cycle has
been supplied to the patient.
The expiration servo unit 4 which is illustrated in FIG. 3 as a
block diagram, may be used in the same manner as the servo unit 2
in order to keep the gas flow from the patient set at any moment in
dependence on an electric signal which is supplied to the
expiration servo unit 4 from the reference voltage generator 3.
Like the inspiration servo unit 2, the expiration servo unit 4 has
a flow meter unit 12 which generates a signal which is dependent on
the gas flow from the patient. The flow meter unit 12 feeds its
flow-dependent signal to an error signal calculator 13 in which the
signal is compared to the signal from the reference voltage
generator and which generates, in dependence thereon, an error
signal which is supplied to a flow control unit 14 in order to set
it in such a way that the desired gas flow from the patient is
maintained at any moment. Like the inspiration servo unit 2, the
expiration servo unit 4 has a device 15 for compensating zero
shift, if any, in the flow meter unit and a linearizing unit 16 in
which the signal from the flow meter unit 12 is linearized in a
conventional manner before it is fed onto the error signal
calculator 13. The linearized signal from the linearizing unit 16
is also fed to the monitoring unit 5 of the lung ventilator, where
the signal is used in a manner described in greater detail in
connection with a description of the monitoring unit 5 in order to
indicate the average flow of the respiration gas leaving the
patient.
For a reason which will be indicated in greater detail in the
following description of the monitoring unit 5 there is also
provided a non-return valve 17 after the flow regulator 14 of the
expiration servo unit 4.
Even if it should consequently be possible to use the expiration
servo unit 4 in order to determine the flow of the expired gas in
basically the same way as the inspiration servo unit 2 determines
the flow of the inspiration gas, i.e., in such a way that the
expiration servo unit at any moment determines the flow in response
to a signal from the reference voltage generator, such a control of
the expiration flow is as a rule not necessary. Instead it is
sufficient to let the expiration servo unit 4 operate with the
control valve completely open during the whole expiration
phase.
However, during free expiration by patients having certain types of
breathing difficulties there may arise a suction action due to high
air flow velocity which causes the walls of the air passages to
stick to each other. This may be prevented by supplying a signal to
the expiration servo unit 4 from the reference voltage generator 3,
said signal indicating the maximum permitted flow during expiration
and setting the flow control unit 14 accordingly.
The reference voltage generator 3 of the lung ventilator will be
described in greater detail in the following with reference to the
block diagram in FIG. 4. The reference voltage generator has a
clock pulse generator 18, such as an astable multivibrator. In the
said clock pulse generator pulses are generated with a frequency
which is one hundred times as great as the breathing frequency, and
hence a breathing cycle will comprise 100 clock pulses. By setting
the clock pulse frequency of the clock pulse generator it is
consequently possible to determine the respiration frequency. The
clock pulses are fed to and counted in a decade counter 19 which is
connected in such a way that it will return to 0 after 100 pulses
and will then resume its counting anew. On the decade counter 19
the length of the inspiration time may be selected by setting the
number of pulses to which the inspiration time is to correspond,
the counter generating a signal when the set number of pulses has
been reached. It is possible to set the counter 19 so that the
inspiration time will amount to 15-33 percent of the whole
ventilation cycle. The signal which is supplied to the inspiration
servo unit 2 from the reference voltage generator 3 is generated in
a signal generator 20. To the signal generator 20 clock pulses are
fed from the clock pulse generator 18 together with a signal
indicating that there is an inspiration phase, from the decade
counter 19. In the signal generator 20 there is generated a voltage
which is inversely proportional to the length of the inspiration
phase through the length of the respiration cycle in a voltage
divider which is ganged to the switch for selecting the length of
the inspiration time. In the signal generator 20 this voltage is
integrated with the clock pulses in such a way that the area of the
reference signal becomes independent of the relation between the
length of the inspiration phase and the length of the respiration
cycle and that the area of the reference signal during the
inspiration phase will vary with the clock pulse frequency so that
the surface of all reference signals will be constant during a
determined period of time. Preferably, the signal generator 20 is
adjustable in such a way that the reference signal which is
supplied to the inspiration servo unit will either correspond
approximately to a semi-period of a sinus wave or is square in
order to provide an inspiration gas flow of the corresponding
shape. It will easily be realized that the signal generator may
also be caused to generate other desired forms of flow, e.g. a
decelerating flow during the inspiration phase. When the
inspiration time is over, i.e., when the number of pulses
corresponding to the inspiration time has been counted, the
inspiration is interrupted and a binary counter 21 is switched on,
which is adapted to determine the length of the pause time in the
same way as when determining the inspiration time, i.e., in such a
way that the pause time corresponds to a predetermined number of
pulses. During the pause both the inspiration valve and the
expiration valve are closed so that the respiration gas is retained
in the lungs of the patient. The pause time can be selected to a
value of between 0 and 20 percent of the ventilation cycle. The
expiration starts when a logic circuit 22 senses that there is
neither inspiration phase nor pause phase, the circuit generating
an expiration signal. The expiration will then continue during the
number of pulses which remains after the number of pulses that
corresponds to the expiration and pause before the decade counter
has counted to one hundred. Thus the expiration time can be
selected to a value of between 47 and 85 percent of the ventilation
cycle. In the described use of clock pulses for controlling the
respiration course the relation between the inspiration time, the
pause time and the expiration time will be maintained independently
of whether the respiration frequency is changed, which is a
palpable advantage.
