Note: Descriptions are shown in the official language in which they were submitted.
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A WEANING AND DECISION SUPPORT SYSTEM
FOR MECHANICAL VENTILATION
FIELD OF THE INVENTION
[0001] This invention relates generally to technology for weaning and
treatment of patients on
mechanical ventilation. More specifically, the invention relates to a method
and apparatus for
open or closed loop control of a mechanical ventilator through monitoring and
analysis of patient
and ventilator input data.
BACKGROUND OF THE INVENTION
[0002] Mechanical ventilation is one of the most widely used techniques of
treatment of
patients particularly in the intensive care units of hospitals. Proper
management of patients on
the ventilator and safe and timely weaning and extubation of patients are the
objectives that serve
as key factors in expediting recovery and reducing the mortality and morbidity
risks associated
with the treatment. A system that can provide expert advice to clinicians for
such treatment and
that can automatically and safely wean the patients from the ventilator can be
a valuable tool in
achieving these objectives.
[0003] Attempts to provide expert advice for mechanical ventilation have been
described.
There have been systems for treatment of ARDS patients only. See D. F. Sittig
et al.,
"Implementation of a Computerized Patient Advice System Using the HELP
Clinical
Information System," Computers and Biomedical Research, Vol. 22, pages 474-
487, 1989.
There have also been expert and control systems developed for use in pressure
support
mechanical ventilation mode only. See M. Dojat et al., "Evaluation of a
Knowledge-Based
System Providing Ventilatory Management and Decision for Extubation," American
Journal of
Respiratory and Critical Care Medicine, Vol. 153, pages 997-1004, 1996.
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[0004] However, the previous techniques were developed for use in specific
ventilation modes
or were designed for specific groups of patients. What is needed is an expert
and control system
for a mechanical ventilator that may be customized according to patient needs
to address a
broader field of respiratory conditions and therapies.
SUMMARY OF THE INVENTION
[0005] A method of providing treatment advice for patients on mechanical
ventilation includes
steps of analyzing patient's and ventilator's data, computing and recommending
optimal
ventilatory requirements, suggesting on whether weaning should be started,
continued or
stopped, and providing appropriate warning messages when necessary. A system
can also be
implemented by using an apparatus including sensors for continuous monitoring
of patient data.
[0006] The system may use data representative of carbon dioxide and oxygen
levels of the
patient, the respiratory mechanics data such as airway resistance and
respiratory compliance, the
patient's ideal body weight and temperature, the settings on the ventilator
such as tidal volume,
respiratory rate, positive end-expiratory pressure, the inspiratory to
expiratory time ratio, the
inspired oxygen concentration, the maximum allowed levels of volume and
pressure on the
ventilator for the patient, measured tidal volume and the spontaneous
breathing rate of the patient
as well as the mode of the mechanical ventilator.
[0007] The system is designed to help regulate patient's arterial blood gases
within an
acceptable range, provide patient's data over time to the clinician, and to
expedite weaning from
the ventilator in a safe and timely manner. It provides advice on how much
ventilation the
patient requires and recommends ventilator settings to minimize the
respiratory work rate. It
determines the optimal levels of inspired oxygen fraction and positive end-
expiratory pressure
for the patient. It provides advice on other settings of the ventilator such
as the inspiratory to
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expiratory time ratio and warns the clinician of any untoward condition of the
patient. It
recommends whether weaning should be started, continued or stopped. It has the
capability of
providing advice in a wide range of ventilation modes and for different
patients on mechanical
ventilation.
[0008] A system of the invention can also be implemented by using an apparatus
including
sensors for continuous monitoring of patient data. The system can provide
automatic control of
the ventilator and automatic weaning from the ventilator. This embodiment
includes an
apparatus for monitoring and analysis of the required data. The apparatus may
include a
computing system, digital readable memory coupled to the computing system, and
transducers
providing input signals to the computing system that may represent data such
as airway
resistance, respiratory compliance, spontaneous breathing rate, and measured
tidal volume.
Control of the ventilator may be achieved by providing the control signals to
the ventilator
automatically. This embodiment of the invention can be used for automatic
weaning of the
patient from the ventilator. The automatic weaning procedure will continue
until the patient is
ready for extubation, or if patient's conditions deteriorate, the system
automatically increases the
ventilator's support for the patient and generates a warning message to the
clinician.
In accordance with an aspect of the present invention there is provided an
apparatus for
providing optimized mechanical ventilation to a patient by delivering a
breathing gas, having a
programmable processor executing instructions stored on data storage medium,
the instructions
comprising steps of: (a) receiving patient input data representing respiratory
airway resistance
(RES), respiratory compliance (COMP), and data indicative of patient's ideal
body weight
(WEIGHT); (b) receiving data representing patient oxygen level (P02); (c)
receiving input data
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representing measured spontaneous breathing rate (FSP), tidal volume (VTMS),
inspired oxygen
fraction (FIO), and positive end-expiratory pressure
(PEEP);
(d) determining optimal ventilation level and optimal respiratory rate for the
patient based on
data received in (a); (e) determining a level of controlled ventilation for
the patient based on the
FSP, the VTMS, the optimal ventilation level, and the optimal respiratory
rate; (f) determining
required levels of FIO and PEEP based on the patient oxygen level data
received in (b) and the
patient FIO and PEEP data received in (c); and (g) generating outputs
representing the required
levels of FIO, and PEEP; and the level of controlled ventilation to control
the mechanical
ventilator.
