Gas delivery method and apparatus

Abstract

An apparatus, method and system for delivering CO.sub.2 into an inspiratory gas stream to formulate a blended respiratory gas in a manner that continuously maintains a target CO.sub.2 concentration in a volume of the inspired respiratory gas, for example, over the course of a breath or a volumetrically definable part thereof or a series of partial or full breaths.

Claims

1. A method for maintaining a targeted concentration (DA.sub.n.sup.T) of a component (DA.sub.n) of an added gas (G.sub.n) over a time period, the added gas (G.sub.n) added to an inspiratory gas (G.sub.0) to formulate a respiratory gas (G.sub.R) for delivery to a subject, the method comprising: obtaining a first cumulative volume of at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over an elapsed portion of the time period; obtaining a second cumulative volume of at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over the elapsed portion of the time period; computing an error signal (e.sub.n) based on the first cumulative volume and the second cumulative volume, the error signal (e.sub.n) representing an adjusted volume of the added gas (G.sub.n) to be delivered over a remainder of the time period to maintain the targeted concentration (DA.sub.n.sup.T) over the time period; and controlling, based on the error signal (e.sub.n), a gas delivery device to deliver the adjusted volume of the at least one added gas (G.sub.n) over the remainder of the time period to maintain the targeted concentration (DA.sub.n.sup.T) over the time period.

2. The method of claim 1, wherein obtaining the first cumulative volume comprises: obtaining an incremental volume of the at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over a most recent time interval; obtaining a previous cumulative volume of the at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over a previously elapsed portion of the time period; and summing the previous cumulative volume and the incremental volume to obtain the first cumulative volume.

3. The method of claim 1, wherein obtaining the second cumulative volume comprises: obtaining an incremental volume of the at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over a most recent time interval; obtaining a previous cumulative volume of the at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over a previously elapsed portion of the time period; and summing the previous cumulative volume and the incremental volume to obtain the second cumulative volume.

4. The method of claim 1, wherein computing the error signal (e.sub.n) comprises: identifying successive time intervals of the remainder of the time period; computing incremental volumes of the added gas (G.sub.n) to be delivered over respective successive time intervals so that a total of the incremental volumes is the adjusted volume of the added gas (G.sub.n) to be delivered over the remainder of the time period; and computing the error signal (e.sub.n) for each of the successive time intervals, each respective error signal (e.sub.n) representing respective incremental volumes of the added gas (G.sub.n) to be delivered over the respective successive time intervals.

5. The method of claim 4, wherein computing the incremental volumes comprises computing one of an average and a weighted average of the incremental volumes to be delivered over the respective successive time intervals in the remainder of the time period.

6. The method of claim 1, wherein computing the error signal (e.sub.n) is based on (i) a current ratio of the first cumulative volume and the second cumulative volume and (ii) a cumulative volume of respiratory gas (CVG.sub.R) including the adjusted volume of the at least one added gas (G.sub.n) to be delivered over the remainder of the time period.

7. A system for maintaining a targeted concentration (DA.sub.n.sup.T) of a component (DA.sub.n) of an added gas (G.sub.n) over a time period, the added gas (G.sub.n) added to an inspiratory gas (G.sub.0) to formulate a respiratory gas (G.sub.R) for delivery to a subject, the system comprising: a delivery conduit to deliver the respiratory gas (G.sub.R) to the subject; an inspiratory gas source coupled to the delivery conduit to supply the inspiratory gas (G.sub.0) to the delivery conduit; an inspiratory volume sensor to detect volumes of one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject; an added gas source coupled to the delivery conduit via an added gas conduit to supply the added gas (G.sub.n) to the delivery conduit; a gas delivery means along the added gas conduit to control a flow of the added gas (G.sub.n) to the delivery conduit; an added gas volume sensor to detect volumes of the at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject; a controller operatively coupled to the inspiratory volume sensor, the added gas volume sensor, and the gas delivery means, the controller to: obtain a first cumulative volume of at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over an elapsed portion of the time period; obtain a second cumulative volume of at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over the elapsed portion of the time period; compute an error signal (e.sub.n) based on the first cumulative volume and the second cumulative volume, the error signal (e.sub.n) representing an adjusted volume of the added gas (G.sub.n) to be delivered over a remainder of the time period to maintain the targeted concentration (DA.sub.n.sup.T) over the time period; and control, based on the error signal (e.sub.n), the gas delivery means to deliver the adjusted volume of the at least one added gas (G.sub.n) over the remainder of the time period to maintain the targeted concentration (DA.sub.n.sup.T) over the time period.

8. The system of claim 7, wherein the inspiratory volume sensor is disposed along the delivery conduit upstream of the added gas conduit.

9. The system of claim 8, further comprising a cross-bridge connecting an inspiratory conduit with an expiratory conduit of the delivery conduit, the cross-bridge to cause the inspiratory volume sensor to detect only volumes of gas destined for inspiration by the subject.

10. The system of claim 7, wherein the inspiratory volume sensor is disposed along the delivery conduit downstream of the added gas conduit, proximal to a patient connection of the delivery conduit.

11. The system of claim 7, wherein the gas delivery means is one of: a proportional flow control valve and a solenoid valve.

12. The system of claim 7, wherein to obtain the first cumulative volume, the controller is to: obtain, from the inspiratory volume sensor, an incremental volume of the at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over a most recent time interval; obtain a previous cumulative volume of the at least one of the inspiratory gas (G.sub.0) and the respiratory gas (G.sub.R) flowed to the subject over a previously elapsed portion of the time period; and sum the previous cumulative volume and the incremental volume to obtain the first cumulative volume.

