Method and Apparatus for Lung Volume Estimation

• US 20080289628A1
• Filed: 12/06/2005
• Published: 11/27/2008
• Est. Priority Date: 12/06/2005
• Status: Active Grant

• Current Assignee: Maquet Critical Care AB (Getinge AB)
• Original Assignee: Maquet Critical Care AB (Getinge AB)
• Inventors: Loncar, Mario, Ahlmen, Christer, Emtell, Par, Hallback, Magnus
• Granted Patent: US 8,371,298 B2
• Time in Patent Office: Days
• US Class Current: 128/203.12
• CPC Class Codes: A61B 5/0813 Measurement of pulmonary parameters by tracers, e.g. radioactive tracers
• CPC Class Codes: A61B 5/091 Measuring volume of inspired or expired gases, e.g. to determine lung capacity
• CPC Class Codes: A61M 16/0051 with alarm devices
• CPC Class Codes: A61M 16/024 including calculation means, e.g. using a processor
• CPC Class Codes: A61M 2230/43 Composition of exhalation
• CPC Class Codes: A61M 2230/435 partial O2 pressure (P-O2)

Abstract

In a method, ventilator and ventilator control unit for determining an end-expertorial lung volume (EELV) for a mechanically ventilated patient, a breathing gas is provided to the patient that has a first fixed N2/O2 gas composition, at least until the N2/O2 gas composition in air expired from the patient is constant. At least once, at a first predetermined point in time, the N2/O2 gas composition in the breathing gas is changed to a second fixed composition. The change in the N2/O2 gas composition exhaled by the patient for each breath is measured until a second point in time at which the level of expired O₂ in at least two subsequent breadths is substantially stable. The measurement is made downstream of the expiratory tube of the ventilator. The total gas volume is determined for each breath, and the EELV of the patient'"'"'s lungs is determined based on the change in O₂ level between the first and second points in time.

Claims

1. A ventilator for ventilation of a patient, comprising an inhalation gas circuit that provides breathing air comprising a mixture of N2 and O2 to the patient and an exhalation gas circuit that receives exhaled air from the patient, and a gas outlet for said exhaled air, and a control unit configured to calculate the end expiratory lung volume, EELV, of a patient being ventilated by the ventilator, based on the expired volume per breathing cycle, and a composition measuring unit that measures a quantity that varies linearly with the N2/O2 composition in the expired gas, said measuring unit being arranged near the gas outlet, the control unit being configured to determine the change in the N₂/O₂ composition in the expired gas between a first point in time (t₁) at which the N2/O2 composition of the breathing gas is changed and a second point in time (t₂) at which the N2/O2 composition of the expired gas in at least two subsequent breaths is substantially stable, based on said measurements, said control unit being further configured to calculate, for each breath between the first and the second points in time (t₁, t₂), the change in the N2/O2 composition exhaled and the total gas volume.

2. A ventilator according to claim 1, wherein the control unit is configured to calculate the end expiratory lung volume, EELV, according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( x n - x after ) x before - x after - V system

where

VCe_n is the volume of expired gas at breath number n,

x is the quantity that varies linearly with the expired N2/O2 composition, and

x_n is the value of the quantity at breath number n in each breath between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆),

x_before is the value of the quantity before the first point in time (t₁;

t₃;

t₅),

x_after is the value of the quantity at or just after the second point in time (t₂;

t₄, t₆).

3. A ventilator according to claim 1, wherein the control unit is arranged to calculate the end expiratory lung volume, EELV, according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( O 



2 

mix n - O 



2 

mix after ) O 



2 

mix before - O 



2 

mix after - V system

where

VCe_n is the volume of expired gas at breath number n,

O2mix_n is the level of O₂ in the expired gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆)

O2mix_after is the level of O₂ in the expired gas at a breath just after the second point in time (t₂;

t₄, t₆) and

O2mix_before is the level of O2 in the expired gas before the first point in time (t₁;

t₃;

t₅).

