Patent Application
20220040427
This application claims the benefit of priority under 35 U.S.C. §119 of German Application 10 2020 120 900.2, filed Aug. 7, 2020, the entire contents of which are incorporated herein by reference.
The present invention pertains to a device and to a process for determining a value indicative of the respective regional compliance of the lung of a patient in a plurality of different regions of the lungs.
The term “respiratory system” of the patient will be used below. The respiratory system comprises the lungs and the chest wall of the patient.
When a patient is breathing spontaneously and/or is ventilated mechanically, the following two undesired situations, each of which may damage the lungs of the patient, shall be avoided:
An approach to controlling a ventilator as a function of the compliance of the lungs is described in Zhanqi Zhao et al.: “Positive End-Expiratory Pressure Titration with Electrical Impedance Tomography and Pressure-Volume Curve in Severe Acute Respiratory Distress Syndrome,” Annals of Intensive Care, 9, No. 1 (December 2019), available at https://doi.org/10.1186/s13613-019-0484-0, downloaded on Jun. 12, 2020. Other approaches to determining mechanical/pneumatic properties of the lungs of a patient and/or to controlling a ventilator are described in
Translational Medicine, 6, No. 19 (October 2018): 392 ff, available at https://doi.org/10.21037/atm.2018.06.05, downloaded on Jun. 12, 2020.
The compliance of the lungs may differ from one region of the lungs to the next. It is known, for example, from DE 10 2005 031 752 B4 (corresponding to U.S. Pat No. 7,941,210 B2) that the lung can be measured by means of impedance tomography, preferably by means of a so-called EIT belt, which is placed around the body of the patient.
It is proposed in Eduardo L. V. Costa et al.: “Bedside Estimation of Recruitable Alveolar Collapse and Hyperdistention by Electrical Impedance Tomography,” Intensive Care Medicine, 35, No. 6 (June 2009), 1132-1137, available at https://doi.org/10.1007/s00134-009-1447-y, downloaded on Jun. 12, 2020, that the compliance or elasticity (compliance) of the respiratory system of a patient be analyzed while a variable end-expiratory pressure (PEEP) is being set. Different values are set on a trial basis and the extent of the reduction of the compliance of the respiratory system is investigated in different regions, and this reduction is interpreted as a hyperdistention or as a collapse of the lungs.
A basic object of the present invention is to provide a device and a process which are capable of measuring the compliance of the lungs better than prior-art devices and processes can.
The object is accomplished by a device having the features of the device according to the invention and by a process having the features of process according to the invention. Advantageous embodiments are described. Advantageous embodiments of the device are, insofar as meaningful, also advantageous embodiments of the process according to the present invention and vice versa.
The present invention pertains to a device and to a process, which are capable of automatically determining a value indicative of (a parameter of) the respective regional compliance of the lungs of a patient in at least two different regions of the lungs. The regional compliance is the compliance of the lungs in a region of the lungs, wherein each lung region is predefined and comprises a respective part of the lung each but not the entire lung. Preferably the lung regions are disjoint, i.e. they do not overlap. Each region of the lungs, whose regional compliance shall be determined, is specified such that the regional compliance in the entire lung region is considered to be equal everywhere at an accuracy sufficient for the application. The determined regional compliance of a lung region may vary over time, for example, because the state of the patient changes or based on different settings of a ventilator, which mechanically ventilates the patient. The respective regional compliance may vary from one lung region to the next at a given time.
The device according to the present invention comprises an EIT measuring device. This EIT measuring device is capable of measuring a value indicative of (a parameter of) the change in the volume of a lung region and to use an electrical impedance tomography (EIT) process for this. Thanks to the EIT measuring device, a value indicative of a change in the volume of each lung region can be measured for the respective change in the volume of each lung region.
The device according to the present invention comprises, furthermore, an airway pressure sensor. This airway pressure sensor is capable of measuring a value indicative of (a parameter) of the pressure, which is variable over time, at or in the airway of the patient, preferably a pneumatic parameter. This pressure results from the intrinsic breathing activity of the patient and/or from a mechanical ventilation of the patient, which is carried out by a ventilator. The intrinsic breathing activity of the patient may result from a spontaneous breathing of the patient as well as from an intrinsic breathing activity stimulated from the outside.
