ANAESTHESIA WORKSTATION FOR GAS DELIVERY

Abstract: The invention provides a workstation for delivering oxygen-enriched air to an anaesthetized patient, comprising: means (1) for entraining ambientair from a room into one or two variable volume working chambers (4) by increasing the volume of the working chamber(s) (4); means (8) for mixing oxygen from a pressurized source with the entrained ambient air; and means (3, 16, 17) for delivering the entrained mixture intermittently or as continuous flow from the working chamber(s) (4) to a patient by decreasing the volume of the working chamber(s) (4). The apparatus lacks the complexity of conventional anaesthesia workstations, provides for the delivery of suitable humid room air to the patient's airways, and is suitable for use with intravenously administered anaesthetics.

Description: TITLE Anaesthesia Workstation For Gas Delivery DESCRIPTION Field of the Invention This invention relates to the delivery of gas to an anaesthetized patient. Background Art Currently available anaesthesia workstations for use in a modern operating theatre environment are designed to generate a continuous flow of a mixture of oxygen, air, nitrous oxide, and a chosen volatile anaesthetic agent. The concentration of each will depend on anaesthetic and surgical factors. All of the gases are delivered from pressurized sources, either pipeline or cylinder, while the volatile agent is introduced from a vapourizer. The patient may breathe the resulting mixture spontaneously via an appropriate breathing system, or the mixture may be fed into a ventilator, and delivered to a paralysed patient under intermittent positive pressure. In either mode of ventilation a device such as endo-tracheal tube is used to provide an air tight seal with the patient's respiratory tract. In the case of spontaneous respiration, gases are normally fed to the patient via a breathing circuit involving a reservoir bag and expiratory outlet. This smooths out fluctuations in available gases, and also serves as a visual monitor of the pattern of respiration. This monitoring function is dependent upon the gas supply to the breathing circuit being effectively continuous. In the case of intermittent positive pressure ventilation, the gases are fed to the patient via a non-rebreathing valve which permits passive exhalation between breaths. Expired gas is then actively scavenged to prevent contamination of the theatre environment . The resulting workstations are complex due to the large number of functions which they perform. Because some of the components of the delivered gas mixture have zero oxygen content, a danger exists that the mixture itself may have very low oxygen content (for example if oxygen or compressed air cylinders are empty or not turned on) . Moreover the gases, being from pressurized sources, will be dry. Inhalation of dry gases has detrimental effects on the human respiratory tract . Recent pharmacological advances dictate that it is increasingly common for anaesthetics to be used which are delivered by the intravenous, rather than inhalational , route. The use of total intravenous anaesthesia does not remove the need carefully to control and monitor the breathing air supplied to the patient. It is the general practice for anaesthetists to make use of conventional anaesthesia workstations to deliver breathing air even when administering anaesthesia by the intravenous route. The conventional workstations are then used to deliver oxygen-enriched air to the patient either as a continuous flow to a spontaneously breathing patient through an appropriate breathing system, or through a ventilator. In such modified usage conditions, however, the conventional workstations are over-specified and still carry the disadvantages discussed above. This invention provides an anaesthesia workstation which overcomes, or substantially mitigates, the above problems for anaesthetists content to restrict themselves to intravenous anaesthesia. The Invention According to the invention, there is provided a workstation for delivering oxygen-enriched air either intermittently or as a continuous or substantially continuous flow to an anaesthetized patient, comprising: means for entraining ambient air from a room into a variable volume working chamber by increasing the volume of the working chamber; means for mixing oxygen from a pressurized source with the entrained ambient air; and means for delivering the entrained mixture intermittently from the working chamber to a patient by decreasing the volume of the working chamber. The workstation of the invention would be used, when treating a patient under total intravenous anaesthesia, in conjunction with an airtight connection to the patient's respiratory tract. Such an airtight seal is necessary even during spontaneous respiration to ensure the effective scavenging of the expired gases. It also carries the advantage of preventing contamination of the accurately mixed gas combination by room air and of ensuring movement of a reservoir bag in response to the patient's breathing effort. It is preferable that the flow generated by the gas delivery system during spontaneous respiration should remain continuous or virtually continuous so as not to interfere