Physical lung model to simulate organ function in health and disease

Abstract

The invention relates to a lung simulator apparatus, as well as to a method to ventilate a lung simulator with a ventilator. The lung simulator apparatus comprises an air chamber with a variable volume for an exchangeable gas, which air chamber is connected in parallel with two air conduits, and a gas exchange element for injecting a tracer gas into the air chamber, wherein the volumes of the air conduits are substantially different. The method of simulating lung function comprises filling a first gas into the air chamber, which has a variable volume and which is connected in parallel with the two air conduits, and injecting a second gas into the air chamber, pressing the first and second gas out of the air chamber, and optionally repeating these steps.

Claims

1. A physical lung simulator apparatus, comprising: an air chamber defining an expandable and contractible air chamber volume for receiving an exchangeable gas; a first air conduit and a second air conduit, each emanating from the air chamber and connecting in parallel to a first end of a common air tube having a single opening at a second end, the first air conduit having a first volume and a second air conduit having a second volume different than the first volume to simulate dead space volume; and a gas exchange element connected to the air chamber for injecting a tracer gas into the air chamber.

2. The apparatus of claim 1, wherein the first volume of the first air conduit differs from the second volume of the second air conduits by at least 1 ml.

3. The apparatus of claim 1, further comprising a controller for affecting or controlling injection of the tracer gas into the air chamber.

4. The apparatus of claims 2, wherein the first volume of the first air conduit differs from the second volume of the second air conduit by between about 10 ml and 100 ml.

5. The apparatus of claim 1, wherein the common air tube comprises at least one sensor for analyzing the tracer gas passing through the common air tube.

6. The apparatus of claims 1, wherein the first volume of the first air conduit and the second volume of the second air conduit are selectively adjustable.

7. The apparatus of claim 1, wherein the gas exchange element comprises a structure that allows penetration of an injecting device.

8. The apparatus of claim 1, further comprising a mechanical actuator for increasing or reducing the air chamber volume of the air chamber.

9. A method of simulating physical lung function, comprising: filling an adjustable volume air chamber with a first gas via a first air conduit and a second air conduit arranged in parallel; injecting a second gas into the adjustable volume air chamber via a gas exchange element connected to the expandable and contractible air chamber; and expelling the first and second gases out of the adjustable volume air chamber via the first and second air conduits in parallel, wherein the first air conduit has a first volume and said second air conduit has a second volume, the first volume being different than the second volume to simulate dead space volume, merging the gases emanating from the gas chamber via the first and second air conduits in a common air tube and expelling the first and second gases via a first opening of the common air tube and further expelling the first and second gases via a second opening of the common air tube.

10. The method of claim 9, wherein a first gas volume of the first gas is at least two times a second gas volume of the second gas.

11. The method of claim 9, further comprising providing the second gas as an inert gas.

12. The method of claim 9, further comprising applying a contracting or expanding force to the adjustable volume air chamber as a function of at least one of time or volume of the adjustable volume air chamber.

13. The method of claim 9, further comprising testing a medical ventilator.

14. A method of simulating physical lung function, comprising: filling a first adjustable volume air chamber with a first gas via a first air conduit and filling a second adjustable volume air chamber with the first gas via a second air conduit; injecting a second gas into the first air chamber via a first gas exchange element connected to the first air chamber; and expelling the first and second gases out of the first and second air chambers via the first and second air conduits in parallel, wherein the first air conduit has a first conduit volume and the second air conduit has a second conduit volume, the first conduit volume being different than the second conduit volume to simulate dead space volume, merging the first and second gases emanating from the first and second air conduits in a common air tube and expelling the gases via one opening at an end of the common air tube.

15. The method of claim 14, wherein a first gas volume of the first gas is at least two times a second gas volume of the second gas.

16. The method of claim 14, further comprising providing the second gas as an inert gas.

17. The method of claim 14, further comprising applying a contracting or expanding force to the first adjustable volume air chamber as a function of at least one of time or volume of the first air chamber.

18. The method of claim 14, further comprising testing a medical ventilator.

19. A physical lung simulator apparatus, comprising: a first adjustable volume air chamber for receiving an exchangeable gas and a second adjustable volume air chamber for receiving the exchangeable gas; a first air conduit and a second air conduit, the first air conduit emanating from the first adjustable volume air chamber, the second air conduit emanating from the second adjustable volume air chamber and the first and second air conduits connected in parallel to a first end of a common air tube having a single opening at a second end, the first air conduit having a first volume and the second air conduit having a second volume different than the first volume to simulate dead space volume; and a first gas exchange element connected to the first adjustable volume air chamber for injecting a tracer gas into the first adjustable volume air chamber.

