System, ventilator and method for real-time determination of a local strain of a lung during artificial ventilation

Abstract

The present invention relates to a system for real-time determination of a local strain of a lung during artificial ventilation. The system comprises a device for electrical impedance tomography (EIT), which device is configured to capture an electrical impedance distribution along at least one two-dimensional section through a human thorax, and further comprises a device for assigning the captured electrical impedance distribution, which device is configured to divide the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels and to assign a specific value of the electrical impedance at a specific time to a specific EIT pixel.

Claims

1. A system for real-time determination of a local strain of a lung during artificial ventilation using a ventilator, wherein the system comprises a device for electrical impedance tomography (EIT), which device is configured to capture an electrical impedance distribution along at least one two-dimensional section through a human thorax, and further comprises a device for assigning the captured electrical impedance distribution, dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels and assigning a specific value of the electrical impedance at a specific time to a specific EIT pixel; wherein the system further comprises a device for determining a local strain value which device is configured to determine at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and an end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel, to form a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between the end-inspiratory electrical impedance (Z.sub.INSP) and the end-expiratory electrical impedance (Z.sub.EXSP), in particular by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP), and to form a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing the local tidal volume reference value (ΔZ) by the end-expiratory electrical impedance (Z.sub.EXSP); and wherein the device for determining a local strain value is further configured to form a relative strain value (STR.sub.RELATIV) of the specific EIT pixel by normalizing the preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF).

2. The system of claim 1, wherein the device for determining a local strain value is configured to normalize the preliminary strain value (STR.sub.VORL) to a quotient of the local tidal volume reference value (ΔZ) and an end-expiratory electrical impedance (Z.sub.EXSP) of a specific EIT pixel at a specified positive end-expiratory pressure (PEEP).

3. The system of claim 1, wherein the device for electrical impedance tomography (EIT) is coupled to the ventilator or is part of the ventilator.

4. A system for automatically setting a value specified by a ventilator, wherein the system comprises the system of claim 1 and a closed-loop control device that is configured to adapt the value specified by the ventilator on the basis of relative strain values (STR.sub.RELATIV) formed.

5. The system of claim 1, wherein the system further comprises the ventilator.

6. The system of claim 5, wherein the ventilator is configured and embodied to perform the local strain of the lungs, which causes a respiration-dependent deformation of the lung tissue, using EIT information at a pixel level.

7. The system of claim 5, wherein the ventilator is configured and embodied to perform a determination of the local strain of the lung using EIT information at a pixel level, by virtue of a tidal volume of each pixel being determined from its respective change in impedance (delta Z) during a respiratory cycle and said tidal volume thereupon being related to its respective end-expiratory volume, which corresponds to its impedance value at the end of expiration (EELI).

8. The system of claim 5, wherein the ventilator comprises at least one controllable respiratory gas source, a control device, and at least one sensor device for determining pressure and/or flow of a respiratory gas, wherein the control device drives the respiratory gas source for specifying a first predetermined ventilation pattern (in respect of pressure, flow, volume, frequency), wherein the control device is configured to communicate with the device for electrical impedance tomography that is spatially separated from the ventilator device, wherein the control device evaluates sensor measured values of the device for electrical impedance tomography for determining a current strain of the lung during ventilation with the first ventilation pattern and wherein the control device drives the at least one respiratory gas source for specifying a second ventilation pattern, which differs from the first ventilation pattern in terms of pressure and/or flow and/or volume and/or frequency, wherein the control device evaluates sensor measured values of the device for electrical impedance tomography determining a resultant strain of the lung during the ventilation with the second ventilation pattern.

9. The system of claim 8, wherein the control device adapts at least one pressure or a ventilation pattern in such a way that the strain is reduced and/or increases PEEP in order to reduce the strain and/or specifies the pressure during expiration based on the strain.

10. A method for determining a local strain of a lung during artificial ventilation, wherein the method comprises: capturing an electrical impedance distribution along at least one two-dimensional section through a human thorax by a device for electrical impedance tomography (EIT), dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels, assigning a specific value of the electrical impedance to a specific EIT pixel at a specific time, determining at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and an end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel, forming a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between the end-inspiratory electrical impedance (Z.sub.INSP) and the end-expiratory electrical impedance (Z.sub.EXSP), in particular by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP), forming a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing the local tidal volume reference value (ΔZ) by the end-expiratory electrical impedance (Z.sub.EXSP), and forming a relative strain value (STR.sub.RELATIV) of the specific EIT pixel by normalizing the preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF).

