U.S. patent application number 11/268117 was filed with the patent office on 2006-03-23 for target drive ventilation gain controller and method.
Invention is credited to Christer Sinderby.
Application Number | 20060060190 11/268117 |
Document ID | / |
Family ID | 23435206 |
Filed Date | 2006-03-23 |
United States Patent
Application |
20060060190 |
Kind Code |
A1 |
Sinderby; Christer |
March 23, 2006 |
Target drive ventilation gain controller and method
Abstract
A gain controller and method for controlling the value of a gain
is used in conjunction with an electrode array for detecting a
signal representative of respiratory drive output of a patient
during inspiration, and a lung ventilator for assisting inspiration
of the patient. The gain controller comprises an input for
receiving the signal representative of respiratory drive output; a
comparator for determining whether the signal representative of
respiratory drive output is higher or lower than a target drive
signal; and a gain adjustment unit for increasing the value of a
gain when the amplitude of the signal representative of respiratory
drive output is higher than the amplitude of the target drive
signal and for decreasing the value of this gain when the amplitude
of the signal representative of respiratory drive output is lower
than the amplitude of the target drive signal. The gain is applied
to the signal representative of respiratory drive output to produce
an amplified respiratory drive output representative signal used
for controlling the lung ventilator. The advantage of target drive
ventilation is that this mode of ventilation does not depend on
pressure, flow or volume measurements. A leaky ventilatory line
will introduce a change in respiratory drive which will change the
ventilatory assist in order to return the respiratory drive to its
target level. Also, changes in the patient's metabolic or
patho-physiological status which result in altered respiratory
drive will be compensated.
Inventors: |
Sinderby; Christer;
(Montreal, CA) |
Correspondence
Address: |
FULBRIGHT & JAWORSKI L.L.P.
600 CONGRESS AVE.
SUITE 2400
AUSTIN
TX
78701
US
|
Family ID: |
23435206 |
Appl. No.: |
11/268117 |
Filed: |
November 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09364592 |
Jul 30, 1999 |
6962155 |
|
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11268117 |
Nov 7, 2005 |
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Current U.S.
Class: |
128/200.14 ;
128/204.18 |
Current CPC
Class: |
A61M 2210/1014 20130101;
A61N 1/05 20130101; A61M 2230/08 20130101; A61M 16/026 20170801;
A61M 2230/60 20130101 |
Class at
Publication: |
128/200.14 ;
128/204.18 |
International
Class: |
A61M 11/00 20060101
A61M011/00; A61M 16/00 20060101 A61M016/00 |
Claims
1. A gain controller for adjusting, in relation to a target drive
signal, the value of a gain applied to a signal representative of
respiratory drive output of a patient during inspiration, to
produce an amplified respiratory drive output representative signal
for controlling a lung ventilator assisting inspiration of the
patient, said gain controller comprising: a first input for
receiving the signal representative of respiratory drive output
having a first amplitude; a second input for receiving the target
drive signal of a second amplitude; a comparator for determining
whether the amplitude of said signal representative of respiratory
drive output is higher or lower than the amplitude of said target
drive signal; and a gain adjustment unit for increasing the value
of said gain when the amplitude of said signal representative of
respiratory drive output is higher than the amplitude of said
target drive signal and for decreasing the value of said gain when
the amplitude of the signal representative of respiratory drive
output is lower than the amplitude of the target drive signal.
2-33. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a target driven inspiratory assist
ventilation system.
[0003] 2. Brief Description of the Prior Art
[0004] The physiological mechanisms which generate myoelectrical
activity when a muscle contracts have been known and understood for
a long time. In particular, how to record signals from the muscles
is one of the most extensively, theoretically described topics in
physiology. Although the theoretical understanding is impressive,
the bio-physiological application of these theories is, in
practice, still deficient As an example, no standardized analysis
procedure has been developed for recording signals produced by
activation of several, different motor units, the so called
interference wave pattern. The interference wave pattern signal
(EMG signal) contains an immense quantity of bio-physiological
information about the given neuro-muscular function. However, as
this EMG signal is very low in amplitude, it is sensitive to
numerous artifacts. The influence of these artifacts varies in
relation to the configuration of recording electrodes, the
digitizing rate of the signal, and the type of recording
technique.