13. The system of claim 7, wherein to obtain the second cumulative volume, the controller is to: obtain, from the added gas volume sensor, an incremental volume of the at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over a most recent time interval; obtain a previous cumulative volume of the at least one of the added gas (G.sub.n) and the component (DA.sub.n) flowed to the subject over a previously elapsed portion of the time period; and sum the previous cumulative volume and the incremental volume to obtain the second cumulative volume.

14. The system of claim 7, wherein to compute the error signal (e.sub.n), the controller is to: identify successive time intervals of the remainder of the time period; compute incremental volumes of the added gas (G.sub.n) to be delivered over respective successive time intervals so that a total of the incremental volumes is the adjusted volume of the added gas (G.sub.n) to be delivered over the remainder of the time period; and compute the error signal (e.sub.n) for each of the successive time intervals, each respective error signal (e.sub.n) representing respective incremental volumes of the added gas (G.sub.n) to be delivered over the respective successive time intervals.

15. The system of claim 14, wherein to compute the incremental volumes, the controller is to compute one of an average and a weighted average of the incremental volumes to be delivered over the respective successive time intervals in the remainder of the time period.

16. The system of claim 7, wherein the controller is to compute the error signal (e.sub.n) based on (i) a current ratio of the first cumulative volume and the second cumulative volume and (ii) a cumulative volume of respiratory gas (CVG.sub.R) including the adjusted volume of the at least one added gas (G.sub.n) to be delivered over the remainder of the time period.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1a is a schematic representation of one embodiment of a CO.sub.2 delivery device in accordance with the invention illustrating, by way of example, optional components and configurations of such a device.

(2) FIG. 1b is a schematic representation of one embodiment of a CO.sub.2 delivery device in accordance with the invention illustrating, by way of example, optional components and configurations of such a device.

(3) FIG. 1c is a schematic representation of one embodiment of a CO.sub.2 delivery device in accordance with the invention illustrating, by way of example, optional components and configurations of such a device.

(4) FIG. 2 is a schematic representation of one embodiment of a CO.sub.2 delivery device in accordance with the invention illustrating an optional scheme for organizing the flow of several added gases G.sub.1 to G.sub.n (directly into the G.sub.0 stream) and relevant inputs into and outputs from a computer programmed to implement this embodiment of the invention.

(5) FIG. 3 is a schematic representation useful for describing key volumetric control considerations underlying the presently disclosed scheme for adding one or more gases (G.sub.n) containing a desired component gas DA.sub.n (e.g. CO.sub.2) to an inspiratory gas (G.sub.0) stream according to one embodiment of the invention.

(6) FIG. 4 is a flow chart illustrating one embodiment of implementing a method according to the invention and related computer processing steps.

(7) FIG. 5 is an illustration in a graph form of advantages of an embodiment of the invention for adding CO.sub.2 to an inspiratory gas stream.

(8) The term “inspiratory gas” denoted G.sub.0 refers to any gas to which a gas consisting of or comprising a gas of interest (a component gas) is added. The G.sub.0 may be a principal gas provided to a subject for inhalation. For example, a ventilated patient may receive oxygen enriched gas as the G.sub.0. The G.sub.0 may also be one or more gases consisting or comprising desired component gases which may be individually or collectively considered a G.sub.0 with reference to another component gas. Accordingly the invention is concerned with but not limited to conditioning an inspiratory gas in the sense only of a principal gas and in one embodiment of the invention several added gases may be channeled into a manifold and each or the combination of several of them may be an inspiratory gas with reference to a particular component gas. Therefore, the invention contemplates that the necessary volumetric information is ascertained to track actual volumes of the component gas(es) of interest and the volume of gas e.g. the G.sub.0 or G.sub.R into which a component gas has been diluted.

(9) The term “volume sensor” (which may also be referred to as a “volume sensing means”) means any device that can be used to directly or indirectly determine the volume of a gas that has passed through a breathing circuit or particular conduit thereof, typically with respect to a reference location in a breathing circuit. The invention contemplates that this can be accomplished by a variety of types of hardware including a flow meter, a gas concentration sensor (for example, a certain amount of any first gas of known composition can be inferred to have been delivered past a reference point if mixed with any second gas of known composition and volume by ascertaining the concentration of the first gas in a mixture of the first and second gases), a pressure transducer (for example, an added gas is sourced from a tank of fixed volume, and a pressure transducer in the tank could be used to determine the volume obtained from the tank).

