Abstract:
A device for detecting the improper adjustment of a ventilatory support machine used on a mammal. The device ( 12 ) includes measuring elements ( 40 ) for taking a measurement, as a function of time, of a neurophysiological signal involved in the respiratory process of the mammal for at least two successive breathing cycles, each breathing cycle comprising a respiratory initiation time; an input for receiving a respiratory initiation signal (t o ) which is different from the neurophysiological signal; elements ( 42 ) for processing the neurophysiological signals, which are configured to process the neurophysiological signals for each respiratory initiation time over a period of time starting from the respiratory initiation time; and elements ( 44 ) for detecting an improper adjustment of the ventilator using the processed signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for detecting the improper adjustment of a ventilatory support machine used on a mammal. 
     2. Description of the Related Art 
     Some people suffer from acute respiratory failure resulting, for example, from pneumonia, pulmonary edema or a secondary infection of chronic respiratory diseases. Mechanical ventilatory support may be required. Ventilatory support machines or ventilators comprise means for detecting the patient&#39;s inspiration and means for helping the patient to inspire by increasing the airflow or the pressure of the air inhaled by the patient. 
     Support thus consists in providing a predetermined volume of gas or pressurizing the airways. In both cases, different settings make it possible to adapt the flow of gas to the needs of the patient. The support machine must therefore be adapted to the respiratory behavior of the patient so as to obtain a “harmonious” relationship therebetween, that is to say that the patient has a satisfactory level of physical comfort and does not encounter any respiratory discomfort when using the support machine. If the settings are inappropriate, for example the airflow is too strong or, conversely, too weak, the patient may be uncomfortable or may even become distressed when breathing. 
     Various means have been used to detect disharmony of this type between the patient and the machine. In particular, it is known to simply ask the patient. However, this is not possible when the patient is asleep or in a coma. 
     It is also known to monitor ventilatory activity, in particular the frequency and use of the various respiratory muscle groups. 
     It is also possible to monitor the coordination between respiratory movements and the response of the ventilator in order to detect any asynchronicity or any ineffective actuation of the ventilator. 
     Lastly, it is known to measure indirect physiological elements which are used to measure ventilatory activity and, if necessary, to detect any desynchronized behavior of the ventilator. Said indirect physiological elements are, for example, occlusion pressure, morphology of the airway pressure curves and ventilatory work. 
     In practice, these elements must be used sensitively and all constitute indirect indicators of the sensations which the patient may be experiencing. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to propose a device which makes it possible to reliably detect an improper adjustment of the ventilation machine. 
     The invention thus relates to a device for detecting the appropriate adjustment of a respiratory support machine used on a mammal, characterized in that it comprises:
         means for measuring, as a function of time, a neurophysiological signal involved in the respiratory process of the mammal for at least two successive respiratory cycles, each respiratory cycle comprising a respiratory initiation time;   an input for receiving a respiratory initiation signal which is different from the neurophysiological signal;   means for processing the neurophysiological signals, which means are configured to process the neurophysiological signals for each respiratory initiation time over a period of time comprising the respiratory initiation time and starting before the respiratory initiation time; and   means for detecting an improper adjustment of the ventilator by means of said processed signals.       

     The invention thus makes it possible to detect any improper adjustment by means of abnormal neuromuscular or neurological activity in a patient on mechanical ventilatory support. A process of this type enables reliable detection, that is to say detection obtained almost directly from the sensations of the patient. 
     In accordance with specific embodiments, the device comprises one or more of the following features:
         the processing means comprise means for back-averaging, point-to-point, the neurophysiological signals measured for all cycles over the same specific period of time;   the measuring means comprise an electroencephalograph and are used to measure possible premotor potential of the mammal before the respiratory initiation time;   the processing means are configured for calculating the slope of the electroencephalographic signal immediately before the respiratory initiation time;   the detection means comprise means for comparing the slope with the reference value;   the reference value is equal to zero;   the means for detecting an improper adjustment comprise means for triggering an indicator when the value of the slope is greater than the reference value;   the indicator is an indicator light;   more than half of said period of time elapses before the respiratory initiation time;   the measuring means comprise an electromyograph and are used to calculate the integral of an electromyographic signal over said period of time;   more than half of the period of time elapses after said respiratory time;   the detection means comprise means for detecting a change over time in the root mean square of the electromyographic signal over said period of time;   it comprises a sensor for detecting the patient&#39;s aspiration, connected to the input for receiving a respiratory initiation signal t o , said sensor being separate from the means for measuring the neurophysiological signal;   the sensor is a load sensor;   the sensor is separate from the measuring means.       