20. The apparatus of claim 19, wherein the first volume of the first air conduit differs from the second volume of the second air conduit by at least 1 ml.

21. The apparatus of claim 19, further comprising a second gas exchange element connected to the second adjustable volume air chamber for injecting the tracer gas into the second adjustable volume air chamber.

22. The apparatus of claim 19, further comprising a controller for affectinq or controlling the tracer gas injection into at least one of the first and second adjustable volume air chambers.

23. The apparatus of claims 19, wherein the first volume of the first air conduit differs from the second volume of the second air conduit by between about 10 ml and 100 ml.

24. The apparatus of claim 19, wherein the common air tube comprises at least one sensor for analyzing the tracer gas passing through the common air tube by measuring a partial pressure of the tracer gas.

25. The apparatus of claims 19, wherein the first volume of the first air conduit and the second volume of the second air conduit are each selectively adjustable.

26. The apparatus of claim 19, wherein the first gas exchange element comprises a structure that allows penetration of an injecting device.

27. The apparatus of claim 19, further comprising a mechanical actuator for increasing or reducing a first air chamber volume of the adjustable volume air chamber.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a graph of a Venegas equation

(2) FIG. 2 shows schematically a simplification of a chest compartment of a human being.

(3) FIG. 3 shows schematically an embodiment of an apparatus according to the invention

(4) FIG. 4 shows schematically a further embodiment of an apparatus according to the invention.

DETAILED DESCRIPTION OF THE FIGURES

(5) The diagram shown in FIG. 1 is related to the Venegas equation, an equation relating mathematically the pressure and the volume of a gas in a compartment, its x-axis containing the pressure values in cm H.sub.2O and its y-axis containing the volume values in ml. The graph changes its form depending on the parameters a, b, c and d.

(6) FIG. 2 shows a simplified schematic of the lung physiology. The reference numeral 91 is related to a chest room within a chest wall 92. Within this chest room 91 are three organs, that is the lung 93 with two lung halves, the heart 95 (naturally also with two ventricles and two atria, that are not shown), and the lung muscles 97 simplified as a kind of diaphragm. The heart 95 is arranged between a venous blood vessel 94 and an arterial blood vessel 96. The pulmonary circulation is not shown. Further an airway 99 is shown, connecting the lung to the exterior of the chest room. The forces relevant for the cardio-pulmonary interaction therefore are defined if the airway resistance, the compliance of the chest wall 92, the muscle activity of the lung muscles 97, the blood pressure, the gas pressure in the lung, the condition of the heart, as well as gas parameters as the O.sub.2 and the CO.sub.2 concentration in the gas filling the lung are known and are interdependently related in a manner that is called human physiology.

(7) Such simplified lung is technically reproduced as shown in FIG. 3.

(8) In the embodiment of FIG. 3 a chest room as well as a chest wall is left out. The technical reproduction as shown in FIG. 3, generally called apparatus 11 with a lung simulator, has at least one lung chamber 13 with air compartments configured as a bellows 15. On such bellows, corresponding tubes 22 and 23 having different volumes are arranged to be joined at the airway 17. The airway 17 comprises one opening 18 to which a ventilator may be connected. In FIG. 3 and FIG. 4 the cross-sectional area of the tube 22 between the tube end points 24 and 24′ and the cross-sectional area of the tube 23 at end point 24 and extending perpendicular to the longitudinal axis of the tube 23 mark the transition from the air conduits, i.e. tube 22 and tube 23, to the airway 17. A base 21 of the bellows 15 can move downwardly when air is introduced through the airway opening 18 to the air chamber 13. Connected to the base 21 there is an actuator, for example a linear motor 33.

(9) Tubes 22, 23 and/or airway tube 17 may be constructed in such a way that their total volume and/or their individual volumes are adjustable, i.e. controllable by an experimenter.

(10) A control unit 43 is provided in the form of a microprocessor board that reads the values of the sensors and controls the position of the base 21 according to the equations given in the text.

(11) Further the embodiments are provided with a mass flow controller 39 to inject CO.sub.2 into the bellows 15 depending on the dead space desired, and conduit 49 connecting a tracer gas source, e.g. a CO.sub.2 tank or supply, with the valves 39′. Conduit tube 49 may advantageously penetrate into the air chamber 15. At least the end 50 of the conduit tube 49, which penetrates into the air chamber 15, is elastic so that it may be pushed by base 21. The opening of end 50 is located close to the base 21 (also during movement of the base 21) and closer to the base than to the outlets 19 and 19′, where the air conduits 22 and 23 contact the air chamber 15.