11. The method of claim 10, wherein the reference strain value (STR.sub.REF) is formed as a quotient of the local tidal volume reference value (ΔZ) and the end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel at a specified positive end-expiratory pressure (PEEP).

12. A method for automatically setting a value specified by a ventilator, wherein the method comprises: determining a local strain of a lung during artificial ventilation by the method of claim 10; and adapting the value specified by the ventilator on the basis of the relative strain values (STR.sub.RELATIV) formed.

13. A ventilator, configured and embodied for carrying out the method of claim 10.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is explained in more detail below with reference to the appended drawings, in which:

(2) FIG. 1 shows a ventilator 1 according to the invention;

(3) FIG. 2 shows a schematic illustration of an EIT measurement during ventilation; and

(4) FIG. 3 schematically shows an ascertainment of the local lung overexpansion on the basis of measurement of deformations caused by respiration in the case of a low PEEP.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

(5) The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description in combination with the drawings making apparent to those of skill in the art how the several forms of the present invention may be embodied in practice.

(6) FIG. 1 shows a ventilator 1 according to the invention, which can be a home ventilator or sleep therapy device or a ventilator 1 used as a clinical ventilator. The ventilator 1 is embodied to carry out the method according to the invention. By way of example, the ventilator 1 is part of a system 10 that comprises an EIT apparatus.

(7) The ventilator 1 comprises at least one controllable respiratory gas source 100 and a programmable control device 21 (closed-loop control device) and at least one sensor device 2 for determining pressure and/or flow of the respiratory gas, wherein the control device 21 drives the respiratory gas source to specify a first predetermined ventilation pattern (in respect of pressure, flow, volume, frequency). The ventilator moreover comprises at least two further sensor devices 3, 30, 40, which are spatially separated from the ventilator 1, for example, and which communicate with the control device 21 of the ventilator 1. A sensor 3 is embodied as an EIT (or EI) apparatus and comprises a multiplicity of individual sensor devices 3, 3′, 3″ . . . , and each sensor device is embodied to generate electrical potentials and/or to non-invasively determine the electrical impedance (EI) of a body section and to ascertain and transmit the sensor measured values to the control unit. The control device 21 evaluates the sensor measured values of the EI sensor device 3 for the purposes of determining the current ventilation or the strain of the lung during the ventilation with a first ventilation pattern. The control device 21 drives the respiratory gas source, for example also to specify a second ventilation pattern, which differs from the first ventilation pattern in terms of pressure and/or flow and/or volume and/or frequency, wherein the control device 21 evaluates the sensor measured values of the EI sensor device 3 for the purposes of determining the resultant ventilation or the strain of the lung during the ventilation with the second ventilation pattern, wherein the control device 21 compares the current ventilation or the strain with the resultant ventilation or the resultant strain, for the purposes of determining the better-suited ventilation pattern. By way of example, the control device adapts at least one pressure, for example the PEEP, or a ventilation pattern in such a way that the strain is reduced. By way of example, the strain is reduced by increasing the EPAP or the PEEP.

(8) The ventilator 1 comprises a respiratory gas source 100 with, for example, a fan device and/or a valve device 101 for generating a respiratory gas flow for the ventilation. Here, a control device 21 is provided for controlling the respiratory gas source 100 and for capturing and processing sensor data. The ventilator 1 is operated and set by way of a user interface 61 comprising operating elements and a display device 11 (display).

(9) The ventilator 1 comprises a respiratory interface 102 for supplying the respiratory gas flow to a patient for ventilation purposes. The respiratory interface 102 shown here is a respiration mask 105, embodied in exemplary fashion as a nasal mask or full mask, or a tube. By way of example, a headgear 106 is provided for securing the respiratory mask 105. By way of example, the respiratory interface 102 can also be configured as a full face mask, as a nasal mask, as a tube or as a laryngeal mask.

(10) At least one connecting tube 109 is provided for connecting the respiratory interface 102 to the respiratory gas source 100, said connecting tube being connected in air-guiding fashion to the respiratory gas source 100 by way of a coupling device 112. The connecting tube 109 is connected to the respiratory interface 102 via a coupling element 107. An exhalation element 108, which comprises a valve or is embodied as such, is disposed between the connecting tube 109 and the coupling element 107. By way of example, the exhalation element 108 is provided to prevent respiration back into the ventilator 1 while the user exhales.