A data processing control device of the device according to the present invention is capable of receiving and processing signals from the EIT measuring device and from the airway pressure sensor and it calculates a respective parameter (a respective value indicative of) each for the regional compliance of the lungs in this area. The control device preferably calculates the respective value indicative of the regional compliance for four lung regions, which are arranged in the manner of four quadrants, or for eight lung regions. The control device calculates a quotient
The control device calculates as the volume difference a value indicative of (a parameter) of the difference between the end-inspiratory volume and the end-expiratory volume of this region of the lungs. The end-inspiratory volume occurs at the end of an inhalation process of the patient, and the end-expiratory volume at the end of an exhalation process. The lungs are known to expand during inhalation and to contract again during exhalation, so that the end-inspiratory volume is larger than the end-expiratory volume. In order to determine these two volumes, the control device uses signals of the EIT measuring device. The volume difference may, of course, differ from one lung region to the next and it may also vary over time.
The control device determines as a pressure difference a value indicative of (a parameter of) the difference between the end-inspiratory transpulmonary pressure and the end-expiratory transpulmonary pressure of the entire respiratory system of the patient. The term “transpulmonary pressure” designates a pressure that acts on the lungs from the inside or also from the outside. This transpulmonary pressure is the difference between
This pleural cavity encloses the lungs of the patient. The respiratory system comprises the lungs and the chest wall of the patient, and the pleural cavity is located between the lungs and the chest wall.
The control device uses signals of the airway pressure sensor to determine the pressure difference. These signals are a parameter of the entire pressure that acts on the respiratory system and is generated
Knowledge of the extent of compliance of different regions of the lungs of a patient can be used for the proper setting of a ventilator for the mechanical ventilation of this patient. The ventilator performs a series of ventilation strokes as a function of at least one operating parameter. The operating parameters that can be set on the ventilator include, for example,
The ventilator shall be set especially such that a hyperdistention of the lungs is avoided during a mechanical ventilation, on the one hand, and that collapse of the lungs is, on the other hand, ruled out. Knowledge of the possible extent of compliance of the lungs of a patient can also be used for a patient monitor and/or for the automatic monitoring of the patient. The patient monitor receives and uses measured values from the device according to the present invention and it preferably displays the received measured values in a form perceptible by a person, especially visually on a display unit.
The compliance of a pneumatic system with variable volume is known to result from the quotient
A pressure difference and a brought-about volume change are determined according to the present invention.
If a patient is supplied with the needed breathing air exclusively by mechanical ventilation, i.e., if the patient is fully anesthetized, the current signal value of the sensor for the airway pressure provides a good value indicative of the pressure and hence for the load that currently acts on the lungs. However, a patient is frequently supplied with breathing air exclusively by mechanical ventilation only for as short a time as possible. As long as possible, the patient also performs an intrinsic breathing activity, namely, a spontaneous breathing and/or a stimulated breathing, i.e., a breathing with his own respiratory muscles, in addition to the mechanical ventilation or even instead of a mechanical ventilation. The mechanical ventilation consequently supports the intrinsic breathing activity. The current signal value of the airway pressure sensor alone is not sufficient in this case to reliably determine the pressure currently acting on the lungs and hence also the acting pressure difference. The end-inspiratory transpulmonary pressure and the end-expiratory transpulmonary pressure are rather determined according to the present invention, and the difference between these two transpulmonary pressures is used as the pressure difference.
The brought-about volume change of the entire lung can be determined by means of a volume sensor or a volume flow sensor. For example, the current volume flow into or out of the lungs of the patient is measured several times one after another, and integrated over the measured values.
However, the compliance of the lungs may differ substantially from one lung region to the next. As a consequence, in many cases a global change in the volume of the lungs does not suffice to determine a regional compliance. The respective compliance of at least two different lung regions and preferably of four lung regions, which are arranged like four quadrants, or even of eight different lung regions, is therefore determined according to the present invention. Signals of the EIT measuring device are used for this according to the present invention. These signals yield a respective value indicative of the regional volume change of the lung region based on the breathing/ventilation for different lung regions. A respective brought-about volume change each is preferably determined for each lung region or at least for each region of a relevant part of the lungs.
The device according to the present invention and the process according to the present invention may, of course, also be used during the time period in which the patient is supplied with breathing air exclusively by mechanical ventilation. It is possible but not necessary thanks to the present invention to switch over between two different modes, namely,
The airway pressure sensor is preferably located outside the body of the patient. For example, a part of the sensor is located in front of the mouth of the patient and branches off air from a breathing air stream or from a ventilation circuit. An analysis unit of the sensor is located at a distance in space from the patient, for example, at or in a ventilator or a patient monitor. Thanks to this embodiment, it is not necessary to bring a part of the airway pressure sensor from time to time into the body of the patient and later to remove it. However, an airway pressure sensor with a reading recorder in the body of the patient may be used for the present invention as well.