[0005] Prior art analysis of interference wave pattern signals
usually comprises a time consuming, tedious manual determination of
the quality of the signal through visual inspection of this signal
in the time domain. This determination is performed by a
"subjective" investigator. Most of the prior art references
describe how to calculate comparison estimates, but present very
few comments on the signal quality. It is therefore not surprising
to find that, in this technical field, independent studies
evaluating the same questions have lead to different or even
contradictory results.
[0006] Also in the prior art, the patient's inspiratory flow and
volume has been used to control inspiratory proportional pressure
assist ventilation. Proper adjustment of the relative contribution
of flow and volume support during the inspiration requires
knowledge of the elastic and viscous properties of the patient's
respiratory system. Since the elastic and viscous properties may
change, these measurements must be repeated at regular intervals.
Correct and repeated measurements of elastance and resistance are
difficult to set up in an intensive care unit. Moreover, in the
presence of intrinsic positive end-expiratory pressure, the
flow-volume controlled proportional assist ventilation may fail to
trigger during whole breaths, and will definitively fail to trigger
during at least the initial part of the inspiration which precedes
the onset of flow; this period can last up to 300 ms in the case of
a patient suffering from obstructive pulmonary disease. Finally
leakage in the system will influence and may disturb the
performance of the flow controlled proportional assist
ventilation.
[0007] Traditionally, the goal of mechanical ventilation has been
to maintain an optimal minute ventilation and respiratory load, and
therefore, has included specific measurements of inspiratory flow
and tidal volume. New concepts in mechanical ventilation allow
patients to take over the control of ventilatory support delivered,
both in terms of magnitude and duration. New technology has also
incorporated new methods of applying ventilatory assist for
example, mask ventilation, uncuffed endotracheal tubes, and
miniature endotracheal tubes. These devices frequently cause
leakage of gases such that measurement of flow and volume become
erroneous.
[0008] Current technology is therefore often limited in its ability
to detect and correct for these gas leaks and patients are at risk
of becoming hyper- or hypo-ventilated.
OBJECTS OF THE INVENTION
[0009] An object of the present invention is therefore to overcome
the above described drawbacks of the prior art.
[0010] Another object of the present invention is to provide a
method and a device capable of adjusting the degree of inspiratory
assist in relation to the real need of the patient, i.e. only to
compensate for the degree of incapacity of the patient.
[0011] A further object of the present invention is to provide a
method and a device for controlling inspiratory proportional
pressure assist ventilation which requires no knowledge of the
elastic and viscous properties of the patient's respiratory system,
is not influenced by intrinsic positive end-expiratory pressure,
altered muscle function, and is not influenced by air leakage of
the lung ventilator unless the leakage exceeds the pumping capacity
of the ventilator.
SUMMARY OF THE INVENTION
[0012] More specifically, in a preferred embodiment of the
invention, there is provided a gain controller for adjusting, in
relation to a target drive signal, the value of a gain applied to a
signal representative of respiratory drive output of a patient
during inspiration, to produce an amplified respiratory drive
output representative signal for controlling a lung ventilator
assisting inspiration of the patient. The gain controller
comprises:
[0013] a first input for receiving the signal representative of
respiratory drive output having a first amplitude;
[0014] a second input for receiving the target drive signal of a
second amplitude;
[0015] a comparator for determining whether the amplitude of the
signal representative of respiratory drive output is higher or
lower than the amplitude of the target drive signal; and
[0016] a gain adjustment unit for increasing the value of the gain
when the amplitude of the signal representative of respiratory
drive output is higher than the amplitude the target drive signal
and for decreasing the value of the gain when the amplitude of the
signal representative of respiratory drive output is lower than the
amplitude of the target drive signal.
[0017] In another embodiment of the invention, there is provided a
method for adjusting, in relation to a target drive signal, the
value of a gain applied to a signal representative of respiratory
drive output of a patient during inspiration, to produce an
amplified respiratory drive output representative signal for
controlling a lung ventilator assisting inspiration of the patient.
The method comprises:
[0018] receiving the signal representative of respiratory drive
output having a first amplitude;
[0019] receiving the target drive signal of a second amplitude;
[0020] determining whether the amplitude of the signal
representative of respiratory drive output is higher or lower than
the amplitude of the target drive signal; and
[0021] increasing the value of the gain when the amplitude of the
signal representative of respiratory drive output is higher than
the amplitude of said target drive signal; and decreasing the value
of the gain when the amplitude of the signal representative of
respiratory drive output is lower than the amplitude of the target
drive signal.
