Note: Descriptions are shown in the official language in which they were submitted.
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This application is a division of Appli-
cation Ser. No. 528,258, filed January 27, 1987.
This invention relates generally to
ventilators for supplying gas to facilita-te and
support human respiration and particularly to
ventilators which employ a high frequency jet of gas
for respiratory therapy. More specifically, the
present invention is directed to enhancing
ventilation at supraphysiologic rates and especially
to maximizing the tidal volume of gas delivered to a
patient during respiration therapy while simul-
taneously minimizing patient discomfort and the
possibility of causing or aggravating trauma.
Accordingly, the general objects of the present
invention are to provide novel and improved methods
and apparatus of such character.
While not limited thereto in its utility,
the present invention is particularly well suited to
high frequency jet ventilation. The use of high
frequency jet ventilation has proven to be quite
beneficial in the treament of certain respiratory
conditions. In high frequency ventilation, rather
than moving gas in bulk quantity into the gas
exchanging areas of the lungs, ventilation is
achieved by enhancing the mass transfer processes in
the lungs through high frequency oscillation of the
supplied gas. However, as the pulsation frequency of
the gas delivered by a ]et ventilator increases,
supplying the necessary tidal volume of inhalation
gas becomes more difficult and is limited by the
response time of mechanisms employed for generating
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the gas pulses. In addition, the requirements of
reliability, ease of maintenance and susceptibility
to sterilization are important design considerations
for a ventilator. Portability is a further desirable
characteristic. Accordingly, the principal
objectives of the present invention are to provide a
new and improved ventilation technique and a
multi-frequency jet ventilator which operates in
accordance with this technique and is compact,
relatively easy to maintain, capable of being easily
sterilized and supplies a maximized tidal volume of
ventilation gas flow over a wide range of frequencies
and duty cycles.
The novel method of the present invention
includes the steps of creating a humidified bias flow
of gas and entraining gas from that bias flow to
create a highly humidiried inhalation gas. The
entrainment consists of subjecting the bias flow to
the effect of high velocity pulses of gas derived
from a high pressure source of entrainment gas. The
invention further contemplates the exercise of
-; control over the entrainment gas to vary the
frequency, duration and width of the gas pulses to
satisfy the requirements of the treatment being
performed.
In accordance with a particular embodiment
of the invention there is provided a method for
generating gas pulses for use in respiration therapy
comprising the steps of:
providing a flow of primary gas;
periodically interrupting the primary gas
flow by means of a normally closed solenoid actuated
valve;
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generating solenoid control voltage pulses
having first and second voltage magnitude levels,
said voltage levels including an initial magnitude in
excess of the rated voltage of the solenoid and a
second contiguous magnitude less than the said rated
voltage and sufficient to hold the solenoid in the
actuated sta-te; and
applying said voltage pulses to the valve
solenoid to cause the generation of gas pulses.
lQ Figure 1 is a functional block diagram of
a multi-frequency jet ventilator
in accordance with the present
invention;
Figure 2 is an enlarged fragmentary
sectional view of a preferred
embodiment of -the entrainment
module of the multi-frequency jet
ventilator of Figure l;
Figure 3 is a functional block diagram of
the control module of the multi-
frequency jet ventilator of
Figure l;
Figures 4a, 4b and 4c are graphical illus-
trations of gas pulse trai.ns
provided to the entrainment
module of Figure 2 in response -to
the control signals generated by
the control module of Figure 3;
and
Figure 5 is a waveform diagram of a
control voltage generated by the
control module of Figure 3.
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DETAILE:D DE:SCRIPTION OF T~ 7ENTION
With reference to the drawing, wherein like numerals
represent like parts throughout the several figures, a ventilator
in accordance with one embodiment of the present invention is
generally designated in Figure 1 by the numeral 10. ~entilator
10 may be selectively employed at conventional ventilation
frequencies or may be utilized as a high frequency jet ventilator.