The pressure in the pleural cavity cannot be measured directly in a person and in many other living beings. In one embodiment, the device additionally comprises a pneumatically operating sensor for the pressure in the esophagus. This sensor provides a value indicative of the pressure in the esophagus of the patient. The measured pressure in the esophagus is a good approximation for the pressure in the pleural cavity. As was already described, the difference between the airway pressure and the pressure in the pleural cavity, which cannot be measured directly, is a value indicative of the transpulmonary pressure. According to this embodiment, the difference between
In a preferred embodiment, this esophageal pressure sensor comprises a catheter and a measuring balloon, both of which are positioned in the esophagus of the patient. The measuring balloon is in a fluid connection with the catheter, and the catheter is in a fluid connection with an analysis unit located outside the patient.
In a preferred embodiment, the present invention is used while the patient is being ventilated mechanically by a ventilator (supporting or mandatory ventilation). The ventilator may be configured as an anesthesia apparatus and it may additionally anesthetize the patient by means of at least one anesthetic. The mechanical ventilation is carried out such that the end-expiratory pressure (PEEP), which shall be brought about by the ventilator, is set at a certain, predefined value. This end-expiratory pressure shall prevail in the respiratory system of the patient at the end of an exhalation process. The airway pressure sensor is capable of measuring the actual end-expiratory pressure, so that a regulation of the mechanical ventilation with the aim of making the actual value for the end-expiratory pressure equal to the predefined value is possible, even as an automatic regulation. At least two, preferably more than two different values are preferably predefined for the end-expiratory pressure to be brought about, and the ventilator is regulated to this respective value.
In a variant of this embodiment, the control device automatically predefines a first required value and at least one second required value for the end-expiratory pressure. The mechanical ventilation is carried out with the regulation goal to ensure that the actual value for the end-expiratory pressure be at first equal to the first predefined value and then to be equal to the second predefined value or to a second predefined value, wherein these two values are different from one another. It is possible to predefine more than two values for the end-expiratory pressure and to set the mechanical ventilation one after another to each of these at least three values.
In one application, a fluid connection is used between the ventilators and the patient in order to automatically determine a desired operating value for the end-expiratory pressure, which shall be brought about by the ventilator during the mechanical ventilation. The regional compliance of a region of the lungs, which is determined according to the present invention, depends on the value set for the end-expiratory pressure. The respective regional compliance of the lungs is determined according to the present invention for each of the at least two values of the end-expiratory pressure and for a plurality of regions. The respective overall lung compliance effected by the different values for the end-expiratory pressure is determined by cumulating over regional compliances. Cumulation is performed in a suitable manner over these regional compliances.
The value of the expiratory pressure that leads to the highest regional compliance of the lung region in question is determined for a plurality of regions of the lungs in one embodiment. This optimal valve varies, as a rule, from one lung region to the next. A relative compliance or stiffness or elastance of the lung region is calculated for each lung region taken into consideration and for a plurality of values of the end-expiratory pressure. The optimal value for a lung region is set automatically as a function of the determined relative compliances. This relative value equals 100% for the optimal value that leads to the maximum regional compliance. Cumulation is then performed over the lung regions whose regional compliance is determined.
The embodiment just described, in which the mechanical ventilation is set one after another at different values for the end-expiratory pressure, may be combined with the above-described embodiment, in which the transpulmonary pressure is determined as the difference between the airway pressure and the esophageal pressure.
In a different embodiment, the necessity to equip the patient with a pneumatic sensor for the pressure in the esophagus is avoided. This alternative embodiment can likewise be used when the patient is ventilated mechanically or when the end-expiratory pressure reached is set one after another at at least two different values. Each value for the end-expiratory pressure leads to a respective resulting end-expiratory lung volume, i.e., to the lung volume present at the end of an exhalation process. The difference between the two end-expiratory lung volumes is calculated. In addition, the difference between the two values for the end-expiratory pressure is calculated. The quotient of the volume difference and the pressure difference provides a value indicative of the difference between the end-inspiratory transpulmonary pressure and the end-expiratory transpulmonary pressure.
The airway pressure sensor and the control device may be parts of a ventilator or of an anesthesia apparatus. A fluid connection, optionally a ventilation circuit, is established at least from time to time between the patient and the ventilator or the anesthesia apparatus. The EIT measuring device is in a data connection with the ventilator or with the anesthesia apparatus.