4. A ventilator according to claim 1, wherein the control unit is configured to calculate the EELV according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( ρ

n - ρ

after ) ρ

before - ρ

after - V system

where

ρ

_n is the density of the gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆)

ρ

_before is the density of the gas at the first point in time (t₁;

t₃, t₅)

ρ

_after is the density of the gas at or just after the second point in time (t₂;

t₄, t₆).

5. A ventilator according to claim 4, wherein the control unit is configured to correct the values ρ

_n, ρ

_before and ρ

_after for temperature before calculating the EELV, to eliminate the risk of a temperature induced drift.

6. A ventilator according to claim 1, wherein the control unit is configured to control the O2 level in the breathing gas supplied from the ventilator to the patient to change the O2 level in the breathing gas at the first point in time (t₁;

t₃, t₅).

7. A ventilator according to claim 1, further comprising measuring unit that measures the volume flow of expired gas, said measuring unit being arranged near the gas outlet.

8. A ventilator according to claim 1, wherein the composition measuring unit comprises at least one ultrasonic sensor which is also used to measure both the volumetric flow and mass flow of the gas.

9. A control unit for a ventilator, said control unit being configured to calculate the end expiratory lung volume, EELV, of a patient being ventilated by the ventilator, based on the expired volume per breathing cycle, the change in N2/O2 composition in the expired gas between a first point in time (t₁) at which the N2/O2 composition of the breathing gas is changed and a second point in time (t₂) at which the N2/O2 composition of the expired gas in at least two subsequent breaths is substantially stable, said control unit being further configured to calculate, for each breath between the first and the second points in time (t₁, t₂), the change in the N2/O2 composition exhaled in each breath and the total gas volume of each breath.

10. A control unit according to claim 9, wherein the control unit is arranged to calculate the end expiratory lung volume, EELV, according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( x n - x after ) x before - x after - V system

where

VCe_n is the volume of expired gas at breath number n,

x is a quantity that varies linearly with the expired N2/O2 composition, and

x_n is the value of the quantity at breath number n in each breath between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆),

x_before is the value of the quantity at the first point in time (t₁;

t₃, t₅),

x_after is the value of the quantity at or just after the second point in time (t₂;

t₄, t₆).

11. A control unit according to claim 9 configured to calculate the end expiratory lung volume, EELV, according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( O 



2 

mix n - O 



2 

mix after ) O 



2 

mix before - O 



2 

mix after - V system

where

VCe_n is the volume of expired gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆),

O2mix_n is the level of O₂ in the expired gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆)

O2mix_after is the level of O₂ in the expired gas at a breath just after the second point in time (t₂;

t₄, t₆) and

O2mix_before is the level of O₂ in the expired gas before the first point in time (t₁;

t₃;

t₅).

12. A control unit according to claim 9, configured to calculate the EELV based on flow data received from an ultrasonic sensor.

13. A control unit according to claim 12, configured to calculate the EELV according to

$\mathrm{EELV}=\frac{\sum _{}^{}}{}$

n = 1 N 



VCe n ·

( ρ

n - ρ

after ) ρ

before - ρ

after - V system

where

ρ

_n is the density of the gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆)

ρ

_before is the density of the gas at the first point in time (t₁;

t₃, t₅)

ρ

_after is the density of the gas at or just after the second point in time (t₂;

t₄, t₆).

14. A control unit according to claim 13, configured to correct the values ρ

_n, ρ

_before and ρ

_after for temperature before calculating the EELV, to eliminate the risk of a temperature induced drift

15. A control unit according to claim 13 further configured to control the O2 level in the breathing gas supplied from the ventilator to the patient to change the O2 level in the breathing gas at the first point in time (t₁;

t₃, t₅).