Ventilator 10 preferrably has a range of operational frequencies
of from 4 breaths~minute (1/15 Hz.) to 3000 breaths/minute (50
la Hz~ and an inspiratory time, i.e., a duty cycle, in the range of
5% to 95% as will be more fully described below. The ventilator
10 will supply respiratory gas to a patient via either a cuffed
or uncuffed endotracheal tube (not illustrated~ and is adaptable
for ventilating with air, air/oxygen, helium/oxygen or any other
suitable gas or combination of gases. Ventilator 10 has a compact,
lightweight construction and may be either battery or line current
powered.
Ventilator 10 is an integrated modular system ~hich generally
comprises a control unit 12, a high pressure gas supply unit 1~, a
20 low pressure gas supply unit lS and an entrainment module 16. The
control unit 12 comprises the electronic controls, safety system
and the electrical power supply for the ventilator. The high
pressure gas supply unit 14 comprises a source of high pressure
gas, a gas pressure regulation system and a valve subassembly for
producing controlled pulses of the gas derived from the high
pressure source. The low pressure gas supply unit 15 comprises a
source of low pressure gas and a humidification system for the
gas. The entrainment module 16 produces, from the pulses of high
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p .sure gas and the humidified low pressure gas, the required
output of the ventilator. The gas flow lines are designated by
heavy lines and the electrical interconnections are designated by
thin lines in the drawing. The control unit 1~ is connected to
the supply unit 14 via conventional se~arable electrical
connectors. The gas supply units 14 and 15 and entrainment
module 16 are interconnected by stan~ard flexible hoses. The
above-mentioned modular units and their sub-units may be easily
connected and disconnected. The modular construction thus
facilitates maintenance of the ventilator and also provides a
ventilator which, to the extent required, may be easily
disinfected and sterilized as will be more fully apparent from
the discussion below.
With reference to Figure 1, the control unit 12 comprises an
electronic control module 20 which generates control pulses for
operating a solenoid actuated valve 22 in supply unit 14. The
control module 20 also provides input signals to an electronic
safety module 2~. The safety module 24, in the manner to be
described below, controls an electrically operated shutoff valve
26 in the primary, i.e., high pressure, gas supply line and is
also connected to an alarm system 28.
A source of pressurized gas 30, which is typically in the
form of plural tanks containing compresse~ dry air,
oxygen/nitrogen, or oxygen~helium, is coupled via shutoff valve
26 and an adjustable pressure regulator 32 to an accumulator 34.
The pressurized gas which appears at the output port of
accumulator 34 has a regulated substantially constant pressure in
the range of between 5 psi and 250 psi. The pr~ssurized gas flows
132~
fr~.~ the accumulator 34 via a flow sensor 36 and valve 22 to
entrainment module 16. The flow sensor 36 provides an information
bearing input signal to safety module 2~ whereby the nature of
the gas flow to the entrainment module 16 derived from high
pressure source 30 may be continuously monitored to provide a
means for actuating the alarm ~8 in the event that the
aforementioned gas flow is not within the selected and required
operational limits of the ventilator. The alarm 28 is preferably
both an audible and a visual alarm. The safety module 24, which
is preferrably a microprocessor, is programmed to monitor the
operation of the ventilator, especially the primary gas flow to
the entrainment module and the pressure downstream of the
entrainment module, in order to determine whether the operati.ng
parameters are within pre-established ranges. Should a monitored
parameter move into a range which is un5afe to the patient,
module 24 will command the closing of shutoff valve 26.
A secondary pressurized gas source 42, within unit 15, is
coupled via a shutoff valve 44 to a humidifier 46. The output of
humidifier ~6 is a bias flow of heated humidified gas which is
continuously supplied to the entrainment module 40 at A relatively
low pressure such as 5 psi. The secondary gas source ~2 is
typically in the form of one or more tanks containing the same
gas as supplied by "high" pressure source 30. Humidifier ~6 is
preferably a cascade bubble humidifier and causes the bias flow
to have approximately 100% relative humidity. In addition, an
ultrasonic nebuli~er 47 may be employed to introduce a vapor mist
to the bias flow of humidified gas. The stream of humidified gas
and vapor mist, which is flowing at low velocity, is entrained in
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dule 16 by high velocity gas pulses, pro~uced in the manner to
be described below, to form an output gas stream. As ncted
above, the output stream is supplied to the patient via an
endotracheal tube (not fully illustrated). The entrainment
module 16 also functions to receive gases exhaled by the patient.