(12) A further simplified lung is technically reproduced as shown in FIG. 4.

(13) In the embodiment of FIG. 4 a chest room as well as a chest wall is left out. The technical reproduction as shown in FIG. 3, generally called apparatus 11 with a lung simulator, has at least two lung chambers 13 and 13′ with air compartments configured as a bellows 15 and 15′. On each bellows, a corresponding tube 22 and 23 having different volume are arranged to be joined at the airway opening 18 to which a ventilator may be connected. A base 21 of the bellow 15 can move downwardly when air is introduced through the airway opening 18 to the air chambers 13 and 13′. Connected to the base 21 there is an actuator, for example a linear motor 33.

(14) A control unit 43 is provided in the form of a microprocessor board that reads the values of the sensors and controls the position of the base 21 according to the equations given in the text.

(15) Further the embodiments are provided with a mass flow controller 39 to inject CO.sub.2 into the bellows 15 and 15′ depending on the dead space desired, and conduits 49 and 49′ connecting a CO.sub.2 tank or supply with the valves 39 and 39′.

(16) Conduit tubes 49 and 49′ may advantageously penetrate into the air chambers 15 and 15′ similarly as depicted in FIG. 3 for conduit tube 49. At least the ends 50 and 50′ of the conduit tubes 49 and 49′, which penetrate into the air chambers 15 and 15′, are elastic so that the tubes 49 and 49′ may be pushed by base 21. The openings of the penetrating conduit tubes 49 and 49′ are located close to the base 21 (also during movement of the base 21) and closer to the base than to the outlets 19 and 19′ of the air conduits 15 and 15′.

(17) In the embodiments according to FIGS. 3 and 4, the disclosed invention encompasses a mechanical frame 31 with bellows 15 and 15′, respectively, attached to that frame 31 and fitted, for example, with a standard 22 mm connector as an airway opening 18, one single linear motor assembly 33 with integrated displacement sensor and long range of motion, typically 30 cm, an oxygen sensor 35 to measure the oxygen content within the bellows 15, a pressure sensor 37 to measure the pressure within the bellows 15, a mass flow controller (not shown) to inject CO.sub.2 into the bellows 15 or 15′, respectively, depending on the valve settings, and a control unit 43 forming at least a part of a central processing unit (CPU) and software the CPU is working with containing a physiological model with a plurality of pathological models. With this software and the CPU 43 the different characteristic equations and parameters of the equations given further down are applied in order to simulate the different patient types and pathologies. The control unit is fitted with at least one communication port 27 to modify the said parameters externally, for example using a PC, in order to create different sets of patients and pathologies.

(18) Above examples are intended to illustrate the art of the present invention and are not intended to limit the scope of the claims below.

(19) Description of Use

(20) The physical lung model described herein can be connected directly to a ventilator or a breathing support device. The results of the ventilation on lung mechanics and gas exchange, including hemodynamic effect, can then be assessed immediately and directly on the physical lung model. A pulse oximeter can be attached to monitor the oxygen saturation and hemodynamic effects of the ventilation. Alternatively, a display unit can be used.

(21) A trainee can judge the result of ventilation immediately, respond to it, and follow the course of a patient in any given curriculum selected by the assigned trainer. Such sophisticated training is hitherto not available due to the lack of a pertinent patient simulator. The present invention fills this gap.

(22) If the attached ventilator is closed-loop controlled, step responses can be measured by selection of a parameter set that simulates lung collapse, loss of blood, or increase in metabolic rate via increase of CO.sub.2 production. Such step response measurements are required by IEC/ISO 60601-1-12 but can currently not be measured because of lack of a suitable physical lung model. The present invention fills this gap too.

(23) Another use of the physical lung model is to test on-airway sensors such as flow transducers or gas analyzers. For this purpose, the devices under test can be connected directly to the physical lung model described herein. Different breathing patterns can be created with the lung model thereby providing reference signals for the devices under test. The signals of the devices under test can be compared with the signal of the lung model which serve as reference, and analyzed for accuracy and precision.