The reference voltage generator has an additional counter 23 which
is adapted to achieve a sigh function. Preferably, the counter 23
is of such a kind that it will bring about after counting 100
respiration cycles that the pulse frequency of the clock pulse
generator during the subsequent inspiration, i.e., during the
subsequent generation of the number of clock pulses corresponding
to the inspiration will generate the clock pulses with a frequency
amounting to only half or one third of the pulse frequency during
the normal respiration course. The length of the sigh function may
be selected and the sigh function may be switched on by means of a
special switch. The result of this is that every hundred breath
obtains twice or three times as large volume as normal, so that the
lung is inflated in a manner corresponding to a sigh. The clock
pulse generator 18 will also feed its clock pulses to the
monitoring unit 3, where the clock pulses together with the said
flow-indicating signal from the expiration servo unit 4 are
utilized in a manner described later in order to bring about a mean
value indication of the gas flow from the patient. The decade
counter 19 for determining the inspiration time is connected with
the monitoring unit in order to permit the inspiration to be
effected or controlled in dependence of a condition sensed by the
monitoring unit 3.
The monitoring unit 5 will be described in greater detail in the
following with reference to FIG. 5. The monitoring unit 5 has a
pressure meter 24 connected to the conduit between the inspiration
servo unit 2 and the patient, said meter measuring the pressure of
the gas supplied to the patient. The pressure meter 24, which also
comprises an amplifier, generates an electric signal in dependence
on the said pressure, and the said signal is fed to an indicating
instrument 25 which indicates the said pressure directly. The
pressure signal from the pressure meter 24 is also fed to two alarm
devices, one alarm device 26 for a lower limit value of the
pressure and one alarm device 27 for an upper limit value of the
pressure. In the alarm devices 26 and 27 the signal from the
pressure meter is consequently compared with the set minimum and
maximum values, respectively, of the gas pressure, the alarm
devices 26 and 27 indicating when the pressure has decreased below
the lower limit value or when the upper limit value has been
exceeded. The alarm device 26 for the lower limit value has an
optical device indicating that the pressure has decreased below the
lower limit value. The alarm device 27 for the upper limit value
also has an optical device for indicating that the upper limit
value has been exceeded and is besides connected to an acoustic
alarm device 28 giving a signal when the upper limit value is
exceeded. The acoustic alarm device 28 is provided with a
disconnecting means 29 in order to make it possible to disconnect
the acoustic alarm during a short period, e.g. 2 minutes, during
intentional, temporary interruptions in the ventilation, e.g. when
the patient is being cared for.