In accordance with another aspect of the present invention there is provided
an
apparatus for optimizing mechanical ventilation of a patient, comprising: a
computing system;
digital readable memory coupled to the computing system; and transducers
providing patient
input signals to the computing system representing respiratory airway
resistance (RES),
respiratory compliance (COMP), and ventilation data representing patient's
spontaneous
breathing rate (FSP), and measured tidal volume (VTMS); wherein the computing
system
executing a program stored in the memory determines output data representing
optimal
ventilation level and optimal respiratory rate of the patient based on data
indicative of patient's
ideal body weight (WEIGHT) stored in the memory and data representing RES and
COMP, and
provides output data representing the optimal ventilation and the optimal
respiratory rate and a
controlled level of mechanical ventilation based on the optimal ventilation
data, the optimal
respiratory rate data, the FSP, and the VTMS.
In accordance with yet another aspect of the present invention there is
provided an
apparatus for providing optimized mechanical ventilation of a patient by
delivering a breathing
3a
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gas, having a processor coupled to the mechanical ventilator that executes
instructions for
performing: (a) receiving patient data representing respiratory airway
resistance (RES) and
respiratory compliance (COMP); (b) receiving patient data indicative of ideal
body weight
(WEIGHT); (c) receiving patient data representing measured oxygen level (P02);
(d) receiving
patient data representing tidal volume (VTMS) and spontaneous breathing rate
(FSP); (e)
receiving patient data representing positive end-expiratory pressure (PEEP);
(f) determining
based on data received in (a) and (b), optimal ventilation and optimal
respiratory rate of the
patient; (g) determining whether weaning should start or continue and a level
of controlled
mechanical ventilation based on the optimal ventilation, the optimal
respiratory rate, and patient
data received in (d); and (h) generating a control signal representing a
controlled ventilation level
based on data from (g) and supplying the control signal to the mechanical
ventilator to
automatically control the level of controlled ventilation of the mechanical
ventilator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For the purpose of illustrating the invention, a presently preferred
form of the invention
is shown in the drawings. However, it is understood that this invention is not
limited to the
precise arrangements and instrumentalities shown.
[0010] FIG. 1 is a block diagram depicting general applications of the
invention.
[0011] FIG. 2 is a block diagram of a Monitor, Analyzer, and Control Apparatus
according to
one embodiment of the invention.
3b
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[0012] FIG. 3 shows a virtual switch board displayed on a computer screen that
provides a user
with initial options for operating software according to the invention.
[0013] FIGS. 4a-4e are a flow chart illustrating one embodiment of a method
according to the
invention, including a preferred sequence of steps executable by a
programmable system
according to the invention.
[0014] FIG. 5 is a process flow diagram of a method for optimizing mechanical
ventilation of a
patient according to the invention.
[0015] FIG. 6 is a process flow diagram of a method for automatically weaning
a patient from
mechanical ventilation according to the invention.
DETAILED DESCRIPTION
[0016] The present invention is designed for a wide range of patients and is
not limited to any
specific mode of ventilation. Unlike previous rule-based techniques, a system
or method
according to the invention operates, or derives many of its rules, on the
basis of the conditions of
individual patients. Therefore the system or method is quite responsive to the
individual
patient's bodily requirements. A system or method according to the invention
may be used to
achieve either open loop or closed loop control. In open loop control, a
clinician may use the
invention as an expert system or decision-support system, and may manually
adjust or operate a
mechanical ventilator based on output automatically provided by the decision-
support system. In
the closed loop scenario, the invention may provide a fully automatic control
system for
monitoring and operating the mechanical ventilator, while still allowing for a
possible manual
override by the clinician.
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[0017] Referring to the drawings, wherein like numerals represent like
elements, there is
illustrated in FIG. 1 a block diagram depicting general application of the
invention. Data from
the mechanical ventilator designated as 10 is provided via switched lines 15
to the Monitor,
Analyzer, and Control apparatus 11. The switch in lines 15 is closed when data
from the
ventilator is sent automatically to apparatus 11. Apparatus 11 also receives
patient data from one
or more transducers through lines 16. Apparatus 11 sends outputs to computer
12 and
communicates back and forth with computer 12 through switched lines 17 which
preferably
represent an RS232 or USB communication port. The control outputs of apparatus
11 are
provided through switched lines 14 to mechanical ventilator 10 when the
invention is used to
automatically control the ventilator.
[0018] Switches in switched lines 14, 15, 16, and 17 may be manual or
automatic switches. If
automatic switches are used, they may include automatic switching means, such
as relays,
transistors, logic gates, and other solid state devices, and may be physically
located at
mechanical ventilator 10, apparatus 11, or computer 12, as desired. In another
embodiment, one
or more of switched lines 14, 15, 16, and 17 may represent a wireless
connection.
