# Patent

## Title
Method and device for determining a level of ventilatory assist to a patient

## Abstract

A method and device determine a level of mechanical ventilatory assist to be delivered to a patient. A patient's neural respiratory drive is measured and multiplied by an existing amplification factor to obtain an existing predicted ventilatory pressure. A mechanical ventilator is controlled accordingly. An existing resulting pressure is measured. The patient's neural respiratory drive is multiplied by a modified amplification factor to obtain a new predicted pressure. The existing and new predicted pressures are compared to determine an anticipated change in pressure. The mechanical ventilator is now controlled according to the new predicted pressure. A new resulting pressure is measured. The existing and new resulting pressures are compared to determine an actual change in pressure. The anticipated and actual changes in pressure are compared. The amplification factor is increased, maintained or decreased in response to the comparison between the anticipated and actual changes in pressure.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of U.S. Provisional Application No. 61/193,987, filed Jan. 15, 2009, and is a U.S. national phase of PCT International Application No. PCT/CA2010/000043, filed on Jan. 14, 2010, both applications are incorporated herein by reference.


FIELD
The present invention is concerned with determining a level of ventilatory assist to be delivered a patient.
BACKGROUND
In practice, an adequate level of ventilatory assist to be delivered to a patient under mechanical ventilation is difficult to determine since unloading of the patient involves compensation for increased respiratory demand in terms of metabolic drive, resistive and elastic loads imposed by the patient's respiratory system, as well as weakness of the inspiratory muscles. There is therefore a need for a technique that facilitates such determination.
SUMMARY
The present invention relates to a method for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator in response to a measure of a patient's neural respiratory drive multiplied by an amplification factor, comprising: calculating an existing predicted ventilatory assist pressure; measuring an existing resulting pressure delivered to the patient by the mechanical ventilator; changing the amplification factor from an existing amplification factor to a new amplification factor; calculating a new predicted ventilatory assist pressure using the new amplification factor; measuring a new resulting pressure delivered to the patient by the mechanical ventilator after the amplification factor has been changed from the existing amplification factor to the new amplification factor; comparing the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator; comparing the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; comparing the anticipated change in pressure with the actual change in pressure; and delivering a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.
The present invention is also concerned with a device for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator in response to a measure of a patient's neural respiratory drive multiplied by an amplification factor, comprising: a first calculator of an existing predicted ventilatory assist pressure; a first sensor of an existing resulting pressure delivered to the patient by the mechanical ventilator; a modifier of the amplification factor from an existing amplification factor to a new amplification factor; a second calculator of a new predicted ventilatory assist pressure using the new amplification factor; a second sensor of a new resulting pressure delivered to the patient by the mechanical ventilator after the amplification factor has been changed from the existing amplification factor to the new amplification factor; a first comparator of the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator; a second comparator of the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; a third comparator of the anticipated change in pressure and the actual change in pressure; and a third calculator of a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the method and device for determining a level of ventilatory assist will become more apparent from reading of the following non restrictive description of illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings in which:
FIG. 1 is a schematic flow chart of a method for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor; and
FIG. 2 is a schematic block diagram of a device for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor.