The present invention pertains, furthermore, to a system which is capable of ventilating a patient mechanically. The system comprises a ventilator, which may be configured as an anesthesia apparatus, as well as a device according to the present invention. The ventilator is capable of carrying out a mechanical ventilation of the patient and it preferably carries out a series of ventilation strokes during the mechanical ventilation. The ventilator uses for this mechanical ventilation the regional compliances of the lungs of the patient, which are determined according to the present invention. Preferred embodiments of the device according to the invention are also preferred embodiments of the ventilating system. The ventilator preferably generates a respective value each for at least one operating parameter of the ventilator as a function of the regional compliances. The operating parameter is, for example,
A predefined calculation rule is preferably used to calculate the value of the operating parameter as a function of the regional compliances. The calculation rule is predefined such that neither is a region of the lungs hyperdistended, nor does it collapse into itself based on an excessively low pressure. In one embodiment, the ventilator outputs the calculated value, and a user can confirm the value or overwrite it with another value. In another embodiment, the ventilator uses the automatically calculated value.
The present invention will be described below on the basis of exemplary embodiments. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
In the drawings:
Referring to the drawings, the present invention is embodied in the exemplary embodiment by means of a device, which comprises
A fluid connection, not shown, connects the patient P to the ventilator 9. A gas or a gas mixture can flow through this fluid connection from the ventilator 9 to the patient P. A ventilation circuit is optionally established between the patient P and the ventilator 9, i.e., breathing air, which has been exhaled by the patient P, flows back to the ventilator 9.
Different sensors measure different vital parameters of the patient P. A pneumatic sensor 3 comprises a transducer 3.1 comprising an opening, which is arranged in the vicinity of the mouth of the patient P and branches off air from the fluid connection between the patient P and the ventilator 9. The branched-off air is transferred via a hose (tube) to a pressure sensor 3.2, which measures a value indicative of the airway pressure Paw (pressure in airway). In one embodiment, the transducer 3.1 is arranged in or at a Y-piece close to the connection piece 11. A sensor 15 at the ventilator 9 optionally measures a parameter of the volume per unit of time of the flow Vol′ of breathing air from the ventilator 9 to the patient P or back from the patient P to the ventilator 9. By analyzing measured values of the sensor 3 and/or of the sensor 15, the ventilator 9 is capable of determining when an inhalation process (inhalation) of the patient P begins and when it ends and when an exhalation process (exhalation) begins and when it ends.
The measuring catheter 14 is placed into the esophagus Sp. The measuring catheter 14 begins in the connection piece 11. A probe 10 in the esophagus Sp of the patient P, preferably comprising a measuring balloon, measures a value indicative of the pneumatic pressure Pes (pressure in esophagus), which is variable over time, in the esophagus Sp. The probe 10 is in a fluid connection with the connection piece 11 via the measuring catheter 14 or is a part of the measuring catheter 14. In one embodiment, the probe 10 is positioned at the transition between the esophagus Sp and the stomach Ma and comprises a measuring balloon and optionally two measuring balloons. The measuring balloon of the probe 10 or a measuring balloon of the probe 10 is located in the lower region of the esophagus Sp and is deformed as a function of the pressure Pes in the esophagus Sp. The optional other measuring balloon is located behind the cardiac orifice in the stomach Ma and is deformed depending on the gastric pressure Pga in the stomach Ma. It is also possible that an additional gastric probe 13 in the form of a measuring balloon is placed into the stomach Ma, cf.
In addition, a plurality of measuring electrodes are preferably attached to the chest of the patient P.
In addition, a belt 7 is placed around the body of the patient P for an electrical impedance tomography (EIT), which will be described below. This EIT belt 7 belongs to an EIT measuring device, which comprises in the exemplary embodiment an additional sensor for the airway pressure Paw (not shown).
The patient P is ventilated mechanically by the ventilator 9 at least from time to time. A fluid connection, optionally a ventilation circuit, is established between the patient P and the ventilator 9. The ventilator 9 carries out ventilation strokes and thereby delivers breathing air through the fluid connection to the mouthpiece 3 and into the lungs of the patient P. This breathing air is optionally mixed with at least one anesthetic, so that the patient P is anesthetized at least partially.
A display unit 12, which is in a data connection with the ventilator 9, displays
The airway pressure Paw is measured by the pneumatic sensor 3, which is arranged in one embodiment in the Y-piece in front of the connection piece 11. The probe 10 in the esophagus Sp measures the pressure Pes in the esophagus Sp. For example, the deformation of the balloon, which belongs to the probe 10, is measured, and it acts as a value indicative of the pressure Pes in the esophagus Sp. Another measuring balloon of the probe 10 or an additional gastric probe 13 makes it possible to measure a value indicative of the pressure Pga in the stomach Ma. For example, the deformation of one measuring balloon, which belongs to the probe 10, is measured and is used as a value indicative of the pressure Pes in the esophagus Sp. The deformation of the other measuring balloon of the probe 10 or the deformation of the separate gastric probe 13 yields a value indicative of the gastric pressure Pga. The other signals are deduced from measured values of these sensors, which will be described farther below.
the gastric pressure Pga in the stomach Ma.