(10) As shown in FIGS. 1a, 1b and 1c, a system for adding CO.sub.2 to an inspiratory gas stream is exemplified by delivering controlled amounts of CO.sub.2 into a G.sub.0 channeling means exemplified in the form of an inspiratory limb 200 of a breathing circuit. A CO.sub.2 supply means, exemplified by tank 202 of pressurized CO.sub.2, is illustrated. The CO.sub.2 is carried by a conduit 204 made of inert tubing (does not react with CO.sub.2 and preferably other optional components of the gas G.sub.n in the tank). Gas leaving the tank of pressurized CO.sub.2 is controlled by one or more flow regulators (206) for reducing the pressure of the gas coming out of the tank 202. For example, the pressure may be first be reduced by a single pressure regulator 206, or may be initially reduced to a range acceptable for a miniature pressure regulator, which in turn reduces the pressure of gas Gn further (e.g. CO.sub.2) to that required at the inlet of a gas delivery means 210, for example a means for delivering variable incremental volumes of CO.sub.2 e.g. a valve that opens proportionally to a control signal (referred to herein as a proportional flow control valve or proportional control valve). Alternatively, a two way solenoid 236 (illustrated in FIG. 1 for a different function) can be turned on an off intermittently to accomplish a form of gas delivery volume control. A pump or blower may serve the purpose in some instances. A computer, optionally in the form of microcontroller 212, optionally incorporates a controller (e.g. a PID controller) for controlling the output signal to the proportional control valve 210. The microcontroller 212 receives input and provides output via signal carrying means shown (with broken lines) as signal lines. Most importantly this input comprises: (1) the output of a sensor means or sensor 214 which serves to determining the actual volume (a “volume sensor”) of CO.sub.2 output by the proportional flow control valve 210. This volume sensor is optionally a flow meter 214 whose instantaneous or total output is integrated to get the incremental volume of inspired CO.sub.2, or the total accumulated volume of inspired CO.sub.2, respectively; (2) the output of a second “volume sensor” exemplified as an inspiratory flow meter 218 (output of the respiratory flow meter is integrated to get the accumulated volume of inspired inspiratory gas G.sub.0) for determining the actual volume of gas being delivered, for example at a selected junction in the inspiratory limb 200. The inspiratory limb 200 leads to a patient connection 220 which is or leads to a mask, endotracheal tube, etc. (not shown) generically referred to for convenience as a subject airway interface. Measurements made by the inspiratory flow meter made at discrete time points T.sub.1 . . . T.sub.current (each in succession, as time passes, a respective T.sub.current) enable the controlled delivery of incremental volumes of CO.sub.2 which are generally coordinately delivered with the inspiratory gas (G.sub.0) stream, in proportioned increments i.e. a calculated amount of CO.sub.2 which achieves the targeted fraction of CO.sub.2 in the blended volume of respiratory gas G.sub.R (FCO2.sup.T) by adjusting the cumulative volume of delivered CO.sub.2 to match the integrated flow of the inspiratory flow meter 218 so that the total volume of CO.sub.2 gas actually delivered matches FCO2.sup.T with respect to the total volume of the delivered respiratory gas of interest (delivered over an incrementally growing time period T.sub.variable). Optionally, a check valve 222 (e.g. Beswick CKVU) prevents back flow through the CO.sub.2 delivery line. One-way check valves 224, 226 (e.g. Hans Rudolph 5610) may be constituted by low resistance one-way valves that prevent the patient from expiring back in to the inspiratory limb 200, and from inhaling via expiratory limb 234. These one way valves (224, 226) also protect the respiratory flow meter 218 and minimize circuit dead space.

(11) As shown in FIGS. 1a, 1b and 1c, for safety purposes a CO.sub.2 analyzer may be used to continuously monitor the inspired and expired fractional concentration of CO.sub.2. As described herein, a gas analyzer may also be indirectly used to a volume of gas passing by its location to the extent that a more diluted or concentrated gas may be used to determine what volumes of gas were mixed.

(12) In one embodiment illustrated, for example in FIG. 1a, the microcontroller 212 reads a target % of CO.sub.2 (FCO.sub.2.sup.T) and all the signals from the sensors and sends suitable control signals to the valve 210 and thereby implements suitable valve control (e.g. PID control). A monitor 230 displays information to the operator and a visible or audible signal generator 232 (e.g. a buzzer) may be used for safety to notify an operator if something goes wrong. In terms of safety features the microcontroller 212 may continuously receive input from the CO.sub.2 analyzer 228. If inspired and/or expired CO.sub.2 is high for a defined period of time then an optional two-way solenoid valve 236 may be closed, the proportional flow control valve 210 may be closed, and the buzzer 232 excited.

(13) FIGS. 1a, 1b and 1c illustrate different configurations related to the placement of flow sensor 218. In a standard configuration, shown in FIG. 1a, when the inspiratory limb is connected to a ventilator (not shown) flow sensor may not accurately reflect the tidal volume of gas actually entering the patients lungs because the flow sensor 218 measures a compressible volume of gas that is compressed in and expands the tubing more proximal to the subject. Gas flowing through the flow sensor 218 may flow around the Y connection through to the expiratory limb 234. On the other hand, as illustrated in FIG. 1b, the placement of a flow sensor 218 at the mouth (proximal to the patient connection 220) adds dead space which may impair carbon dioxide elimination, especially in children and small adults. Furthermore, it adds bulk and weight to the patient airway interface. As illustrated in FIG. 1c, a cross-bridge 208, connecting the inspiratory 200 and expiratory 234 limbs, causes the flow sensor 218 to see only gas destined for inspiration by the patient as the compressible volume flows through the cross-bridge preventing passage through the flow sensor 218 or in a sense by-passing the flow sensor 218. Optionally, the G.sub.0 gas channeling means includes a flow-sensor by-pass means, such as described above, for obtaining an accurate measurement of G.sub.0 tidal volume actually flowing to the patient. This form of cross-bridge or by-pass means between inspiratory and expiratory sides can also be present in the ventilator such that a flow meter downstream thereof (towards the patient) in the tubing leading out to the connection to the inspiratory limb of a breathing circuit measures accurate tidal volumes flowing to the patient.