16. A method of determining an end-expiratory lung volume, EELV, for a mechanically ventilated patient, where a breathing gas is provided to the patient through an inspiratory tube and removed from the patient through an expiratory tube, said method comprising the following steps:

providing a breathing gas comprising a first fixed N2/O2 gas composition to the patient at least until the N2/O2 gas composition in air expired from the patient is constant;

changing, at least once, at a determined point in time (t₁;

t₃, t₅), the N2/O2 gas composition to a second fixed composition in the breathing gas;

measuring the change in N2/O2 gas composition exhaled by the patient for each breath until a point in time (t₂;

t₄, t₆) at which the level of expired O₂ in at least two subsequent breaths is substantially stable, said measurement being made downstream of the expiratory tube;

determining the total gas volume of each breath; and

determining the EELV of the patient'"'"'s lungs based on the change in O₂ level between the first and second points in time (t₁, t₂;

t₃, t₄;

t₅, t₆).

17. A method according to claim 16, comprising changing the N2/O2 composition by reducing the level of O2.

18. A method according to claim 16, comprising changing the N2/O2 composition by increasing the level of O2.

19. A method according to claim 16 comprising changing the level of O2 by changing the level of O2 by a unit of between 5% and 35% of the total volume.

20. A method according to claim 19 comprising determining the total gas volume of each breath using a volume sensor downstream of the expiratory tube.

21. A method according to claim 16 comprising determining EELV according to the following equation:

$\mathrm{EELV}=\frac{\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br><br><br>{\mathrm{VCe}}_n·<br><br>\left(x_n-x_{\mathrm{after}}\right)}{x_{\mathrm{before}}-x_{\mathrm{after}}}-V_{\mathrm{system}}$

where

VCe_n is the volume of expired gas at breath number n,

x is a quantity that varies linearly with the expired N2/O2 composition, and

x_n is the value of the quantity at breath number n in each breath between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆),

x_before is the value of the quantity at the first point in time (t₁;

t₃, t₅),

x_after is the value of the quantity at or just after the second point in time (t₂;

t₄, t₆).

22. A method according to claim 16 comprising determining EELV according to the following equation:

$\mathrm{EELV}=\frac{\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br><br><br>{\mathrm{VCe}}_n·<br><br>\left(O<br><br><br><br>2<br><br>{\mathrm{mix}}_n-O<br><br><br><br>2<br><br>{\mathrm{mix}}_{\mathrm{after}}\right)}{O<br><br><br><br>2<br><br>{\mathrm{mix}}_{\mathrm{before}}-O<br><br><br><br>2<br><br>{\mathrm{mix}}_{\mathrm{after}}}-V_{\mathrm{system}},$

where

VCe_n is the volume of expired gas at breath number n,

O2mix_before is the level of O2 in the expired gas at the first point in time (t₁;

t₃, t₅),

O2mix_n is the level of O2 in the expired gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆), and

O2mix_after is the level of O₂ in the expired gas at a breath just after the second point in time (t₂;

t₄, t₆).

23. A method according to claim 16 comprising determining EELV according to the following equation:

$\mathrm{EELV}=\frac{\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br><br><br>{\mathrm{VCe}}_n·<br><br>\left({\mathrm{\rho <br><br>}}_n-{\mathrm{\rho <br><br>}}_{\mathrm{after}}\right)}{{\mathrm{\rho <br><br>}}_{\mathrm{before}}-{\mathrm{\rho <br><br>}}_{\mathrm{after}}}-V_{\mathrm{system}},$

where

ρ

_n is the density of the gas at breath number n between the first and second points in time (t₁, t₂;

t₃, t₄, t₅, t₆)

ρ

_before is the density of the gas at the first point in time (t1;

t3, t5)

ρ

_after is the density of the gas at or just after the second point in time (t2;

t4, t6).

24. A method according to claim 23, further comprising the step of correcting the values ρ

_n, ρ

_before and ρ

_after for temperature before calculating the EELV, to eliminate the risk of a temperature induced drift.