(11) The control device 21 is connected to a sensor device 2, 3, 30, 40, not illustrated in any more detail, which comprises one or more sensors for capturing appliance parameters and/or patient parameters and/or other quantities characteristic for ventilation. The control device 21 can be embodied as a central control device which processes all sensor values, for example in the ventilator. The control device 21 can be configured in the form of a plurality of decentralized control devices, for example one in the ventilator and respectively one assigned to one sensor (or the sensors), having quantities characteristic for ventilation.

(12) By way of example, the control device 21 comprises a pressure sensor 2, not shown in any more detail here, which ascertains the pressure conditions in the respiratory gas. To this end, the pressure sensor is operatively connected to the respiratory gas, for example via a pressure measuring tube 110 to the respiratory interface 102. The pressure measuring tube 110 is linked to the control device 21 via an input nozzle 111. By way of example, the control device 21 also comprises a flow sensor 2, not shown in any more detail here, which ascertains the flow conditions in the respiratory gas.

(13) Moreover, the control device 21 is used here to drive the fan device and/or the valve device 101. The control device 21 provides a necessary minimum pressure and compensates pressure variations that are caused by the respiratory action of the user. By way of example, the control device 21 also captures the current pressure in the respiratory mask 105 and accordingly updates the power of the respiratory gas source until a desired ventilation pressure sets in.

(14) The appliance parameters required to set the respiratory gas source 100 and the appliance configuration and/or appliance software are stored in a memory device 31.

(15) The control device 21 can also regulate the oxygen content in the respiratory gas by appropriate driving of the fan device and/or the valve device 101, wherein the valve device has or contacts at least one oxygen source (pressurized gas mask or hospital line). By way of example, oxygen is admixed to the respiratory gas flow downstream or upstream of the fan device. In this case, the valve device is an oxygen mixing valve. Oxygen admixing into the respiratory gas flow is implemented, for example, by a valve unit that guides and regulates the respiratory gas flow and the oxygen flow.

(16) The control device 21 can also be embodied to capture patient parameters captured by sensors. To this end, the control device 21 can be equipped with sensors for measuring the respiratory excursion, for measuring an oxygen supply and/or a carbon dioxide supply and/or for measuring an EEG, EMG, EOG or ECG activity.

(17) In particular, the system 10 comprises at least two sensor devices 3, 30, 40 that are spatially separate from the ventilator 1 and that communicate with the control unit 21, wherein a sensor 3 has a multiplicity of individual sensor devices 3, 3′, 3″ . . . , and each sensor device is embodied to generate electrical potentials for non-invasively determining the electrical impedance EI of a body section. By way of example, the sensor devices 3 are embodied as adhesive plasters, with an outer layer distant from the skin and an adhesive layer facing the skin, or combined as a belt that holds the sensor devices.

(18) By way of example, the ventilator also comprises at least one further sensor device 30, which is embodied to non-invasively determine a carbon dioxide (CO2) value 30. By way of example, the sensor device 30 is embodied as a clamped sensor or as an adhesive plaster, with an outer layer distant from the skin and an adhesive layer facing the skin. The ventilator 10 comprises at least one further sensor device 40, which is embodied to non-invasively determine an oxygen (O2) value 40. By way of example, the sensor device 40 is embodied as a clamped sensor or as an adhesive plaster, with an outer layer distant from the skin and an adhesive layer facing the skin. According to the invention, the CO2 value 30 and/or the O2 value 40 can be determined invasively or non-invasively in the tissue of the patient and/or non-invasively in the respiratory gas flow.

(19) By way of example, the sensors 3, 30, 40 comprise power and transmission-means means for ascertaining and wirelessly transmitting the sensor measured values. Alternatively, the sensors 3, 30, 40 comprise, e.g., communication means for transmitting the sensor measured values to the controller 21.

(20) FIG. 2 shows a schematic illustration of the EIT measurement during ventilation.

(21) In exemplary fashion, the ventilator also comprises at least one further sensor device 3 for performing the EIT measurement.