[0019] Whether control is manual or automatic, a user interface 13 may be
provided to allow a
human user or clinician to enter data into computer 12 through a communication
line 18. User
interface 13 may be a keyboard, mouse, touchscreen, microphone, or other input
device.
[0020] Computer 12 may be any computing system having a processor coupled to
memory,
and may include operating system software. Computer 12 may be or may include a
customized
digital controller such as an FPGA or ASIC or other processor programmed to
execute the
specific algorithms or methods of the present invention as disclosed herein,
or computer 12 may
be a personal computer system such as a desktop or laptop unit running an
operating system such
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as WindowsTM, Mac OSTM, UnixTM or LinuxTM, and also having loaded in its
memory a custom
software application for executing the algorithms or methods of the present
invention. In one
embodiment, computer 12 may include a display unit such as a CRT or LCD
monitor. The
display unit may display options provided to a clinician by the custom
software, and may display
data received from a patient, data received from mechanical ventilator 10,
data received from
apparatus 11 such as instructions or warnings, or data input by the clinician
through user
interface 13.
[0021] The human operator may control the system using the computer 12 and
user interface
13. At least three preferred modes of application of this invention may be
recognized: (i)
Decision Support mode, (ii) Open Loop manual mode, and (iii) Closed Loop
automatic mode.
[0022] In Decision Support mode, the system is installed on computer 12 and
the invention is
used as a decision support tool.
[0023] In Open Loop manual mode, the system is installed on either computer 12
or on a
processor incorporated in apparatus 11 that receives data from ventilator 10
and the patient on
lines 15 and 16, respectively. There is no automatic control of ventilator 10
by the system of the
invention and lines 14 are open.
[0024] In Closed Loop automatic mode, the system is installed on either
computer 12 or on a
processor incorporated in apparatus 11 that receives data from the ventilator
10 and the patient
on lines 15 and 16, respectively. Ventilator 10 is controlled automatically by
the invention
through preferably lines 14. The human operator supervises the system and can
change the data
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or intervene at any time by using computer 12 and user interface 13. One of
the applications of
this mode is in automatic weaning of the patient from the ventilator. The
control outputs to
ventilator 10 are updated automatically at predefined intervals and weaning is
continued until the
patient is ready for extubation or needs to have increased ventilatory support
again in which case
the system generates a warning message to the clinician and increases the
level of support by
ventilator 10.
[0025] FIG. 2 shows a block diagram of the preferred Monitor, Analyzer, and
Control
apparatus 11. It includes a Digital Processor and Control Unit 20, hereinafter
control unit 20.
This unit may have one or more digital controllers which may preferably be
Micromint Brand
BCC 52 BASIC controllers. The input data from ventilator 10 is applied to an
analog to digital
converter (A/D) board 33 through lines 15. The outputs of A/D 33 are applied
to control unit 20
through lines 37. This data may include ventilatory data such as set tidal
volume, measured tidal
volume, respiratory rate, the spontaneous breathing rate, the peak inspiratory
pressure, the
positive end-expiratory pressure, the inspiratory to expiratory time ratio,
the maximum allowed
levels of volume and pressure, and data indicative of the mode of ventilation.
[0026] Patient oxygen level at 24 is preferably measured by using a transducer
such as a pulse
oximeter 21. Output 27 of pulse oximeter 21 is applied to an A/D converter 30,
and the digital
output from A/D converter 30 is applied to control unit 20 at line 34. The
patient's CO2 level at
may be measured by using a transducer such as an end-tidal CO2 analyzer 22.
Output 28 of
20 analyzer 22 is applied to an A/D converter 31, and the digital output
from A/D converter 31 is
provided at line 35 to control unit 20. The patient's respiratory compliance
and airway resistance
may be measured by using a transducer such as a respiratory mechanics monitor
23, which
receives its inputs from the patient at 26 and provides its outputs at line 29
to an A/D board 32.
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The outputs of A/D board 32 are applied at line 36 to control unit 20. The
control unit 20
communicates with computer 12 through a communication link 17, which is
preferably RS232 or
USB. If the system is used for automatic control of the ventilator, such as in
a closed loop
automatic mode, the output of control unit 20 may be applied at line 38 to one
or more D/A
converters 19, and the outputs of D/A converters 19 may be applied to
ventilator 10 preferably
through lines 14 as shown.
100271 It should be noted that if the system is installed on computer 12 and
computer 12 is
equipped with a software package such as LabViewTM for data collection and
analysis, the
system inputs can be applied to computer 12 and the digital processor and
control unit 20 may
not be needed. Also, if the Monitor, Analyzer, and Control Apparatus 11 is
incorporated in the
Mechanical Ventilator 10, AID boards 33 and D/A boards 19 may not be needed.
It is also clear
that apparatus 11 may not be needed if the system is installed on computer 12
and all the controls
and adjustments are done by a human operator.
