Abstrict A flow meter system having a disposable mouthpiece including symmetrically
disposed pitot tubes for measuring the velocity of the respiratory
gases flowing through the mouthpiece and an electronic module including
sensitive pressure transducers coupled to the mouthpiece and operative
over a wide dynamic range provides input signals to a signal processing
network for converting the pressure readings to flow information
useful in cardiopulmonary performance analyzing equipment.
Claims What is claimed is:
1. A respiratory gas flow measuring and indicating system comprising
a tubular mouthpiece member of generally circular cross-section
and including two symmetrically disposed cruciform ribs extending
transverse to the longitudinal dimension of said tubular mouthpiece
member, said ribs each including a pair of lumens with one lumen
of each pair in one rib in fluid communication with a corresponding
lumen of the other pair of lumens in the other rib, each of said
ribs including a plurality of symmetrically located apertures passing
through the walls thereof into said pair of lumens and a further
pair of apertures passing through the wall of said tubular mouthpiece
member and individually aligned coaxially with said pair of lumens
of one of said ribs, the passage of respiratory gases through said
tubular mouthpiece member and over said cruciform ribs creating
pressure differential in said pair of lumens of said one of said
ribs.
2. The respiratory gas flow measuring device as in claim 1 and
further including a pair of hollow needle probes insertable into
said pair of apertures in said tubular mouthpiece member.
3. The respiratory gas flow measuring and indicating system as
in claim 2 and further including pressure transducing means coupled
in fluid communication with said hollow needle probes.
4. The respiratory gas flow measuring and indicating system as
in claim 3 wherein said pressure transducing means includes a first
differential pressure transducing means covering a first predetermined
range of relatively high pressures and a second differential pressure
transducing means covering a second predetermined range of relatively
low pressures overlapping in part with said range of relatively
high pressures.
5. The respiratory gas flow measuring system as in claim 4 and
further including means coupled to said first and second transducing
means for translating the output of said first and second transducing
means into a single signal corresponding to gas flow rate through
said mouthpiece member.
6. The respiratory gas flow measuring system as in claim 5 wherein
said means for translating comprises a signal processing channel
for each of said first and second transducing means, said signal
processing channels including means for producing an analog voltage
proportional to a pressure difference between said pair of lumens
and means for subtracting from said analog voltage a further analog
voltage obtained form said first and second transducing means when
said pressure difference is zero.
7. The respiratory gas flow measuring system as in claim 6 and
further including circuit means in each of said channels and coupled
to the output of said subtracting means for obtaining an analog
voltage proportional to the square root of said output of said subtracting
means.
8. The respiratory gas flow measuring system as in claim 6 and
further including polarity selecting means coupled to said circuit
means in each of said channels and responsive to the direction of
flow of respiratory gases through said tubular mouthpiece for affixing
an algebraic sign to said analog voltage proportional to the square
root of said output of said subtracting means.
9. The respiratory gas flow measuring system as in claim 1 and
further including temperature sensing means operatively coupled
to said mouthpiece member for measuring the temperature of respiratory
gases flowing through said mouthpiece member.
10. The respiratory gas flow measuring system as in claim 7 and
further including temperature sensing means operatively coupled
to said mouthpiece member for producing an analog signal proportional
to the temperature of the respiratory gases flowing through said
mouthpiece member.
Description BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to medical electronic equipment
for assessing cardiopulmonary performance during exercise and for
evaluating pulmonary function during static testing. More particularly,
the invention relates to an improved flow measuring system which
is operative over a wide dynamic range of respiratory flows as is
seen in patients with differing degrees of respiratory impairment.
II. Discussion of the Prior Art
Differential pressure pneumotachographs have been in use for several
decades. Typically, these devices consist of a tubular, open-ended
frame with a known value flow resistive element inserted in the
lumen thereof. The resistive element is generally either one or
more screens positioned transverse to the direction of gas flow
or a grouping of parallel capillary tubes within the gas flow. Under
conditions of gas flow, this creates a pressure drop across the
resistive element which can be assessed by connecting pressure taps
at sequential points along the tube with a differential pressure
transducer. As an example of this type of pneumotachograph is described
in the Anderson et al. U.S. Pat. No. 4463764 the Rudolph U.S.
Pat. No. 3626755 and published results of Fleisch (Pfluegers Arch.
209: 713-722 1925), Lilly (Methods of Medical Research. Chicago,
IL; Yearbook, 1950 2:113-121), Pearce et al. (J. Appl. Physiol:
Respirat. Environ. Exercise Physiol. 42: 968-975 1977) and Osborn
(Crit. Care, Vol. 6 No. 5: 349-351 1978).
