Abstract:
In a method for controlling an expiratory valve in a ventilator during expiration, the expiratory valve is opened substantially fully for a first interval. Fully opening the expiratory valve has the advantage of minimizing the expiratory resistance a patient needs to overcome. Keeping the expiratory valve open as long as possible during expiration, without losing control of positive end expiratory pressure (PEEP), is therefore advantageous. An optimal system is achieved. In a method wherein pressure in the expiratory section of the ventilator is measured during a second interval, the expiratory valve is regulated so a predetermined end pressure is obtained in the expiratory section, and at least one parameter, directly or indirectly related to control of the expiratory valve, is determined, and a determination is made from that parameter as to whether the next first interval for the next expiration should be longer than, shorter than or as long as the first interval.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for controlling an expiratory valve in a ventilator. 
     2. Description of the Prior Art 
     In normal circumstances in respiratory care, the patient is 10 allowed to exhale as normally as possible, sometimes against an elevated positive end expiratory pressure (PEEP). The tubes (the tracheal tube in particular) and devices (e.g. a dehumidifier, bacterial filter and the ventilator&#39;s expiratory valve in particular) in the path of flow of expired gas pose a resistance to expiration. The patient is forced to overcome this unnatural resistance, which can become tiring. 
     One way to reduce such resistance is to open the expiratory valve to a maximum for a specific period of time. East German Patentschrift 293 268 describes one such method for controlling a ventilator, wherein the expiratory valve consists of an on/off valve with only two positions, fully open or fully closed. 
     This known control of the expiratory valve causes the expiratory valve to open at the onset of expiration. It is kept open for a specific period of time and then closed. The pressure (end pressure) on the valve (on the patient side) then corresponds to the pressure in the patient&#39;s lungs. The period of time in which the valve is kept open for the next consecutive breathing cycles is set according to the difference between the determined end pressure (actual value) and a preset value for PEEP (reference value). The time the valve is kept open is increased if the measured value exceeds the reference value. The time the valve is kept open is reduced if the measured value is less than the reference value. In this way, a convergence toward the reference value is obtained. 
     A disadvantage of this known control system is that the patient risks exposure to an end pressure that is less than PEEP during an initial phase of treatment (when maintenance of PEEP is particularly important in preventing the collapse of alveoli in the lungs) 
     Another disadvantage of this known control system is that the patient is subjected to varying end pressures, at least during the adjustment phase of treatment, since an end pressure greater than the target PEEP pressure could develop. 
     An additional disadvantage of this known control system is that the patient&#39;s lungs and the tubing do not constitute a static system. Any change in the patient&#39;s physical position could alter parameters for the gas mechanics of the lungs/tubing system, for which the control system is unable to compensate. In a worst case scenario, this could result in an end pressure much lower (or higher) than the reference value. 
     Yet another disadvantage is the fact that bias flows cannot be used, since the known valve is an on/off valve. Bias flows have the advantage of making flow-triggering possible for the patient. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for controlling an expiratory valve in a ventilator during expiration wherein the aforementioned problems are avoided. 
     The above object is achieved in accordance with the principles of the present invention in a method for controlling an expiratory valve in a ventilator during expiration, wherein the expiratory valve is open substantially completely in a first interval and wherein, during a second interval following the first interval, pressure is measured in the expiratory system of the ventilator, the expiratory valve is controlled so that a predetermined end pressure is achieved in the expiratory section including determining at least one parameter, directly or indirectly related to control of the expiratory valve, and determining from that parameter whether the duration of the next-following first interval for the next expiration should be longer than, shorter than, or the same as the duration of the first interval during the current expiration. 
     The valve can be kept fully open during a first interval by the use of an adjustable expiratory valve and then regulated toward a reference value (PEEP) during a second interval when expiration has largely subsided. A parameter related in some way to control of the expiratory valve is determined and utilized for establishing the first interval for the next expiration. Flow through the expiratory valve is one such parameter, as are the required regulatory force on or the regulatory current to the expiratory valve in achieving the preset end pressure (PEEP). Pressure is also one such parameter, of course. 
     It should be clearly noted that, in contrast to an on-off valve, “fully open” as used in the context of the valve according to the invention means sufficiently open to avoid the resistance to gas flow that the valve causes during control of PEEP. Whether this requires the valve to be truly fully open or only open to a certain degree (50%, 70% or other degree) depends more on the physical properties (flow through area, etc.) of the valve than the control function (to which the invention is directed to). 
     To prevent development of an unstable system, a known integration-type control method can be used over a number of breathing cycles. A specific number of preceding expirations then serve as the basis (average value formation) for determining whether the next first interval should be prolonged or reduced. 
     If the first interval is too long and the end pressure is too low at the start of the second interval, the correct end pressure (PEEP) could still be achieved during expiration as the result of an existing bias flow of breathing gas. Such bias flows are known and used for e.g. the flow triggering function in the Servo Ventilator 300, Siemens-Elema AB, Sweden. If no bias flow is employed or if it is insufficient to produce the desired end pressure, a supplementary flow of breathing gas can be supplied. 