[0022] Target drive ventilation is based on the assumption that the
patient's respiratory centers are intact and the patient is able to
control minute ventilation as long as he/she has sufficient
respiratory muscle. In a preferred embodiment of the invention,
determination of respiratory drive is made by measuring the
electrical activation of the diaphragm during an inspiration. Of
course, any other signal representative of respiratory drive output
may be used in other embodiments of the invention. Electrical
activity of the diaphragm has previously been demonstrated to
reflect global respiratory drive. The inspiratory electrical
activation of the diaphragm can be quantified as the mean, median,
total, peak, etc. and the trend of the previous breaths is used to
adjust ventilatory assist for the present breath.
[0023] The invention is aimed to control ventilatory assist levels
in order to maintain the respiratory drive (determined by diaphragm
electric activation) at a sustainable target level. The lung
ventilator can use a pressure/flow/volume generating device with a
control unit which operates to maintain the mean (could also be
median/peak/total, etc.) pressure/flow/volume in the ventilatory
line sufficient for maintaining a constant target diaphragm
electrical activity. The diaphragm electrical activity during a
breath will be calculated in order to determine the mean (could
also be median/peak/total, etc.) neural drive to the diaphragm for
that particular breath. The trend for respiratory drive can be
obtained from diaphragm electrical activity of previous breaths
such that one can determine whether respiratory drive increases,
decreases, or remains constant. A trend for a change in diaphragm
electrical activity indicating an increase in respiratory drive
will result in a progressive increase ventilatory assist until
diaphragm electrical activity, i.e., respiratory drive has returned
to its target level. Similarly, the decrease in diaphragm
electrical activity, indicating reduced respiratory drive, will
produce a progressive decrease in ventilatory assist until
diaphragm electrical activity i.e. respiratory drive has returned
to its target level.
[0024] Target Drive Ventilation would be more efficiently used in
combination with Neurally Adjusted Proportional Pressure Assist
(U.S. Pat. No. 5,820,560 to Sinderby et al., 1998), where
ventilatory assistance will be proportional to the patient's
respiratory drive throughout the breath and the average respiratory
drive would remain constant over time. For proportional assist
ventilation or other modes which deliver varying levels of support,
the increasing or decreasing levels of mean/total ventilatory
assist will be adjusted by increasing or decreasing the gain factor
applied in the respective functions.
[0025] Target drive ventilation can also be applied with other
modes of ventilatory assist. For use with ventilatory support modes
that provide constant levels of support, for example pressure
support, the increasing or decreasing levels of mean/total
ventilatory assist will be achieved by relative increases or
decreases of the pressure support, the increasing or decreasing
levels of mean/total ventilatory assist will be achieved by
relative increases or decreases of the pressure support level.
Extreme pressure support levels will be avoided by introducing
safety limits.
[0026] The advantage of Target drive ventilation is that this mode
of ventilation does not depend on flow or volume measurements. A
leaky ventilatory line will introduce a change in respiratory drive
which will change the ventilatory assist in order to return the
respiratory drive to its target level. Also, changes in the
patient's metabolic or patho-physiological status which result in
altered respiratory drive will be compensated. In contrast with
present methods of controlling mechanical ventilators, an increase
in respiratory assistance using a signal representative of
respiratory drive output (e.g., an EMG signal) does not affect the
efficiency with which these signals reliably control the ventilator
(unless of course the disease affects the neuro-muscular
function).
[0027] A combination of Target Drive Ventilation and Neurally
Adjusted Proportional Pressure Assist (U.S. Pat. No. 5,820,560),
would provide partial correction for leaks within breaths and
compensation for leaks over long periods of time. The use of neural
triggers would also overcome issues related to intrinsic PEEP.