(14) As shown in FIG. 2, according to one embodiment of an apparatus according to the invention several added gases G.sub.1 to G.sub.n sourced from gas sources 410, 420 and 430, respectively, may optionally be directly added into the G.sub.0 stream. A volume sensor (VS.sub.0) 370 is operatively associated with the G.sub.0 delivery conduit 55 and is optionally located in the G.sub.0 conduit 55, optionally along with respective gas analyzers GS.sub.1,0 to GS.sub.n,0 (340, 350, 360) for each of the respective added gases which gases may be present in the inspiratory gas G.sub.0. Relevant inputs into and outputs from a computer 300 programmed to implement this embodiment of the invention include: inputs from optional gas analyzers GS.sub.1,0 to GS.sub.n,0 (340, 350, 360) and GS.sub.1 to GS.sub.n (380, 390, 400) (needed where the fractional concentration of gas of interest in an added gas is unknown or optionally for enhanced safety) and inputs from volume sensors 310, 320, 330, (as broadly defined herein) optionally located in the respective added gas delivery conduits 315, 325 and 335. Inputs of fractional concentrations of the respective component gases F.sub.DA1,R-F.sub.DAn,R enable a controller 345, optionally integrated as part of the computer 300, to direct outputs to respective gas delivery means GD.sub.1 to GD.sub.n (355, 365, 375) based on the actual incremental volumes of gas delivered by the gas delivery means as determined via the respective volume sensors VS.sub.0 (370) and VS.sub.1 to VS.sub.n (310, 320, 330). It will be appreciated that a component gas of interest does not need to be CO.sub.2 and may be any gas such O.sub.2, Xe, He, H.sub.2, NO, N.sub.2O, H.sub.2S, CO, SF6, an anesthetic, etc. In, for example, a compressed gas form, one or more such gases can be added to the G.sub.0 gas to compose a respiratory gas G.sub.R. The term DA.sub.n is generically used herein to refer to a component gas of interest and any reference herein to carbon dioxide, except where the context necessarily implies that carbon dioxide is being referred to specifically, may be replaced by a reference to DA.sub.n or any other specific component gas.

(15) A device according to the invention may therefore comprise a wider or narrower variety of components that may be particularly useful or more readily substitutable for implementing a particular application of the invention. Furthermore, depending on a suitable range of deliverable volumes of gas in question, optionally attuned to a particular size range of a breath (e.g. for a premature human infant or small animal) the sizes and ranges of accuracy of components may be differently selected according to well understood design criteria.

(16) In terms of optional embodiments, the GD.sub.n is optionally a proportional solenoid valve metering the release of G.sub.n from a pressurized gas source. The invention also encompasses intermittently turning on and off a two-way solenoid valve metering the release of G.sub.n from a pressurized gas source. Optionally, the GD.sub.n can be a gas pump, blower, or injector connected to a pressurized or unpressurized reservoir of G.sub.n.

(17) Input of CVG.sub.n actually delivered during T.sub.1 . . . T.sub.now is obtained via a volume sensor (VS.sub.n) operatively associated with a G.sub.n delivery conduit. Input of CVG.sub.0 actually delivered during T.sub.1 . . . T.sub.now may be obtained via a second volume sensor (VS.sub.0) operatively associated with a G.sub.0 delivery conduit. A volume sensor can be constituted by any hardware for directly or indirectly measuring a volume of gas, for example, a spirometer, or a flow transducer or gas analyzer from which the flows of gases can be deduced (and then computing the integral of the flow).

(18) The inputs, computations, and outputs described in the aforementioned method can be carried out by a variety of signal processing means including, but not limited to, a programmable processor, programmable microcontroller, dedicated integrated circuit, programmable integrated circuit, discrete analog or digital circuitry, mechanical components, optical components, or electrical components.

(19) The VC.sub.n can be implemented by a variety of signal processing means including, but not limited to, a programmable processor, programmable microcontroller, dedicated integrated circuit, programmable integrated circuit, discrete analog or digital circuitry, mechanical components, optical components, or electrical components.

(20) A G.sub.n gas channeling means, optionally in the form of a G.sub.n delivery conduit, is optionally adapted to be operatively associated with or directly fluidically connected to a G.sub.0 channeling means, for example, a G.sub.0 delivery conduit such as the inspiratory limb of a breathing circuit, a patient airway interface, a manifold for receiving multiple gas connections, or a connector interconnecting one or more of the above.

(21) In one embodiment of the invention, all inputs, computations, and outputs are performed on a general purpose microcontroller. The G.sub.n is a pressurized gas containing a known, fixed concentration of DA.sub.n. A rapid DA.sub.n analyzer is operatively associated with the G.sub.0 delivery conduit to ascertain the concentration of DA.sub.n in G.sub.0. A VS.sub.n is implemented by integrating the output of a flow transducer operatively associated with the G.sub.n delivery conduit. A VS.sub.0 is implemented by integrating the output of a flow transducer operatively associated with the G.sub.0 delivery conduit. The VC.sub.n may be a PID controller implemented on a general purpose microcontroller. For each T.sub.current, GD.sub.n receives a weighted sum of the output of VC.sub.n and G.sub.n.sup.P. The incremental G.sub.0.sup.P predicted to be delivered in the time interval ΔT between the respective T.sub.current and a ensuing time point T.sub.current+ is equated with the incremental volume of G.sub.0 delivered in the time interval ΔT ending at T.sub.current. The GD.sub.n is implemented with a series of pressure regulators and a proportional solenoid valve metering the release of G.sub.n from the pressurized source. The G.sub.n delivery conduit is directly fluidically connected to the G.sub.0 channeling means so that the G.sub.n is directly delivered into the G.sub.0 stream.

