Abstract:
A respiratory aid system has a device arranged for connection to a living creature in order to facilitate, support and/or control the respiration of the living creature. The device can be a ventilator or a neuroelectrical stimulation device. For faster and more efficient and more accurate respiratory aid the system contains a respiration detector, devised to pick up neuroelectrical signals from the living creature, identify respiration-related signals and send a control signal, related to the identified signals, to the device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a method for identifying physiological attempts at respiration, as well as to a respiration detector and a respiratory aid system operating in accordance with the method. 
     2. Description of the Prior Art 
     Disease and injuries can make it necessary for a patient to receive respiratory aid. In principle, this respiratory aid can cover everything from facilitating a patient&#39;s spontaneous breathing to complete control of the patient&#39;s breathing. The type and degree of respiratory aid which is supplied depends, on the nature and scope of the disease/injury and the patient&#39;s treatment needs. Patients receiving anaesthesia (regardless of whether it is supplied because of disease/injury, for cosmetic surgery or for some other reason) may also need respiratory aid. 
     It should be noted that “patient” in this context relates to both people and animals. 
     Today, the most common way of providing respiratory aid is with ventilators (respirators) delivering a pressurised gas to the lungs. A number of ventilation modes are known, depending on which respiratory aid to supply. See for instance the article “Mechanical Ventilation”, Chest 1993; 104; 1833-1859, which includes a discussion relating to objectives and recommendations regarding mechanical ventilation. 
     Another way to provide respiratory aid is to apply extracorporeal pressure around the entire thorax and midriff (e.g. the classic ‘iron lung’ used for e.g. patients whose respiratory muscles have been damaged by polio). A device of this kind is disclosed in U.S. Pat. No. 5,222,478. 
     Another way to induce breathing is by means of electrical stimulation. Either by direct stimulation of the muscles of the diaphragm in particular or by indirect stimulation via the nervous system, in particular the phrenic nerve. A diaphragmatic pacer is disclosed in U.S. Pat. No. 5,056,519. A nerve stimulator for treatment of respiratory disorders is disclosed in PCT publication WO 93/01862. 
     Stimulation of the respiratory system can also be made magnetically, as disclosed in U.S. Pat. No. 5,857,957. 
     One problem shared by all respiratory aid is to obtain information on when the patient needs to breathe and how much the patient needs to breathe. This is particularly the case for completely controlled respiratory aid with mechanical ventilation. 
     In completely controlled respiratory aid the physician bears the main responsibility for programming appropriate respiration parameters, such as respiratory rate, tidal volume, inspiration duration etc. 
     Measurement of physical parameters related to metabolism, such as oxygenation of blood, the carbon dioxide content of blood etc can be of help for the physician when programming the respiration parameters. Some of the physical parameters can be estimated from accurate analysis of the contents of expired gas. The measurements of physiological are too imprecise to provide enough information for individual breath to breath control but can supply important information for the overall regulation of respiratory rate etc. 
     For patients that can breath spontaneously, at least in part, information of the patients breathing can be determined. When pressure, flow and/or changes in temperature inside or outside the patient&#39;s airways are sensed, the patient&#39;s spontaneous attempts to breathe can be detected and used for triggering the respiratory aid. The magnitude of each attempt to breathe can also be determined and used to affect the magnitude or type of respiratory aid. This option is mainly useful in supported respiration aid. 
     One problem in supported respiration aid is that the patient&#39;s spontaneous attempts of breathing come in conflict with the supportive respiration aid. This is normally referred to as competition. One way of avoiding competition is to sedate the patient and use completely controlled respiratory aid only. 
     The above mentioned physical parameters can of course also be used for spontaneously breathing patients for determining when a patient needs to breathe. 
     Lung movements can also be measured by measuring the impedance across the chest (and lungs). Lung movements are also indicative of attempts to breathe. 
     Direct measurement of the musculature participating in respiration can also provide information relating to attempts to breathe. 
     Common to all these procedures is the circumstance that none of them provide an accurate physiological picture of the patient&#39;s true respiratory needs. 
     Breathing is part of the body&#39;s autonomic system. One important known factor affecting this autonomic system is the body&#39;s own chemical receptors that sense carbon dioxide content. But the autonomic system is also affected by a number of other physiological factors. Some of these factors are probably still unknown to the medical expertise. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a respiratory aid system designed for determining, more reliably and efficiently than in known systems, when a patient wishes to or needs to breathe. 
     Another object of the invention is to provide a respiration detector capable of more rapid and efficient sensing of a patient&#39;s attempts at breathing than in known detectors. 
     Another object of the invention is to provide a method for identifying physiological attempts to breathe. 
     A patient&#39;s true respiratory need can be established by registering neuroelectrical signals and extracting the signal components related to respiration. In other words, the nerve impulses are detected and used to extract information. The nerve impulses contain information about a patient&#39;s respiratory needs that automatically comprises all physiological functions with an impact on respiration. True respiratory needs can be utilised for controlling a device supplying respiratory aid. In particular, the point in time for each individual breath (initiated by the patient) can then be determined from the neurological signals, but these signals also contain about all the information essential to individual breaths. 