     The duration of the first interval can advantageously be maximized to e.g. 0.5 second. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a ventilator, in which the method according to the invention can be implemented. 
     FIG. 2 is a diagram showing the use of flow as a parameter in the method according to the invention. 
     FIG. 3 is a diagram illustrating the use of regulatory force as a parameter in the method according to the invention. 
     FIG. 4 is a diagram illustrating the use of regulatory current as a parameter in the method according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a ventilator  2 , connected to a patient  4 , for providing respiratory care. Breathing gas is carried to the patient  4  during inspiration in an inspiration line  6  and back to the ventilator  2  during expiration in an expiratory line  8 . 
     The breathing gas is mixed from gases supplied to the ventilator  2  through a first gas connection  10 A and a second gas connection  10 B. A mixer unit  12  regulates each gas with respect to pressure and flow so the mixed final breathing gas has the pressure and flow set by the physician. The mixer unit  12 , which contains e.g. valves, is controlled by a control unit  14  in the ventilator  2 . The mixer unit  12  can also be regulated to supply a continuous bias flow of breathing gas during expiration in addition to the inspiration flow of gas. 
     Expired breathing gas passes a flow sensor  16  and a pressure  5  sensor  18  in the expiratory section of the ventilator  2  before it is discharged into the atmosphere through an expiratory valve  20 . Measurement signals are sent to the control unit  14  which controls the expiratory valve  20 . 
     The control unit  14  controls the expiratory valve  20  so that the expiratory valve  20  is sufficiently open for a first interval during expiration in order to minimize expiratory resistance. A second interval then commences during which the expiratory valve  20  is regulated so the system achieves a preset end expiratory pressure (PEEP) at the end of expiration. This end pressure is an over-pressure in relation to prevailing atmospheric pressure and can vary upwardly from 0 cmH 2 O. 
     In the transition to or during the second interval, a parameter, directly or indirectly related to regulation of the expiratory valve  20 , is determined. The pressure measured by the pressure sensor  18  is one such parameter, as is the flow measured by the flow sensor  16 . Other parameters are the regulatory force on the expiratory valve  20  or the regulatory current to the expiratory valve  20 . The latter parameter is obviously only applicable to an expiratory valve  20  which displays a relatively simple correlation between force and current. 
     The control unit  14  then utilizes the established parameter for determining whether the expiratory valve  20  should be kept fully open for a shorter, longer or equally long period of time in the next expiration. The determination can be made from one or a number of breathing cycles. A number of breathing cycles, combined with an integrative control mode, makes the system stable and safe. 
     FIGS. 2,  3  and  4  are diagrams illustrating the way in which three different parameters can be used for determining the first interval for the next expiration. 
     FIG. 2 shows flow as a function of time during expiration. Flows are depicted here as being negative, since the nomenclature usually designates flows to the patient as positive. A first curve  22 , a second curve  24  and a third curve  26  show three possible flow sequences during expiration in relation to the method according to the invention. A vertical line  28  designates the end of the first interval ti, during which the expiratory valve is sufficiently open, and the start of the second interval t 2 , during which the expiratory valve is controlled so the preset end pressure is achieved. The duration of the second interval t 2  is mainly controlled by the difference between the preset expiratory duration and t 1 . In all normal conditions, this leaves enough time to achieve the correct end pressure. The duration of t 2  will be governed by the time it takes the system to achieve the correct end pressure only in specific circumstances. The horizontal line  30  designates the level of a bias flow that is always present during expiration (and which can even be 0, i.e. no flow at all). 
     The first curve  22  designates a situation in which the first interval ti is too long, i.e. the expiratory valve stays open too long. Flow before the end of the first interval ti (the vertical line  28 ) has dropped to the level of the bias flow  30 . The pressure is then less than the preset end pressure. In this situation, the expiratory valve can be fully closed, and the bias flow causes the pressure to rise toward the preset end pressure. If the valve is not fully closed, it will take longer time to build up the pressure. 
     In the method according to the invention, it is determined that the next first interval in the next expiration will be shorter. In a control context, this can be accomplished by reductions in specific steps, a reduction to the duration the first interval ti should have had (i.e. up to the intersection of the first curve  22  and the bias flow  30 ) or an averaged duration for a number of preceding breathing cycles. Known conventional control methods can be used here. 
     Certain safety features can be incorporated into the control system in order to keep pressure in any part of the first interval ti from dropping too far below the preset end pressure. 
     One such safety feature is to limit the longest duration of the first interval ti, e.g. to a maximum of 0.5 second. Excessive amounts of breathing gas then will not have time to flow out of the patient. 
     Another safety feature is to measure flow throughout an entire expiration (as is common) and to terminate the first interval ti prematurely if flow drops to the level of the bias flow  30 . A safety margin can also be incorporated, for example the first interval t 1  can be terminated if the measured flow drops to a certain level, e.g. 20% above the level of bias flow  30 . 
     Combinations of the two safety features are obviously possible. 
     The second curve  24  illustrates a situation in which the first interval t 1  has been regulated to the correct duration. In principle, flow reaches the level of bias flow  30  at the same time the first interval ti elapses. The expiratory valve then mainly regulates bias flow only, and the end pressure (pressure in the patient&#39;s lungs) is, in principle, the end pressure previously set. This is the situation the control method strives to achieve, and it is maintained with great accuracy in relatively constant breathing cycles. 