[0028] The objects, advantages and other features of the present
invention will become more apparent upon reading of the following
non restrictive description of a preferred embodiment thereof,
given by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] In the appended drawings:
[0030] FIG. 1 is a schematic representation of a set-up of an EMG
analysis system;
[0031] FIG. 2 is a section of oesophageal catheter on which an
array of electrodes of the EMG analysis system of FIG. 1 is
mounted;
[0032] FIG. 3 illustrates a section of oesophageal catheter on
which a second embodiment of the array of electrodes is
mounted;
[0033] FIG. 4 is a graph showing a set of EMGdi signals of the
diaphragm detected by pairs of successive electrodes of the array
of FIG. 2;
[0034] FIG. 5A is a flow chart showing a method for conducting
double subtraction technique of the EMGdi signals;
[0035] FIG. 5B is a flow chart showing a method for controlling a
gain value in accordance with an embodiment of the invention;
[0036] FIG. 6 is a graph showing the distribution of correlation
coefficients calculated for determining the position of the center
of the depolarizing region of the diaphragm along the array of
electrodes of FIG. 2;
[0037] FIG. 7 is a schematic diagram illustrating in the time
domain a double subtraction technique for improving the
signal-to-noise ratio and to reduce an electrode-position-induced
filter effect;
[0038] FIG. 8a is a graph showing the power density spectrum of
electrode motion artifacts, the power density spectrum of ECG, and
the power density spectrum of EMGdi signals;
[0039] FIG. 8b is a graph showing an example of transfer function
for a filter to be used for filtering out the electrode motion
artifacts, ECG, and the 50 or 60 Hz disturbance from electrical
mains;
[0040] FIG. 9 is a schematic diagram illustrating in the frequency
domain stabilization by the double subtraction technique of the
center frequency upon displacement of the center of the
depolarizing region of the diaphragm along the array of electrodes
of FIG. 2;
[0041] FIG. 10 is a schematic block diagram of a lung ventilator
showing control of inspiratory proportional pressure assist
ventilation by means of an EMG signal obtained with the above
mentioned double subtraction technique; and
[0042] FIG. 11 is a schematic block diagram showing a structure for
implementing the steps of the method described in FIG. 5B.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0043] To measure EMG activity of the diaphragm 11 (EMGdi) of a
human patient 14, an array of electrodes such as 12 (FIGS. 1 and 2)
are mounted on the free end section 15 of an oesophageal catheter
13, with a constant inter-electrode distance d (FIG. 2). As shown
in FIG. 1, the catheter 13 is introduced into the patient's
oesophagus through one nostril or the mouth until the array of
electrodes 12 are situated at the level of the gastro esophageal
junction. The diaphragm 11 and/or the oesophagus slightly move
during breathing of the patient 14 whereby the array of electrodes
12 also slightly moves about the diaphragm 11. As will be explained
in the following description, automatic compensation for this
displacement is provided.
[0044] To mount an electrode 12 on the free end section 15 of the
catheter 13, stainless steel wire (not shown) may be wound around
the catheter 13. The wound stainless steel wire presents a rough
surface smoothed out by solder, which in turn is electroplated with
nickel, copper and then gold or silver. Of course, EMG signals from
other muscles and other constructions of electrodes can be
implemented.
[0045] Electric wires (not shown) interconnect each pair of
successive electrodes such as 1-7 (FIG. 2) with a respective one of
a group of differential amplifiers 16. This defines an overlap
array. Obviously, these electric wires follow the catheter 13 from
the respective electrodes 12 to the corresponding amplifiers 16,
and are preferably integrated to the catheter 13. Preferably, the
electric wires transmitting the EMGdi signals collected by the
various pairs 1-7 of electrodes 12 are shielded to reduce the
influence of external noise, in particular disturbance from the 50
or 60 Hz current and voltage of the electrical mains.
[0046] The group of differential amplifiers 16 amplifies (first
subtraction step of the double subtraction technique) and band-pass
filters each EMGdi signal. This first subtraction step may also be
carried out in the personal computer 19 when the amplifiers 16 are
single-ended or equivalently designed amplifiers (monopolar
readings).
[0047] In the example illustrated in FIGS. 1 and 2, the free end
section 15 of the catheter 13 is provided with an array of eight
electrodes 12 defining seven pairs 1, 2, 3, 4, 5, 6 and 7 of
successive electrodes 12 respectively collecting seven different
EMGdi signals. Although it has been found that EMG activity of the
diaphragm (EMGdi) can be measured accurately with an oesophageal
catheter 13 provided on the free end section 15 thereof with an
array of eight electrodes 12, a different number and/or
configuration of pairs of electrodes 12 can be contemplated
depending on the patient's anatomy and movement of the diaphragm.
Also, the pairs 1-7 do not need to be pairs of successive
electrodes; FIG. 3 illustrates an array of nine electrodes to form
seven overlapping pairs of electrodes 1-7.
[0048] A major problem in recording EMGdi signals is to maintain
the noise level as low and as constant as possible. Since the
electric wires transmitting the EMGdi signals from the electrodes
12 to the differential amplifiers 16 act as an antenna, it is
crucial, as indicated in the foregoing description, to shield these
electric wires to thereby protect the EMGdi signals from additional
artifactual noise. Also, the package enclosing the differential
amplifiers 16 is preferably made as small as possible
(miniaturized) and is positioned in close proximity to the
patient's nose to decrease as much as possible the distance between
the electrodes 12 and the amplifiers 16.