DETAILED DESCRIPTION
The method and device for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor will now be described.
The level of ventilatory assist to the patient may be controlled by applying to the mechanical ventilator a control signal representative of a predicted ventilatory assist pressure Ptarget determined as a function of the patient's neural respiratory drive and an amplification factor AF. For example, the electrical activity of the patient's diaphragm EAdi may be measured as the patient's neural respiratory drive and may be multiplied by the amplification factor AF to describe the predicted ventilatory assist pressure Ptarget.
An example of technique for measuring the electrical activity of the patient's diaphragm EAdi is described in U.S. Pat. No. 5,671,752 (Sinderby et al.) issued on Sep. 30, 1997. Although in the herein described embodiment the electrical activity of the patient's diaphragm EAdi is used as a representation of the patient's neural respiratory drive, it should be kept in mind that any other signal, for example the electrical activity of another respiratory related muscle can be used as the representation of the patient's neural respiratory drive.
The method and device for determining the level of ventilatory assist to be delivered to the patient uses a prediction (predicted ventilatory assist pressure Ptarget) of the pressure to be delivered to the patient by the mechanical ventilator in response to a given change of the amplification factor AF and a comparison between the predicted ventilatory assist pressure Ptarget and a resulting pressure Presult actually delivered to the patient by the mechanical ventilator in response to the given change of the amplification factor AF. For example, if the measured electrical activity of the patient's diaphragm EAdi is 10 μV and the amplification factor AF is increased from 0 cm H2O/μV to 1 cm H2O/μV, it can be expected that the predicted ventilatory assist pressure Ptarget will be 10 cm H2O. If the resulting pressure Presult turns out to be 10 cm H2O, this means that the patient has absorbed/accepted the whole ventilatory assist with no reduction of the patient's neural respiratory drive since the electrical activity of the patient's diaphragm EAdi remained unchanged. If, on the other hand, the electrical activity of the patient's diaphragm EAdi reduced from 10 μV to 1 μV, the resulting pressure Presult becomes 1 cm H2O although the predicted ventilatory assist pressure Ptarget was 10 cm H2O, showing that the patient does not require the increase of ventilatory assist and rather down regulates his/her neural respiratory drive (EAdi).
If the patient already receives ventilatory assist and, for example, the amplification factor AF is 1 and the electrical activity of the patient's diaphragm EAdi is 10 μV, it is possible to calculate an expected increase of the predicted pressure Ptarget if, for example, the amplification factor AF is increased to 1.5. The predicted ventilatory assist pressure will then be Ptarget=1.5 cm H2O/μV×10 μV=15 cm H2O. If the increase of the amplification factor AF from 1.0 cm H2O/μV to 1.5 cm H2O/μV increases the resulting pressure Presult from 10 cm H2O to 15 cm H2O, this indicates that the patient welcome the increase of ventilatory assist (increase in pressure) and maintains his/her neural respiratory drive (EAdi).
If, on the other hand, the amplification factor AF is increased from 1.0 cm H2O/μV to 1.5 cm H2O/μV but the resulting pressure Presult increases from 10 cm H2O to 12 cm H2O while the predicted ventilatory assist pressure Ptarget indicates a 5 cm H2O increase, the patient does not welcome the whole increase in ventilatory assist and rather down regulates his/her neural respiratory drive (EAdi).
The method 100 and device 200 for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of a patient's neural respiratory drive multiplied by the amplification factor will now be described with reference to FIGS. 1 and 2.
Operation 101 (FIG. 1)
At a stable level of ventilatory assist, a calculator 201 (FIG. 2) calculates an existing predicted ventilatory assist pressure Ptarget old by multiplying the existing amplification factor AFold by the existing electrical activity of the patient's diaphragm EAdiold, in accordance with the following relation:

Ptarget old=AFold·EAdiold  (1)

Operation 102 (FIG. 1)
At a stable level of ventilatory assist, a pressure sensor 202 (FIG. 2) measures the existing resulting pressure Presult old delivered to the patient by the mechanical ventilator. This can be made, for example, by measuring the pressure in the mechanical ventilator or mechanical ventilator circuit.
When the neurally controlled ventilator system has been operated at a given amplification factor for a certain time, the existing predicted pressure Ptarget old and the existing resulting pressure Presult old will assume very similar if not the same values.
Operation 103 (FIG. 1)
A modifier 203 (FIG. 2) of amplification factor, for example a keyboard, is used by the medical personnel, for example a caregiver, to change (increase or decrease) an existing amplification factor AFold to a new amplification factor AFnew.
Alternatively, the modifier 203 can be implemented by an automatic computerized modifier requiring no intervention from the medical personnel.
Operation 104 (FIG. 1)
A calculator 204 (FIG. 2) calculates a new predicted ventilatory assist pressure Ptarget new by multiplying the new amplification factor AFnew by the existing electrical activity of the patient's diaphragm EAdiold, in accordance with the following relation:

Ptarget new=AFnew·EAdiold  (2)

Operation 105 (FIG. 1)
A comparator 205 (FIG. 2) compares the new predicted ventilatory assist pressure Ptarget new to the existing predicted ventilatory assist pressure Ptarget old using, for example, the following relation:

ΔPtarget new-old=Ptarget new−Ptarget old  (3)

to show an anticipated change (increase or decrease) in pressure (ΔPtarget new-old) that will be delivered to the patient by the mechanical ventilator.