The monitoring unit also has a mean value integrator 30 which as
mentioned previously is supplied with a signal indicating the
instantaneous flow from the expiration servo unit 4 and is supplied
with clock pulses from the clock pulse generator 18 of the
reference voltage generator 3. In that connection the clock pulses
control the mean value integrator 30 in such a way that the time
constant of the integrator is inversely proportional to the
ventilation frequency. This is brought about by prolonging the
clock pulses and making them open a field effect switch energizing
the mean value integrator 30 during the duration of the prolonged
clock pulse. The mean value integrator 30 consequently operated
during a respiration cycle during the same time independently of
the length of the respiration cycle so that the time constant of
the integrator, as mentioned, will be inversely proportional to the
respiration frequency. As a result of this, the integrator will
always react with suitable speed. The output signal from the mean
value integrator 30 is fed on one hand to an indicating instrument
31 directly showing on an operating panel the average flow in e.g.
liters per minute and on the other is fed to a limit value alarm
device 32 in which the signal from the mean value integrator 30 is
compared to the maximum and minimum values of the minute volume. If
the minute volume determined by the mean value integrator exceeds
the maximum minute volume or is less than the minimum minute volume
an optical alarm is given on the limit value alarm device 32 and a
signal is fed on to the acoustic alarm device 28 which generates an
acoustic signal when the said deviation from the set minute volume
occurs.
When the pressure drops below the lower limit value the signal from
the alarm device 26 for the lower limit value of the pressure is
fed to the reference voltage generator 3 and more particularly to
its decade counter 19 in order to zeroize said counter and cause
the decade counter to start a new respiration cycle at once. The
signal from the alarm device 26 is fed, when the upper limit value
of the pressure is exceeded to the decade counter 19 of the
reference voltage generator in order to interrupt the respiration
cycle and start expiration immediately.
The lower alarm limit of the pressure may also be utilized for
letting the patient control the lung ventilator. If the patient
during the expiration course makes an attempt to inspire a
subpressure is generated in the patient circuit, whereby the
pressure drops below the lower alarm limit and a new inspiration
cycle is begun in accordance with the course described above. The
non-return valve 17 described in connection with the expiration
servo unit 4 will be closed when the patient makes spontaneous
attempts at inspiration so that the necessary subpressure may be
generated in the circuit.
As mentioned the inspiration and expiration servo units 2 and 4,
respectively, are provided each with one flow meter which is
preferably of the kind shown in FIG. 6. The respiration gas flows
to two parallel passages 33 and 34 into the flow meter. The major
passage 33, through which the main portion of the gas flows, has a
fine-mesh netting 35 (450 mesh per inch). The netting 35 causes a
pressure drop, which is essentially directly proportionally to the
flow through the passage. In the minor passage 34 there is disposed
a round thin disc 36 which is carried at one end by a rigid thread
37. The disc 36 is disposed at right angles through the direction
of flow of the gas. The rigid thread 37 rests against a pressure
sensitive member 38 which is preferably constituted by a thin
silicon plate. At either sides of the silicon plate 38 there are
two resistors diffused into the device and said resistors are
connected in a bridge connection. The gas flowing through the minor
passage 34 exerts a pressure on the silicon plate 38 via the rigid
thread 37 in such a way that said plate is bent, the resistance of
one resistor increasing while the resistance of the other resistor
decreases. As a consequence of this the flow meter will give an
output signal which is in a predetermined relation to the flow. As
mentioned, the output signal is then linearized in the unit 10 in
the inspiration servo unit 2 and in the unit 16 in the expiration
servo unit 4. In order to prevent condensate from forming when the
respiration gas is moistened it is preferred to heat the flow meter
by means of a resistor cast into it. The great advantages of the
flow meter described is that it can measure frequencies as rapid as
200 c.p.s. and also that it is autoclaveable.
In addition, each of the inspiration servo unit and the expiration
servo unit has a servo valve which is part of the respective
control unit and which is preferably of the kind shown in FIG. 7
The servo valve consists of a step motor 40 the axle 41 of which is
provided with a metal plate 42. In the plate 42 there is shaped a
body provided with a cam surface 43 contacted by a cam follower 44
fixed to a lever 45. The leaver 45 is moveably mounted on a shaft
46 and rests at the edge of one of its ends against a hose 47 of
silicon rubber through which the respiration gas flows to and from
the patient, the hose being connected into the conduits leading to
and from the patient. When the step motor operates the hose 47 will
be squeezed or opened in dependence on the direction of rotation of
the motor. A position indicator (not shown), which consists of a
lamp and a photo cell senses the position of the valve and gives a
signal when the valve is completely closed. This signal is utilized
in a manner described later in order to bring about zeroizing of
the flow meter. The cam surface 43 of the valve is preferably
designed in such a way that the flow within the whole flow region
during normal control will be changed 10 percent from the previous
value for each displacement of one step by the step motor. Thus, a
logarithmic control of the flow takes place, which also entains
that the loop amplification in the servo system becomes constant
within the whole control region. This is required in order that
stability in the servo system may be obtained. This gives a
resolution of .+-. 5 percent but this value will be improved by the
motor reciprocating during control above the rate value. The times
during which the flow is too large or too small are controlled in
such a way that the average flow becomes correct. The step motor is
driven at 500 steps per second and from a completely open to a
completely closed valve there is required a time of about 1/10
second. An advantage in the described regulator is that the parts
thereof contacting the respiration gas are autoclaveable.