[0028] FIG. 3 shows the main options of the software used in a preferred
method of the
invention. In one embodiment, where computer 12 provides a clinician with a
graphical user
interface on a display unit, the main options may be displayed as a virtual
switchboard, as
indicated in the figure. To access these options, a user starts up the
software in an initial step
100. At step 100 at the start of the program, the user is provided with four
options, any one of
which may be directly selected after step 100. From left to right, the first
option 110 is a Help &
Guidance option, which allows a user to view the software guidelines and get
help. The second
option 120 is a Create New Patient File option, which allows a user to create
a new file for a new
patient. The third option 130 is Follow Up on An Existing Patient. This option
allows a user to
continue use of the software for a patient whose personal history may already
be logged in the
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computer memory or who has had prior respiratory therapy on the system
according to the
invention. The fourth option 140 is View Patient's Data, which allows a user
to view a patient's
accumulated data stored in system memory.
[0029] In one embodiment, selecting option 120 allows a user to enter patient
data such as
name, ID number, gender, ideal body weight, etc., and begin respiratory
therapy for the patient.
For example, in Decision Support mode, the user selecting option 120 may then
enter ventilator
data by reading data from the mechanical ventilator 10 and entering that data
into computer 12
via user interface 13. The user may then wait for computer 12 to output an
instruction,
responsive to patient and ventilator input data, for display on the display
unit. The user may
receive the instruction by reading the display unit, and then take appropriate
action, for example,
by adjusting a control on mechanical ventilator 10, adjusting a sensor
connected to the patient, or
by taking some action to stop the respiratory therapy. In Open Loop Manual
mode, the user
selecting option 120 may need only enter patient nominal data such as name,
ideal body weight,
etc. Computer 12 will then automatically provide decision support to the
clinician by displaying
instructions for respiratory therapy, and the clinician may then act on the
instructions by
manually controlling mechanical ventilator 10 in accordance with the
instructions. In Closed
Loop Automatic mode, the user selecting option 120 may need only enter patient
nominal data,
then allow computer 12 to automatically administer respiratory therapy to the
patient (including
weaning the patient) through closed-loop control as described above with
reference to FIGS. 1 &
2, and according to automated processes described below in further detail. In
this mode, the user
may supervise the automatic therapy, and may at any time intervene through
user interface 13.
For existing patients, a user selecting option 130 need not enter nominal data
for the patient, as
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that data may already be logged in computer memory. After selecting option
130, respiratory
therapy or weaning may be administered in any desired mode.
[0030] FIGS. 4a-4e show a process flow chart of one embodiment of a preferred
sequence of
steps executed to carry out a method of the invention. Those skilled in the
art will appreciate
that the sequence of steps may be easily reduced to source code instructions
for input to and
execution by a processor or computer using standard programming languages such
as Basic,
Visual BasicTM, or other languages. As can be seen after the start of the flow
chart at 200, the
input data are read at 202. The input data may include patient and ventilator
data such as
patient's ideal body weight (WEIGHT), body temperature (TEMP), respiratory
airway resistance
(RES), respiratory compliance (COMP), CO2 level (PCO2), arterial oxygen
saturation (SP02),
as well as positive end-expiratory pressure (PEEP), the inspired oxygen
fraction (FIO), total
respiratory rate (F2), the inspiratory to expiratory time ratio (TI/TE), the
spontaneous breathing
rate (FSP), the peak inspiratory pressure (SPN), tidal volume (VTS), measured
tidal volume
(VTMS) which may be spontaneous tidal volume for spontaneously breathing
patients,
maximum allowed tidal volume (VMAX), and maximum allowed pressure (PMAX). Data
indicative of the mode of ventilation (VM) is also provided at this stage
(e.g. VM is 1 in the
pressure control/assist modes and is zero in the volume control/assist modes).
[0031] At step 204 that follows, each data is compared to a predefined
acceptance range and if
it is not within the specified range, its value is rejected and an error
message is generated. In the
next step, 206, the maximum allowed respiratory rate can be calculated as:
FMM = 60/(5xRESxCOMP)
where FMM is the maximum allowed respiratory rate in breaths/minute, RES is
respiratory
airway resistance in cm H20/liter/second, and COMP is respiratory compliance
in liter/cmH20.
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[0032] At the next step, 208, the system checks whether patient's PCO2 data is
provided. This
data can be provided by many different means and methods of measurement known
to those
skilled in the art. For example, a gas analyzer may be used to measure the end-
tidal CO2
pressure. The patient's arterial pressure of CO2 may be obtained as:
PaCO2 = PCO2 +K1
where PaCO2 is the partial pressure of CO2 in the patient's arterial blood,
PCO2 may be the end-
tidal pressure of CO2, and K1 is the difference between Pam and PCO2 which can
also be used
to set a desired Pam level for the patient based on his/her conditions. The
unit for the variables
in this equation can be mmHg.
