Apparatus for determining when a partially or completely collapsed lung has been opened up

Abstract: In the course of opening up an alveolus by supplying a gas pressure the alveolus will generate a crackle sound. By listening to the crackle sounds via microphones (6) an apparatus (4) can determine when the lung is completely open. The apparatus (4) comprises a filter (10) for filtering out the relevant frequencies for the crackle sounds and a determination unit (12) for identifying the crackles and for determining when the lung is open based on the filtered out sounds.

Description: The present invention relates to an apparatus for determining when a partially or completely collapsed lung has been opened up. In SE-501 560 the importance of opening up a collapsed lung and keeping it open is described. A ventilator which can calculate an opening pressure for a lung based on measured pressure and volume values is also disclosed. When correlating pressure and volume for a collapsed lung during the opening procedure an inflection point will normally be present in the curve obtained. This inflection point can be determined mathematically from the obtained curve and represent an opening pressure for the lung. However, determining the inflection point is difficult. The calculated opening pressure will vary depending on the mathematical method used, this in particular since the inflection point is not sharp and easy to identify. The reason for this is that the alveoli do not open up all at once, but rather in portions forming small clusters or compartments within the lungs. Therefore, the opening up of small compartments of alveoli at different pressures will result in several smaller inflection points in the pressure -volume curve. Some of these compartments (or alveoli) can require a higher pressure in order to be opened up. When such compartments are present and treatment is given based on the calculated opening pressure, such compartments of the lungs remains closed during the entire respiratory cycle, resulting in an insufficient or at least impeded gas exchange for the patient. The pressure - volume curve analysis described above does therefore not indicate when the lung is opened up. There are no known apparatus which can determine when the lung is opened up or at which pressure the lung is opened up. An object of the present invention is to achieve an apparatus for determining when the lung of a subject is opened up in a new and more accurate manner. Yet another object of the invention is to achieve a ventilator incorporating such an apparatus. The first object is achieved in accordance with the invention in that the apparatus is devised as disclosed in claim 1. A closed alveolus is very stiff. When a gas is supplied to the alveolus at an steadily increasing pressure, the pressure must be increased up to a certain threshold, which is specific for each alveolus. Then the alveolus starts to open up. As the alveolus opens up, the compliance will increase rapidly and the alveolus fills with gas until a steady state is reached. The steady state will be at the same pressure as when the alveolus started to open up (if gas is supplied - otherwise the pressure will have decreased), but the alveolus will now comprise a specific volume of gas. During the rapid increase in the compliance, as the alveolus opens up, a crackle sound is generated. This crackle sound is very specific for the alveoli as they open up. By recording, measuring or in any manner picking up the sounds from the lung, these crackles can be filtered out and identified by a suitable calculation means, such as a computer or similar device. When the crackle sounds have ebbed away, the lung is open and the supplied pressure corresponds to the opening pressure. More specifically, a more correct opening pressure can be determined by applying a suitable pressure ramp for the gas supplied to the lungs and register both pressure and crackle sounds (similar to when forming a pressure - volume diagram). Since the crackle sounds are much more definite than volume changes as to both presence and source, it is much easier to read out the opening pressure from a correlated pressure - sound recording. Lung sound detection systems are known per se, as e. g. disclosed in EP - 491 781. However, these systems are generally directed to the picking up of adventitious lung sounds for diagnosing different lung diseases and do not recognise or suggest the possibility of determining when a collapsed lung has been opened up. Improvements of the apparatus are disclosed in the dependent claims to claim 1. In particular, by using a plurality of microphones at different locations around the lung region, the location of the crackle sounds and information regarding which compartments that are being opened up can also be determined. It is particularly advantageous to use the apparatus above in combination with a ventilator, whereby the supply of respiratory gas from the ventilator to the lung can be controlled based on, inter alia, the determining of at which supplied pressure the lung is opened up. The use can be improved by determining when different compartments of the lung