In addition, the measurement positions (left bottom) as well as current values of the vital parameters (right-hand column) are shown. The view in
Different operating parameters, for example,
The ventilator 9 preferably carries out a regulation, wherein a volume or a pressure is predefined in the fluid connection and it acts as a command variable of the control circuit (here of the ventilation circuit). The command variable may be variable over time.
When a patient P is breathing spontaneously and/or is ventilated mechanically, the lungs of the patient P expand and contract again. Each ventilation stroke of the ventilator 9 feeds air to the lungs and brings about an expansion of the lungs. The exhalation brings about a flow of air out of the lungs. This expansion and contraction of the lungs can be compared in a simplified manner to the process of pumping up the bicycle tube and then letting air again out of the tube. In case of considerably damaged or diseased lungs, the exhalation may lead to a collapse of the lungs into themselves.
Two situations critical for the patient P must be avoided, because each these situations may damage his lungs:
Both situations may occur simultaneously especially in case of damaged lungs, doing so in different regions of the lungs, which are damaged to different extents and/or in different manners.
The lungs and the chest wall of the patient P form together an expandable respiratory system.
If the patient P is fully anesthetized and is not carrying out any spontaneous breathing, the pressure that acts on the respiratory system of the patient P is exerted exclusively by the ventilator 9. The pressure Paw, which is measured by the sensor 10 in the vicinity of the mouth of the patient P, is a good value indicative of the pressure that acts on the respiratory system in case of full anesthesia (the patient is not performing any intrinsic breathing activity at all). However, the situation in which the patient P is not fully anesthetized but breathes spontaneously at least from time to time during the mechanical ventilation or in which his intrinsic breathing activity is stimulated and the mechanical ventilation is superimposed to the intrinsic breathing activity (spontaneous breathing and/or stimulated intrinsic breathing activity) of the patient P should be taken into consideration. The intrinsic breathing activity is not taken sufficiently into account in many cases solely by the measured pressure Paw.
The ventilation strokes of the ventilator 9 as well as the intrinsic breathing activity contribute to the expansion of the respiratory system. The entire pressure generated by the ventilator 9 brings about both a compliance of the lungs and an expansion of the chest wall. The jacketing of the lungs, called the pulmonary pleura, slides along the inner side of the chest wall during the breathing. The thin pleural cavity is located between the lungs and the chest wall. This pleural cavity is filled with a liquid and holds the two surfaces of the lungs and of the chest wall, which adjoin one another, together by capillary forces.
The difference between the total pressure, which is present at the respiratory system, and the pressure in the pleural cavity is a good value indicative of the pneumatic pressure that acts on the lungs during a mechanical ventilation, and it is thus a value indicative of the transpulmonary pressure. The pressure at the airway, preferably the airway pressure Paw measured by the sensor 3, is used as the value indicative of the total pressure that is present. The pressure at the pleural cavity occurs between the outer side of the lungs (pulmonary pleura) and the inner side (costal pleura) of the chest wall. This difference takes into account quantitatively both the mechanical ventilation by the ventilator 9 and the intrinsic breathing activity of the patient P. Due to this difference being taken into consideration, the pressure acting on the lungs is separated by calculation from the pressure acting on the chest wall in the total pressure that acts on the respiratory system and the pressure acting on the lungs is determined as a result in an isolated manner.
The pleural cavity is a closed system. It is therefore impossible to measure this pleural pressure directly. A good approximation to the pleural pressure is the pressure Pes, which is measured by the probe 10 in the esophagus Sp of the patient P, providing that the probe 10 is positioned correctly in the esophagus Sp.
In order to prevent the lungs of the patient P from collapsing into themselves and/or from becoming hyperdistended, a value indicative of the mechanical compliance or elasticity of the lungs is measured. This elasticity is the quotient of the brought-about change ΔVol (increase or decrease) of the lung volume Vol and the change ΔP of the pressure present at the lungs, which is caused by this volume change, i.e., compliance=ΔVol/ΔP.
A change in the transpulmonary pressure Ptp is used in the exemplary embodiment as the pressure difference that acts on the lungs and brings about a change in volume. Consequently, the quotient ΔVol/ΔPtp is used as the value indicative of the elasticity of the lungs.