Operation 106 (FIG. 1)
A pressure sensor 206 (FIG. 2) measures the new resulting pressure Presult new actually delivered to the patient by the mechanical ventilator following change from the existing amplification factor AFold to the new amplification factor AFnew. This can be made, for example, by measuring the pressure in the mechanical ventilator or mechanical ventilator circuit.
Operation 107 (FIG. 1)
A comparator 207 (FIG. 2) compares the new resulting pressure Presult new to the existing resulting pressure Presult old using, for example, the following relation:

ΔPresult new-old=Presult new−Presult old  (4)

to show an actual change (increase or decrease) in pressure (ΔPresult new-old) delivered to the patient by the mechanical ventilator.

Operation 108 (FIG. 1)
A comparator 208 (FIG. 2) compares the anticipated change (increase or decrease) in pressure ΔPtarget new-old to the actual change (respectively increase or decrease) in pressure ΔPresult new-old using, for example, calculation of the following ratio:

ΔPresult new-old/ΔPtarget new-old  (5)

to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator. The ratio ΔPresult new-old/ΔPtarget new-old will range between 0 and 1, such that the response to change (increase or decrease) of the amplification factor AF can be divided into classes as described hereinafter.

When the amplification factor is increased in operation 103 (FIG. 1), a ratio ΔPresult new-old/ΔPtarget new-old equal to 1 suggests that the increase in ventilatory assist from the mechanical ventilator was welcomed by the patient, whereas a ratio ΔPresult new-old/ΔPtarget new-old of 0.5 suggests that only 50% of the increase in ventilatory assist delivered by the mechanical ventilator was welcomed by the patient and that the patient down regulated his/her neural respiratory drive (EAdi).
Operation 109 (FIG. 1)
A calculator 209 (FIG. 2) includes a decision algorithm implemented to take a decision as to increase or decrease the amplification factor AF for example as described hereinafter.
When the amplification factor AF is increased in operation 103 (FIG. 1), the ratio ΔPresult new-old/ΔPtarget new-old is positive. For example, the decision can then be as follows:

    
    
        In response to a ratio ΔPresult new-old/ΔPtarget new-old=0.8 to 1, the decision algorithm of the calculator 209 (FIG. 2) delivers a decision to further increase the amplification factor AF;
        In response to a ratio ΔPresult new-old/ΔPtarget new-old=0.5 to 0.8, the decision algorithm of the calculator 209 (FIG. 2) delivers a decision to maintain the amplification factor AF unchanged; and
        In response to a ratio ΔPresult new-old/ΔPtarget new-old.<0.5, the decision algorithm of the calculator 209 (FIG. 1) delivers a decision to decrease the amplification factor AF.
    
    


When the amplification factor AF is decreased in operation 103 (FIG. 1), the ratio ΔPresult new-old/ΔPtarget new-old is negative. For example, the decision can then be as follows:

    
    
        In response to a ratio ΔPresult new-old/ΔPtarget new-old=−0.8 to −1, the decision algorithm of the calculator 209 (FIG. 2) delivers a decision to increase the amplification factor AF;
        In response to a ratio ΔPresult new-old/ΔPtarget new-old=−0.5 to −0.8, the decision algorithm of the calculator 209 (FIG. 2) delivers a decision to maintain the amplification factor AF unchanged; and
        In response to a ratio ΔPresult new-old/ΔPtarget new-old>−0.5, the decision algorithm of the calculator 209 (FIG. 2) delivers a decision to further decrease the amplification factor AF.
    