(22) As shown in FIG. 3, implementation of the invention, preferably takes into account a cumulative volume of all of the several components of a respiratory gas G.sub.R 20 delivered to a subject including the respective volumes of one or more added gases V.sub.1, V.sub.2 . . . V.sub.n which are coordinately delivered with an inspiratory gas (G.sub.0) stream, in proportioned increments i.e. a calculated amount of G.sub.n 10 which achieves the targeted fraction of DA.sub.n 30 in a volume of G.sub.R (DA.sub.n.sup.T) is consistently added to keep step with one or more recent incremental amounts of G.sub.0 and other added gases flowed to the subject. The term “coordinately” means involving coordination so as to update the fraction of DA.sub.n in a volume G.sub.R to a desirable extent in terms of frequency and accuracy. Optionally, keeping step with new incremental volumetric amounts of delivered G.sub.0 takes full advantage of the capabilities of the hardware of choice. For example, typical cost-effective hardware may be capable of delivering a proportioned amount of G.sub.n (relative to an updated cumulative volume of G.sub.0 including the last confirmed incremental volume of G.sub.0 flowed to the subject) containing a known/determined fraction DA.sub.n to match DA.sub.n.sup.T—every millisecond. The curved arrow is used to imply controlled and confirmed (via some form of sensor) volumetric addition (e.g. via an appropriately controlled gas metering device e.g. a proportional solenoid under PID control) of a discrete corrective volume of a gas G.sub.n 10 containing a component gas of interest (DA.sub.n) to a confirmed total volume of delivered gas (including V.sub.1 . . . V.sub.n, V.sub.0 and optionally DA.sub.n)−the confirmed cumulative G.sub.R volume (confirmed via cooperative action of a plurality of sensors of a broadly varying type and location of placement). It is understood that G.sub.n may be made up entirely (100%) of DA.sub.n. As described herein, incremental amounts of DA.sub.n flowed to the subject are preferably at least “retrospectively corrective” to target DA.sub.n.sup.T, and as described below may optionally be a “predictively corrective” component (inasmuch as a predictive strategy can be termed “corrective” when serving the optional purpose of facilitating a retrospectively corrective strategy).

(23) V.sub.DAn (represented by the oval area 40) is an accumulated volume of the component gas of interest that forms part of the volume of G.sub.R 20 delivered in virtue of being added at full concentration (G.sub.n is 100% DA.sub.n) or blended (as illustrated) into one or more of V.sub.1, V.sub.2 . . . V.sub.n. DA.sub.n may optionally also be a component of inspiratory gas G.sub.0. An error signal e.sub.n according to a simple embodiment of the invention is the volume 10 of gas G.sub.n that is needed to be delivered (based on what on has been “definitively” already delivered) to carry with it enough of the component gas of interest to ensure that the resulting amount of this component gas in a volume of the respiratory gas G.sub.R is incrementally maintained (subject to hardware time lags) and therefore consistently (to the extent practicable or desired) in the same proportion vis-à-vis the total volume of “definitively” delivered G.sub.R regardless of the changing total volume of gas that has been delivered to the subject. “Definitively” delivered amounts of G.sub.n and G.sub.R are not amounts that the hardware would have delivered if responding perfectly to flow settings but volumes that requisite “sensors” determine have indeed flowed through the pertinent gas delivery conduits. The term “definitively” is used superfluously in the present description of FIG. 3 for extra emphasis to distinguish amounts theoretically flowed to the subject (the term “delivered” when used to refer to a volume of a gas implies a volume measured by some form of device—generically called a “volume sensor”—that directly or indirectly measures or enables volume computation—“delivered” is considered to sufficiently distinguish a “flow setting” or other “unverified delivery” modality used in prior devices). Variations on the above-described basic approach of “incrementally” correcting to a desired relative proportion—delivered DA.sub.n in delivered G.sub.R (retrospective) include adjunct “predictive correction” strategies for optimizing the error signal. Sub-optimal correction on the other hand may vary the size range of volumetric increments of G.sub.0 being corrected—the invention is best exploited by correcting small increments of newly delivered gas, optionally, but not necessarily, the smallest increments possible having regard to inherent limitations the hardware available or selected (e.g. cost effective) for use. In one embodiment, adjunct predictive correction involves predicting how much the volume of G.sub.R will grow from delivery of G.sub.0 in one or more ensuing increments of time (typically by predicting one or more ensuing incremental volumes of G.sub.0 based one or more recent G.sub.0 sensor readings) and computing an accommodative error signal even before the new gas is definitively delivered into the cumulative volume of G.sub.R. According to another embodiment, the computed e.sub.n may take into account, only amounts of G.sub.0 and G.sub.n definitively delivered. Variations on “computing” the error signal, involving a predictive correction factor are understood as adjunct strategies that are not inconsistent with baseline retrospective volumetric correction that broadly defines the invention in one aspect. Aside from “computing” a baseline error signal retrospectively, a controller, for example a PI or PID controller, would conventionally be used to appropriately implement the corrective e.sub.n. Accordingly, compensating for hardware limitations inherent in delivering e.sub.n may be seen to represent a distinct aspect of the totality of the volumetric corrective measure that would be expected to be implemented by an engineer in implementing the invention.