(24) Description of the Function of the Embodiments and Examples

(25) The control unit does not only read the sensor values several times per second. The control unit also controls the motor or motors at the same rate it reads the sensor samples with an electrical current driver according to the equations given above depending on the lung pathology and respiratory muscle activity chosen by the user. Any attached ventilator, whether it is a high level medical ventilator with closed loop technology or a CPAP device or another respiratory support device, will interact with the settings of the invention and create gas flow into or out of the lung simulator. The result of this gas movement is recorded by virtue of the displacement sensor (integrated displacement sensor) and converted into flow and volume by multiplication of the displacement by the active area within the bellows as follows:
V.sub.L(t)=x(t)*A.sub.L+V.sub.RC  [19]

(26) Where x(t) is the current position of the bellows. Flow is simply the first derivative of the above equation. Instantaneous pressure within the bellows P.sub.bellows is measured. Volume, flow, and pressure are stored and used for the following calculations. The values can be exported to an attached PC for documentation. Next, the instantaneous pressure within the bellows is measured and the instantaneous value for P.sub.cardio is calculated as given in Equation 17 and the resulting instantaneous value for the current amplitude for the pulse oximeter plethysmogram is calculated according to Equation 18. The instantaneous A.sub.PO values are sent to the peripheral circulation compartment to control the emitted light from that circulation compartment.

(27) In one embodiment, the control unit controls the value of dead space by injecting the CO.sub.2 into either one of the at least two air compartments. The control unit also adjust the mass flow controller which delivers pure CO.sub.2 gas into the bellows to simulate CO.sub.2 production V′.sub.CO2.

(28) For each breath, the actual value of V.sub.Lee and P.sub.O2 is measured and the resulting oxygen saturation S.sub.aO2 is calculated according to equation 15. The control unit sends the S.sub.aO2 to the peripheral circulation compartment which then adjusts the light intensity accordingly and thereby simulates oxygen saturation.

(29) A breath is defined, for example, as a change in lung volume that is larger than the dead space Vd.

(30) In a further embodiment, the control unit controls a restrictor at the entrance to the bellows to control the resistance to flow, R.sub.aw. R.sub.aw is calculated from measured levels of pressure across the restrictor and measured displacement of the bellows.

(31) The control unit reads the values of the lung model parameters from an internal memory (default values), from an attached user interface as for example a keyboard, or from a data interface, for example an RS232 or ETHERNET connection. The parameters of the model are C.sub.W, V′.sub.CO2, level of hemodynamic stability, respiratory activity (P.sub.0.1), form and amplitude of P.sub.musc(t), minimal Venegas parameters a.sub.min, b.sub.min, c,d, collapse/recruitment threshold P.sub.threshold, RC.sub.lh, RC.sub.c FRC.sub.pred, T.sub.delay, dead space Vd, recruitment factor Cr, and the parameters inside table1. Typically, also the time and pressure dependency of the Venegas parameters “a” and “b” are transmitted. Barometric pressure and temperature can be either transmitted via the data interface or measured directly by the lung model by using appropriate sensors. The bellows size, type, and the associated residual volume of the collapsed bellows V.sub.RC, can be read via the data interface or by virtue of an inherent code that can be read by the control unit from the bellows label. The parameter A.sub.L, i.e. the active surface of the bellows, can be derived thereof.

(32) The present invention allows to model normal respiration, the lung in disease, as well as a combination thereof by adjusting said lung model parameters, as illustrated by the following examples.

(33) Normal lungs: by injection of CO.sub.2 into air chamber which connects to the airway opening with low volume tube, for example 50 ml for adults. This yields a total of 100 ml dead space.

(34) Pulmonary embolism: by injection of CO.sub.2 into air chamber which connects to the airway opening with high volume tube, for example 100 ml for adults. This yields a total of 200 ml dead space.

(35) Paralysis (no muscle activity) or week breathing: by setting P.sub.musc(t) to zero or to very low values, for example −3 hPa for 1 second and to 0 for 2 seconds.

(36) Strong respiratory activity: by letting P.sub.musc(t) increase to −10 hPa within 0.1 second and keeping it there 1 second and thereafter increasing P.sub.musc(t) to 10 hPa for 1 second and repeating this process yielding 30 strong breaths per minute.

(37) Stiff lungs: by setting the Venegas parameters a, b, and c to low values, for example 20% of predicted FRC, 40% of predicted FRC, and 30 hPa, respectively.

(38) Lung collapse: by setting the Venegas parameters “a” and “b” to low values, for example 1000 ml and 2000 ml, respectively and/or by letting “a” and “b” collapse with time down to a minimal value of, for example 700 ml and 1500 ml, respectively. As a result of such manipulation of the basic parameters, the oxygenation of the arterial blood will become worse, since S.sub.aO2 depends on the level of V.sub.Lee. The degree of such deterioration is highest without external respiratory support and will improve with external respiratory support, for example with positive end expiratory pressure.