     Finally, the third curve  26  describes a situation in which the first interval ti is too short. Flow in the third curve  26  has not yet dropped to the level of bias flow  30 , and pressure in the patient&#39;s lungs is higher than the preset end pressure. The expiratory valve is kept open more than needed to regulate the bias flow  30  in order to discharge more breathing gas, thereby making it possible to maintain the preset end pressure. 
     In the corresponding manner described above, the next first interval for the next expiration is prolonged. The magnitude of this prolongation can be preset or calculated from one or a number of breathing cycles according to normal automatic control theory. 
     A number of additional features can be incorporated to keep expiratory resistance from becoming unnecessarily high, i.e. control of the expiratory valve starts too soon. One such feature is to exercise overriding control that prolongs (or shortens if necessary) the time at which the first interval t 1  elapses. This control can be based on measured flow. For example, if flow at the end of the first interval t 1  is more than e.g. 200% of the bias flow, the first interval t 1  is prolonged by a preset increment. 
     As an alternative to manipulating the first interval t 1 , the expiratory valve can be regulated to remain fully open for the second interval as long as the flow exceeds bias flow  30  by a sufficient degree. 
     Flow is not the only parameter that indicates whether the first interval t 1  is too long, too short or about right. Pressure can be measured immediately after control of the expiratory valve starts. The measured pressure value then indicates whether the first interval t 1  needs to be changed. Regulation to the desired end pressure is performed with the aid of the measured pressure. Flow in the first interval t 1  can then be measured and used for the supplementary features above. 
     An additional parameter is indicated in the diagram in FIG. 3. A first curve  32 , a second curve  34  and a third curve  36  illustrate three typical situations for the requisite regulatory force on the expiratory valve in the transition to control in the second interval t 2 . A horizontal line  38  designates the regulatory force required to maintain the bias flow of breathing gas passing the expiratory valve. A vertical line  40  designates the transition between the first interval t 1  and the second interval t 2 . 
     The first curve  32  illustrates a situation in which the first interval was too short. The slow change in the regulatory force required by the expiratory valve indicates that the pressure exceeds the preset end pressure and that a certain amount of breathing gas needs to be discharged from the system. 
     The second curve  34  illustrates a situation in which the first interval was proper. Regulatory forces increase relatively steeply but without overshoot. 
     The third curve  36  illustrates a situation in which the first interval was too long. Heavy overshoot in requisite force indicates that the pressure dropped below the preset end pressure and that a pressure build-up by means of bias flow is necessary. 
     Determination of the next first interval for the next expiration can be made in a manner analogous to the procedure outlined in the description of FIG.  2 . The introduction of safety features based on flow etc. is also possible here. 
     A diagram in FIG. 4 describes another parameter, viz. requisite current. In contrast to the examples above, this example requires the use of an electrically operated expiratory valve employing e.g. a traction magnet. The required current is then largely proportional to the requisite force described in FIG.  3 . The curves  32 ,  34 ,  36  in FIG. 3 can therefore also be said to correspond to the current in the various situations. Signal processing can be performed in such a way that the various situations arising can be identified by e.g. comparison of the signal level at the first change in sign for the derivative of the required current with the level of the requisite current at the time t 2  elapses (curve morphology has been idealized somewhat in FIG. 3) 
     FIG. 4 illustrates further alternative curve morphologies occurring, viz, a fourth curve  42  and a fifth curve  44  which illustrate situations in which a first expiratory interval ti too short and too long respectively. These curve morphologies are mainly found at fast breathing rates. A first vertical line  46  designates the transition from the first interval ti to the second interval t 2 . A second vertical line  48  designates the end of the second interval. 
     The fourth curve  42  illustrates a situation in which the first interval ti was too short. If this curve morphology is registered without registration of any of the curves according to FIG.  3  and the duration of the first interval ti is less than the maximum permissible duration, this duration can be increased. The end derivative of the fourth curve  42  can be used for determining the increase. Alternately, the variance between measured pressure and end pressure can be determined and used for determination of the increase. 
     In the corresponding manner, the fifth curve  44  illustrates a situation in which the first interval t 1  was too long. Even here the end derivative and the maximum permissible duration for the first interval ti can be used for determination of the decrease. Alternately, the variance between measured pressure and end pressure can be determined and also used for determination of the decrease. 
     Even in the situation in which current is used as a parameter, the aforementioned safety and control functions can be utilized in both the first interval and the second interval. Combinations of the various embodiments, in which a number of criteria and parameters are weighed together in determining the duration of a next first interval, are possible. 
     The parameter can even be other signals in the control system to the extent that they reflect e.g. force or current. A control signal reflecting requisite current is therefore one conceivable parameter, as is requisite current itself. 
     The ventilator in FIG. 1 illustrates only one type of ventilator to which the method according to the invention can be applied. But ‘ventilator’ also refers to other devices for supplying breathing gas, such as respirators and anesthetic machines. 
     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.