     The invention also relates to a ventilatory support assembly comprising:
         a ventilatory support machine and   a device for detecting improper adjustment, as described above.       

     The invention also relates to a method for detecting the improper adjustment of a ventilatory support machine used on a mammal, characterized in that it comprises the steps of:
         measuring, as a function of time, a neurophysiological signal involved in the respiratory process of the mammal for at least two respiratory cycles; each respiratory cycle comprising a respiratory initiation time;   receiving a respiratory initiation signal t o  which is different from the neurophysiological signal;   processing the neurophysiological signals for each respiratory initiation time over a period of time comprising the respiratory initiation time and starting before the respiratory initiation time; and   detecting an improper adjustment of the ventilator by means of said processed signals.       

     The invention also relates to computer software comprising instructions which, when said software is loaded onto a computer connected to the means for measuring a neurophysiological signal, carries out the above method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood upon reading the following description, given purely by way of example and with reference to the drawings, in which; 
         FIG. 1  is a schematic view of a ventilatory support assembly used by a patient; 
         FIG. 2  is a perspective view of the rear three quarters of a human head showing an example arrangement of the electrodes; 
         FIG. 3  is a series of curves showing point-to-point averaging over a series of respiratory cycles of the electroencephalographic signal obtained; 
         FIG. 4  is an example of a series of curves showing the encephalographic activity detected and the electromyographic activity of the scalene muscle of a human being on ventilatory support; 
         FIG. 5  is an identical view to that of  FIG. 4  in the case of disharmony and is an example of a series of curves showing the encephalographic activity detected and the electromyographic activity of the scalene muscle of a human being on ventilatory support; 
         FIG. 6  is an identical view to that of  FIG. 1  of a variation of an assembly according to the invention; 
         FIG. 7  is an example of a series of curves showing the ventilatory activity (above) and the electromyographic activity (below) for a plurality of respiratory cycles; and 
         FIG. 8  is a curve showing the ventilatory activity (above) and the mean envelope of electromyographic activity (below). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an example of a ventilatory support assembly  8  which uses a device according to the invention. 
     Said assembly  8  comprises a mechanical ventilatory support machine  10  and a device  12  for detecting an improper adjustment of the support machine  10 . 
     The machine  10  comprises, as is known per se, a turbine  14  for providing an airflow to a patient at a predetermined airflow rate and at a given pressure. At the output of the turbine  14  a valve  16  is provided for feeding or not feeding the pressurized air produced by the turbine  14  to the patient. The turbine  14  and the valve  16  are connected to a control unit  18  which, in turn, is connected to a load sensor for detecting the patient&#39;s aspiration. 
     The turbine  14  is connected downstream of the valve  16  to a mask  22  which is arranged over the patient&#39;s upper airways. The load sensor  20  is arranged, for example, inside the mask  22  of the patient. 
     In a variation, the mask  22  may be replaced with an endotracheal probe. 
     The control unit  18  is connected to an adjustment unit  24  for modifying the functioning parameters of the machine  18  and, in particular, the airflow rate produced by the turbine  14 , airflow pressure, the switchover times of the valve  16  and any other parameter known from the prior art. 
     In addition, the control unit  18  comprises an output for providing a respiratory initiation signal t o  which represents the start of the patient&#39;s inspiration. 
     The device  12  for detecting improper adjustment comprises a unit  40  for measuring a neurophysiological signal which represents inspiration and is used to provide a neurophysiological signal as a function of time. 
     It also comprises a processing unit  42  connected to the measuring unit  40  for receiving the neurophysiological signal. Said processing unit  42  comprises an input for receiving the respiratory initiation signal t o , said input being connected to the corresponding output of the machine  10 . 
     The device  12  further comprises means  44  for providing a doctor with information showing an improper adjustment of the machine  10 . 
