Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1999-06-15
2001-02-27
Weiss, John G. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204210, C128S204180
Reexamination Certificate
active
06192885
ABSTRACT:
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'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'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's lungs and the tubing do not constitute a static system. Any change in the patient'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.
REFERENCES:
patent: 4611591 (1986-09-01), Inui et al.
patent: 5002050 (1991-03-01), McGinnis
patent: 5072729 (1991-12-01), DeVries
patent: 5572993 (1996-11-01), Kurome et al.
patent: 293 268 (1991-08-01), None
Patel Mital
Schiff & Hardin & Waite
Siemens-Elema AB
Weiss John G.
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