    


If the decision algorithm of the calculator 209 (FIG. 2) requests a change (increase or decrease) of the amplification factor, the complete, above described procedure including operations 101-109 is then repeated.
In operation 103 (FIG. 1), the modifier 203 (FIG. 2) of amplification factor will be used by the medical personnel to change (increase or decrease) or automatically changes (increase or decrease) the amplification factor AF as indicated. As a non limitative example, the modifier 203 (FIG. 2) may be designed to automatically change the amplification factor AF as a function of the ratio ΔPresult new-old/ΔPtarget new-old according to a predetermined relation, for example by predetermined steps. Alternatively, it can be left to the medical personnel to decide to what extent the amplification factor should be changed through the modifier 203 (FIG. 2).
The calculator 201, calculator 204, comparator 205, comparator 207, comparator 208 and calculator 209 can be implemented by a computer or computers, for example a single, suitably programmed general purpose computer.

## Claims

1. A method for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator, comprising:
measuring a patient's neural respiratory drive;
using a first calculator to multiply the patient's neural respiratory drive by an existing value of an amplification factor to obtain an existing predicted ventilatory assist pressure;
applying to the mechanical ventilator a first control signal representative of the existing predicted ventilatory assist pressure;
measuring an existing resulting pressure delivered to the patient by the mechanical ventilator as a result of the first control signal;
using a modifier to increase or decrease the amplification factor to obtain a new value of the amplification factor;
using a second calculator to multiply the patient's neural respiratory drive by the new value of the amplification factor to obtain a new predicted ventilatory assist pressure;
applying to the mechanical ventilator a second control signal representative of the new predicted ventilatory assist pressure;
measuring a new resulting pressure delivered to the patient by the mechanical ventilator as a result of the second control signal;
using a first comparator to compare the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator;
using a second comparator to compare the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator;
using a third comparator to compare the anticipated change in pressure with the actual change in pressure; and
using a third calculator to deliver a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.

2. A method for determining a level of ventilatory assist as defined in claim 1, wherein measuring the existing resulting pressure comprises measuring a pressure in patient's airways.

3. A method for determining a level of ventilatory assist as defined in claim 1, wherein comparing the new predicted ventilatory assist pressure Ptarget new and the existing predicted ventilatory assist pressure Ptarget old comprises using the following relation:


ΔPtarget new-old=Ptarget new−Ptarget old 


to show an anticipated change in pressure ΔPtarget new-old that will be delivered to the patient by the mechanical ventilator.

4. A method for determining a level of ventilatory assist as defined in claim 3, wherein:
comparing the new resulting pressure Presult new and the existing resulting pressure Presult old comprises using the following relation:


ΔPresult new-old=Presult new−Presult old 



to show an actual change in pressure ΔPresult new-old delivered to the patient by the mechanical ventilator; and
comparing the anticipated change in pressure ΔPtarget new-old to the actual change in pressure ΔPresult new-old comprises calculating a ratio:


ΔPresult new-old/ΔPtarget new-old 



to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator.

5. A method for determining a level of ventilatory assist as defined in claim 4, wherein:
the amplification factor is increased;
the ratio ΔPresult new-old/ΔPtarget new-old is positive;
delivering a decision comprises:
in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in an upper range, delivering a decision to further increase the amplification factor;
in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in a middle range, delivering a decision to maintain the amplification factor unchanged; and
in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in a lower range, delivering a decision to decrease the amplification factor.

6. A method for determining a level of ventilatory assist as defined in claim 4, wherein:
the amplification factor is decreased;
the ratio ΔPresult new-old/ΔPtarget new-old is negative;
delivering a decision comprises:
in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in a lower range, delivering a decision to increase the amplification factor;
in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in a middle range, delivering a decision to maintain the amplification factor unchanged; and
in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in an upper range, delivering a decision to further decrease the amplification factor.

7. A method for determining a level of ventilatory assist as defined in claim 1, wherein measuring the new resulting pressure comprises measuring a pressure in patient's airways.

8. A method for determining a level of ventilatory assist as defined in claim 1, wherein comparing the new resulting pressure Presult new and the existing resulting pressure Presult old comprises using the following relation:


ΔPresult new-old=Presult new−Presult old 


to show an actual change in pressure ΔPresult new-old delivered to the patient by the mechanical ventilator.