Although widely accepted for use, these types of pneumotachographs
suffer from several problems as it relates to accurately measuring
inspiratory and expiratory flow. To maintain a linear relationship
between flow and the pressure drop, the resistive element must maintain
laminar flow. Failure to maintain laminar flow in these types of
pneumotachographs generates unpredictable linearity. These resistive
elements create a back resistance to flow which can distort the
measurements, particularly in patients with significant respiratory
impairment. Moreover, back resistance to flow can distort the measurements.
The frequency response of the pneumotachograph is important since
if the change in pressure across the resistive element is out of
phase with the actual flow signal, it has important significance
when performing phase alignment for gas analyzers and flow signals
during gas exchange measurements.
Further, after a short period of use, the screens or capillary
tubes become coated with condensation and/or saliva which invariably
alters the resistance value. Heating the pneumotachograph to prevent
condensation helps somewhat, but complicates the calculations by
cooling the resistive element unpredictably as gas flow changes.
Because it is a wetted surface, the device must be replaced or decontaminated
between patients. The design of the resistive elements creates a
relatively high cost item which is at odds with disposability. Decontamination
is both time-consuming and inconvenient as the resistive element
must be thoroughly dired after cleansing.
Devices which do not employ resistive elements produce changes
in pressure as a function of the square of the flow. Pressure measurement
over the dynamic range dictated by patients having varying degrees
of respiratory impairment.
As recommended by the various organizations such as the American
Thoracic Society, American College of Chest Physicians and the National
Institute for Occupational Safety and Health, measurements made
from patients should be corrected to a standard environmental condition,
specifically fully saturated, body temperature and pressure. Traditionally,
this has been accomplished by assuming that the respiratory gases
cool to ambient room temperature and applying a fixed correction
of approximately 8%. It is widely known that gas cools dynamically
depending upon the expiratory flow. Gas measured during high flows
will more closely approximate body temperature than during low flows.
This means that during high flow, the correction will be smaller
than during low flow. The magnitude of this error can approach 5%.
It is apparent to those skilled in the art, that a dynamic correction
based upon the actual measured temperature is preferable.
Another drawback of the Hans Rudolph pneumotachograph mouthpiece
is that it includes a significant dead-space leading to inaccuracies
due to the patient rebreathing previously expired gas sample. This,
too, distorts the readings obtained from any O.sub.2 or CO.sub.2
analyzer which may be coupled to the mouthpiece.
A further drawback of the prior art mouthpiece is that it tends
to be a relatively high cost item. Because it embodies wetted surfaces,
i.e., the screen(s) and tubular housing, it is treated as a disposable
unit to avoid the possibility of the spread of harmful virus from
patient-to-patient. High cost and disposability run at odds to each
other.
Those skilled in the art will appreciate that when applied to respiratory
gas analysis systems used in the evaluation of cardiopulmonary performance,
the flow measuring system must be capable of operating over a broad
dynamic range so as to be operative with patients with both healthy
and sick pulmonary organs and with adults as well as infants and
children.
SUMMARY OF THE INVENTION
In accordance with the present invention, a specially designed
mouthpiece which can be made at low cost in a simple molding process
comprises a generally tubular open-ended barrel and midway along
the length of the barrel is disposed a pair of integrally molded
ribs intersecting to form a cross, the ribs each including a pair
of lumens which are separated from one another by a dividing wall
but with the corresponding lumens of the crossing ribs being in
fluid communication with one another. Each of the ribs includes
a series of minute apertures at symmetrically spaced locations on
opposed sides thereof. An additional pair of apertures extend through
the thickness dimension of the barrel so a to intersect with the
separate lumens on one of the two crossing ribs whereby tubular
probes may be inserted into those lumens.
The minute apertures formed through the ribs and into their respective
lumens function as pitot tubes while the probes are coupled to pressure
transducers for measuring the pressure differential in the paired
lumens of the ribs as respiratory gases pass over the exterior of
the ribs during inspiration and expiration.
By eliminating the resistive element, the problems related to back
pressure and phase lag are minimized. Further, this design permits
economical manufacturing for disposability and low cost.
The flow meter mouthpiece of the present invention is adapted to
be used with an electronics module which is operative over a wide
dynamic range of flows, typically from, say, 20 ml/sec to 20 l/sec.
While this represents a dynamic range of 10.sup.3 in terms of flow,
it must be capable of handling a dynamic range of 10.sup.6 in terms
of the pressure differential which is the quantity being measured
in arriving at the flow value, it being understood that flow is
proportional to the square root of the pressure differential.