(22) By way of example, the sensor device has 32 high-resolution sensors 3, 3′ . . . in a textile belt (not illustrated). An integrated relative position sensor (not illustrated) determines the position of the patient. The sensors are interconnected by communication or line means 4 and disposed in such a way that they can successively receive signals from all directions. Therefore, the control unit 21 can spatially assign the sensor measured values of the EIT measurement. Using this, it is possible to ascertain the local tissue resistances from different directions and then convert these into moving images. The control unit 21 processes the sensor measured values of the EIT measurement in such a way that a specific electrical impedance (EI) is ascertained for individual lung sections L1, LA2. The sensor device 3 is connected to the control unit 21 via wired or wireless communication means. The control unit 21 stores and processes the sensor measured values for use by the ventilator. The memory device 31 can be a constituent part of the control unit 21 or a separate component.

(23) FIG. 2 also shows a schematic illustration of the results of the EIT measurement on the display of the ventilator. The control unit 21 processes the sensor measured values for use by the ventilator in such a way that, for example, a visualization of the patient thorax P with the two lungs L is presented on the display of the ventilator 10. The control unit 21 processes the sensor measured values of the EIT measurement 3 in such a way that a specific electrical impedance (EI) of the lung L is ascertained for individual lung sections L1, LA2. For the presentation on the display, the specific electrical impedance (EI) of the lung sections L1, LA2 is prepared in such a way that lung sections with a high electrical impedance LA2 are represented differently from a graphical point of view than the lung sections with a low electrical impedance L1. The control unit 21 processes the sensor measured values of the EIT measurement 3 in respect of time in such a way that a specific electrical impedance EI of the lung L is ascertained per breath for individual lung sections L1, LA2 and prepared for presentation on the display.

(24) The sensor device 3 is also configured and embodied (together with the controller and the memory) to form an EIT summed signal, for example, which is a measure for the ventilation of the lung or of lung sections. The sensor device 3 is also configured and embodied (together with the controller and the memory) to ascertain a frequency of the EIT change frequency. Consequently, in summary, the sensor device 3 can ascertain the current ventilation.

(25) Within the meaning of the invention, the EIT summed signal is understood to mean that the impedance distribution or impedance change distribution in the sectional plane determined by the sensors or in the volume spanned by the sensors need not necessarily be presented pictorially, for example on a display, for the purposes of performing the method according to the invention but at least that the calculated result values of the impedance distribution or impedance change distribution are available for performing the method in the apparatus according to the invention.

(26) Accordingly, when summing all impedance values of the sensors, there is a modified summed signal during inspiration in comparison with the summed signal during expiration. Accordingly, the summed value represents an adequate measure for determining the ventilation of the lung.

(27) The EIT change frequency can be determined from the respiratory phase-dependent change in the impedance values of the sensors. The impedance values of the sensors vary in respiratory phase-dependent fashion with inspiration and expiration. Accordingly, the EIT change frequency represents an alternative or complementary measure for determining the (frequency of the) ventilation of the lung.

(28) By way of example, the sensors are integrated, together with a relative position sensor, in a textile and breathable and stretchable electrode belt. The sensor 3 and the control unit are configured to generate a frame rate of, e.g., up to 50 frames per second. Lung regions LA are visualized in the process. According to the invention, patient-related inputs are possible, such as height, weight, sex, chest size.

(29) The EIT data ascertained and prepared thus facilitate a temporally and spatially resolved differential analysis and representation of the electrical impedance EI of the lung.

(30) FIG. 3 shows the ascertainment of the local lung overexpansion on the basis of measurement of deformations caused by respiration in the case of a low PEEP.

(31) All calculations were performed within the individual lung region of interest (ROI), which was formed by the pixels whose amplitude was greater than x % of the amplitude of the pixel with the maximum respiratory-caused impedance change in the case of PEEP 30.

(32) According to the invention, the strain of the whole lungs, which causes the deformation of the lung tissue caused by breathing, can be estimated as follows:
strain=VT/FRC

(33) The calculation of the local strain using this novel EIT-based approach is implemented at a pixel level, by virtue of the tidal volume of each pixel being determined from its respective change in impedance (delta Z) during the respiratory cycle and said tidal volume thereupon being related to its respective end-expiratory volume, which corresponds to its impedance value at the end of expiration (EELI). The corresponding formula therefore reads:
Strain=Zend-expiration—Zend-inspiration/EELI=Delta Z/EELI

(34) Zend-expiration, Zend-inspiration and EELI were ascertained for each pixel and each breath, the respective mean value was calculated therefrom over all measured breaths at a respective PEEP level and the strain of each pixel was then calculated therefrom.