     In principle, all kinds of known apparatuses for respiratory aid, such as ventilators, anaesthetic machines, nerve stimulators, muscle stimulators and magnetic stimulators, can be used, where or when appropriate with regard to the conditions in each specific case. 
     Neuroelectrical signals (nerve impulses) related to respiration originate in the respiratory centre of the medulla oblongata, and these signals can, in principle, be collected from the respiratory centre or picked up along the nerves carrying the signals to the respiratory musculature. The phrenic nerve has proved to be particularly suitable for use as a result of its location. Signals can be picked up with an extracorporeal sensor arrangement placed on the skin, with a needle-shaped sensor means adapted to puncture the skin to get close to (or in contact with) the nerve or with a sensor designed to be placed on a surgically exposed nerve. An implanted sensor arrangement could also be considered, in particular for long-term treatment. 
     One version of the respiratory aid system utilizes sensor signals related to breathing for triggering an inspiration by the patient. The triggering can be adapted so inspiration occurs at a phase in which the patient would have breathed spontaneously. In completely controlled respiratory aid, this would provide a patient-related respiratory rate. In supported respiration, this would also lead to a much faster response to the delivery of respiratory assistance than in prior art technology. Competition between the patient and the ventilator can be avoided. 
     In cases in which the patient has a varying capacity or ability to breathe spontaneously, a respiratory sensor can be used for determining whether a respiratory signal from the respiratory centre really leads to a spontaneous inspiration or an attempt at inspiration. This kind of respiratory sensor can also be used for determining the strength of an inspiration/attempt at inspiration. 
     If a spontaneous inspiration does occur, triggering is not always necessary and can be completely inhibited in some instances. The respiratory aid system could instead be devised to provide supplementary support (possibly according to information derived from the neurological signals) for a spontaneous inspiration when the latter is inadequate. For example, the patient&#39;s respiratory musculature might be too weak to sustain physiological inspiration duration. 
     With a respiration detector for sensing the respiration-related part of the patient&#39;s nervous system, attempts at respiration can be detected at the earliest possible stage and used for controlling a device for supplying respiratory aid. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of a respiratory aid system constructed and operating in accordance with the principles of the present invention. 
     FIG. 2 shows one embodiment of a respiration detector and a ventilator according to the invention. 
     FIG. 3 shows a second embodiment of a respiration detector and a ventilator according to the invention. 
     FIG. 4 shows a third embodiment of a respiration detector and a ventilator according to the invention. 
     FIG. 5 shows a fourth embodiment of a respiration detector and a ventilator according to the invention. 
     FIG. 6 shows a fifth embodiment of a respiration detector and a ventilator according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a patient  2  on a bed  4 . The patient  2  is connected to a ventilator  6  by a tubing system  8  in order to receive respiratory aid from the ventilator  6 . In principle, the ventilator  6  can be a known ventilator, such as the Servo Ventilator  300 , Siemens-Elema AB, Sweden, or some other known respirator (including home-care breathing aid, CPAP-machines, resuscitation machines etc.) or anaesthetic machine. In principle, respiratory aid can consist of any conventional respiratory assistance given to patients who have or stand a risk of getting difficulty in achieving or are unable to achieve adequate spontaneous respiration. 
     A respiration detector  10  is connected to the ventilator  6  and, via a signal line  12 , to the patient&#39;s  2  phrenic nerve. Here, the signal line  12  can be transcutaneously connected to the phrenic nerve or indirectly connected to the phrenic nerve from the surface of the skin. Neuroelectrical control signals for respiration, generated in the respiratory centre of the medulla oblongata, are carried along the phrenic nerve to the diaphragm (in particular). These signals are picked up by the signal line  12  and sent to the respiration detector  10 . 
     The respiration detector  10  extracts the relevant signals related to respiration. Identification can be made using known signal processing methods. Signals concerning respiration or, the function of the diaphragm in this case, usually arrive as a train of impulses. The respiration detector  10  then generates a control signal, sent to the ventilator  6 , according to respiration signals detected. More advanced analysis of the nerve impulses can be made with more advanced methods, inter alia including pattern recognition systems and artificial neural networks (ANN). 
     These respiratory signals are the earliest indication that the patient  2  wants to breathe. These signals can also supply information on the physiological need of the patient  2  to breathe. The latter is in particular valid for patients  2  whose inability to breathe sufficiently is unrelated to injuries to or some impact on the respiratory centre. 
     One advantage of obtaining the respiratory signal at an early stage is that the respiratory aid (provided by a ventilator  6  in this instance) can be supplied according to the  2  true physiological needs and not according to estimated needs or needs calculated in some other way. Moreover, respiration will have the most natural rhythm for the patient  2 . Thus the triggering of inspiration phases is an essential part of the invention, but the neurological signals contain more intelligence than just information on when the patient should take a breath. In principle, they contain all the information essential to each individual breath, especially respiratory depth, inspiration duration and any inspiration pause. Some physiological respiratory control also concerns expirations, even if expiration is a normally passive process occurring when the respiratory musculature relaxes. 