Depending on the state of a t~Jo-way flow control valve ~8, the
exhaled gas is either vented to the ambient atmosphere or
delivered to a reclamation unit 49. A pressure sensor 50 may be
interposed in the gas path which extends from the entrainment
lo module to the endotracheal tube for sensing the pressure
immediately upstream of the endotracheal tube and providing a
corresponding input signal to the safety module 24 for insuring
safe operation of the ventilator.
With reference to Figure 2~ the entrainment module 16
comprises a housing 51 which interiorly forms an entrainment
chamber 52. ~ousing 51 is a generally T~shaped cylindrical
member which has an open output end. A fitting 54 at the output
end fluidically couples chamber 52 to a conduit 56 which leads to
or comprises the end of the endotracheal tubeO Gas pulses,
produced by modulating the gas exiting accumulator 3~ by means of
valve 22, are injected into the entrainment chamber 52 through a
nozzle 58. Nozzle 58 is a convergent or convergent-divergent
nozzle and thus the velocity of the gas downstream of the nozzle
throat is high. Nozzle 58 extends axially into chamber 52
through an end wall of the housing 51 along the central axis of
the chamber. Nozzle 58 is aerodynamically shaped to enhance
entrainment by directing the low pressure bias flow in the
downstream direction in chamber 52. Nozzle 58 thus preferrably
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a forwardly tapering convergen~ external profile. An inlet
leg 62 and an outlet leg 64 protrude radially at diametrically
opposite locations of housing 51. Legs 62 and 64 are
substantially identical and are equidistant from the centrally
disposed nozzle 58. Inlet leg 62 functions as a connector
structure for coupling to a conduit for supplying the low
velocity bias stream of humidified gas to the entrainment chamber
52 via port 66 as illustrated by the arrows in Figure 2. The
humidified gas is continuously supplied to the entrainment
chamber. During an inspiratory phase of the ventilation cycle,
humidified gas is entrained by a high velocity gas pulse injected
into chamber 52 via the nozzle 58 and propelled axially through
the chamber to conduit 56 and thence to the patient via the
endotrachial tube. The ventilating gas pulses delivered to the
patient will be comprised primarily of humidified gas supplied
via inlet leg 62~entrained by the pulses of dry gas supplied via
nozzle 58. Accordingly, the patient will receive gas having the
highest possible relative humidity.
During the expiratory phase of the ventilation cycle, the
gas exhaled by the patient returns via conduit 56 to the
entrainment chamber 52. The exhaled gas is entrained by the low
velocity bias flow and is thus discharged through discharge port
68 which leads -to outlet leg 64. Leg 6~ is coupled to a conduit
- for conducting the exhaled gas and excess humidified gas to valve
48. The expired carbon dioxide from the patient is discharged
through port 68 in part due to the driving force of the bias flow
of humidified gas which prevents the exhaled gases from entering
port 66.
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- The entrainment of the humidified gas by the hi~h velocity
pulses or slugs of primary gas is facilitated by the convergent
exterior shape of nozzle 58 which, as mentioned above, functions
as a flow control surface. The entrainment of the humidified gas
is improved by the placement of the outlet 60 of nozzle 58 at an
axial location of the chamber which is proximate the downstream
axial terminus of the inlet port 66. Consequently, the high
velocity pulse is injected into the chamber at a location
slightly do~nstream from the entry of the humidified gas. As
should now be obvious, the continuous supply of the low pressure
humidified secondary gas functions to alternately supply
humidified gas for entrainment and to remove the expired carbon
dioxide from the ventilator unit without the use of any mechanical
valves which would otherwise tend to deteriorate the entrainment
effects and, thus, would result in lower tidal volumes.
