A PATIENT VENTILATION SYSTEM WITH GAS IDENTIFICATION MEANS

Technical field

The present invention relates to a patient ventilation system comprising flow regulating and gas mixing means connected to an inspiratory channel of the system wherefrom a gas mixture comprising oxygen and at least a second gas is delivered to the system's proximal tubing, which proximal tubing further is connected to an expiratory channel and connectable to a patient, said system further comprises at least two gas inlets connected to said flow regulating and mixing means, and gas identification means by which said at least second gas supplied to the system via one of said gas inlets can be identified. Such a system is disclosed in EP 1 455 876 Bl.

Background of the invention

Patient ventilation systems are employed in the administration of breathing gas to a patient, particularly in a hospital environment, and operate to control either or both the amount and the composition of the administrated breathing gas. As such, the term "ventilation system" shall encompass in the present context ventilators, respirators and anesthesia machines as well as on-demand type face masks employed in medical environments.

Patients in need of frequent respiratory treatment often show a severe increase in airway resistance. To overcome that resistance, a certain gas pressure is needed for moving gas into and out of the lungs of the patient. The pressure in the airway is directly related to the dynamic pressure gradient during the respiratory cycle, the flow rate of the gas, the density and viscosity of the gas, and the caliber and length of the airway.

It is well known to mix air with oxygen to increase the overall oxygen concentration delivered to the patient. To decrease the pressure required for moving gas through the airways, air can be substituted by "heliox", a mixture of helium and oxygen. As an inert gas, helium does not participate in any biochemical process of the body. However, as helium is the second lightest gas, it decreases the density and by that the required driving pressure. Typically, helium is mixed with at least 21% oxygen but depending on the specific conditions of the patient, this mixture can be altered.

Prior art ventilation systems normally have at least two gas inlets, one of which is connected to an oxygen source and the other to a second gas source such as an air source or a heliox source. If heliox is used, the distribution between helium and oxygen in the heliox mixture is typically 80 % helium and 20 % oxygen (heliox 80/20), or 70 % helium and 30 % oxygen (heliox 70/30). These external gas sources may be provided locally by pressurized bottles. Typically, there are often more gas supplies available for connection to the gas inlets than are required and care must be taken to ensure that the correct supplies are connected, especially as conventional gas sources are supplied with standardized pneumatic connection terminals. The prior art mentioned above discloses a gas identifier, which comprises a voltage divider adapted to provide an electrical interface to the ventilation system and a lookup table. The voltage divider includes a resistor having a resistance value unique for each gas supply. For a specific gas supply, a corresponding voltage drop will result as measured across the resistor. The lookup table comprises a list of voltage drops for the various gases, so that the gas mapping with the voltage drop is obtained from the lookup table.

With such an identification system, there may be an uncertainty if the correct voltage divider has been introduces or not. Therefore, the safety of such a system is deficient and barely provides more certainty than manually identifying the gas supply by simply looking at it and making the correct input to the ventilation system via the interface. In both cases and having in mind the stress situation in an ICU, there is no absolute knowledge about the gas, which actually is delivered to the ventilation system and there is no check up or safety control.

As is similar known, e.g. from the prior art mentioned above, flow meters provide output signals which are dependent on the type of gas, i.e. if a flow meter is calibrated for measuring air, the meters output signal would deviate from the actual flow for another gas type like heliox 80/20. This is true even for other gases like zenon or other gas mixtures. The prior art therefore suggests means for correcting the calibration of any flow meter based on gas supply, which is identified in the above described way.

To increase the safety of any gas supply to a patient ventilation system, EP 1 441 222 A2 discloses monitoring means using a acoustic transceiver detecting the amplitude of the emitted acoustic energy propagated through a measurement chamber and generating a control signal from a comparison of the detected signal with a reference signal for the target gas, and generating a control signal to inhibit the gas flow through the system if the wrong gas is supplied. This prior art gives no hint to use the detected signal for anything else but inhibiting the gas flow.

It is further known to use an oxygen sensor, e.g. an oxygen cell, to measure the oxygen concentration in the ventilation system. Such an oxygen sensor cannot be used to identify other gases or gas mixtures like air or heliox to be mixed with pure oxygen.

Brief summary of the invention

It is an object of the present invention to improve the safety of the identification of any gas or gas mixture connected to the ventilation system via a gas inlet and to provide an automatic correction of the flow meter(s) based on online measured gas identification.

It is another object of the invention to ensure that any possible human mistake in connecting a gas source to an inlet has no influence for the correct functioning of the system.

It is another object of the invention to generate an alarm signal in case the intended and programmed gas is not detected or a deviating gas mixture is detected. To further simplify the overall ventilation system, it is another object of the invention to use already existing components in the ventilation system for the identification purpose.