     According to a first embodiment, the means  40  for measuring a neurophysiological signal are formed by an electroencephalograph. Said electroencephalograph comprises, for example, three electrodes  46 A,  46 B,  46 C arranged on a patient&#39;s scalp and, in particular, at the supplementary motor area, that is to say at the premotor cortex. More precisely and preferably, the electrodes  46 A,  46 B,  46 C are arranged at the C3-A+, C4-A+ and Cz-A+ leads of the scalp, as shown in  FIG. 2 , these positions being defined in the illustration of the international  10 - 20  system. 
     As is known per se, the electroencephalograph comprises means  50  for receiving the signal, filtering and amplification means  52  and sampling means  54  for digitalizing the signal, for example with a sampling frequency of 2,000 Hz. It also comprises means  56  for electronically storing the sampled values with their corresponding sampling time. 
     The processing means  42  comprise means  60  for back-averaging the sampled values stored over a plurality of respiratory cycles. They are formed, for example, of a microcomputer which uses suitable software. More precisely and as is shown in  FIG. 3 , the processing means are used to obtain the arithmetic mean, point-to-point, between various successive periods of time I 1 , I 2 , I n , of the recorded signal, each period of time I 1 , I 2 , I n  corresponding to a respiratory cycle. Each period of time I 1 , I 2 , I n  is defined relative to the respiratory initiation time t o  of the corresponding respiratory cycle and comprises said time t o . Respiratory cycle means the period of time constituted by a complete expiration and inspiration. 
     In  FIG. 3 , the curve P illustrates the pressure measured by the sensor  20  whilst the curve S illustrates the electroencephalographic signal received at the same times. 
     The successive periods of time I 1 , I 2 , I n , from which the signals are averaged, are shown. 
     They each comprise the respiratory initiation time t o  and have the same duration. 
     The mean signal labeled I m  is obtained by averaging the signals of the periods I 1 , I 2 , I n . A mean signal I m  of this type is determined for each of the three electrodes  46 A,  46 B,  46 C. The average is preferably obtained over a number n of successive cycles greater than 10 and, for example, between 50 and 100 and advantageously equal to 80. 
     The duration of the averaging periods I 1 , I 2 , I n  is at most equal to the duration of a respiratory cycle. It is preferably between 2 s and 4 s and preferably substantially equal to three seconds. 
     The averaging period I 1 , I 2 , I n  includes the respiratory initiation time t o . More than half of this period advantageously elapses before the respiratory initiation time t o . Preferably more than two thirds elapse before the initiation time t o . More precisely, the period starts between 1.5 s and 3.5 s before the respiratory initiation time t o . It preferably starts substantially 2.5 s before. 
     The period finishes between 0.2 s and 0.7 s and preferably 0.5 s after the respiratory initiation time t o . 
     The processing means  42  further comprise means for calculating the slope of the averaged electroencephalographic signal observed before the respiratory initiation time t o . 
     The means  44  for detecting an improper adjustment comprise means  70  for comparing the slope of the average signal I m  with a reference value and means  72  for triggering an indicator, such as an indicator light, when the value of the slope is greater than the reference value. 
     In practice, said reference value is, for example, equal to zero. 
     As well as the indicator, the detection means  44  advantageously comprise means for storing and displaying the averaged signals I m  and the values of the slope calculated for each signal I m . 
       FIGS. 4 and 5  show an example of the three averaged signal values S A , S B , S C  obtained from the sensors  46 A,  46 B,  46 C on a patient in harmony with the ventilator in  FIG. 4  and, in  FIG. 5 , a patient in disharmony. 
     It has been found that if the patient and the support machine are in harmony there is no premotor potential before the respiratory initiation time t o , the premotor potential being the electroencephalographic potential measured during the phase immediately before the respiratory initiation time. 
     In contrast, in the case of disharmony, the premotor potential progressively increases before the respiratory initiation time t o . It is thus assumed that, during disharmony, premotor potential which translates into cortical preparation for movement is established before the initiation time t o . 
     Since the device is able to detect this premotor potential, it is able to detect disharmony between the respiratory machine and the patient. 
     From the information provided by the detection means, the doctor is able to modify the settings of the support machine  10  from the adjustment unit  24 . In a variation, a control loop is established between the detection means and the support machine  10 , the settings being automatically modified as a function of the value of the detected slope of premotor potential. 