with the monitoring function of the reservoir bag. The means for mixing oxygen into the entrained air preferably adds the oxygen as the entrained air flows into the increasing volume working chamber. There should be a control over the oxygen content of the entrained mixture . For example, oxygen could be supplied through a flowmeter into a length of reservoir tubing, from whence it is drawn into the flow of entrained ambient air as it flows into the working chamber . The use of a single working chamber generates an intermittent flow of oxygen-enriched air to the patient. That may be adequate if the working chamber is constructed such as to be capable of being recharged with fresh gas sufficiently rapidly to provide flow which is effectively continuous to permit easy visual monitoring of patient respiratory effort . To generate a more genuinely even and continuous flow of oxygen-enriched air to the patient, however, the invention preferably provides two such working chambers working in alternation, so that as one is filling with oxygen-enriched air, the other is delivering its oxygen-air mixture to the patient. The or each working chamber is preferably a variable volume working chamber such as a bellows chamber. For example a balloon or diaphragm or movable partition could divide an outer chamber into two sub-chambers of which the first is the variable volume working chamber for the oxygen-air mixture (referred to herein as the patient gas sub-chamber) and the second is a driving gas sub-chamber. Variation of the pressure in the driving gas sub-chamber causes alternate inflation and deflation of the balloon or movement of the diaphragm or movable partition, to draw an air/oxygen mixture into or expel it from the patient gas sub-chamber. The or each driving gas sub-chamber is provided with inlet means for introducing that driving gas into said driving gas sub-chamber, and outlet means for venting the gas from said driving gas sub-chamber to atmosphere. The partitions form airtight seals between the sub-chambers of each outer chamber such that introduction of a driving gas into the driving gas sub-chamber causes movement of the partition with expulsion of oxygen-enriched air from the patient gas sub-chamber . The or each patient gas sub-chamber is connected to a patient-gas circuit which includes two limbs. The upstream portion of each limb provides inlet means for the related patient gas sub-chamber, and the downstream portion provides outlet means for the related patient gas sub-chamber. Each portion of each limb is provided with a one-way valve to establish the direction of flow. Where there are two working chambers operable alternately the limbs unite upstream of the patient gas sub-chambers in a reservoir tubing, into which is drawn room air, and which is provided with a pressurized source of oxygen. The limbs unite downstream of the patient gas sub-chambers to supply the patient. The design of the or each working chamber may be such as to promote a tendency, in the absence of pressure difference between the two sub-chambers of each outer chamber, for each patient gas sub-chamber to expand in volume, with an accompanying reduction in volume of the corresponding driving gas sub-chamber. Most preferably, the partitions take the form of self-inflating bags, the patient gas sub-chambers being the volume contained within these bags. Alternatively, the partitions may be bags with a natural tendency to collapse, in which case the driving gas sub-chambers are the volumes within the bags and the patient gas sub-chambers are the volumes outside of the bags . In a further embodiment, the two sub-chambers of each chamber are separated by rigid partitions sliding vertically within their respective outer chambers. The patient gas sub-chambers have a natural tendency to expand in this case, due to the effect of gravity on the mass of the partitions . The pair of chambers may be constructed, according to a yet further embodiment, not to rely on any natural tendency for the patient gas sub-chambers to expand. Rather, the partitions may be operably linked, such that the partitions are displaceable within their respective chambers in unison, the arrangement being such that displacement of the partitions causes a reduction in volume of one of each pair of sub-chambers, and a corresponding increase in volume of the other of each pair of sub-chambers. Conveniently to this embodiment, the partitions may be pistons, slidably mounted within the outer chambers. Most preferably, the chambers may be cylindrical and coaxial, in which case the pistons are preferably connected by a rigid rod. The chambers are preferably formed integrally with a common casing including a dividing wall to define the two chambers . A ratio other than 1:1 may be made to exist between the volume of driving gas introduced and the volume of patient gas thereby delivered. An alternative ratio may be achieved, by way of example, if pistons are connected by a rod which is of substantial cross-sectional diameter. The resulting mechanical advantage would be such that less driving gas is used for a given flow of patient gas, thereby reducing wastage of the driving gas . Drawing The invention will now be described in greater detail, with