[0033] If PCO2 data is provided, the next step at 218 is followed in which
alveolar ventilation
is computed. In this computation, if PaCO2 is less than a predefined threshold
level (e.g. 33
mmHg), then the effect of PCO2 on ventilation is set to zero. Otherwise, the
net effect of PCO2
on alveolar ventilation is calculated as:
VAC = C1xPaco2 ¨ C2
where VAC is the ratio of alveolar ventilation as the net effect of Pam to the
resting value of
alveolar ventilation and Cl and C2 are constants (examples of Cl and C2 are
0.405 and 14.878,
respectively, where PaCO2 is in mmHg). Also at step 218, the net effect of
patient's oxygen level
on ventilation is computed. There are many techniques known to those skilled
in the art to
measure a patient's blood oxygen level. If the non-invasive method of pulse
oximetry is used for
such a measurement, the patient's arterial partial pressure of oxygen, Pa02,
may be found from
arterial oxygen saturation data, SP02, from the pulse oximeter as:
Pa02 = ¨141- (SP02) 51+ C3
0.046
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where Paw is in mmHg and C3 is a constant added to shift and correct Paw based
on the patient's
blood pH level. If the patient's blood pH level is in the 7.45-7.55 range, C3
is set to zero.
Otherwise C3 is adjusted by +/- 3.5 mmHg per every -1+ 0.1 deviation in blood
pH from the
above range.
[0034] At step 218, if Pa02 is greater than a predefined value (e.g. 104 mm
Hg), then the net
effect of oxygen on ventilation is zero. Otherwise:
VA0 = (4.72x10-9) x (104-Pao2)4.9
where VA0 is the ratio of alveolar ventilation as the net effect of oxygen to
the resting value of
alveolar ventilation.
[0035] At the end of the calculations in step 218, the total alveolar
ventilation is calculated as:
VALV = (VAO + VAC)xVALV(rest)
where VALV and VALV(rest) are alveolar ventilation and alveolar ventilation at
rest in
liters/min respectively. VALV(rest) may be found as:
VALV(rest) = (0.056333/66)xWEIGHTx60
where WEIGHT is the patient's ideal body weight in Kg. WEIGHT may be input to
the system
or patient's height may be used to determine WEIGHT. If this input is not
provided, a default
value may be used. Also, if ventilation at rest is provided as input, it will
indicate WEIGHT and
works the same way as described above.
[0036] At step 220 that follows, the computed alveolar ventilation found at
step 218 is
compared to a predefined minimum and if it is too low, it's value is
increased.
[0037] At the next step at 222, the patient's respiratory dead space volume is
calculated if not
provided. The following empirical equation may be used to calculate the dead
space volume:
VD = 0.1698x(VALV/60) +0.1587
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where VD is respiratory dead space volume in liters.
100381 In the next step, 224, the optimum respiratory rate to minimize the
respiratory work rate
is calculated as:
¨ K'xVD+ (K'xVD)2 + 4 x K'xRESx1-12 x(VALV)xVD
Fl = 60 x 60
2xRESx1-12xVD
where F 1 is the optimum total respiratory rate in breaths/minute and I(' is
the respiratory
elastance (reciprocal of compliance, 1/COMP). The above equation is a modified
version of an
equation derived for optimum frequency of breathing by A. B. Otis et al.,
"Mechanics of
Breathing in Man," Journal of Applied Physiology, Vol. 2, pages 592-607, 1950.
[0039] In the next step, 226, the computed breathing frequency is compared
with a predefined
minimum rate and the maximum rate found in step 206, and it is adjusted if
found outside the
range. Then at step 228 that follows, minute ventilation is calculated as:
MV = VALV +F 1 x(VD + VED)
where VED is the added dead space due to tubes and connections to the
ventilator and MV is
minute ventilation in liters/minute.
[0040] In the next step at 230, the expiratory time is compared to a minimum
value and the
TI/TE ratio is adjusted if the expiratory time is too short, in order to
prevent build up of intrinsic
PEEP. The minimum expiratory time may be defined as:
TEMIN = 2.5xRESxCOMP
where TEMIN is the minimum expiratory time in seconds.
[0041] Back to step 208, if patient's PCO2 data is not provided, its value is
assumed to be
normal (e.g. 39 mmHg), but the program passes to step 210 in which minute
ventilation, MV, is
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calculated by using the patient's ideal body weight and temperature (if
temperature input is not
provided, a default value such as 37 Celsius may be assumed). The following
equations may be
used for this calculation:
= (6.7/66)xWEIGHT
If TEMP > 37 Celsius, then MV = V + 0.08xV x(TEMP ¨37)
Otherwise if TEMP < 370 Celsius, then MV =
[0042] In the next step at 212, the patient's respiratory dead space is
calculated. This may be
done by using the ideal body weight as:
VD = 0.0026xWEIGHT
[0043] In the next step at 214, the optimum respiratory rate is computed for
minimum
respiratory work rate by using the same equation that was discussed in step
224. In order to
solve the equation, VALV will be substituted by (MV ¨F 1 xVD) in the equation
and since MV
and VD have already been found at steps 210 and 212, the equation can be
solved for Fl by
using an iterative trial and error procedure. Then at the next step, 216, the
computed respiratory
rate is compared to a predefined minimum rate and the maximum rate found in
step 206, and is
adjusted if it is out of range. Then control passes to step 230 in which the
inspiratory to
expiratory time ratio is adjusted if necessary as discussed before.