open up. When used over a plurality of consecutive respiratory cycles, the combined use of the apparatus and the ventilator can be utilised to determine which parts of the lung close during each respiratory cycle. This is of particular importance when the lungs only collapse partially at the end of each respiratory cycle. It is, in a similar manner, also possible to determine a lowest acceptable pressure at which the lungs are sufficiently open during the entire respiratory cycle. It is in this case also an advantage to incorporate the apparatus according to the invention completely into a ventilator. In the following the apparatus according to the invention shall be described in more detail with reference to the figures, where Fig. 1 shows a patient connected to the apparatus according to the invention and to a ventilator, and Fig. 2 shows a pressure and volume curve for an alveolus. Fig. 1 shows a patient 2 connected to an apparatus 4 adapted to determine when the lungs of the patient 2 have opened up. A number of microphones 6 are attached at the chest wall on the patient 2 for picking up sound from same. The microphones 6 are connected to the apparatus 4 via signal lines 8. The microphones can be connected with a small air space, approximately 5 - 15 ml of volume, to the chest wall. This air space provides an air coupling to the chest wall. The apparatus 4 comprises a filtering unit 10 and a determination unit 12. The filtering unit 10 filters out sound which is related to the crackle sound which occurs when an alveolus or several alveoli open up. The filtering unit 10 can include e. g. a third order Butterworth high pass filter with a cut off frequency of 50 Hz or any other suitable filter. The determination unit 12 be devised as an analogue or digital unit. The determination can be carried out by means of software or hardware within the determination unit 12. When using digital systems, the filtered signal can be amplified and digitised using a sampling frequency of, for instance, 20 kHz. A suitable algorithm for detecting the crackles is also implemented in the determination unit. For instance, Fourier analysis can be applied. The crackle sounds are known per se, and there exist algorithms which selectively can identify the corresponding signals. Also connected to the patient 2 is a ventilator 14. The ventilator 14 comprises a tubing system 16 for supplying respiratory gas to and removing respiratory gas from the patient 2. Both the apparatus 4 and the ventilator 14 are connected to a control unit 18, which comprises a screen 20 for displaying information about the treatment and condition of the patient 2 as well as the measurements made on the patient 2. Via the control unit 18 the ventilator 14 can be controlled based on, inter alia, the analysis of the lung sounds made by the apparatus 4. In fig. 2 a pressure and volume curve of an individual alveolus is shown. As a pressure is supplied to the alveolus, as shown in the first part 22 of the curve, pressure will increase relatively rapid while only a very small volume of gas enters the alveolus. As the supplied pressure reaches a certain pressure, the opening pressure P O , a rapid change occurs, indicated by a second part 24 of the curve. The pressure will drop significantly as the compliance of the alveolus rapidly increases and gas will flow into the alveolus. Then both the pressure and the volume increases until, at point 26, an equilibrium between the pressure in the alveolus and the supplied pressure has been reached. If a further pressure is supplied at this stage, the volume will increase in a more or less linear fashion until the limit of expansion for the alveolus is reached. At that point, the pressure will again increase rapidly in relation to the volume increase. However, such pressures should be avoided since they not only can damage the alveolus and rupture the tissue, but can also compress the important capillaries near the alveolus and impede the gas exchange. The determination of when the lung should be considered open can be made by awaiting the complete ebbing away of crackle sounds. Such an approach could be particularly useful when the lung is only partially collapsed and otherwise in a fairly good condition. As an alternative, crackles from different compartments of the lung can be identified and used as an indication of when a sufficient number of important compartments have opened up. This is then a marker for when the lung is sufficiently open. Another way of utilising the sound for determining when the lung is open is to set a threshold either for a frequency or a number of crackles to be reached. Yet another way of utilising the sound, is for determining the opening pressure in a more accurate manner. For instance, gas displaying a pressure ramp is supplied to the partially collapsed lung and pressure and sound are registered (similar to obtaining a pressure - volume, or pressure - flow diagram). Based on the registrations, which can be made for one or several respiratory cycles, the pressure at which basically all alveoli has opened up can be determined. The crackle sounds are distinct and only related to alveoli