The foregoing broad-range differential sensing can be achieved
by utilizing two separate differential transducers, one for a high
range of pressures, such as 0.1 to 40.0 inches H.sub.2 O, and a
second for a low range from about 0.00005 inches to 0.5 inches H.sub.2
O. While the transducers themselves used in the system are commercially
available, the electronics module of the present invention translates
the output from these two transducers into a single signal representative
of the flow through the mouthpiece. The pressure lines from the
pitot tubes comprising the mouthpiece are brought in simultaneously
to the low pressure transducer in a first signal processing channel
and the high pressure transducer in a second signal processing channel.
The output from the transducers are each applied to a first stage
of amplification and to an autozero circuit which then sends a command
to a second stage amplifier in each channel to effectively remove
from the output of the first stage amplifier any offset existing
at a time that the instrument should be reading zero.
In that the pressure reading is proportional to the square of the
velocity of the respiratory gases passing over the pitot tubes and
because flow is proportional to velocity to the first order, the
electronic module includes an absolute value and square root circuit
which is coupled to the output from the second stage amplifier and
thus receives the zeroed value of the pressure reading from the
low and high pressure transducers, respectively, and produces an
output proportional to flow. The output from the second stage amplifiers
for low pressure channel is also applied to a zero-crossing detector,
which is configured to provide a logic level signal indicative of
whether the input signal thereto is positive or negative relative
to a threshold voltage. The output from the absolute value/square
root circuit in each channel is applied to a follower/invertor which
functions to buffer the output from the square root circuit while
providing both a positive and a negative quantity corresponding
to output from the square root stage. These two values go to a selector
circuit which, in responding to the outputs of the zero-crossing
detector used to restore the sign value, determines whether the
positive or negative outputs from either of the follower/invertors
is to be selected.
DESCRIPTION OF THE DRAWINGS
The foregoing features, objects and advantages of the invention
will become apparent to those skilled in the art from the following
detailed description of the preferred embodiment, especially when
considered in conjunction with the accompanying drawings in which
like numerals in the several views refer to corresponding parts.
FIG. 1 is a perspective view of the mouthpiece portion of the flow
meter of the present invention;
FIG. 2 is a side elevation of the mouthpiece member of FIG. 1;
FIG. 3 is a right end view of the mouthpiece member of FIG. 1;
FIG. 4 is a left end view of the mouthpiece member of FIG. 1;
FIG. 5 is a cross-sectional view taken along the line 5--5 in FIG.
4;
FIG. 6 is a cross-sectional view taken along the line 6--6 in FIG.
4;
FIG. 7 is a block diagram of the electronics module which, along
with the mouthpiece of FIG. 1 comprises the respiratory gas flow
meter of the present invention;
FIG. 8 is a schematic of the circuit used for monitoring the temperature
of the respiratory gas flow stream and providing a signal for allowing
compensation of the flow value with temperature; and
FIG. 9 is a block diagram of the digital circuit for computing
and displaying respiratory flow information.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1 there is indicated generally by numeral
10 the disposable mouthpiece portion of the flow meter system of
the present invention. It is seen to comprise a generally tubular,
open-ended barrel or sleeve 12 which is preferably molded or otherwise
formed from suitable medical grade plastic or a metal such as stainless
steel. Plastic is preferred if the mouthpiece is to be treated as
a disposable and which is generally symmetric relative to a plane
passing through the midsection thereof. From the view of FIG. 2
it can be seen that the wall thickness of the tubular member 12
tapers slightly from a minimum at its opposed ends 14 and 16 to
a maximum proximate the midpoint 18 the slope or taper appearing
on both the outer wall surface 20 and the inner wall surface 22.
Centrally disposed relative to the midpoint 18 of the tubular barrel
12 are a pair of hollow tubular ribs 24 and 26 which intersect at
their midpoints to form a cross. As illustrated in the cross-sectional
view of FIG. 5 each of the ribs has a pair of lumens as at 28 and
30 separated from one another by a median wall 32. While the lumens
28 and 30 of rib 26 are isolated from one another, they are individually
in fluid communication with the two lumens of rib 24. Moreover,
a pair of apertures 31 and 33 pass through the side wall of the
tube 12 and separately into the two lumens of one of the ribs 24
or 26. The ribs have a rhombic cross-sectional configuration and
formed along the opposed vertices facing the ends 14 and 16 are
a series of fine apertures as at 34 and 36 in the left end view
of FIG. 4 and as at 38 and 40 in the right end view of FIG. 3.