     Since an inspiration phase can be started in synchrony with the actual inspiration of the patient  2 , breathing effort can be greatly reduced for patients  2  retaining some ability to breathe spontaneously. 
     A respiratory sensor can be used in parallel with patients  2  with a greater ability to breathe spontaneously so the control signal from the respiration detector  10  can be inhibited. This is illustrated in FIG. 2 in which the ventilator  6  and respiration detector  10  are depicted in greater detail. 
     The respiration detector  10  is connected to the patient  2  by an electrode  18  for sensing the neuroelectrical signals. Sensed signals are sent to a signal processing unit  20  for filtration and amplification in some suitable fashion. The processed signals are then sent to an analyzer  22  for signal identification. Appropriate signal analysis can establish when the patient  2  wishes to breathe spontaneously and the extent to which the patient  2  wishes to breathe. Respiratory signals mainly consist of a train of impulses to the respiratory musculature (especially to the diaphragm in this example, since the phrenic nerve is being sensed). 
     Information on the presence of respiratory signals is sent to a signal generator  24  that generates a control signal depending on the respiratory signals identified. This control signal is sent via a control line  26  to a control unit  28  in the ventilator  6 . 
     The control unit  28  regulates the ventilator  6  according to parameters set by a physician and parameters measured by the ventilator  6 . 
     A breathing gas is mixed to the right composition, pressure and flow in a valve unit  30 . Different gases can be connected in the known fashion via a first gas connector  32 A and a second gas connector  32 B. 
     Measurement of pressure and flow can be performed in different parts of the system by a first flow meter  34 , a first pressure meter  36 , a second flow meter  38 , a second pressure meter  40 , a third flow meter  42  and a third pressure meter  44 . When pressure and/or flow is/are measured, an initiated breath can be sensed as a change in pressure and /or flow. The control unit  28  plus one or more of aforementioned meters thereby constitutes a respiration sensor. These meters are therefore only shown to illustrate that measurement of pressure and flow can be made at one or a number of locations in the system. 
     If the patient  2  commences a breath by drawing in air, the control signal&#39;s effect on the control device  28  is reduced, delayed or inhibited. A sufficiently strong spontaneous breath within a specific period of time after the respiration detector  10  senses a respiratory signal means that the patient  2  does not need respiratory aid during that breath, other than a supply of breathing gas. In these circumstances, the ventilator  6  can operate as a normal ventilator  6  and support the patient  2  according to a pre-determined pattern or according to the strength of the spontaneous breathing by the patient  2 . 
     In principle, the respiration detector  10  can also sense when the patient  2  wishes to exhale and then send a signal to the ventilator  6  to permit the start of an exhalation (expiration). This spares the patient  2  the inconvenience of actively building up enough pressure in the lungs to activate the expiratory phase. During expiration, an expiratory valve  46  can be regulated to maintain an appropriate positive end expiratory pressure (PEEP) for the patient  2 . 
     FIG. 3 shows an alternative design for the respiratory aid system according to the invention. A patient  2  placed on a bed  4  is connected to a respiration detector  10  by an electrode line  12 . In this instance, the electrode line  12  is devised to sense signals in the respiratory centre itself or in nerve pathways that mainly carry respiratory signals to the respiratory musculature around the thorax. 
     A myostimulation device  14  is connected to the musculature at the diaphragm of the patient  2  via stimulation line  16  in order to stimulate the diaphragm&#39;s muscles with electrical signals and accordingly induce an inspiration. 
     Alternately, as shown in FIG. 4, a nerve stimulating device  48  can be used for stimulating the respiratory nerves via a stimulation line  50 . In this instance the phrenic nerve is stimulated. This can be used in cases where there is a fault in the nervous system which prevents a natural passage of impulses from the respiratory centre to the muscle. A nerve stimulation “downstream” the fault can then maintain as normal respiration as possible. In the alternative, the impulses can be enhanced by superimposing the stimulation signal from the nerve stimulating device  48 . 
     FIG. 5 shows another alternative, where a magnetic stimulation device  52  is arranged over the lung region of the patient  2  for magnetic stimulation of the respiratory system. The magnetic stimulation device  52  is controlled and powered by a control unit  54  via a line (or set of lines)  56 . In the specific embodiment of FIG. 5, the nerve impulses of the patient  2  are detected at several places via a multitude of electrodes  12 . 
     Finally, FIG. 6 shows another alternative, where a casing  58  placed around the upper part of the patient  2  provides the respiratory aid. Pressure generated by the casing  58  is controlled and supplied by control unit  60  via pressure line  62 . 
     The respiratory aid devices and nerve impulse detection devices shown in the embodiments can be combined where suitable. For instance, a multitude of sensing electrodes (FIG. 5) can be used to sense the nerve impulses in the embodiment showing a ventilator (FIG.  1 ). 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.