which are opened up. The correlating opening pressure is therefore much easier to determine than when using volume changes, which depend more on the volume change in the opened up alveoli than the opening up of alveoli - at least for the partially collapsed lung. The determined opening pressure can then be used for setting an appropriate peak inspiratory pressure, PIP, which will ensure that the lungs open up sufficiently during each respiratory cycle. The treatment can thus be optimised so that the pressure supplied to the lungs (PIP) can be minimised while obtaining a non-impeded gas exchange in the lungs. The apparatus 4 can also be used for controlling the ventilator 14 in other ways. For instance the end pressure of respiratory gas supplied by the ventilator (PEEP) may be increased over a consecutive number of respiratory cycles, until the apparatus 4 no longer detects any crackle sounds. In other words, until the lung is completely open during the entire respiratory cycle. Hereby, the lowest pressure at which the lung remains open at all times can be determined by means of the apparatus 4 and used in the control of the ventilator 14 as a set value for PEEP. The apparatus 4 can also be used in a reverse manner for controlling the ventilator 14. I. e. the supplied end pressure respiratory gas can be decreased in small steps over a number of respiratory cycles (maintaining a sufficiently high peak pressure for all cycles) and when crackles begin to appear during inspiration phases it can be established that some alveoli collapse at the end of the respiratory cycle. By allowing a certain amount of alveoli in non-dependent compartments to collapse, a lower end pressure can be supplied. This systematic lowering of the pressure can also be used as the lung recovers and more alveoli remains open at lower pressures. The upper pressure can normally also be decreased in these cases. Since crackles normally begin in the zone between the dependent and the non-dependent parts of the lungs, the determination of where the alveoli open up is important. By registering when alveoli in the important dependent parts of the lungs have started to open up, a different threshold can be set for determining when the lung is open. It is possible to use the apparatus 4 as a completely separate unit for determining when the lung has been opened up, i. e. without the control unit 18. The apparatus can also be completely integrated with the ventilator 14 (forming a new advanced ventilator) or any other device intended for opening up partially or completely collapsed lungs.

Claims: Apparatus (4) for determining when a partially or completely collapsed lung of a subject (2) has been opened up by a gas pressure supplied from a ventilator (14) or similar device, comprising a sound receiver (6) for detecting audible sound signals from the lung, a filtering unit (10) for selectively filter out sound signals related to the sound generated when an alveolus is opened up and a determination unit (12) for, based on the filtered out sound signals, determining when the lung is sufficiently open. Apparatus according to claim 1, characterized in that the sound receiver (6) comprises a plurality of microphones and that the determination unit (12) determines the location of the alveoli that are opened up. Apparatus according to claim 1 or 2, characterized in that the determination unit (12) is adapted to determine that the lung is opened up when alveoli in specific compartments of the lung are opened up. Apparatus according to claim 1 or 2, characterized in that the determination unit (12) is adapted to determine that the lung is opened up when the sound generated by the opening up of alveoli reaches a specific threshold, in particular a threshold related to frequency of generated sounds or number of generated sounds. Ventilator (14) for supplying a respiratory gas to the lung of a subject, characterized in that the ventilator comprises an apparatus according to any of the claims 1-4. Ventilator according to claim 5, characterized in that the ventilator further comprises a control unit (18) for controling the supply of respiratory gas from the ventilator (14) to the lung of the subject (2) based on, inter alia, the information relating to when the lung is opened up, obtained from the apparatus (4). Ventilator according to claim 6, characterized in that the control unit (18) is adapted to control the ventilator (14) to supply an increasing gas pressure until a specific sound response from the lung is detected by the apparatus (4). Ventilator according to claim 6 or 7, characterized in that the control unit (18) is adapted to determine a lowest gas pressure (PEEP) which keeps the lungs sufficiently open during an entire respiratory cycle by controlling the ventilator (14) to deliver a sequence of respiratory cycles with varying PEEP and correlating this with the information obtained from the apparatus (4).

Inventors: BOEHM STEPHAN, HUYGEN PAUL, LACHMANN BURKHARD, BOEHM, STEPHAN, HUYGEN, PAUL, LACHMANN, BURKHARD, Böhm, Stephan
Applicants: MAQUET CRITICAL CARE AB
Application year: 1997
Main classification: A61B5
IPC codes: A61B8, A61B7, A61B5