At the time of manufacture, the ports 31 and 33 formed through
the side wall of the barrel or tube 12 are sealed with a dam or
membrane. At the time of use, a pair of hollow needle probes (not
shown) are inserted through the membrane and into the separate lumens
28 and 30 of the rib 26. The hollow needle probes are then coupled
by appropriate tubing to the pressure transducers contained within
the electronics module yet to be described. When the needle probes
are inserted through the membrane, the material tightly surrounds
the exterior of the needles creating a tight seal between the needles
and the mouthpiece. Once the membranes are punctured, it provides
a visual indication that the mouthpiece member has previously been
used and should be discarded. It is only when the dams are intact
that one can be assured that the mouthpiece has not previously bee
used.
When the mouthpiece is inserted into the mouth of a patient undergoing
cardiopulmonary analysis, as he or she breaths in and out, the respiratory
gases pass over the cruciform rib structure and the fine apertures
34 36 38 and 40 function as pitot tubes, causing a pressure differential
across the barrier 32 separating the pairs of intersecting lumens
of the ribs 24 and 26. As is well known in the art, the pressure
difference is proportional to the square of the velocity of the
fluid (respiratory gases) flowing past the ribs through the central
opening of the barrel or tube 12. Because of the symmetrical construction
of the mouthpiece member 10 the pressure differential is found
to be independent of the rotational position of the mouthpiece relative
to the patient's mouth. That is to say, the pressure differential
readings do not change depending upon how the ribs are positioned
as the mouthpiece is inserted into the patient's mouth.
Referring to FIG. 7 the mouthpiece 10 is shown as having needle
probes 42 and 44 entering the sampling ports 31 and 33. The needle
probes 42 and 44 are coupled by appropriate tubular lines 46 to
pressure transducers in a low pressure channel and a high pressure
channel.
Two transducers 50 and 51 are used to cover a pressure range of
from 0.0001 to 40 inches water column. The low pressure transducer
50 has a full scale reading of about 0.5 inches water column while
the high pressure transducer 51 may have a full scale of about 40
inches water column. The output of each of these transducers, after
appropriate gain adjustment and offset compensation by circuits
52 and 53 comprises an analog signal with full scale output of each
transducer corresponding to positive or negative 10 volts, depending
upon which side of the pressure transducer is at a higher pressure
than the opposite side. That is to say, during inspiration, the
side of the mouthpiece ribs 24 or 26 closest to the person's mouth
with be at a lower pressure than the opposed side, but during expiration,
the side closest to the subject's mouth will be at a higher pressure
than the downstream side.
When there is a zero pressure differential between the two sides
of the mouthpiece ribs, the output of each of the transducers should
be at zero volts. Should small deviations from zero occur, they
can be compensated for by means of an autozero circuit 54 for the
low pressure side and a corresponding circuit 55 for the high pressure
side. The autozero circuits are configured so that the signal input
thereto is amplified and converted to a digital value proportional
to pressure which is then stored in a buffer circuit. The contents
of the buffer are then converted back into an analog signal form.
The analog signal is inverted and attenuated to an appropriate level
so that when it is summed with the original signal in a summing
amplifier, as at 56 and 57 the result will be zero volts. The contents
of the buffer in the autozero circuits 54 and 55 are updated, on
command, during a time interval when it is known that there is a
zero pressure difference across the two needle probes. This condition
is established by means of a shunt valve 58 coupled between the
opposed sides of the mouthpiece ribs 24 and 26 and which is opened
by the same "zero" command. As indicated above and in
the drawing of FIG. 7 each of the pressure transducers 50 and 51
includes its own autozero circuit as at 54 and 55.
The output of one or the other of the transducers 50 and 51 is
passed along to the next stage through a switch 60 referred to as
the "crossover select switch". Switch 60 is controlled
by a signal produced by a comparator 59 which is configured to monitor
the output of the low pressure transducer 50. When the output of
that transducer is nearly full scale in either direction, the signal
from the comparator 59 changes state, so that the output of the
high pressure transducer 51 will be passed through the cross-over
select switch 60 instead of the low pressure signal. It should also
be noted that the output of the comparator 59 is used to select
a gain value at a later amplifier stage.
The amplified, zero-corrected output of the low pressure transducer
at the output of summing amplifier 56 is also connected to a zero-crossing
detector 61 whose output is used to re-introduce the appropriate
algebraic sign in the signal at a later stage.