[0049] The amplified EMGdi signals are supplied to a personal
computer 19 through respective isolation amplifiers of a unit 18.
Unit 18 supplies electric power to the various electronic
components of the differential and isolation amplifiers while
ensuring adequate isolation of the patient's body from such power
supply. The unit 18 also incorporates bandpass filters included in
the respective EMGdi signal channels to eliminate the effects of
aliasing. The EMGdi signals are then digitally processed into the
personal computer 19 after analog-to-digital conversion thereof.
This analog-to-digital conversion is conveniently carried out by an
analog-to-digital converter implemented in the personal computer
19. The personal computer 19 includes a monitor 40 and a keyboard
31.
[0050] It is believed to be within the capacity of those of
ordinary skill in the art to construct suitable differential
amplifiers 16 and an adequate isolation amplifiers and power supply
unit 18. Accordingly, the amplifiers 16 and the unit 18 will not be
further described in the present specification.
[0051] An example of the seven EMGdi signals collected by the pairs
1-7 of successive electrodes 12 (FIGS. 1 and 2) and supplied to the
computer 19 is illustrated in FIG. 4.
[0052] As the diaphragm is generally perpendicular to the
longitudinal axis of the oesophageal catheter 13 equipped with an
array of electrodes 12, only a portion of the electrodes 12 are
situated in the vicinity of the diaphragm. It is therefore
important to determine the position of the diaphragm with respect
to the oesophageal electrode array.
[0053] The portion of the crural diaphragm 11 which forms the
muscular tunnel through which the oesophageal catheter 13 is passed
is referred to the "diaphragm depolarizing region" (DDR). The
thickness of the DDR is 20-30 mm. It can be assumed that, within
the DDR, the distribution of active muscle fibers has a center from
which the majority of the EMGdi signals originate, i.e. the
"diaphragm depolarizing region center" (DDR center). Therefore,
EMGdi signals detected on opposite sides of the DDR center will be
reversed in polarity with no phase shift; in other words, EMGdi
signals obtained along the electrode array are reversing in
polarity at the DDR center.
[0054] Moving centrally from the boundaries of the DDR, EMGdi power
spectrums progressively attenuate and enhance in frequency.
Reversal of signal polarity on either side of the electrode pair 4
with the most attenuated power spectrum confirms the position from
which the EMGdi signals originate, the DDR center.
[0055] Referring to FIG. 5A, the first task of the computer 19 is
to determine the center of the DDR. The center of the DDR is
repeatedly determined at predetermined time intervals.
[0056] For that purpose, slow trend is first removed from each
EMGdi signal (step 500). To carry out such trend removal, the
processing conducted by the computer 19 on each EMGdi signal is
equivalent to high-pass filtering each EMGdi signal at a transition
frequency of about 20 Hz. In particular, step 500 will remove the
direct current component of the EMGdi signals to enable the
computer 19 to evaluate the polarities of the EMGdi signals
relative to each other.
[0057] In step 501, the EMGdi signals are cross-correlated in
pairs. As well known to those of ordinary skill in the art,
cross-correlation is a statistical determination of the phase
relationship between two signals and essentially calculates the
similarity between two signals in terms of a correlation
coefficient r (step 502). A negative correlation coefficient r
indicates that the cross-correlated signals are of opposite
polarities.
[0058] FIG. 6 shows curves of the value of the correlation
coefficient r versus the midpoint between the pairs of electrodes
from which the correlated EMGdi signals originate. In this example,
the inter-electrode distance is 10 mm. Curves are drawn for
distances between the correlated pairs of electrodes 12 of 5 mm
(curve 20), 10 mm (curve 21), 15 mm (curve 22) and 20 mm (curve
23). One can appreciate from FIG. 5A that negative correlation
coefficients r are obtained when EMGdi signals from respective
electrode pairs situated on opposite sides of the electrode pair 4
are cross-correlated. It therefore appears that the change in
polarity occur in the region of electrode pair 4, which is
confirmed by the curves of FIG. 4. Accordingly, it can be assumed
that the center of the DDR is situated substantially midway between
the electrodes 12 forming pair 4.