The transpulmonary pressure Ptp is a value indicative of the mechanical stress to which the lungs are exposed based on the expansion and contraction occurring during breathing. The transpulmonary pressure Ptp is preferably related to a reference value, for example, to the ambient air pressure, and may therefore assume negative values. The end-inspiratory transpulmonary pressure EIPtp is a value indicative of the maximum expansion of the lungs, which occurs during breathing and during mechanical ventilation, and which must not become too great. The end-expiratory transpulmonary pressure EEPtp is a value indicative of the minimal expansion and can indicate the risk that the lungs will collapse, especially at negative values of the expansion with respect to the environment.
In one embodiment, the current values of these three parameters are displayed on the display unit 12.
The transpulmonary pressure Ptp cannot be measured directly. In one embodiment, the transpulmonary pressure Ptp is calculated as the difference between the pressure Paw at the airway and the pressure Pes in the esophagus Sp, i.e.,
P
tp=Paw−Pes.
Correspondingly,
EIP
tp=EIP−EIPes and
EEP
tp=PEEP−EEPes
apply to the parameters shown in
An alternative to determining the transpulmonary pressure Ptp does not need the pressure in the esophagus Sp and therefore it makes do without a probe 10. Such a process is described in EP 2 397 074 B1 (corresponding U.S. Pat. No. 8,701,663 B2 and U.S. Pat. No. 9,655,544 B2 are incorporated herein by reference). The difference ΔPtp between the end-inspiratory transpulmonary pressure EIPtp and the end-expiratory transpulmonary pressure EEPtp is determined in this alternative. The ventilator 9 carries out the mechanical ventilation such that two different values peep1 and peep2 are generated for the end-expiratory pressure PEEP one after another, wherein peep2 is the higher value. Each value leads to a respective value eelv1, eelv2 for the end-expiratory lung volume EELV. The pressure change peep2−peep1 brings about a change in volume, eelv2−eelv1. The difference ΔPtp being sought is calculated, for example, according to the calculation rule
ΔPtp=(eelv2−eelv1)/(peep2−peep1).
The compliance of the respiratory system comprising the lungs and the chest wall will hereinafter be designated by Cresp, the compliance of the lungs by CLung and the compliance of the chest wall by Cew. The reciprocal value 1/C is also called elastance E. Here, 1/Cresp=1/CLung+1/Cew as well as Eresp=ELung+ECew.
The compliance of a system is known to be the quotient of
The respiratory system of the patient P is expanded by the pressure present during inhalation and it contracts again during exhalation. The tidal volume VT can be used as the volume difference AVol. The average compliance of the respiratory system can be described approximately by means of the following lung mechanical model equations:
C
resp=VT/ΔPaw,
C
Lung=VT/ΔPtp,
C
ew=VT/ΔPes.
In one embodiment, the sensor 3 located in front of the mouth of the patient P measures, in addition to the airway pressure Paw, the volume rate of flow Vol′ into and out of the airway of the patient P, i.e., the quantity of gas moving per unit of time. In another embodiment, the optional flow sensor 15 at the ventilator 9 measures this volume rate of flow Vol′. A volume change and especially the tidal volume VT, i.e., the volume that is taken up by the lungs during an individual inhalation process, can be deduced from this volume rate of flow Vol′. The size of the lungs increases by this volume.
It is possible to deduce the compliance of the lungs by means of this volume rate of flow Vol′ and the transpulmonary pressure P. This yields an average compliance. However, the compliance of the lungs varies, as a rule, from one region of the lungs to the next. Individual regions of the lungs may have a markedly lower compliance than other regions. This global method will not therefore lead, as a rule, to satisfactory results. The EIT measuring device with the EIT belt 7 is used in the exemplary embodiment to measure the local compliance of the lungs, i.e., the compliance in certain regions.
The basic idea behind such a measuring device is the following: A series of at least four, for example, 16 electrodes are placed on the skin of the patient P. An alternating current (feed current) with a predefined current intensity or with a current intensity known by measurement is fed between two electrodes of this series. These two electrodes are also called a stimulating electrode pair. The voltage, which results from the feed, is measured at the other electrodes. The impedance Z of the tissue between the two electrodes of the stimulating electrode pair is deduced according to Ohm's law as a quotient of the measured values for the voltage at the other electrodes and the known feed current intensity. The muscles and the blood in the body of the patient P can conduct the measuring current fed better than the pulmonary tissue can because muscles and blood contain more unbound ions. The two respective electrodes used as the stimulating electrode pair are changed in the course of time at a high frequency, and each electrode belongs to the stimulating electrode pair during a respective part of the measurement time period.