The signal selected by the cross-over switch 60 is applied to an
analog circuit 62 that has a voltage output equal to the absolute
value of the input voltage. This signal is passed to a square root
circuit 63 whose voltage output is equal to the square root of the
voltage applied to its input. The output of the square root circuit
is amplified by a variable gain stage 64 whose value of gain is
controlled by the same signal that is used to control the state
of the cross-over select switch 60. The gains are adjusted so that
when the pressure is just sufficient to change the signal being
acted upon by the absolute value and square root circuits 62 and
63 the output of this gain stage 64 will be piecewise continuous.
Next, the signal is acted upon by a follower invertor stage 65.
The output of this stage is either equal to the input to the stage,
or equal to the negative of the input to this stage, depending on
the output of the zero-crossing detector 61.
From the output of the follower invertor stage 65 the signal is
buffered by a unity gain amplifier 66 before it is provided as the
value of the flow measured in units of milliliters-per-second or
liters-per-second, again depending upon whether it is the low pressure
transducer 50 or high pressure transducer 51 whose output is being
processed.
The flow meter 10 in FIG. 7 is shown as being partially broken
away to reveal a temperature probe 68 which extends into the respiratory
gas flow path through the flow meter 10. The temperature probe 68
preferably comprises a thermistor mounted through the wall of the
flow meter and having electrical leads 70 leading to a circuit for
producing a voltage proportional to temperature. The thermistor
68 comprises a bead of temperature sensitive resistance material
which is small enough to have a short reaction time, e.g., about
150 milliseconds.
Referring to FIG. 8 the thermistor 68 comprises one leg of a Wheatstone
bridge which also includes fixed resistors 72 74 and 76 and a variable
resistor 78 connected in parallel with the fixed resistor 74. A
fixed potential +V is derived from a regulated DC source including
a zener diode 80 in series with a fixed resistor 82. The bridge
output is obtained across the junction points 84-86 and is applied
across the inputs of an operational amplifier 88 by way of DC coupling
resistors. A feedback resistor 94 is coupled between the output
from the operational amplifier 88 and the inverting input thereto.
The operational amplifier 88 is configured as a variable gain amplifier
with the variable resistance of the thermistor 68 controlling the
overall gain of the amplifier whereby its output is directly proportional
to temperature change. Specifically, an increase in temperature
sensed by the thermistor element results in a decrease in its resistance
which has the overall effect of increasing the output of the operational
amplifier 88. Likewise, a drop in temperature sensed by the thermistor
element 68 results in a increase in its resistance causing the output
of the operational amplifier 88 to decrease.
From the ideal gas law, it is known that the volume of a given
amount of gas varies linearly with temperature. This implies that
if a volume, or flow, of gas is stated, a temperature must also
be stated or implied. For a flow measuring device, such as the present
invention where the temperature of the gas being measured is not
a constant, it is desirable to be able to measure the temperature
of the gas so that an appropriate correction to a given condition
(such as "standard temperature" or "body temperature")
may be calculated. Also, those skilled in the art will appreciate
that for a pitot tube type of device, the pressure generated across
the needle probes varies inversely with the density of the gas being
measured. Gas density, in turn, varies linearly with temperature.
Hence, to accurately measure a volume of gas or a flow, it is important
to know the temperature of the gas in question.
As shown in FIG. 9 the flow information output from the circuit
of FIG. 7 and the temperature information from the circuit of FIG.
8 are applied to an A/D converter 96 where, on a time multiplexed
basis, the flow information and temperature information are digitized
and fed over a bus 98 to a microprocessor 100 which is appropriately
programmed to produce an output to a display terminal 102 or to
a hard copy printer 104 relating to respiratory flow in which dynamic
compensation based upon temperature variations is provided for on
a real-time basis.
It can be seen, then that the circuit of FIG. 7 when coupled to
the mouthpiece of FIG. 1 in the fashion described, permits two separate
transducers covering discrete ranges of relatively low and relatively
high pressure to be used together so as to create an extremely wide
dynamic range from, for example, 0.005 to 40 inches of water column
and allowing the conversion of the pressure measured in this range
to an analog signal proportional in amplitude to the respiratory
flow through the mouthpiece, and indicating by an appropriate polarity,
whether the flow is due to inspiratory or expiratory breathing.
This invention has been described herein in considerable detail
in order to comply with the Patent Statutes and to provide those
skilled in the art with the information needed to apply the novel
principles and to construct and use such specialized components
as are required. However, it is to be understood that the invention
can be carried out by specifically different equipment and devices,
and that various modifications, both as to the equipment details
and operating procedures, can be accomplished without departing
from the scope of the invention itself. |