[0059] For example, the center of the DDR can be precisely
determined by interpolation (step 503 of FIG. 5A) using a square
law based fit of the three most negative correlation coefficients
of curve 21 obtained by successive cross-correlation of the EMGdi
signals from each electrode pair to the EMGdi signals from the
second next electrode pair. Association of the center of the DDR to
a pair of electrodes 12 provides a "reference position" from which
to obtain EMGdi signals within the DDR. Such control is essential
in overcoming the artifactual influence on the EMGdi power
spectrum.
[0060] It has been experimentally demonstrated that EMGdi signals
recorded in the oesophagus are satisfactory as long as they are
obtained from electrode pairs (with an inter-electrode distance
situated between 5 and 20 mm) positioned at a distance situated
between 5 and 30 mm on the opposite sides of the DDR center (the
inter-pair distance being therefore situated between 5 and 30 mm).
Although EMGdi signals obtained from these positions offers a clear
improvement in acceptance rate, the signal-to-noise ratio during
quiet breathing still tends to remain unsatisfactorily low.
[0061] In another embodiment of the invention, step 500 can be
eliminated and steps 501, 502 and 503 could be implemented
immediately after step 505.
[0062] For example, in FIG. 4, the EMGdi signals originating from
the electrode pairs 3 and 5 situated respectively 10 mm below and
10 mm above the DDR are strongly inversely correlated at zero time
delay. In contrast to the inversely correlated EMGdi signals, the
noise components for electrode pairs 3 and 5 are likely to be
positively correlated. Hence, as illustrated in FIG. 7, subtraction
of the EMGdi signals 24 and 25 from electrode pairs 3 and 5 will
result into an addition of the corresponding EMGdi signals (signal
26 of FIG. 6) and into a subtraction, that is an elimination of the
common noise components. This technique will be referred to as "the
double subtraction technique" (step 504 of FIG. 5A).
[0063] Subtraction step 504 (second subtraction step of the double
subtraction technique) can be carried out either in the time
domain, or after conversion of signals 24 and 25 in the frequency
domain. Double subtraction technique can be performed by
subtracting other combinations of signals, for example by
subtracting the EMGdi signal from electrode pair 2 from the EMGdi
signal from electrode pair 5 (FIG. 4), by subtracting signal from
electrode pair 6 from the signal from electrode pair 3 and by
adding these differences, etc. Other means for reducing the effect
of electrode filtering can be applied.
[0064] The double subtraction technique is carried out in step 504
on the pair of EMGdi signals (for example the signals from
electrode pairs 3 and 5 shown in FIG. 4) identified in step 503,
after appropriate filtering of these EMGdi signals in step 505.
Filtering step 505 will remove from each EMGdi signal the motion
artifacts, the electrocardiogram (ECG) component, and the
disturbance from the electrical mains. Motion artifacts are induced
by motion of the electrodes. More generally, motion artifacts are
defined as a low frequency fluctuation of the EMGdi signals' DC
level induced by mechanical alterations of the electrode metal to
electrolyte interface i.e. changes in electrode contact area and/or
changes in pressure that the tissue exerts on the electrode.
[0065] The graph of FIG. 8a shows the power density spectrum of the
above defined electrode motion artifacts, the power density
spectrum of ECG, and the power density spectrum of EMGdi signals.
The graph of FIG. 8b shows an example of transfer function for a
filter (the dashed line showing the optimal transfer function, and
the solid line the transfer function implemented by the inventors)
to be used in step 505 for filtering out the electrode motion
artifacts, ECG, and the 50 or 60 Hz disturbance from the electrical
mains. Processing of the EMGdi signals by the computer 19 to follow
as closely as possible the optimal transfer function of FIG. 8b
will conduct adequately filtering step 505.
[0066] Referring back to FIG. 5A, step 506 calculates the RMS
(Root-mean-square) value of the double-subtracted signal produced
in step 504. The increase in amplitude obtained with the double
subtraction technique is associated with a twofold increase in RMS
values. RMS values obtained with the double subtraction technique
are closely and linearly related to the original signals. The RMS
value can be replaced by any other value representative of the
strength of the double-subtracted signal, for example mean, median,
peak or total signal amplitudes.
[0067] The double subtraction technique compensates for the changes
in signal strength and frequency caused by movement of the
diaphragm 11 (FIG. 1) and/or the oesophagus during breathing of the
patient 14 causing movement of the array of electrodes 12 with
respect to the diaphragm 11. Referring to FIG. 9, off center of the
array of electrodes 12 (electrode-position-induced filter effect)
causes a variation of center frequency values (see curves 27 and
28) for the EMGdi signals from the electrode pairs 3 and 5. The
double subtraction technique eliminates such variation of center
frequency values as indicated by curve 29 as well as variation of
signal strength. Therefore, the reciprocal influence of the
position of the DDR center on the EMGdi signal frequency content is
eliminated by the double subtraction technique.