The higher the air content in a region of the lungs, the higher is the measured impedance in that region. The air content and hence the impedance increase in a region, as a rule, during the inhalation (generated by intrinsic breathing activity and/or by mechanical ventilation), and they decrease again during exhalation. The regional difference between the impedance at the end of the inhalation and the impedance at the end of the exhalation is correlated with the regional change in the air content in the lungs. A linear relationship, which is determined, e.g., in advance during a calibration, may be used as this correlation. The EIT measuring method therefore yields an image of the regional changes of the air content in the lungs in the body.
Each measuring electrode 1 is connected via a respective cable 2 to a switch or multiplexer 60. The switch or multiplexer 60 applies an a.c. signal to two respective measuring electrodes 1, which will then act as a stimulating electrode pair. The selection of the stimulating electrode pair rotates around the body of the patient P. The remaining 14 measuring electrodes 1 act as measuring electrodes. The voltage signals of the measuring electrodes 1 are fed via the switch or multiplexer 60, via a difference amplifier 62 and via an analog-digital converter 64 to a control and analysis unit 20. The control and analysis unit 20 generates from the voltage signals an image of the regional distribution of the air content in the lungs. To generate this image, the control and analysis unit 20 uses a suitable method for image reconstruction. The generated image can be called an electrical impedance tomography image (EIT image).
The two respective activated measuring electrodes 1 are supplied with alternating current by an a.c. power source 22. The control and analysis unit 20 is connected via a digital-analog converter 21 to the a.c. power source 22 and it actuates same. The alternating current of the a.c. power source 22 is separated galvanically from the switch or multiplexer 60 by means of an isolation transformer or transformer 40.
For example, a measuring electrode 4, which measures the common mode signal on the body of the patient P, is attached to the right leg of the patient P. The signals of the measuring electrode 4 are fed to the control and analysis unit 20 via a measuring amplifier 6 and an analog-digital converter 8 to the control and analysis unit 20. The control and analysis unit 20 comprises one or more processors and memory and is connected to a compensation a.c. power source 30 via a digital-analog converter 29. The control and analysis unit 20 actuates the compensation a.c. power source 30 as to phase and amplitude such that the symmetry of the primary a.c. power source 22 is detuned on the secondary side of the isolation transformer 40, the detuning being such that the common mode signal on the body of the patient P is minimized. The control and analysis unit 20 uses for this regulation the common mode signal, which is measured by the measuring electrode 4 and is subsequently amplified.
The regional change in the impedance and hence the regional change in the air reserve in the lungs of the patient P are determined by means of such an EIT measuring device 17. The regional compliance CLung[Reg] is sought, and the regional compliance is related to a region Reg of the lungs of the patient P. The calculation of the respective compliance shall be carried out for a plurality of regions or even for each region of the lungs. The region Reg or each region Reg is selected to be such that the compliance in this region at a given time can be considered to be equal for the entire region. The regional compliance of the respiratory system of the patient P is designated by Cresp[Reg]. The regional compliance of a lung region may vary over time. In one embodiment, each pixel of the EIT image generated for the lungs is a respective region Reg. Larger regions, i.e., regions with a plurality of pixels of the EIT image, are possible as well. As was already described above, the compliance depends on the change in pressure and the brought-about volume change. The brought-about change in pressure is thus determined, as was just described, by the EIT measuring device 17.
The following lung mechanical model equations are used in the exemplary embodiment:
C
resp[Reg]=ΔZ[Reg]/ΔPaw,
C
Lung[Reg]=ΔZ[Reg]/ΔPtp,
C
ew[Reg]=ΔZ[Reg]/ΔPes.
Here, ΔZ[Reg] designates the change in the electrical resistance (impedance) in the region Reg, i.e., the change relative to a predefined reference value. This regional change in the impedance is determined, as was just described, by the EIT measuring device.
To determine the regional compliance CLung[Reg] of the region Reg of the lungs, measurements are carried out at different settings of the ventilator 9. In one embodiment, the ventilator 9 carries out a maneuver automatically. It is possible that a command variable for this maneuver, which variable is variable over time, is predefined by a person, who is monitoring the maneuver. The end-expiratory pressure PEEP is set in this maneuver at first at a relatively high value, e.g., at the highest possible value, at which the respiratory system of the patient P will not be damaged with certainty. As a result, the respiratory system of the patient P is opened. The end-expiratory pressure PEEP is then set step by step, i.e., incrementally, at a value that is lower than the value set before. Even at the minimal set value the risk is low that the lungs collapse. A determination process is carried out at each set value for the end-expiratory pressure PEEP. For example, the end-expiratory pressure PEEP is set one after another at the eleven values 25 cm H2O, 23 cm H2O, . . . , 5 cm H2O.