(39) Weak circulation: if there is not enough blood in the vessels, the heart, and particularly a weak heart, will not be able to completely counteract the forces caused by a ventilator. This deficiency can be made visible by making the Pulse Oximetry Plethysmogram variation POP.sub.v dependent on intra-thoracic pressure. Intra-thoracic pressure is a combination of the forces within the thoracic cavity and the forces applied to the thoracic cavity. Intra-thoracic pressure is created by the recoil forces of lung and chest wall plus the action of the respiratory muscles and the pressure applied by a ventilator. Lung recoil pressure, chest wall elasticity and respiratory muscle activity are determined by the lung model itself. They can said to be “internal forces”. In contrast, the pressure applied by a ventilator or respiratory support device is an “external force”, which is independent on the lungs. The advantage of the illustrated embodiment of the present invention is that the two sources of force interact with each other as described by equation 18. The pressure P.sub.cardio, derived from the low pass filtered pleural pressure surrogate P.sub.pl, can therefore be taken as predictor of POP.sub.v in different hemodynamic conditions, as described in the Table 1:

(40) TABLE-US-00001 TABLE 1 Example of link between level of hemodynamic stability and POP.sub.v for three different levels of simulated hemodynamic stabilities: “Stable hemodynamics”, “Moderate instability”, and “Severe instability” Stable Moderate hemodynamics instability Severe instability P.sub.cardio POP.sub.v P.sub.cardio POP.sub.v P.sub.cardio POP.sub.v 10hPa 6% 10hPa 10% 10hPa 15% 20hPa 7% 20hPa 13% 20hPa 20% 30hPa 8% 30hPa 30% 30hPa 40%

(41) Although the present invention has been described in considerable detail and with reference to certain versions thereof, other versions are possible.

(42) In summary, the invention relates to a lung simulator apparatus, as well as to a method to ventilate a lung simulator with a ventilator. The invention solves the problem of such apparatuses, that physiology is not accurately represented, by the new step of calculating a change of values of the physiological parameters dependent on the measured values and based on a physiological model defining the dependencies between the values of the physical and the physiological parameters. An embodiment of such apparatus has an adjustable dead space, a motor driven set of bellows and controls a cardio-vascular interface presenting cardio-pulmonary parameters.

(43) Definitions of Abbreviations: a end expiratory volume of the lung compartment in the Venegas equation A.sub.L is the active surface of the bellows A.sub.PO(t) the instantaneous effect of the actual lung pressure on the actual amplitude of the pulse ARDS acute respiratory distress syndrome b breathing volume of the air chamber (lung) in the Venegas equation c parameter of the Venegas equation C.sub.a the oxygen content of the arterial blood C.sub.c the oxygen content of the capillary blood, C.sub.v the oxygen content of mixed venous blood C.sub.L lung part of C.sub.rs, C.sub.r determines how much recruitment can be done C.sub.rs total respiratory compliance C.sub.w chest wall part of C.sub.rs, COPD chronic obstructive lung disease d parameter of the Venegas equation natural constant FRC functional residual capacity, FRC.sub.pred predicted FRC, typically for a healthy lung of a certain patient size i.sub.corr corrected voice coil current i.sub.vc voice coil current k a constant for a particular voice coil ln the natural logarithm P.sub.aw pressure at the airway opening P.sub.bellows pressure within the compressible compartments P.sub.cardio low-pass filtered pleural pressure P.sub.diff partial pressure at which the blood is completely saturated P.sub.L theoretical lung pressure P.sub.musc muscular activity P.sub.O2 partial pressure of O2 in the air chamber P.sub.pl pleural pressure P.sub.threshold collapse/recruitment threshold pressure P.sub.vc pressure of the voice coil POP.sub.v Pulse-Oximetry Plethysmogram variation Q.sub.s the ml/min of blood not exchanging gas with the lung Q.sub.t the total blood flow R.sub.aw airways resistance RC.sub.c the time constant of collapse and recruitment RC.sub.lh the time constant of the lung-heart transfer function or the time constant with which the lung pressure impacts the blood pressure S.sub.aO2 oxygen saturation in the arterial blood S.sub.c oxygen saturation in the capillaries, S.sub.v oxygen saturation in the venous blood S.sub.c oxygen saturation in the alveolar capillaries (t) as a function of time V′.sub.CO2 CO.sub.2 release V.sub.dS, Vd. dead space V.sub.L volume inside the lungs V.sub.Lee actual lung volume at the end of exhalation V.sub.RC the volume within the bellows when they are fully compressed x(t) is the current position of the bellows as a function of time z.sub.lh =dt/(RC.sub.lh+dt) defining the responsiveness on a recruitment maneuver