     According to another embodiment of the device labeled  112  and shown in  FIG. 6 , the means for measuring a neurophysiological signal are replaced with an electromyograph  140 . Said electromyograph comprises one or more electrodes  146  which are placed on the respiratory muscles, that is to say those muscles activated by respiration, for example the muscles of the neck or the face of the patient (in particular the outer walls of the nostrils). These electrodes are able to detect the electrical signals of these muscles. The electrodes are placed, for example, in accordance with a first embodiment on the scalene muscle or, in accordance with a second embodiment, on the alae nasi muscle which controls the opening of the nostrils. In the first case, the electrode  146  is advantageously arranged directly inside the mask  22  ensuring the patient&#39;s air supply. The lower signal S SC  in  FIGS. 4 and 5  shows the recorded electromyographic activity of the scalene muscle. 
     As before, the device  112  comprises a processing unit  142  connected to the electromyograph  140  and means  144  for revealing information which indicates an improper adjustment. 
     As is known per se, the electromyograph  140  comprises means  150  for receiving the signal, filtering and amplification means  152  and sampling means  154  having, for example, a frequency of 10,000 Hz, filtering being carried out between 20 Hz and 3 kHz. As before, the electromyograph  140  comprises means  156  for storing sampled values with their corresponding sampling time. 
     The processing means  142  comprise means  160  for back-averaging the sampled values. 
     With reference to  FIG. 7 , in this embodiment the processing means  142  are used to calculate the square root of the signal and then the arithmetic mean, point-to-point, of said square root for a number n of pre-determined cycles over periods of time J 1 , J 2 , J 3 , . . . , J n  defined relative to the respiratory initiation time t o  of each cycle and comprising said time. 
     In fact, since in this embodiment the signal is symmetrical relative to the x-axis, it is necessary to make it asymmetrical by squaring it so as to calculate a non zero mean. 
     The root mean square is advantageously obtained for each period of time J 1 , J 2 , J 3 , . . . , J n  over a mobile time window, preferably lasting one millisecond, covering said period so as to obtain an envelope for each period. 
     The duration of the average period J 1 , J 2 , J 3 , . . . , J n  is at most equal to the duration of the respiratory cycle. It is preferably between 2 s and 4 s and preferably substantially equal to 3 s. The period comprises the respiratory initiation time t o . Said period advantageously lasts for more than half and advantageously for more than three quarters after the respiratory initiation time t o . More precisely, the period starts between 0.5 s and 1.5 s before the respiratory initiation time t o . It preferably starts substantially one second before. 
     The period finishes between 1 and 3 seconds and preferably substantially 2 seconds after the respiratory initiation time t o . 
     The processing unit  142  further comprises means  162  for calculating a mean envelope representing the pace of the electromyograms of the preceding n cycles. This mean envelope is shown in  FIG. 8  with the scale indicated (1 sec). The envelopes of the various periods J 1 , J 2 , J 3 , . . . , J n  are thus averaged in order to obtain the mean envelope. The mean envelope is preferably obtained for a number n of cycles, that is to say periods J 1 , J 2 , J 3 , . . . , J n  greater than 10 and between 50 and 100 and advantageously equal to 80. 
     The means  144  for detecting an improper adjustment comprise means  170  for detecting a change over time in the value of the mean envelope calculated. 
     The mean envelope is thus stored for various measuring times. Said detection means  170  are used to compare the various values of the mean envelopes stored between periods J 1 , J 2 , J 3 , . . . , J n . 
     A mean envelope is preferably calculated at a frequency of 2 to 15 times per hour and preferably substantially equal to 5 times per hour. 
     In a variation, the mean envelope is calculated on demand, for example by hospital staff. 
     If there is no change over time in the value of the envelope, the means  172  do not trigger the indicator for showing an improper adjustment. 
     In contrast, if there is a change over time in this value and, in particular, an increase in the integral of the mean envelope (denoted as A in  FIG. 8 ), the triggering means are adapted to trigger an indicator. 
     It has been found that in the case of harmony the value of the integral remains constant. In contrast, in the case of disharmony the integral tends to increase. 
     Of course the invention is not limited to the embodiment described. 
     In particular, the way in which the airflow is delivered to the patient is not limited to that which has been described and may be achieved as a volume of administered gas or by pressurizing the airways.