(35) Subsequently, the pixel strain at each PEEP level was related to the individual pixel strain in the case of the reference PEEP of 15 cm H2O, as result of which a normalization was obtained, the latter facilitating the creation of comparable relative strain images, which formed the basis for the calculation of numerical values of the total strain for the respective PEEP level. To this end, the respective pixel values were summed and normalized with respect to the number of all pixels in the lung ROI of each pig. Finally, EIT images were generated from the aforementioned strain images, said EIT images representing the mean relative strain of all subjects at the respective PEEP level.

(36) According to the invention, the measurement and closed-loop control technology of the ventilator is linked to electrical impedance tomography (EIT). This allows the function of the lung to be continuously presented in imaging fashion. The measured values of the ventilator are combined with the results of the electrical impedance tomography examination. Thus, this technology allows very different clinical questions to be assessed and the therapy to be adjusted accordingly.

(37) To sum up, the present invention provides: 1. A system for real-time determination of a local strain of a lung during artificial ventilation using a ventilator, wherein the system comprises a device for electrical impedance tomography (EIT), which device is configured to capture an electrical impedance distribution along at least one two-dimensional section through a human thorax, and further comprises a device for assigning the captured electrical impedance distribution, for dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels, and for assigning a specific value of the electrical impedance at a specific time to a specific EIT pixel. 2. The system of item 1, which further comprises a device for determining a local strain value, which device is configured to determine at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and one end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel. 3. The system of item 2, wherein the device is configured to form a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between an end-inspiratory electrical impedance (Z.sub.INSP) and an end-expiratory electrical impedance (Z.sub.EXSP). 4. The system of item 2, wherein the device is configured to form a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between an end-inspiratory electrical impedance (Z.sub.INSP) and an end-expiratory electrical impedance (Z.sub.EXSP) by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP). 5. The system of any one of the preceding items, wherein the device is configured to form a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing a local tidal volume reference value (ΔZ) by an end-expiratory electrical impedance (Z.sub.EXSP). 6. The system of any one of the preceding items, wherein the device is configured to form a relative strain value (STR.sub.RELATIV) of a specific EIT pixel by normalizing a preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF). 7. The system of any one of the preceding items, wherein the device is configured to normalize a preliminary strain value (STR.sub.VORL) to a quotient of a local tidal volume reference value (ΔZ) and an end-expiratory electrical impedance (Z.sub.EXSP) of a specific EIT pixel at a specified positive end-expiratory pressure (PEEP). 8. The system of any one of the preceding items, wherein the system comprises a device for electrical impedance tomography (EIT), which device is configured to capture an electrical impedance distribution along at least one two-dimensional section through a human thorax, and further comprises a device for assigning the captured electrical impedance distribution, for dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels and for assigning a specific value of the electrical impedance at a specific time to a specific EIT pixel; wherein the system further comprises a device for determining a local strain value, which device is configured to determine at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and an end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel, to form a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between the end-inspiratory electrical impedance (Z.sub.INSP) and the end-expiratory electrical impedance (Z.sub.EXSP), in particular by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP), and to form a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing the local tidal volume reference value (ΔZ) by the end-expiratory electrical impedance (Z.sub.EXSP); and wherein the device for determining a local strain value is further configured to form a relative strain value (STR.sub.RELATIV) of the specific EIT pixel by normalizing the preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF). 9. The system of any one of the preceding items, wherein the device for electrical impedance tomography (EIT) is coupled to the ventilator or is part of the ventilator. 10. A system for automatically setting a value specified by a ventilator, in particular a pressure, preferably a positive end-expiratory pressure (PEEP), wherein the system comprises the system of any one of the preceding claims and a closed-loop control device that is configured to adapt a value specified by the ventilator on the basis of relative strain values (STR.sub.RELATIV) formed. 