reference to the accompanying drawing, which is a schematic side view of a ventilator according to the invention. In the drawing, a ventilator comprises a pair of chambers (1, 2) , each chamber containing a partition dividing the chamber into two sub-chambers (3, 4 and 5, 6) . The partitions in this example of the invention take the form of self-inflating bags. A volume of driving gas from a pressurized source (17) is initially introduced via a valve (15) into a sub-chamber (5) , causing expulsion of an equal volume of the contents of sub-chamber (6) through an outlet (13) . Valve (15) then opens sub-chamber (5) to atmosphere, and sub-chamber (6) expands by drawing gas through outlet (13) . In a similar manner but acting 180° out of phase, driving gas is introduced via valve (16) into sub-chamber (3) , causing expulsion of the contents of sub-chamber (4) through outlet (14) . Valve (16) then opens sub-chamber (3) to atmosphere, and sub-chamber (4) expands by drawing gas through outlet (14) . With reference now to the patient gas circuit, in relation to which sub-chambers (4) and (6) have been alternately expanded and compressed, room air is entrained through reservoir tubing (7) and mixed with oxygen from a pressurized source (8) via a flowmeter (not shown) . A series of one-way valves (9, 10; 11, 12) dictates that the resulting mixture will, in response to the actions of the driving gas on chambers (4) and (6) , be directed toward the patient (18) . The operation of valves (15, 16) is subject to control by electronic, fluidic, or mechanical means. It will be seen that generation of patient gas flow is achieved alternately by chambers (1) and (2) . The chambers may be made to pause between their contributions, for the purpose of generating intermittent positive pressure, in which case the patient gas is fed to the patient via a valve which permits passive exhalation, and some form of airtight seal with the patient's respiratory tract. Alternatively, there may be no pause, with continuous flow being generated and offered to a spontaneously breathing patient preferably via some form of reservoir.

Claims: CLAIMS 1. A workstation for delivering oxygen-enriched air either intermittently or as a continuous or substantially continuous flow to an anaesthetized patient, comprising: means (1) for entraining ambient air from a room into a variable volume working chamber (4) by increasing the volume of the working chamber (4) ; means (8) for mixing oxygen from a pressurized source with the entrained ambient air; and means (3, 16, 17) for delivering the entrained mixture intermittently from the working chamber (4) to a patient by decreasing the volume of the working chamber (4) . 2. A workstation according to claim 1, wherein the mixing means comprises means for mixing oxygen from the pressurized source with the ambient air as it is drawn towards the working chamber (4) . 3. A workstation according to claim 1 or claim 2, wherein the means (1) for entraining ambient air into the variable volume chamber (4) is capable of sufficiently rapid operation to establish substantially continuous delivery to the patient. 4. A workstation according to claim 1 or claim 2, wherein the entraining means comprises means (1, 2) for entraining ambient air from a room alternately into first and second variable volume working chambers (4, 6), the mixing means comprises means (8) for mixing oxygen from a pressurized source with the ambient air entrained into each of the first and second working chambers (4, 6); and the delivering means comprises means (3, 16, 17; 5, 15, 17) for delivering the entrained mixture alternately from first one and then the other of the working chambers (4, 6) to provide a substantially constant supply of oxygen-enriched air to the patient. 5. A workstation according to any preceding claim wherein the or each working chamber (4, 6) comprises a self-inflating bag within an outer chamber, and the delivering means comprises means for delivering a driving gas under pressure intermittently to the space (3, 5) between the bag and the outer chamber. 6. A workstation according to any of claims 1 to 4 , wherein the or each working chamber (4, 6) comprises a piston chamber, within which a piston can reciprocate under pneumatic, electronic or mechanical control for the alternate entrainment and delivery of the oxygen-enriched air . 7. A workstation according to any preceding claim, wherein the delivering means comprises means for delivering the oxygen-enriched air to the patient for spontaneous ventilation via a breathing circuit and means are provided for providing an airtight seal with the patient's respiratory tract. 8. A ventilator comprising a workstation according to any of claims 1 to 6, wherein the delivering means comprises means for delivering the entrained mixture to the patient via a valve which permits passive exhalation, and means are provided for providing an airtight seal with the patient's respiratory tract, for establishment of an intermittent positive pressure ventilated supply to the patient .

Inventors: RUSSELL JOHN ALEXANDER, RUSSELL, JOHN, ALEXANDER
Applicants: RUSSELL JOHN ALEXANDER, RUSSELL, JOHN, ALEXANDER
Application year: 1998
Main classification: A61M16
IPC codes: A61M16