(24) It may be appreciated that the strategy of the invention may be implemented to a useful extent if the incremental gas proportioning is consistent in the first part of inspiratory cycle (see FIG. 5) even if done in less than optimally small sequential as opposed to sporadic and/or wider spaced time increments. The first part of the inspiratory cycle of interest is the volume that destined to enter the alveoli as opposed to dead space volumes. Furthermore, in theory, the desired or chosen fraction of DA.sub.n in G.sub.R may be so small that computational inclusion of DA.sub.n as a necessary part of G.sub.R could be obviated for small cumulative volumes of DA.sub.n. Accordingly, the formulas and strategic approaches presented herein address embodiments of and options with respect to best and/or most universal practices and don't purport to cover all sub-optimal or circumventive strategies of exploiting the invention.

(25) As shown in FIG. 4, one embodiment of a method (and related algorithm) according to the invention, may be expressed as a series of steps carried out with respect to each time increment T.sub.current (optionally a time increment that is optimized having regard to time delays of the computer, controllers, gas delivery means and sensors (volume, gas analyzer), wherein the computer: 1. Reads inputs as exemplified in FIG. 4 (70) 2. Calculates (90) the accumulated inspired volume of G.sub.0 (CVG.sub.0) by adding the volume of G.sub.0 inspired at the respective T.sub.current (to the accumulated volume of G.sub.0 determined as at the previous respective T.sub.current (CVG.sub.0-)) as measured by VS.sub.0 3. Calculates (90) the accumulated inspired volume of each G.sub.n (CVG.sub.n) by adding the volume of G.sub.n inspired for the respective T.sub.current (to the accumulated volume of G.sub.n determined as at the previous respective T.sub.current (CVG.sub.n-)) as measured by VS.sub.n 4. For each DA.sub.n, determines (either from measurement or otherwise) F.sub.DAn,0 (80) 5. For each DA.sub.n, determines (either from measurement or otherwise) F.sub.DAn,n (80) 6. For each DA.sub.n, calculates (100) the accumulated inspired volume of pure DA.sub.n (CVG.sub.DAn) by adding the volume of G.sub.0 inspired the respective T.sub.current as measured by VS.sub.0 multiplied by F.sub.DAn,0 at T.sub.current, and adding the volume of G.sub.n inspired during the time interval ending in T.sub.current as measured by VS.sub.n multiplied by F.sub.DAn during the respective T.sub.current (it will be appreciated that this type of computation may accomplished by calculating (90) the accumulated inspired volume of each G.sub.n (CVG.sub.n) by adding the volume of G.sub.n inspired during the respective T.sub.current especially where fractional concentration of DA.sub.n, in G.sub.n is constant or is consistently 100%). 7. For each DA.sub.n, calculates (110) an error signal (e.sub.n) equal to the volume of G.sub.n that must be added to the accumulated inspired volume of respiratory gas (CVG.sub.R) so that the volume of inspired DA.sub.n composes the target concentration of DA.sub.n (DA.sub.n.sup.T or F.sub.DAn,R) in CVG.sub.R (optionally the projected total inspired volume of respiratory gas at T.sub.current+)

(26) $\frac{{\mathrm{CVG}}_{\mathrm{DAn}}+F_{\mathrm{DAn}}.Math.e_n}{{\mathrm{CVG}}_0+{\mathrm{CVG}}_n+e_n}=F_{\mathrm{DAn},R}$$e_n=\frac{F_{\mathrm{DAn},R}\left({\mathrm{CVG}}_0+{\mathrm{CVG}}_n\right)-{\mathrm{CVG}}_{\mathrm{DAn}}}{F_{\mathrm{DAn}}-F_{\mathrm{DAn},R}}$ 8. Delivers a signal to each GD.sub.n generated from the weighted sum of the current value of e.sub.n, the derivative of e.sub.n, and the integral of e.sub.n (not shown).

(27) The term DA.sub.n (and, as applicable, terms which reference DA.sub.n such as F.sub.DAn and CVG.sub.DAn) may for convenience be understood to be used expansively to reference embodiments of the invention in which there is one DA.sub.n as well as multiple DA.sub.ns (more precisely DA.sub.1-n) since DA.sub.n in the latter scenario can be understood, if interpreted in a restrictive sense, to mean the last in the series 1 to n. In context, where more than one “DA.sub.n” is delivered as part of the cumulative volume of respiratory gas G.sub.R, with respect to each “DA.sub.n” (strictly speaking each respective DA.sub.1 . . . DA.sub.D), each other “DA.sub.n” can be understood to be taken into account as part of the G.sub.0 and hence the aforementioned generic formula for the error term e.sub.n should be understood broadly to be generically applicable to each respective “DA.sub.n” (DA.sub.1 . . . n) based on the assumption that the volumes of the added gases other than the one for which the computation is being made are taken into account as part of the G.sub.0. The (+) sign with reference to a time point is used herein to refer to a future time point (not yet a T.sub.current) for which actual delivered (output by a G.sub.n specific gas delivery device or ventilator) volumes of a component gas and G.sub.0/G.sub.R have not been ascertained. A minus (−) sign may be used to refer to a time point before a respective T.sub.current e.g. the most recent T.sub.current for which actual delivered (output) volumes have been ascertained.

(28) It will be appreciated from the foregoing description that e.sub.n may be a corrective volume to bring the concentration of DA.sub.n in line with DA.sub.n.sup.T as at T.sub.current without taking into account the incremental volume of G.sub.0 expected to be delivered by the next respective T.sub.current+1. Alternatively, the computation may take into account the expected incremental volume of G.sub.0 expected to be delivered by the next respective T.sub.current+ and hence the projected total of volume of respiratory gas expected to be delivered by the next T.sub.current including the e.sub.n and its DA.sub.n content.