A low compliant tube connects nozzle 58 to the solenoid
actuated control valve 22. Valve 22 is a bi-state val~e having
an open and closed position. The command signals generated by
control module 20 and applied to the solenoid of valve 22
determine the frequency and duration of the gas pulses delivered
to nozzle 58. Thus, valve 22 is cyclically opened and closed for
selected time intervals to interrupt the flow of pressurized gas
to nozzle 58 to thereby produce the desired gas pulse train
characteristics to provide optimum treatment for the patient.
The characteristics of the train of pressurized gas pulses
produced by valve 22 may best be appreciated by reference to
Figures 4a, 4b and 4c. The horizontal axes represent the time in
milliseconds and the vertical axes represent the flow rate of the
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h lh velocity ventilation gas exiting nozzle 58. The letter T
represents the time of one ventilation cycle, i.e., the time of
a inspiratory phase plus the time of a following expiratory
phase. The symbol tl represents the time interval during which
valve 22 is open. For each of the graphs of Figure ~, the time
interval in which valve 22 is opened, i.e., the inspiratory time,
is 30 percent of the ventilation cycle T. The graph of Figure 4a
represents the pulse train characteristics when valve 22 is
opened and closed at a 5 Hz. frequency. Graph 4b represents the
pulse characteristics when valve 22 is opened and closed at a 10
Hz. frequency. Figure 4c represents the pulse charactistics when
valve 22 is opened and closed at a 20 Hz. frequency.
The volume of gas supplied by t~e valve per breath is equal
to the area under the flow rate-time curve of the graphs of
Figure 4. The solid lines represent the flow characteristics for
ventilator 10. The broken lines represent the flow
characteristics for a ventilator which does not incorporate a
feature for redocing the time required for the valve to change
states in accordance ~ith the present invention. It will be
appreciated that the depicted curves have a trapezoidal shape
rather than a square wave shape due to the incremental time
interval required for valve 22 to change from one state to
another, i.e., from a fully closed state to a fully open state
and vice versa. In the prior art, at high pulse frequencies
there was insufficient time for the valve to open completely
before receipt of a "closen command. ~ccordingly, the triangular
flow pattern indicated by the broken lines of Figure ~c resulted.
flow pattern as represented by the broken line showing of
Fi re 4c leads to a drastic reduction in the tidal ~olume, i.e.,
the volume of gas s~pplied to the patient, during the inspiratory
phase. Consequently, in order that sufficient tidal volumes be
supplied at high ventilation frequencies, the valve must be
caused to react quickly to "open" commands and should remain open
for a significant portion of the inspiratory phase.
With reference to Figure 3, valve 22 is opened and closed by
means of solenoid 70 which is responsive to command signals
generated by the control module 20. Control module 20 includes a
lo square wave generator 72. A resistance capacitance network 73 is
adjustable in the conventional manner to vary the time constant
of and thus the output frequency of square wave generator 72.
The square wave output signal of generator 72 is applied to a
timer circuit 74. Referring jointly to Figures 3 and 5, an
adjustable voltage magnitude selection circuit and an adjustable
duty cycle selection circuit are coupled to timer 74 to cause the
timer to provide an output waveform having a selected amplitude
(voltage Vl), pulse width ttime tl) and frequency f. Voltage v
is selected to be the minimum solenoid holding voltage required
to sustain valve 22 in the opened position. This voltage is
typically lower than the voltage necessary to cause the solenoid
to open the valve. ~se of a low voltage to maintain the valve
open reduces the closing time for the valve. The closing time is
further reduced ~y a short duration large negative voltage spike
2 which is generated at the end of the inspiratory phase of the
cycle upon removal of the timer 74 output voltage from the valve
solenoid. Time tl is selected to provide the optimum inspiratory
time per ventilation cycle. Frequency f is selected to provide
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th optimum ventilation frequency in accordance with the
condition of the patient. The square wave from circuit 72 is
also applied to an overdrive timer circuit 76. The overdrive
timer circuit is also adjustable to generate a second waveform
having a second amplitude (voltage V3-Vl) and second pulse width
(time t2) with the same frequency f as and in phase with the
waveform provided by timer 74. Voltage V3-V1 and tiTne t2 are
selected to reduce the valve opening time as detailed
hereinafter. The two waveforms are combined, as represented
lo schematically by summing circuit 77, and applied to solenoid 70.