[0044] After step 230, the program passes to step 232 in which the patient's
oxygen data that
may be the arterial oxygen saturation measured by a pulse oximeter (SPO2) is
examined. At step
234 that follows, SPO2 is compared to a high threshold value (e.g. 0.95). If
SPO2 is greater than
or equal to the high threshold value, at step 236, FIO is compared to a low
value (e.g. 0.25). If
FIO is found to be higher than 0.25 for example, then its value may be reduced
at step 238 as:
New FIO = 0.25 + (FIO -0.25)x0.65
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and then control passes to step 276 which will be described later. Otherwise,
if at step 236, FIO
is not found to be higher than 0.25, its value is not changed at step 240, and
then control passes
to step 276.
[0045] Back to step 234, if SPO2 is less than the high threshold value (e.g.
0.95), the next step
at 242 is performed in which SPO2 is compared to a second threshold value
(e.g. 0.94). If SPO2
is greater than or equal to the second threshold value, the next step at 244
is performed in which
FIO is compared to another relatively low level (e.g. 0.3). For example, if it
is greater than 0.3,
then its value may be reduced at step 246 by:
New FIO = 0.3 + (FIO -0.3)x0.65
and then control passes to step 276. Otherwise, if at step 244, FIO is not
found to be greater than
0.3, its value is not changed in step 248, and then program passes to step
276.
[0046] Back to step 242, if SPO2 is less than the second threshold value (e.g.
0.94), then it is
compared to a third threshold value (e.g. 0.92) at step 250. If SPO2 is
greater than or equal to
the third threshold value, then at the next step at 252, FIO is compared to a
predefined value (e.g.
0.4). For example, if FIO is less than 0.4, then it is raised to 0.4 at step
254 and program passes
to step 276. But if at step 252, FIO is found to be greater than or equal to
0.4, its value is not
changed in step 256, and control is transferred to step 276.
[0047] Back to step 250, if SPO2 is less than the third threshold value (e.g.
0.92), then at the
next step, 258, it is compared to a fourth threshold value (e.g. 0.91). If
SPO2 is found to be
greater than or equal to the fourth threshold value, at the next step at 260,
FIO is compared to
another predefined value (e.g. 0.45). If FIO for example is less than 0.45,
then its value is raised
to 0.45 at step 264 and program passes to step 276. Otherwise FIO is not
changed at step 262
and control transfers to step 276.
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[0048] Back to step 258, if SPO2 is less than the fourth threshold value (e.g.
0.91), then at the
next step at 265 a warning message is generated and in the step that follows
at 266, FIO is
compared to a predefined high value that can be up to 1. In the flow chart a
conservative value
of 0.65 is used for this high FIO value as an example. If FIO is less than
this high value, it is
raised to that level at step 268 and then step 270 is performed in which PEEP
is increased by a
predefined increment (e.g. 2 cm H20), and control passes to step 276. But if
at step 266, FIO is
not found to be less than the predefined high level, its value is not changed
in step 272 that
follows and at the next step, 274, PEEP is increased by a predefined increment
(e.g. 2 cm H20),
and then control passes to step 276.
[0049] At the next step, 276, the "RATIO" of PEEP to FIO is calculated as
follows:
RATIO = PEEP / (FI0x100).
This RATIO is compared to a predefined minimum (e.g. 0.12) at step 278. If it
is less than the
minimum value, PEEP is raised at step 280 and control passes to step 286. If
at step 278,
RATIO is not found to be low, then at step 282 that follows, it is compared to
a predefined
maximum value (e.g. 0.24). If RATIO is greater than the maximum value, PEEP is
reduced at
step 284 and then control transfers to step 286. However, if at step 282,
RATIO is not found to
be too high either, then no adjustment is done to PEEP and control transfers
to step 286. It
should be noted that between two successive increases in PEEP, it is preferred
that a certain time
gap is allowed so that the PEEP change affects the oxygenation status of the
patient. Therefore,
at steps 270, 274, and 280, PEEP is increased only if adequate time has passed
since the last
change in PEEP was made, and the time gap may be specified by the clinician.
Imposition of the
time gap between two successive increases in PEEP is particularly important if
the invention is
used to automatically control the ventilator.
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[0050] At step 286 that follows next, minimum and maximum comparison levels
for
respiratory rate and tidal volume are defined. These levels will be used to
determine the status of
the patient, and whether the patient should be considered for weaning or not.
As examples, these
levels may be defined as:
FIN = 0.45xF1
F2M = 1.8xF1
VTM = 0.7xtidal volume
VTL = 1.6xtidal volume.
[0051] In the above equations, FlN and VTM are minimum comparison levels for
respiratory
rate and tidal volume, and F2M and VTL are maximum comparison levels for
respiratory rate
and tidal volume respectively. It should be noted that "tidal volume" used in
the above equations
may be the data provided by the ventilator as the set tidal volume on the
ventilator (VTS), or
maybe adjusted later based on the new calculated value of optimal tidal
volume. Also, in
ventilation assist modes that all breaths are triggered by the patient such as
pressure support
mode, F 1N may need to be defined at a higher level such as 0.75xF1.