[0068] It has been found that the double subtraction technique may
improve the signal-to-noise ratio by more than 2 dB ratio and
reduce an electrode-position-induced filter effect. Double
subtraction technique is also responsible for a relative increase
in acceptance rate by more than 30%.
[0069] Cross-talk signals from adjacent muscles are strongly
correlated at zero time delay and equal in polarity between all
pairs of electrodes 12. Hence, these cross-talk signals appear as a
common mode signal for all electrode pairs and therefore, are
eliminated by the double subtraction technique.
[0070] Referring to FIG. 5B, a target drive signal is set by an
operator at step 601. The output of block 601 is therefore the
target drive signal 602. The value of the target drive signal 602
is determined by a person skilled in the art. The target drive
signal 602 can be any signal which is representative of respiratory
drive output. In a preferred embodiment of the invention, this
signal can be any signal representative of the electrical activity
of a muscle (i.e., electromyographic signal) that reflects the
global respiratory drive. The target drive signal 602 can therefore
be quantified as, among others, the mean, the median, the total or
the peak of the EMGdi signal. The target drive signal 602 is then
compared to the RMS value on line 508 in block 604. If the present
breath RMS value on line 508 is greater than the target drive
signal 602, this indicates that there is a trend for a change in
diaphragm electrical activity (EMGdi) indicating an increase in
respiratory drive and requiring a progressive increase in
ventilatory assistance. The result of the decision block 604 will
be positive until the EMGdi 508 returns to the target level 602. In
this case, the stored gain (k) will be increased in block 606. The
gain (k) 610 will then be recorded in block 611 and outputted as
signal 613.
[0071] Returning now to block 604, if the present breath RMS value
on line 508 is not greater than the target drive signal 602, it may
be equal or smaller. In block 605, it is determined if the present
breath RMS value 508 is equal to the target drive signal 602. If it
is equal, the gain value (k) does not change.
[0072] If the present breath RMS value 508 is not equal to the
target drive signal 602, then it is smaller than the target drive
signal 602, and this indicates a decrease in diaphragm electrical
activity (EMGdi) resulting in reduced respiratory drive. It will
therefore be necessary to progressively decrease the ventilatory
assist until the diaphragm electrical activity (EMGdi) 508 returns
to the target level 602. In this case, the result from decision
block 604 will be negative resulting in a decrease in the stored
gain (k) in block 608. The gain (k) 609 will then be stored at step
611 and outputted as signal 613.
[0073] Those skilled in the art will understand that the amount of
the change (increase, step 606, or decrease, step 608) in
ventilatory assistance is derived from experience, the patient's
condition, the environment, etc. The amount of change can therefore
be adjusted on a case by case basis. Also, in a particular
embodiment of the invention, the increase or decrease in
ventilatory assistance could be a relative value; that is, a
fraction or percentage multiplied by, for example, the target drive
signal, the signal representative of respiratory drive output, or a
difference between the amplitude of the signal representative of
respiratory drive output and the amplitude of the target drive
signal.
[0074] Those skilled in the art will also understand that the test
at steps 604 and 605 can include a certain range of amplitudes for
the target drive signal 602 (whether they are absolute or relative
amplitudes); that is, for example, the target drive signal could be
X plus or minus a predetermined value. Therefore, at step 604, the
present breath RMS value 508 needs to be greater than X plus the
predetermined value in order to proceed to step 606. In the same
way, at step 605, the present breath RMS value 508 needs to be
smaller than X minus the predetermined value in order to proceed to
step 608.
[0075] Finally, the present RMS value on line 508 will be
multiplied by the gain (k) 613 in block 612 to produce a control
signal 614. The control signal 614 will be the input to lung
ventilator 54 of FIG. 10.
[0076] FIG. 11 illustrates a possible physical embodiment of steps
604, 605, 606, 608, 611, and 612 of FIG. 5B. A gain controller 620
and a gain multiplier 628 are provided. A first input to gain
controller 620 is the RMS value on line 508, a second input to gain
controller 620 is target drive signal 602, and the output of gain
controller 620 is gain (k) 613 value. The gain (k) 613 value is
then inputted to the gain multiplier 628 where it is multiplied by
the RMS value on line 508 for the present inspiration (step 612 of
FIG. 5B) resulting in a control signal 614.