The waveform applied to solenoid 70 is illustrated in Figure 5.
The period of one opening and closing phase or cycle of valve 22,
and hence the ventilation cycle, is given by time T. By applying
the overdrive voltage V3-Vl to the solenoid, the overdrive
voltage having an amplitude which is at le~st three times as
great as the holding voltage V1, a greater electromagnetic force
is generated, and the opening time of the valve is significantly
reduced. Thus, the tidal volumes produced by the ventilator at
high frequencies is not substantially reduced by the time
required for the valve to change its state. As noted above, in
the graphs o~ Figure 4, the broken lines illustrate generally the
pulse characteristics without application of the overdrive
voltage to the solenoid and the solid lines represent the pulse
characteristics of the ventilator when the foregoing described
overdrive voltage is applied from the control module.
It will be appreciated that the ventilator 10 is operated by
selecting an optimum ~requency and duty cycle, i.e., the ratio of
inspiration time to ventilation cycle time, for the condition of
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th ?atient. The tidal volume oE ventilation gas supplied to the
patient is a function of pulse frequency and duration as well as
gas pressure. Pressure regulator 32 regulates the pressure by
conventional means. The control module functions to
electronically control valve 22 to provide the optimum ventilation
characteristics. The latter characteristics may chang~ over the
treatment period and the ventilator of the present invention is
capable of manual or automatic readjustment in accordance with
varying patient requirements. In actual practice, the control
o and safety modules may be a single subassembly including a
programmable microprocessor and the operational mode may be
entered from a keyboard and/or selected from preprogrammed data.
Since this can be accomplished without disconnec~ing the patient
from the ventilator, trauma is avoided that could otherwise
occur. It should be appreciated that since the ventilator is of
modular construction, sterilization and maintenance of the unit
can be relatively easily achieved. The entrainment module 16 has
no moving components and thus may be easily disconnected from the
ventilator for sterilization and/or replacement.
The present invention has the flexibility, particularly
operational parameters which are adjustable over broad ranges,
which enables its use in a synchronous intermittent mandatory
~entilation (IMV) mode. The IMV mode will be selected, via the
microprocessor based control module 20, when it is desired to
attempt to wean a patient from the ventilator. In the IMV mode a
pulse, at a frequency less than the normal breathing rate, will
be provided by a clock in the microprocessor to trig~er the
generation of command signals for the valve 22 solenoid. A
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s~~sor 80, which could be a pressure sensor in the endotracheal
tube, will sense spontaneous breathing by the patient and provide
signals commensurate therewith which are inputted to control
module 20. The valve 22 will open at the selected frequency
except each time spontaneous exhalation is sensed, in which case
the opening of the valve ~ill be delayed until the end of
exhalation and the clock will be reset to zero. The present
invention may also, with the removal of the entrainment module 16
and low pressure gas supply unit 15, be employed in the case of a
transcutaneous cricothyroidalostomy. In emergency situations,
for example under battlefield conditions or in the case of medical
technicians at the scene of an accident, a patient experiencins
breathing difficulty cannot be provided with an endotracheal tube.
That is, the proper insertion of an endotracheal tube may require
as long as one-half hour. requires good lighting and requires a
highly trained medical professional. The present invention, wi-th
the entrainment module removed but a nozzle similar to nozzle 58
retained, can be utilized by medical technicians in the following
manner. A needle with associated catheter will be inserted into
the trachea, the needle will then be withdrawn and the nozzle
then inserted into the trachea via the catheter. Jet ventilation
may then be started with e~halation being via the patient's mouth
and/or nose.
While preferred embodiments of the invention have been set
forth for purposes of illustration, the foregoing description
should not be deemed a limitation of the invention disclosed
herein. Accordingly, various modifications, adaptations and
alternatives may occur to one skilled in the art without departing
from the spirit and the scope of the present invention.
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