(29) As shown in FIG. 5, panel A, a device according to the invention implements a form of closed loop volume control that enables the flow of G.sub.1 to maintain a target concentration of G1 in the total inspired volume, as early as possible in the breath. As shown in Panel B, this is especially important and advantageous with respect to the first part of a breath that fills the alveoli. As seen in Panel B, belated matching of the theoretical flow without later compensation accumulates volumetric error that will not achieve the targeted concentration of DA.sub.n in G.sub.R in the volume of G.sub.R destined to be part of volume of gas entering the alveoli. FIG. 5 is described in more detail below.

(30) In one aspect, the volume of gas containing DA.sub.n is ideally delivered as part of the expanding volume of gas that is alveolar gas and not dead space gas. This is explained with reference to FIG. 5 described in more detail below.

(31) The invention can therefore be understood to be broadly directed to a method for adding at least one added gas (G.sub.n) to an inspiratory gas G.sub.0, to formulate a respiratory gas (G.sub.R) for delivery to a subject, and to maintain a targeted concentration of at least one component of an added gas G.sub.n (DA.sub.n) in a volume of the G.sub.R comprising the steps of:

(32) for each of a series of time points of interest (generally time points in which G.sub.n is being coordinately delivered with G.sub.0):

(33) (a) obtaining input of confirmed incremental volumes of G.sub.0 or G.sub.R flowed to a subject; (b) obtaining input of confirmed incremental volumes of G.sub.n or pure DA.sub.n flowed to a subject; c) computing for any respective T.sub.current an error signal (e.sub.n) equal to the volume of G.sub.n that must be coordinately delivered to the subject with the G.sub.0 so that the cumulative volume of DA.sub.n equals DA.sub.n.sup.T; d) for any respective T.sub.current providing an output to GD.sub.n based on the e.sub.n computed for the respective T.sub.current whereby the actual cumulative volume of DA.sub.n (generally coordinately delivered with CVG.sub.0 as part of CVG.sub.R) is controlled to target DA.sub.n.sup.T.

(34) Thus in a broader aspect, the present invention relates to a device, method and system for delivering at least one added gas into an inspiratory gas stream to formulate a blended respiratory gas in a manner that continuously maintains a target concentration of the added gas in a volume of inspired respiratory gas, for example, over the course of a breath or a volumetrically definable part thereof or a series of partial or full breaths. The inspiratory gas may be a principal gas stream delivered to a patient such as air optionally having an enhanced oxygen content or air and oxygen combined with an anesthetic gas delivered by a ventilator or anesthetic machine but may also be comprised of several additive gases delivered individually or in blended form according to a method/device according to the invention.

(35) A goal of most respiratory gas blenders is to deliver a target concentration of an additive gas into the inspired stream. Previously, this has been done by measuring the inspiratory flow and sending a signal to a flow controller to provide a flow of additive gas proportional to the inspired stream. Such a “flow-based control” system essentially tries to maintain the instantaneous concentration of the additive gas in the inspired gas at the target value. However, due to practical limitations of flow transducers and flow controllers, most notably finite response times, it is not possible for a flow controller to exactly “track” the inspired gas stream. Therefore, at any time during the breath, the instantaneous concentration of additive gas in the inspired stream may not be equal to the target concentration. Moreover, the overall concentration of additive gas in the accumulated inspired volume will not be equal to the target concentration. Furthermore, the design of previous respiratory gas blending systems overlooks that it is the concentration of additive gas, by volume, in the volume of inspired gas that reaches the alveoli that is the most important factor in many physiologic, therapeutic and/or diagnostic contexts.

(36) The simplest gas blending systems try to match the flow of additive gas to the inspired stream with an open loop signal to a flow controller. That is, the actual flow of additive gas delivered by the flow controller is not monitored. If there is a systematic error/offset in the flow delivered by the flow controller, the flow of additive never reaches the target flow rate. Therefore, the instantaneous concentration of additive in the inspired stream never reaches target, and the concentration by volume in the accumulated inspired gas is always in error.

(37) More complex blending systems try to match the flow of additive gas into the inspiratory gas stream with a closed loop signal to a flow controller. That is, the actual flow of additive gas delivered by the flow controller is monitored. If there is a systematic error/offset in the flow delivered by the flow controller, the signal to the flow controller is adjusted until the flow of additive reaches the desired flow rate. Therefore, the concentration of additive in the inspired stream may reach the target value as the breath proceeds, but because of an obligatory delay in response of the flow controller for the additive gas, the overall concentration by volume will always be less than the target concentration set at the beginning of the breath.

(38) In one aspect, the present invention contemplates a control system in which the overall concentration of additive gas, in the volume inspired gas entering into the alveoli, reaches the desired value.