[0077] The gain controller 620 further comprises a comparator 624
and a gain adjustment block 626. The comparator 624 implements
steps 604 and 605 and the gain adjustment block 626 implements
steps 606, 608 and 611 of FIG. 5B.
[0078] FIG. 10 illustrates a lung ventilator 54 capable of being
controlled by the multiplied, RMS value 614 of the
double-subtracted signal produced in step 612 of FIG. 5B. Although
an air-flow-based pressure ventilator is illustrated as an example
in FIG. 10, it should be kept in mind that the RMS value of the
double subtracted signal can be used for controlling any other lung
ventilator.
[0079] Ventilator 54 shown in FIG. 10 as an illustrative example
only comprises a flow control unit 53, a flow pump 55, a patient's
respiratory (inspiratory and expiratory) implement 56 such as a
mask, a tracheal tube connector, or any other respiratory
implement, a pressure sensor 57, a pressurizing valve 58, and a
depressurizing valve 59.
[0080] The flow pump 55 produces a constant air flow and supply of
this air flow to the patient's respiratory accessory 56 is
controlled through the pressurizing valve 58. The patient is
allowed to breathe out through the respiratory accessory 56 and the
depressurizing valve 59. The pressurizing and depressurizing valves
58 and 59 are controlled by the flow control unit 53.
[0081] The pressure sensor 57 is connected close to the respiratory
implement 56 through a line 60. The pressure sensor 57 produces a
corresponding respiratory pressure representative signal 61
supplied to the flow control unit 53. Accordingly, the pressure
sensor 57 provides feedback of actual respiratory pressure close to
the respiratory implement 56. The flow control unit 53 is also
supplied with the multiplied, RMS value 614 of the
double-subtracted signal delivered on line 62 (FIG. 10) by step 612
of FIG. 5B.
[0082] Those of ordinary skill in the art know that the amplitude
of the multiplied, RMS value 614 of the double-subtracted signal
delivered on line 62 is a representation of the demand to breathe
from the brain.
[0083] When the RMS value 614 supplied to the flow control unit 53
is higher than the amplitude of the pressure representative signal
61, this indicates that the demand to breath from the brain is
higher than the air actually breathed by the patient. Inspiratory
assist is then required and the flow control unit 53 will open
pressurizing valve 58 to supply air flow from the pump 55 to the
patient's respiratory accessory (depressurizing valve 59 being
closed) until the amplitude of the pressure representative signal
61 is equal to the multiplied, RMS value 614. The flow control unit
53 will continue to control the position of valve 58 to maintain
the amplitude of the pressure representative signal 61 equal to the
multiplied, RMS value 614 during all the inspiratory cycle.
[0084] During the inspiratory cycle, when the multiplied, RMS value
614 falls slightly below the amplitude of the pressure
representative signal 61, depressurizing valve 59 can be opened to
correct the situation and maintain the amplitude of the pressure
representative signal 61 equal to the multiplied, RMS value
614.
[0085] When the multiplied, RMS value 614 drops below a given
threshold, this indicates the beginning of an expiratory cycle.
Then, the flow control unit 53 closes pressurizing valve 58 and
opens depressurizing valve 59 to allow the patient to breath out
through the respiratory accessory 56 and the depressurizing valve
59.
[0086] In another example embodiment of the invention, in order to
obtain correct proportionality between the pressure representative
signal 61 and the multiplied, RMS value 614, a gain adjustment is
introduced for example in sensor 57 or on the line 62 to adequately
control pressure assist to the respiratory implement 56 in function
of the multiplied, RMS value 614.
[0087] Accordingly, the subject invention presents a major
advantage over the prior art. Indeed, the degree of inspiratory
assist is adjusted in relation to the real need of the patient. In
other words, assist is proportional to the difference between the
pressure representative signal 61 and the multiplied, RMS value
614. Inspiratory assist is therefore provided only to compensate
for the degree of incapacity of the patient. The patient still
contributes to inspiration as a function of his capacity to prevent
the lung ventilator to further reduce the patient's inability to
breathe. Requiring breathing efforts from the patient usually
accelerates recovery of the patient and faster disconnection of the
patient from the lung ventilator.
[0088] Although the present invention has been described herein
above with reference to preferred embodiments thereof, these
embodiments can be modified at will, within the scope of the
appended claims, without departing from the spirit and nature of
the subject invention.
* * * * *