The inspiration servo unit 2 and the expiration servo unit 4
contain, as mentioned, a device 9 and 15, respectively, which is
adapted to compensate for zero-shift, if any, in the flow meter
unit 6 and 12, respectively, consisting of a flow meter 48 and an
amplifier 49. The said device is shown in FIG. 8 and comprises a
capacitor 50, a field effect switch 51 and a sensing means 52
disposed in the servo valve which has previously been mentioned and
is adapted to indicate when the servo valve is closed and the flow
through the flow meter is consequently 0. When the flow is 0, which
is the case during the expiration phase, the servo valve is
consequently closed, and in that case the field effect switch 51 is
short-circuited in such a way that the capacitor 50 is charged, the
input voltage to the amplifier 49 becoming 0. During the
inspiration phase, when the field effect switch 51 is consequently
open, there can consequently not take place any change worth
mentioning in the charge of the capacitor 50 due to the fact that
the subsequent amplifier has a very high input impedance.
In the respirator according to the invention there are unique
possibilities of determining the resistance and compliance of the
patient. For this purpose the electric output signals from the flow
meter units and the pressure meter of the lung ventilator are
utilized. The signals are supplied to a recorder and the curves
obtained in that connection may be used for determining the said
resistance and compliance.
The described servo system of the lung ventilator according to the
invention is completely flow-dependent but it is also possible to
provide the lung ventilator with a servo system operating in
dependence on the pressure or a system where control is achieved by
means of the product of the pressure and the flow.
By connecting an underpressure source to the expiration passage the
emptying of the lungs of certain patients may be facilitated.
When using expensive narcoses gases the lung ventilator may operate
with a circle system, i.e., the expired gas is fed back to the
inspiration side via a compressor after being supplied with a
substitute for the consumed parts of the gas and removing for
instance carbon dioxide.
Referring to FIG. 9 in the drawing the lung ventilator shown
therein comprises an over-pressure source 61 such as a motor driven
pump or a reservoir for respiratory gas supplying such gas to the
lung ventilator under a pressure preferably in agreement with the
maximum pressure compatible with the respiratory passages and lungs
of the patient. Source 61 is connected by a flexible tube or hose
62 for supplying respiratory gas to the patient whose respiratory
system including the respiratory passages and the lungs is
diagrammatically indicated in FIG. 9 by a volume 63. In tube 62
there is provided a flow regulator 64 controlling the flow of
respiratory gas to patient 63 so as to keep the flow rate at
constant level during the inspiration period, pressure changes
during said period on each side of flow regulator 64 having no
influence on the constant flow rate. Flow regulator 64 is
preferably of the type described in U.S. Pat. No. 3,502,100
comprising a valve member 64, actuated by a flow rate responsive
differential pressure appearing over a restriction 64" in tube 62
having the flow regulator incorporated therein, in order to
throttle the flow in the passage for the respiratory gas through
the flow regulator if said differential pressure exceeds a
predetermined value, said flow rate thereby being maintained at the
level providing said differential pressure. Further details
regarding the construction of flow regulator 64 may be had from the
Patent referred to above.
However, in general, it can be said that should there be an
increase in the rate of flow from the over-pressure source 61
through conduit 62 to the inlet side of regulator valve 64, the
pressure difference between this conduit and the cylinder 64a
within which the valve piston 64' operates, communicated to the
cylinder via the pipe connection 64b extending from a junction with
conduit 62 upstream from the restriction 64", will increase. When
the pressure difference reaches a certain value depending on the
size of the restriction 64", the valve piston 64' will be raised
and flow through the opening 64c is restricted to such a degree
that the flow will be exactly of such a size that the pressure
difference will balance the gravity of valve piston 64' evenly
whether or not the pressure of the fluid in tube 62 at the inlet
side of regulator 64 increases further. Thus, the resistance to
flow of the fluid through the regulator 64 is automatically
modulated in response to the instantaneous value of the flow
rate.
