Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1998-11-16
2001-05-22
Dawson, Glenn K. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180
Reexamination Certificate
active
06234170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas pressure generator and in particular to a gas pressure generator of the type used to generate a substantially constant pressure of gas in a ventilator, especially fresh gas in an anesthesia ventilator.
2. Description of the Prior Art
Anesthesia ventilators are well known and generally contain a so-called “closed” or “partially closed” respiratory circuit, dependent on whether all or some of the exhaled respiratory gases are reused. These circuits reduce the consumption of fresh gas, for example respiratory air enriched with oxygen or anesthetic gas. In order to compensate for gas losses during a breathing cycle, a supply of fresh gas is provided for introduction into the respiratory circuit. The constituents of the fresh gas are often supplied from a high pressure source, such as a gas bottle or a hospital's centralized high pressure system, and its pressure must be reduced to around 100 cm H
2
O before being supplied to a patient.
It is also desirable that the fresh gas supply to the respiratory circuit be maintained at a substantially constant over-pressure, not least because the amount of fresh gas introduced can be more easily monitored if it is delivered at a substantially constant pressure. In order to obtain such a substantially constant pressure fresh gas source, it is usual to provide a source of fresh gas connectable to a gas pressure generator having a variable volume container with an output that is connectable to the respiratory circuit. The volume of the container is variable in a manner so that the input pressure of fresh gas can be reduced to a usable level and fresh gas at a substantially constant pressure can be supplied as an output.
Although referred to as a “constant” pressure source in reality the output pressure may be allowed to vary within limits (typically 10%-20% of the desired pressure) dependent on the circumstances of intended use in a manner known to those skilled in the art.
One known variable volume container is described in U.S. Pat. No. 3,824,902 and includes an expansion bellows made of soft plastic material attached between two rigid plates. One plate is spatially fixed and the other is operably connected to a shaft which, when rotated, collapses the container by means of springs. Through suitable choices of spring and spring attachment points the container can be made to collapse so that over a certain range of volumes the force exerted on the container is constant, thereby producing a constant pressure fresh gas output. Such a container, however, is mechanically complex which results in relatively high construction costs and likelihood of malfunction during use.
A further variable volume container is described in European Application 0 744 184 which also includes a collapsible bellows arrangement. This bellows is formed, at least in part, of an elastic material. Through careful choice of material and constructional configuration the bellows is made to produce, over a limited range of volume change, a constant pressure on the fresh gas contained in it as its volume reduces. Although a great simplification over the aforementioned container, a problem still remains that, as with the previously described bellows, the constant pressure gas can only be generated over a limited range of volume changes, which range tends to be removed from the zero volume condition.
This means that the container usually has a relatively large “dead volume” of fresh gas in it which is not removed during the breathing cycle of a patient. This may cause problems when a change in the constituents of the fresh gas is needed during an operation because a relatively large, often expensive, volume of gas has to be flushed out of the container. This is time consuming and fresh gas cannot be supplied, or is supplied but with an unknown constituency, to the patient during this time.
SUMMARY OF THE INVENTION
It is an object of the present to provide a gas pressure generator which supplies a gas at a constant (as defined above) pressure wherein the above-described problems associated with conventional generators of this type are avoided.
This object is achieved in accordance with the principles of the present invention in a gas pressure generator for a ventilator having a variable volume container provided with an inlet connected to a fresh gas supply, and an outlet connectable to a respiration circuit of the ventilator, a driving fluid-actuated piston with a shaft and a piston head for transmitting a force to the variable volume container at one end, and having an opposite end, at which the force is generated, which is movable in a chamber containing the driving fluid, the piston being movable so as to reduce the volume of the container to maintain a substantially constant output pressure of fresh gas contained therein, wherein the driving fluid container contains driving fluid at a substantially constant pressure, and wherein the piston shaft and the chamber are dimensioned relative to each other so that a percentage volume change within the chamber as the shaft moves therein is no larger than air allowable percentage variation in the output pressure of the fresh gas.
By a suitable choice of dimensions of the piston shaft and driving fluid (hydraulic or pneumatic) chamber, a piston can be provided in which the driving fluid pressure is maintained substantially constant throughout the stroke length of the piston which tends to collapse the variable volume container. In this manner a substantially constant output pressure of contained gas can be provided by a gas pressure generator which is relatively mechanically simple and reliable. Such a generator can be readily constructed so as to be capable of operating to provide fresh gas which is supplied at a substantially constant pressure over the entire volume of the variable volume container.
Advantageously the piston shaft is movable in a driving fluid chamber dimensioned so the percentage change in its internal volume is no greater than the allowable percentage change in fresh gas output pressure. This provides for a simple construction of the driving fluid section of the gas generator, which may even be formed as a sealed unit.
For ease of construction the gas generator may be formed from a single piston housing which includes compartments to contain the driving fluid chamber and the variable volume container.
REFERENCES:
patent: 3336920 (1967-08-01), Thomas
patent: 3530856 (1970-09-01), Bird et al.
patent: 3824902 (1974-07-01), Olsson
patent: 3918447 (1975-11-01), Inkster et al.
patent: 4010761 (1977-03-01), Tipple
patent: 4836198 (1989-06-01), Gates
patent: 5484270 (1996-01-01), Adahan
patent: 0 112 979 (1984-07-01), None
patent: 0 283 141 (1988-09-01), None
patent: 0 744 184 (1996-11-01), None
patent: 2 054 387 (1981-02-01), None
Dawson Glenn K.
Schiff & Hardin & Waite
Siemens Elema AB
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