[0052] In the next step at 288, the required tidal volume is calculated as:
tidal volume = MV/F1
[0053] In the next step, 290, the minimum allowed tidal volume and a
comparison level for
minute ventilation maybe defined as:
VTMIN = 2xVD +VED
VREQ = 0.85xMV
[0054] In the above example equations, VTMIN is the minimum allowed tidal
volume, and
VREQ is a comparison value for minute ventilation.
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[0055] In the next step, 292, the calculated tidal volume in step 288 is
compared to VTMIN
and VMAX, and if it is outside this range, its value is adjusted. In the step
that follows at 294,
the total required peak inspiratory pressure in the pressure control/assist
modes is calculated as:
Peak Inspiratory Pressure = [tidal volume/COMP] + PEEP
[0056] Then control passes to step 296 in which the calculated peak
inspiratory pressure is
compared to a maximum allowed pressure which may be defined as (PMAX -8 cm
H20) and a
minimum level that may be defined as PMIN = PEEP + 5 cm H20. If the peak
inspiratory
pressure is not within this range, its value is adjusted at step 296.
[0057] At the next step at 298, the comparison levels for tidal volume, VTM
and VTL, which
were defined in step 286, are redefined preferably in the pressure
control/assist modes of the
ventilator based on the new calculated value of the tidal volume by using the
following example
equations:
VTM = 0.7xtidal volume
VTL = 1.6xtidal volume
[0058] At the step 300 that follows next, the spontaneous breathing rate, FSP,
and the
measured tidal volume, VTMS, are simultaneously examined. If FSP is higher
than or equal to
F2M and VTMS is lower than VTM, then the possibility of dyspnea is detected
and a warning
message is generated at step 302 and control passes to step 318 which will be
described later.
However, if at step 300, FSP is not found to be too high with VTMS being too
low, the next step
at 304 is performed. At this step, measured tidal volume, VTMS, is compared to
VTM. If
VTMS is less than VTM, at step 306, the possibility of apnea is detected, a
warning message is
generated, and then control passes to step 324 which will be described later.
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[0059] However, if at step 304, VTMS is not found to be less than VIM, the
next step at 308
is performed in which FSP is compared to F2M. If FSP is higher than or equal
to F2M, then
rapid breathing is detected at step 310, a warning message is generated, and
control passes to
step 318. However, if at step 308, FSP is not found to be too high, the next
step at 312 is
performed in which measured tidal volume, VTMS, is compared to VTL. If VTMS is
higher
than or equal to VTL, a warning message is generated at the next step, 314,
and program
transfers to step 318. If at step 312, VTMS is found to be less than VTL, then
step 316 is
performed in which multiple conditions are checked. Those conditions may be
the following:
Is FSP greater than FIN and less than F2M?
Is FIO less than a predefined value (e.g. 0.5)?
Is PEEP less than a predefined level (e.g. 6 cm H20)?
Is PaCO2 less than a predefined value (e.g. 53 mmHg)?Is SPO2 greater than
or equal to a minimum safe value (e.g. 0.9)?
[0060] If the answers to all or selected ones among the above questions are
yes at step 316 (for
example for more stable patients, it may be sufficient that only the first
condition be checked)
then at step 328 that follows, weaning is recommended to be started, or
continued if started
already, and a message is generated to convey the recommendation to the
clinician. Next, at step
330, recommendation is made to reduce controlled level of ventilation by a
certain predefined
percentage (e.g. 15% to 20%), and control passes to step 332. Reduction of the
controlled level
of ventilation may be done in different ways. For example, the minute
ventilation supplied by
the ventilator may be reduced as compared to the optimal required minute
ventilation calculated
by the program by a prescribed percentage. Or, as another example, in the
pressure support
mode, the level of pressure support supplied by the ventilator may be reduced
by a certain
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percentage (e.g. 15% to 20%) as compared to the required peak inspiratory
pressure computed by
the program.
[0061] At step 332, the optimal ventilatory parameters for the patient and the
messages
generated by the system are sent to display. If the invention is used to
control the mechanical
ventilator automatically, the required outputs are sent to the ventilator to
control its outputs at
this step. It should be noted that in the automatic weaning mode, while the
output signal to the
ventilator for controlling the level of ventilation such as pressure support
is reduced periodically,
but the control signals for FIO and PEEP are adjusted only according to the
calculated required
values of these outputs and are not subjected to any reduction due to weaning.
100621 In the next step at 334, the system is instructed to wait for a
predetermined period of
time if it is used to control the mechanical ventilator automatically, and
after the wait time is
over, control passes to step 200 at the start of the program and continues
again. Otherwise, if the
invention is not used to control the ventilator automatically, it stops at
step 334 until it is
restarted by the human operator.
[0063] Back to step 316, if the necessary conditions for weaning are not
satisfied, then the next
step at 318 is performed. At step 318, measured minute ventilation is compared
to a minimum
required level, VREQ, which was defined in step 290. If measured ventilation
is lower than
VREQ, possibility of apnea is detected at step 320, a warning message is
generated, and control
passes to step 322. But if at step 318, measured ventilation is not found to
be low, then step 320
is not performed and control is transferred to step 322.