A flexible tube or hose 65 is connected to the respiration system
of the patient 63 and also therein is incorporated a flow regulator
66 with a valve member 66' controlling the flow rate in tube 65
during the expiration period so as to prevent said flow rate to
exceed a predetermined level. Flow regulator 66 is of the same type
as and may be identical with flow regulator 64 and is responsive to
the differential pressure over a restriction 66" in tube 65.
The switching between the inspiration and expiration phases is
controlled by solenoids 67 and 68 incorporated into flow regulators
64 and 66 in the manner described in the above mentioned Patent.
When energized solenoid 67 or 68 actuates the valve member in the
associated flow regulator the flow through the flow regulator
thereby being cut off. As will be understood flow regulator 64 is
open and flow regulator 66 is closed during the inspiration period
the opposite conditions prevailing during the expiration period. A
respiration pause may be introduced by closing both flow regulators
simultaneously.
Solenoids 67 and 68 are energized and de-energized in response to
an electronic and preferably transistorized unit 69 in order to
achieve a desired respiration frequency and a desired ratio between
inspiration and expiration periods and also a desired pause, if
any, between such periods. Associated with unit 69 is a transducer
70 connected to the respiration circuit and responsive to the
pressure therein to produce electrical signals supplied to unit 69
and affecting the control of solenoids 67 and 68 provided by such
unit. Unit 69 and transducer 70 are shown in more detail in FIG. 10
and now will be described with reference to that figure.
Referring to FIG. 10 electronic unit 69 comprises an astable
multivibrator 80 which controls the respiration frequency, i.e.,
the number of respirations per minute. This multivibrator
incorporates an RC-circuit in the conventional manner and an
element in such RC-circuit is adjustable to vary the respiration
frequency. Multivibrator 80 is of a type which may be triggered to
be momentarily returned from one of its quasistable states to the
other of said states although the period for the multivibrator to
be in said one state as determined by the RC-circuit has not
lapsed. Multivibrator 80 produces an output trigger pulse which is
supplied by a line 81 to one of two trigger inputs of a monostable
or one-shot multivibrator 82 said latter multivibrator being
arranged to supply an output pulse in a line 83 of a predetermined
duration when triggered to the quasistable state by the output
trigger pulse from multivibrator 80. The duration of such pulse is
determined by a conventional RC-circuit in multivibrator 82 an
element of such circuit being adjustable to determine the length of
the pulse from multivibrator 82. Line 83 is connected to solenoid
67 in inspiration flow regulator 64 and the output pulse of
multivibrator 82 controls the energization of solenoid 67. If there
is maintained in line 83 a predetermined potential energizing
solenoid 67 to maintain valve member 64' associated therewith in
the closed position then multivibrator 82 produces in line 83, when
triggered, a pulse of the opposite polarity in order to de-energize
solenoid 67. Thus, this solenoid will be de-energized during the
output pulse from multivibrator 82 and flow regulator 64 will be
kept open during this pulse and will operate to maintain the flow
rate through the inspiration tube at a constant predetermined
level. It will be seen that multivibrator 82 governs the period of
inspiration.
Multivibrator 62 is arranged to produce in dependence of the rear
or lagging edge of the pulse supplied to line 83 a trigger pulse in
a line 84 connecting multivibrator 82 to a further monostable
multivibrator 85 said trigger pulse being used to trigger
multivibrator 85. Thus, when the output pulse from multivibrator 82
in line 83 disappeares multivibrator 85 is triggered to its
quasistable state and a pulse is supplied to a line 86 connecting
multivibrator 85 to one of the inputs of an AND gate 87 another
input thereof being connected by a line 88 to line 83. The output
of gate 87 is connected by a line 89 to solenoid 68 associated with
expiration flow regulator 66. Gate 87 is arranged to be in open
state when both multivibrators 82 and 85 are in their stable state.