9. A method for determining a level of ventilatory assist as defined in claim 1, wherein the patient's neural respiratory drive is represented by the electrical activity of the patient's diaphragm.

10. A device for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator, comprising:
means for measuring a patient's neural respiratory drive;
a first calculator for multiplying the patient's neural respiratory drive by an existing value of an amplification factor to obtain an existing predicted ventilatory assist pressure used as a first control signal applied to the mechanical ventilator;
a sensor of an existing resulting pressure delivered to the patient by the mechanical ventilator as a result of the first control signal;
a modifier of the amplification factor configured to provide a new value of the amplification factor;
a second calculator for multiplying the patient's neural respiratory drive by the new value of the amplification factor to obtain a new predicted ventilatory assist pressure used as a second control signal applied to the mechanical ventilator;
a sensor of a new resulting pressure delivered to the patient by the mechanical ventilator as a result of the second control signal;
a first comparator of the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator
a second comparator of the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator;
a third comparator of the anticipated change in pressure and the actual change in pressure; and
a third calculator of a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.

11. A device for determining a level of ventilatory assist as defined in claim 10, wherein the sensor of the existing resulting pressure comprises a sensor of a pressure in patient's airways.

12. A device for determining a level of ventilatory assist as defined in claim 10, wherein the first comparator of the new predicted ventilatory assist pressure Ptarget new and the existing predicted ventilatory assist pressure Ptarget old uses the following relation:


ΔPtarget new-old=Ptarget new−Ptarget old 


to show an anticipated change in pressure ΔPtarget new-old that will be delivered to the patient by the mechanical ventilator.

13. A device for determining a level of ventilatory assist as defined in claim 12, wherein:
the second comparator of the new resulting pressure Presult new and the existing resulting pressure Presult old uses the following relation:


ΔPresult new-old=Presult new−Presult old 



to show an actual change in pressure ΔPresult new-old delivered to the patient by the mechanical ventilator; and
the third comparator of the anticipated change in pressure ΔPtarget new-old to the actual change in pressure ΔPresult new-old calculates a ratio:


ΔPresult new-old/ΔPtarget new-old 



to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator.

14. A device for determining a level of ventilatory assist as defined in claim 13, wherein:
the modifier is configured to increase or decrease the amplification factor so that, when the amplification factor is increased, the ratio ΔPresult new-old/ΔPtarget new-old is positive; and
the third calculator is configured to deliver, when the ratio ΔPresult new-old/ΔPtarget new-old is positive:
a decision to further increase the amplification factor in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in an upper range;
a decision to maintain the amplification factor unchanged in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in a middle range; and
a decision to decrease the amplification factor in response to the ratio ΔPresult new-old/ΔPtarget new-old situated in a lower range.

15. A device for determining a level of ventilatory assist as defined in claim 13, wherein:
the modifier is configured to increase or decrease the amplification factor so that, when the amplification factor is decreased, the ratio ΔPresult new-old/ΔPtarget new-old is negative; and
the third calculator is configured to deliver, when the ratio ΔPresult new-old/ΔPtarget new-old is negative:
a decision to increase the amplification factor in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in a lower range;
a decision to maintain the amplification factor unchanged in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in a middle range; and
a decision to further decrease the amplification factor in response to a ratio ΔPresult new-old/ΔPtarget new-old situated in an upper range.

16. A device for determining a level of ventilatory assist as defined in claim 10, wherein the sensor of the new resulting pressure comprises a sensor of a pressure in patient's airways.

17. A device for determining a level of ventilatory assist as defined in claim 10, wherein the second comparator of the new resulting pressure Presult new and the existing resulting pressure Presult old uses the following relation:


ΔPresult new-old=Presult new−Presult old 


to show an actual change in pressure ΔPresult new-old delivered to the patient by the mechanical ventilator.

18. A device for determining a level of ventilatory assist as defined in claim 10, wherein the patient's neural respiratory drive is represented by the electrical activity of the patient's diaphragm.