100641 At step 322, the following conditions are checked:
IS PaCO2 greater than or equal to a predefined high value (e.g. 53 mmHg)?
Is SPO2 less than or equal to a predefined low value (e.g. 0.86)?
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CA 02651287 2012-04-27
Is the spontaneous breathing rate too low (e.g. below F 1N)?
[0065] If the answer to any of the above questions at step 322 is yes, then at
the next step at
324, a message is generated that it is too early to start weaning, and if
already started, weaning
should be stopped and the patient should be switched back to full ventilatory
support (e.g. assist
control), and program transfers to step 332. However, if at step 322, the
answers to all questions
are no, then control passes to step 326 in which a message is generated that
weaning should not
be started, but if it has already begun, it may continue with increased
controlled ventilation (e.g.
raised to the previous higher level), and then program passes to step 332 (if
the amount of
ventilation checked at step 318 was not low, controlled ventilation may not
need to be increased
at step 326 if measured tidal volume is higher than the acceptable range).
[0066] In the automatic mode if breaths are patient triggered, in addition to
the above-
described procedure, the system also watches for prolonged apnea, and if no
breath is triggered
by the patient for a predefined period of time (e.g. 25 seconds), it
automatically switches back to
mandatory breathing with full ventilatory support.
[0067] It should be noted, that many of the constant parameters in the
equations described
above were examples for use in the treatment of adult patients which may be
modified for
different patients. Also, if the invention is used for pediatric or neonatal
treatments, many of the
constant parameters will need to be changed.
[0068] With the foregoing apparatus and processes in mind, various other
embodiments of
methods are possible within the scope of the invention. One such embodiment is
a method 500
illustrated in the process flow diagram of FIG. 5. This is a general
application method for
optimizing mechanical ventilation of a patient. The method begins at step 502,
in which patient
input data representing RES, COMP and WEIGHT is received, for example, at the
input
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terminals of a processor such as computer 12. In the next step 504, input data
representing
patient oxygen level P02 is received, for example, from a pulse oximeter or
other transducing
source. In the next step 506, additional input data is received. This input
data may include FSP,
VTMS, FIO, and PEEP, and may be received directly from a mechanical ventilator
such as
ventilator 10.
100691 The next three steps are computational steps, preferably performed
automatically by a
digital processor executing algorithms according to the invention. In step
508, optimal
ventilation level and optimal respiratory rate are determined based on all or
a portion of the
patient input data (RES, COMP, and WEIGHT) that was received in step 502. In
the next step
510, a level of controlled ventilation is determined for the patient based on
FSP, VTMS, the
optimal ventilation level, and the optimal respiratory rate. In the third
computational step 512,
the method determines required levels of FIO and PEEP based on P02 data
received in step 504
and on FIO and PEEP data received in step 506. In the final step 514, one or
more output signals
are generated. These outputs may represent the required levels of FIO, PEEP,
ventilation,
respiratory rate, or the controlled ventilation needed for a next breath of
the patient.
[0070] FIG. 6 illustrates another generalized method according to the
invention. This method
600 may be practiced to determine whether weaning is appropriate for a
patient, and if so, to
provide a controlled way to safely wean the patient from reliance on a
mechanical ventilator.
[0071] Method 600 begins at step 602, which includes providing patient input
data
representing RES and COMP. This data is provided, for example, to a computing
system
executing the control algorithms of the present invention. At the next step
604, additional patient
input data indicative of the patient's weight is provided. This data may be
patient's ideal body
weight, height, ventilation level at rest, or any other data that may be used
as indicative of
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patients's ideal body weight by those skilled in the art. In the next step
606, data for the patient's
oxygen level is provided. In the next step 608 patient input data representing
VTMS and FSP is
provided, and in step 610, patient input data representing PEEP is provided.
Patient data
provided in steps 602-610 may originate from one or more individual
transducers connected to
the patient, from direct outputs received from a mechanical ventilator, or
from a control circuit
that derives the patient data from transducers, ventilator outputs, or user
inputs (the data
indicative of WEIGHT is preferably stored in software).
100721 A calculation step 612 is next in the process sequence. In step 612, an
optimal
ventilation and an optimal respiratory rate are determined for the patient,
based on data provided
in steps 602 & 604. Next, in step 614, a decision is made whether to start
weaning, or whether to
continue weaning, if weaning has already started. If weaning is to start or
continue, step 614 also
determines a controlled ventilation level based on data provided in step 608
and also on results
determined in step 612. Finally, in step 616, a control signal is provided to
the ventilator to
automatically control a next breath of the patient. The control signal may be
based on data from
step 614.
[0073] There has been described a method and apparatus that can be used for
automatic
respiratory control and weaning of patients on mechanical ventilation as well
as a decision
support system in the treatment of such patients. Those skilled in the art
appreciate
modifications to the specific parameters described herein that can be made
without departing
from the spirit of the invention, and such modifications are included in the
invention. The scope
of the claims should not be limited by the preferred embodiments set forth in
the examples, but
should be given the broadest interpretation consistent with the description as
a whole.
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