When in its open state gate 87 supplies a pulse in line 89. In line
89 there is normally maintained a potential sufficient to energize
solenoid 68, and the pulse supplied by gate 87 is of opposite
polarity to de-energize solenoid 68. When gate 87 receives a pulse
from multivibrator 85 through line 86 as a consequence of said
multivibrator being triggered to its quasistable state by the
trigger pulse from multivibrator 82 no pulse is supplied by gate 87
to line 89. Thus, both solenoids 67 and 68 will be energized when
multivibrator 85 is in its quasistable state. The duration of this
condition is determined by an RC-circuit in multivibrator 85 and
this duration determines in turn a pause during which both flow
regulators 64 and 66 are positively closed by solenoid 67 and 68,
respectively. An element in the RC-circuit of multivibrator 85 is
adjustable to adjust the duration of the pause introduced in the
inspiration cycle by multivibrator 85.
When multivibrator 85 returns to its stable position after the
period established by the RC-circuit therein has lapsed no pulse
will be supplied to gate 87 through line 86. Thus, a pulse will be
supplied by gate 87 through line 89 in order to de-energize
solenoid 68 starting the expiration phase. This phase 66 will
continue until a new inspiration trigger pulse is supplied by
multivibrator 80 in line 82 bringing multivibrator to its
quasistable state, since the conditions for the gate to be in its
open state then are no longer satisfied.
The operation of electronic unit 69 and associated solenoids 67 and
68 would be clear from the description of the construction thereof.
However, befor describing further components connected to unit 69
the function thereof will be briefly described.
The breathing frequency is controlled by astable multivibrator 80
which triggers multivibrator 82 by its output trigger pulse. When
triggered multivibrator 82 changes to its quasistable state, and in
this position solenoid 67 is deenergized. The inspiration period is
started. When the time interval set in multivibrator 82 has lapsed
this multivibrator returns to its stable state and solenoid 67 is
energized to close flow regulator 64. At the same time a trigger
pulse is supplied to multivibrator 85 which switches to its
quasi-stable state to start a pause both solenoids 67 and 68 being
energized by multivibrator 82 and gate 87, respectively. After the
time interval preset in multivibrator 85 has lapsed this
multivibrator returns to its stable state. The conditions for gate
87 to open thereby are satisfied (both multivibrators 82 and 85 are
in the stable state) and solenoid 68 is deenergized. Thereby, the
expiration period is started and will continue until a new pulse
for trigging multivibrator 82 is supplied by multivibrator 80. The
conditions for gate 87 to be open thereby are no longer satisfied;
solenoid 68 will be deenergized to close the expiration period.
Thereafter the operation cycle as described will be repeated.
As mentioned above the period of the complete respiration cycle is
controlled by an RC-element in multivibrator 80, the period of
inspiration in relation to the period of the complete cycle is
controlled by an RC-element in multivibrator 82, and the period of
pause in relation to the complete cycle is controlled by an
RC-element in multivibrator 85. A second RC-element of each of
multivibrators 82 and 85 may be ganged to the adjustable RC-element
in multivibrator 80 to be adjusted simultaneously therewith. In
this way the ratio between the inspiration period and the complete
cycle period and the ratio between the pause period and the
complete cycle period, respectively, is adjusted by the first
RC-element in each of these multivibrators. When the respiration
frequency is changed by adjusting the RC-element of multivibrator
80, the second RC-elements in multivibrators 82 and 85 are adjusted
to maintain the ratios set therein by said first RC-element.
As an example the lungs of a patient are to be ventilated by a flow
of 10 liters/min. and at a frequency of 20 respiration cycles/min.
The frequency is set in multivibrator 80 and the inspiration period
is set in multivibrator 82 and is chosen to be e.g. 1/3 of the
complete cycle period. The inspiration flow rate is adjusted to 3
.times. 10 = 30 liters/min. by adjusting flow regulator 64. Thus,
the patient will be supplied with 30 liters/min. of respiratory gas
during 1/3 of the complete cycle period, i.e., 10 liters/min. The
expiration flow may be adjusted analogically if desired.
Referring to FIG. 10 transducer 70 comprises a pressure sensitive
apparatus 90 providing an electric signal which is proportional to
the pressure in the respiration system. This apparatus may be a
pressure transducer of type EMT 94, supplied by Elema-Schonander
AB, Solna, Sweden. The pressure sensitive apparatus 90 supplies a
pressure responsive signal through a line 91 to a meter 92 of the
Parker type, "Two Set Point Control" ER 35, supplied by Parker
Instrument Corporation, Samford, Conn., U.S.A.