These objects of the present invention are provided by a patent ventilation system in accordance with the claims.

There are a number of characteristics which differ for different types of gases or gas mixtures, e.g. the speed of sound through the gas or the thermal conductivity. The speed of sound can be measured with an ultrasonic transducer and the thermal conductivity can be measured with a heated thermistor or thermal resistor. However, the invention is not limited to the use of these particular gas characteristics for identifying the gas or gas mixture. Any properties or characteristics that differ from gas to gas, or gas mixture to gas mixture, to an extent that is measurable with the gas identification means, may be used. Since the flow rate measured by conventional flow meters also depends on the gas characteristics, the present invention provides means for automatically correcting the calibration of any flow regulating and gas mixing units and/or flow meters in the ventilation system depending on the online measurement of the type of gas or gas mixture connected to the gas inlet. Since the gas supply is actively measured and identified, the system is not limited to gas bottles with a predefined gas mixture, for example Heliox 70/30 or 80/20, as provided from the suppliers, but will function properly with an arbitrary gas mixture. Thus the system will function very well even in rebreathing setups where expensive gases like Zenon are used, and where the expired gas is directly reused after CO2 has been removed by a filter. In such a situation, the supplied gas will differ in its mixture over time, but the system will always identify the mixture and correct the flow regulation and gas mixing units and/or flow meters accordingly. In addition, the ventilation system will detect if an erroneous gas bottle is unintentionally connected

to the ventilation system. Thereby the ventilation system according to the present invention is less vulnerable to human errors than most prior art ventilation systems.

The gas identification means can be arranged anywhere in the gas flow after the gas inlet, i.e. in the inspiration channel, the proximal tubing, which is connectable to a patient, or the expiration channel. Depending on the actual placement of the identification means, other factors like CO2 or humidity may have to be taken into account. None the less, identification is possible at all places.

In a preferred embodiment of the invention, the output signal from the identification means is displayed on an interface connectable to the ventilation system, to show the user of the system which gas has been identified.

In another preferred embodiment of the invention, the output signal of the identification means will generate an alarm if the connected gas is not identified or if the identifies gas or gas mixture is not allowed, e.g. if 100% helium is identified.

In yet another preferred embodiment of the invention, flow meters already existing in the system are simultaneously used as identification means, preferably flow meters using transit time technology. By that, the number of components in the system can be minimized.

Brief description of the drawings

A more complete appreciation of the invention disclosed herein will be obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying figures briefly described below.

Fig. 1 shows the principal of the gas identification using transit time technology. Fig. 2 shows the same principal for additional gas mixtures and conditions.

Fig. 3 shows a first embodiment of a ventilation system with identification means for gases connected to a gas inlet. Fig. 4 shows a second embodiment of such a system. Fig. 5 shows a flow diagram for a gas exchange in a standby situation. Fig. 6 finally shows a flow diagram for a gas exchange during ventilation.

Detailed description of the invention

Fig. 1 shows the principle of the gas identification using e.g. an ultrasound transceiver which measures the time of flight (TOF) for a sound pulse passing through the gas to be identified or, if it is done in the expiration line of the system, the expired gas including the gas to be identified. In this latter case, humidity and CO2 concentration can be estimated. Fig. 1 shows the TOF over the 02 concentration in percent for air as a solid line and for heliox as a broken line, starting on the left side of the diagram with 20% 02. Since helium concentration has a big influence on the speed of sound, there is a great difference in the time of flight between the sound pulses traversing heliox and the sound pulses traversing an equal distance in air. With a temperature of 37 degrees Celsius, a dry gas, and a specific measurement setup, the TOF for the sound pulse is approximately 122 μs in heliox 79/21 (21% 02), and approximately 222 μs in air. As can be seen from this diagram, increasing the 02 concentration changes the TOF for air only slightly, but for heliox substantially. Over the interval between 21% 02 and 100% 02 the TOF for heliox varies with 110 μs. As a result, the TOF measurements are equal to having a sensitive helium concentration meter and the composition of heliox, i.e. the mixture of helium and oxygen, can be identified with great accuracy. If the measured TOF stays within predefined limits, e.g. ± 5 μs from the expected value for the gas mixture to be supplied, then the gas mixture has been identified. A greater deviation indicates that the wrong gas mixture has been connected to the gas inlet or that the identification does not work properly.

Fig. 2 shows the same diagram as Fig. 1 for compositions where heliox is mixed with 5% C02 and/or has 100% relative humidity (RH). As can be seen from this diagram,

if the CO2 concentration and/or humidity is known or can be estimated, the system still functions in a satisfactory way to identify the correct gas mixture.