11. A method for determining a local strain of a lung during artificial ventilation, in particular to be performed in the system of any one of the preceding items, wherein the method comprises: capturing an electrical impedance distribution along at least one two-dimensional section through a human thorax by a device for electrical impedance tomography (EIT), dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels, assigning a specific value of the electrical impedance to a specific EIT pixel at a specific time, determining at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and an end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel, forming a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between the end-inspiratory electrical impedance (Z.sub.INSP) and the end-expiratory electrical impedance (Z.sub.EXSP), in particular by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP), forming a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing the local tidal volume reference value (ΔZ) by the end-expiratory electrical impedance (Z.sub.EXSP), and forming a relative strain value (STR.sub.RELATIV) of the specific EIT pixel by normalizing the preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF). 12. The method of item 11, wherein the reference strain value (STR.sub.REF) is formed as a quotient of the local tidal volume reference value (ΔZ) and the end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel at a specified positive end-expiratory pressure (PEEP). 13. The method of item 11 or item 12, wherein a relevant lung area is initially identified. 14. A method for automatically setting a value specified by a ventilator, in particular a pressure, preferably a positive end-expiratory pressure (PEEP), wherein the method comprises: determining a local strain of a lung during artificial ventilation by the method of any one of items 11 to 13; and adapting the value specified by the ventilator based on the relative strain values (STR.sub.RELATIV) formed. 15. A ventilator, configured and embodied for use in the system of any one of items 1 to 11 and/or for carrying out the method of any one of items 11 to 14. 16. A ventilator, in particular according to item 15, wherein the ventilator is configured and embodied to perform the global strain of the lungs, which causes the respiration-dependent deformation of the lung tissue, using the EIT information at a pixel level. 17. A ventilator, in particular according to item 15, wherein the ventilator is configured and embodied to perform a determination of the local strain of the lung using the EIT information at a pixel level, by virtue of the tidal volume of each pixel being determined from its respective change in impedance (delta Z) during the respiratory cycle and said tidal volume thereupon being related to its respective end-expiratory volume, which corresponds to its impedance value at the end of expiration (EELI). 18. A ventilator, in particular according to item 15, wherein the ventilator comprises at least one controllable respiratory gas source, a (programmable) control device (closed-loop control device), and at least one sensor device for determining pressure and/or flow of a respiratory gas, wherein the control device drives the respiratory gas source for specifying a first predetermined ventilation pattern (in respect of pressure, flow, volume, frequency), wherein the ventilator further comprises at least one sensor device that is spatially separated from the ventilator and communicates with the control device of the ventilator, wherein the sensor device is configured to generate electrical potentials and/or to non-invasively determine the electrical impedance (EI) of a body section and to determine and transfer the sensor measured values to the control unit, wherein the control device evaluates the sensor measured values of the EI sensor device for the purposes of determining the current strain of the lung during the ventilation with a first ventilation pattern and wherein the control device drives the respiratory gas source for specifying a second ventilation pattern, which differs from the first ventilation pattern in terms of pressure and/or flow and/or volume and/or frequency, and wherein the control device evaluates the sensor measured values of the EI sensor device for the purposes of determining a resultant strain or strain of the lung during the ventilation with the second ventilation pattern. 19. The ventilator of item 18, wherein the control device adapts at least one pressure, for example the PEEP, or a ventilation pattern in such a way that the strain is reduced. 20. The ventilator of item 18 or item 19, wherein the control device increases the PEEP in order to reduce the strain. 21. The ventilator of any one of items 18 to 20, wherein the control device specifies the pressure during expiration based on the strain. 22. The ventilator of any one of claims 15 to 21, wherein the ventilator is configured and embodied to determine a local strain of a lung during artificial ventilation and wherein the following steps are carried out: capturing an electrical impedance distribution along at least one two-dimensional section through a human thorax by means of a device for electrical impedance tomography (EIT), dividing the captured electrical impedance distribution at different times during the artificial ventilation into a multiplicity of EIT pixels, assigning a specific value of the electrical impedance to a specific EIT pixel at a specific time, determining at least one end-inspiratory electrical impedance (Z.sub.INSP) of the specific EIT pixel and an end-expiratory electrical impedance (Z.sub.EXSP) of the specific EIT pixel, forming a local tidal volume reference value (ΔZ) of the specific EIT pixel as a difference between the end-inspiratory electrical impedance (Z.sub.INSP) and the end-expiratory electrical impedance (Z.sub.EXSP), in particular by subtracting the end-expiratory electrical impedance (Z.sub.EXSP) from the end-inspiratory electrical impedance (Z.sub.INSP), forming a preliminary strain value (STR.sub.VORL) of the specific EIT pixel by dividing the local tidal volume reference value (ΔZ) by the end-expiratory electrical impedance (Z.sub.EXSP), and forming a relative strain value (STR.sub.RELATIV) of the specific EIT pixel by normalizing the preliminary strain value (STR.sub.VORL) to a reference strain value (STR.sub.REF).