This meter is arranged to supply a signal at a preset lower
pressure limit through a line 93 connecting the meter to
multivibrator 80 and at a preset upper pressure limit through a
line 94 connecting the meter to multivibrator 82. In line 94 there
is provided a gate 95 this gate 95 being assumed to be permanently
in its open position for the time being. When the signal supplied
by apparatus 90 to meter 92 through line 91 is of a value lying
between the upper and lower limits set in meter 92 no signals are
supplied through lines 93 and 94 to multivibrators 80 and 82,
respectively, and the lung ventilator operates as described above.
However, if meter 92 indicates that the lower pressure limit is
underpassed a pulse is supplied through line 93 to multivibrator 80
trigging said multivibrator to immediately return to its
inspiration position in order to start an inspiration period. If
meter 92 indicates that the upper pressure limit is exceeded a
pulse is supplied through line 94 to multivibrator 82 triggering
said multivibrator to immediately return to its stable position
starting the pause set by multivibrator 85 as explained above. This
means that the expiration period is extended to the same degree as
the inspiration period is shortened.
If the pressure in the respiration system sinks under the lower
pressure limit set in meter 92 the expiration period is interrupted
and an inspiration period of normal length is started. This enables
the flow regulator to be adjusted so as to prevent the pressure in
the pipes 62 and 65 to be lower than e.g. +7 cm H.sub.2 O such as
during surgical operations in the thorax. Thereby, the lungs are
prevented from being completely empty and to collapsing. In
connection with respiratory treatment of other type when the
patient is able to breath spontaneously the same system may be
utilized for controlling the lung ventilator by the activity of the
patient. In that case the lung ventilator is adjusted so as to
supply to the patient a predetermined gas volume at a frequency
securing the ventilation necessary for the patient when resting.
The lower pressure limite is adjusted to start the inspiration
period at a pressure in the respiration system of -0,5 cm H.sub.2
O. If the patient wishes to increase the ventilation, he starts the
spontaneous inspiration before the inspiration phase is started by
the lung ventilator. The pressure in the respiration system thereby
will sink to a pressure under -0,5 cm H.sub.2 O and the lung
ventilator will start the inspiration period. In this case the
expiration period is shortened, the respiration frequency and the
ventilation being intensified.
Optical or sound indicating apparatus may be connected to meter 92
to indicate pressures over and under the upper and lower pressure
limits, respectively, established by said meter.
The breathing of a human being is not a train of identical
respiration cycles even if the breathing takes place under constant
conditions. In the breathing are incorporated deep breaths known as
sighs at different intervals. The lung ventilator described herein
may incorporate means ascribing a sigh feature to the lung
ventilator. Referring to FIG. 19 such means incorporates an astable
multivibrator 100 the output of which is connected through a line
101 to a counter 102 and through a line 103 to gate 105.
Multivibrator 100 supplies a pulse at preset intervals for example
every fifth minute through lines 101 and 103 to counter 102 and
gate 105, respectively. This pulse conditions counter 102 for
counting pulses supplied thereto from the trigger output of
multivibrator 82 through a line 104 and closes gate 95. When the
next trigger pulse is supplied by multivibrator 82 this pulse is
counted by counter 102 and simultaneously a trigger pulse is
supplied by counter 102 through a line 105 and through line 93 to
multivibrator 80 to immediately start a new inspiration period
without an intervening pause and expiration period. Counter 102 is
arranged to be deactivated after a predetermined number of pulses
supplied by multivibrator 82 through line 104, e.g. three pulses,
and thus four inspirations are performed integrally during an
extended inspiration period. When counter 102 is deactivated the
normal respiration cycle is continued until next pulse is supplied
by multivibrator 100 through lines 101 and 103.
As shown in FIG. 9 a subpressure source 71 may be connected to tube
65 of the respiration system to facilitate the emptying of the
lungs. In that case meter 92 may be used to start an inspiration
period when the pressure in the respiration system underpasses a
preset value, e.g. -10 cm H.sub.2 O.
It should be noted that the lung ventilator according to the
invention advantageously may be adapted for autoclave treatment
contrary to most prior art lung ventilators. A further important
advantage of the lung ventilator according to the invention is that
a sigh feature may be incorporated therein in a simple manner as
described above.
* * * * *