Fig. 3 shows a first embodiment of a patient ventilation system with identification means for gases connected to a gas inlet. The system has two gas inlets 1 and 2, one for oxygen and one for air/heliox. From the inlets, the gases are let via inspiratory valves 3 and 4 and flow meters 5 and 6 to an inspiration channel 7, and further via a proximal tubing 8 to the airways of a patient. The expired gas passes through the expiration channel 9, gas identification means 10 and a flow meter 11. The gas identification means 10 can as well be arranged in the inspiration channel or the proximal tubing, without deviating from the general principal of the invention. Gas identification means 10a and 10b are depicted in these places in dashed lines.

In a ventilation system without gas identification means, the output signal from the flow meter 6 is fed to a flow control 16 as actual value. The flow control 16 compares this value with a set value and generates a control signal for the inspiration valve 2. The same closed loop flow control is provided for the 02 supply, but not depicted in the figure.

According to the present invention, the gas identification means generates a signal representative for the measured gas mixture, e.g. air 21/79, heliox 20/80 or heliox 30/70. This signal is fed to means 12 and 13 for correcting the flow value directly measured by the flow meters 6 or 11. Normally, the flow meters are calibrated for air and their output signal would deviate from the actual value for other gases like heliox. The means for correcting 12 or 13 compensate for such a deviation and make sure, that the flow control 16 receives a corrected actual value. In addition, the corrected flow signals are fed to an alarm and/or display (not shown), as indicated by arrows 14 and 15.

In this embodiment, the correction of the flow takes place in the correction means 12, 13. Without deviating from the present invention, these means 12, 13 can be part of

the flow meters 6, 11 so that the output signal from the gas identification means 10 corrects the calibration of the flow meters 6, 11.

Fig. 4 shows a second embodiment of the present invention, in which the same reference numerals as in Fig. 3 are used for similar components.

The only but important difference between the embodiments shown in Fig. 3 and Fig. 4 consists of a specific flow meter 11, which uses transit time technology such as ultra sound propagation to measure the flow. This measurement technology can simultaneously be used to identify the gas passing through the flow meter 11, either by utilizing the speed of sound or the damping of a sound pulse traversing the gas flow, as is generally known in the art. As an advantage, no separate gas identification means is necessary. The output signal from this combined gas identification means/flow meter 10, 11 is fed via line 18 to the flow meter 6 to correct its calibration. It is also possible to include a means for correction between the flow meter 6 and the flow control 16 as in Fig 1. Again, the flow meter 6 could be combined with the gas identification means if the technology for measuring the flow is suitable to measure a value, which depends on a characteristic of the gas to be identified. As mentioned before, other characteristics of the gas to be identified, e.g. the thermal conductivity thereof, could also be used in the identification process without deviating from the principle of the invention.

In all embodiments, the possibility to generate an alarm if the identified gas deviates from the gas the user has chosen, or if no gas is identified, increases the overall safety of the ventilation system. The display on an interface facilitates the understanding of what is going on in the system. Another advantage of this automatic gas identification and flow correcting system according to the invention lies in the possibility to check the gas supply in a pre-use check when a new gas supply is connected to one gas inlet under standby, or even during ventilation. Figs. 5 and 6 show possible flow diagrams for these two cases.

Fig. 5 illustrates a flow diagram for a gas exchange in a standby situation. In step 50, a user changes the gas in the ventilation system and, in step 51 , the gas identification means 10, 10a, 10b detects and identifies the new gas. If the detected gas or gas mixture is not allowed, the procedure proceeds to step 52 in which the system warns the user by means of a suitable alarm signal, e.g. by displaying an alarm symbol on the interface or by generating a sound alarm. If, on the other hand, the detected gas or gas mixture is allowed, the procedure proceeds to step 53 in which the system compensates the set volume, i.e. the breathing gas volume provided to a patient during ventilation, in dependence of the properties of the new gas. Finally, in step 54, the system confirms the gas detection by, e.g., a notification displayed on the interface, and further prompts the user to review the ventilation settings in order to ensure a correspondence between the ventilator settings and the new gas.

Fig. 6 shows a flow diagram for a gas exchange during ventilation. The procedure is identical to the procedure illustrated in Fig. 5, in the case where an allowed gas or gas mixture is detected by the gas identification means 10, 10a, 10b. Consequently, in step 60, a user changes the gas used in the ventilation system whereupon the system detects the new gas in step 61. In step 62, the system compensates the set volume based on the detected gas and, in step 63, the system gives the user feedback on the gas detection and prompts the user to review the ventilator settings.

It should be evident from the foregoing that various modifications can be made to the embodiments of this invention without departing from the scope thereof, which would be apparent to those skilled in the art.