It is, of course, possible to reverse the procedure and to start at first with a lower value and then to increase the value step by step. However, the respiratory system of the patient P is not opened at the beginning of the maneuver in this alternative.
Each determination process at a set value for the end-expiratory pressure PEEP comprises the following steps, which are carried out by the control device 16 automatically:
The top part of
The bottom part of
It can be seen that the regional compliance CLung[RegA], CLung[RegB] assumes a maximum CLung,max[RegA], CLung,max[RegB] as a function of the set end-expiratory pressure PEEP.
This maximum varies, as a rule, from one region of the lungs to the next. An essential reason for this is that the effects of the intrinsic weight of the body of the patient P on these regions are different. Other influencing factors may be the regional inflammation, inactive surfactant or edemas. The value of the end-expiratory pressure PEEP at which the regional compliance CLung[Reg] assumes the maximum for this region will be designated by peepmax[Reg] below. The maximum varies, as a rule, from one region to the next.
In one embodiment a respective value for the relative stiffness SLung,rel[Reg] is calculated relative to the maximum, for example, according to the formula
S
Lung,rel[Reg](peep)={CLung,max[Reg]−CLung,max[Reg](peep)}/CLung,max[Reg]*100%={1−ELung,max[Reg]/ELung,max[Reg](peep)}*100%
for reach region Reg of the lungs and for each set value peep of the end-expiratory pressure PEEP.
CLung,max[Reg](peep) is the regional compliance and ELung,max[Reg](peep) is the regional elastance at the peep value for PEEP.
The lower the regional compliance, the higher is this relative stiffness.
In one embodiment of mechanical ventilation, collapse of the lungs of the patient P shall primarily be prevented. A regional collapse value KLung,rel[Reg] is calculated in this application according to the calculation rule
Overdistention of the lungs shall be primarily prevented in another embodiment. A regional hyperdistention value is calculated in this other application according to the calculation rule
These two embodiments may be combined with one another.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
1 16 measuring electrodes of the EIT belt 7
2 Cables, which connect the measuring electrodes 1 to the multiplexer 60
3 Pneumatic sensor in front of the mouth of the patient P; it measures the airway pressure Paw and optionally the volume rate of flow Vol′; it acts as an airway pressure sensor
3.1 Transducer of the sensor 3; it branches off air from the ventilation circuit
3.2 Pressure sensor proper of the sensor 3; it receives fluid streams from the transducer 3.1
4 Measuring electrode at the leg of the patient P; it measures a common mode signal of the body of the patient P against ground
5.1.1, 5.1.2 Pericardial pair of measuring electrodes on the skin of the patient P
5.2.1, 5.2.2 Pair of measuring electrodes on the skin of the patient P near the diaphragm
6 Measuring amplifier
7 EIT belt, placed around the body of the patient P; it comprises a plurality of (e.g., 16) measuring electrodes 1 and a respective cable 2 each per measuring electrode 1; it belongs to the EIT measuring device 17
8 Analog-digital converter
9 Ventilator; it ventilates the patient P mechanically; comprises the display unit 12
10 Probe in the esophagus Sp of the patient P; it measures the esophageal pressure Pes and optionally the gastric pressure Pga
11 Connection piece in the mouth of the patient P, connected to the measuring catheter 14 in the esophagus Sp
12 Display unit of the ventilator 9; it comprises a display screen
13 Gastric probe in the stomach Ma of the patient P; it measures the gastric pressure Pga
14 Measuring catheter in the esophagus Sp of the patient P; connected to the connection piece 11
15 Sensor at the ventilator 9; it measures the volume rate of flow Vol′
16 Control device; it receives signals from the EIT measuring device 17 and from the sensors 3, 10, 15
17 EIT measuring device; it comprises the EIT belt 7—EIT means electrical impedance tomography
20 Control and analysis unit of the EIT measuring device 17
21 Digital-analog converter
29 Digital-analog converter
30 Compensation a.c. power source
60 Multiplexer, to which the cables are connected
Cew Average compliance of the chest wall of the patient P
CLung Average compliance of the lungs of the patient P
CLung[Reg] Regional compliance of the lungs in region Reg
CLung[Reg](peep) Value for the peep value of the regional compliance of the lungs in the region Reg
CLung,max[Reg] Maximum regional compliance of the lungs in region Reg
Cresp Average compliance of the respiratory system comprising the lungs and the chest wall of the patient P
Cresp[Reg] Regional compliance of the respiratory system in a region Reg
ΔP Change in the pressure present at the lungs
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 120 900.2 | Aug 2020 | DE | national |