(39) The invention contemplates that one or more additional gas delivery conduits may be operatively connected to or otherwise operatively associated with (via coordinated delivery into an airway interface) the G.sub.0 delivery conduit for coordinately delivering a controlled volume of an added gas G.sub.n (or controlled volumes of a plurality of added gases G.sub.1 to G.sub.n) with the G.sub.0 stream, wherein each G.sub.1-n is at least partially composed of a respective desired additive gas (DA.sub.1-n). In one embodiment, each gas delivery conduit carrying an added gas is operatively associated with a volume controller for controlling the volume of gas coordinately delivered with the G.sub.0, a volume sensor operatively associated with the added gas delivery conduit for continuously measuring the accumulated volume of the added gas. Optionally, where the fractional concentration of the added gas in G.sub.0 is not known, a means (for example a gas analyzer), operatively associated with the G.sub.0 delivery conduit, may be employed to measure, optionally continuously, the fraction of the added gas in the G.sub.0 delivery conduit. Optionally, for example for a given G.sub.n, where the fractional concentration of the desired added gas in G.sub.n (F.sub.DAn) is not known, a means, operatively associated with the G.sub.n delivery conduit, for example a gas analyzer may be employed to measure, optionally continuously, F.sub.DAn in the G.sub.n delivery conduit (GS.sub.n). For example, in one embodiment, for a gas G.sub.n, the device may comprise: (1) A volume controller (VC.sub.n) for controlling the volume of G.sub.n added to G.sub.0 (2) A volume sensor (VS.sub.n) operatively associated with the G.sub.n delivery conduit for continuously measuring the accumulated volume of inspired G.sub.n Optionally, where the fractional concentration of DA.sub.n in G.sub.0 (F.sub.DAn,0) is not known, a means, operatively associated with the G.sub.0 delivery conduit, to measure continuously F.sub.DAn,0 in the G.sub.0 delivery conduit (GS.sub.n,0) (3) A computer that takes input of: A target concentration of each DA.sub.n in the accumulated inspired volume (F.sub.DAn,l) The output of VS.sub.0 The output of each VS.sub.n Each F.sub.DAn,0 (either known or measured) Each F.sub.DAn,n (either known or measured) And provides output to each VC.sub.n.

(40) On inspiration, gas entering the mouth or nose is conducted to the lung through a series of conduits consisting nasopharynx, oropharynx, trachea and bronchi. From the point of view of gas exchange, these are considered conducting vessels directing gas to the alveoli. As these conducting vessels do not contribute substantially to gas exchange, they are termed anatomical deadspace. In an average adult, they consist of about 2 ml per kg of body mass, or about 150 ml for the average adult. The alveoli are small saccules where the gas comes into close contact with blood and gas exchange takes place.

(41) The distribution of gas during inspiration is well understood. At end expiration, the lung volume is smallest. In the course of inhalation gas is drawn through the anatomical deadspace into the alveoli. At end inspiration, the inspired gas is distributed between the alveoli and the anatomical deadspace. Note the last inhaled gas is retained in the anatomical deadspace. Physiologically the alveoli acts like a mixing chamber where the accumulated gases are mixed. The physiologic effect of an inhaled gas is determined by its net concentration once it has mixed in the alveolar space, that is, its fractional volume in the alveoli. Its instantaneous inhaled concentration is only important to the extent that it affects the net volume of that gas in the inspired volume. Although the flow-based controllers can reach an instantaneous target concentration of additive in the inspired stream, they are not directed towards providing a net concentration of a gas in the alveolar space where gas exchange takes place and where the net concentration of the added gas exerts its pharmacologic effect. This is illustrated in FIG. 5. The model is that of a subject being ventilated by a ventilator that provides a square wave inspiratory flow of G.sub.0 (this simplest of cases is used for illustrative purposes; the principle herein described applies to any inspiratory flow pattern by a ventilator or via spontaneous ventilation) and a target concentration of component gas 1 (G.sub.1). A required flow of G.sub.1 is required to attain this target concentration of G.sub.1 in the total inspired gas. At the beginning of inspiration the response delay in the flow controller results in a ramp up to target flow. The effect on the G.sub.1 concentration of the gas in the alveoli—the cumulative volumetric error resulting from the shortfall in the instantaneous concentration of additive G.sub.1 in the inspired stream during inhalation—is illustrated in the lower part of the figure. Note that the difference in the volume of G.sub.1 delivered to the alveoli, as represented by the difference between the between the curves “target flow G.sub.1” and that of the flow-based controllers, results in the volumetric concentration of G.sub.1 in the inspired gas reaching the alveoli always being less than the desired concentration. The instantaneous concentration of additive in the inspired stream is in greatest error at the start of the breath. Therefore, early terminations of the breath increase the discrepancy between desired concentration and actual concentration of G.sub.1 in the volume of inspired gas reaching the alveoli.

(42) Previous devices have been designed on the premise that a response delay of a flow-based controller at the start of the breath is mirrored by an overshoot in G.sub.1 flow at the termination of the breath. Over the whole breath, this may result in an average alveolar G.sub.1 concentration at the desired level if the acceleration and deceleration profile of the breath are symmetrical. However with respect to ventilation, as illustrated in the lower part of FIG. 5, only the initial part of the breath reaches the alveoli. The overshoot in G.sub.1 flow at the termination of the breath in our model of a square wave inspiratory flow occurs during exhalation and provides no compensation to the lung concentration. With more sinusoidal inspiratory gas flow, there may be excess flow of G.sub.1 as the inspiratory flow of G.sub.0 slows down. However, at least part of the terminal aspect of the breath where the compensation takes place with flow-based control always resides in the anatomical deadspace and the increased compensatory flow of G.sub.1 does not reach the alveoli.

(43) By contrast, a closed loop volume-based control provides early reconciliation of the G.sub.1 volume and the inspired volume of G.sub.0 such that the net inspired concentration of G.sub.1 is at the desired level very early in the breath. In practical terms, a volume controller according to the invention can provide fully compensated alveolar concentrations of G.sub.1 within as little as 10 ml of inspired volume and typically within 20-50 ml. Premature terminations of breaths after this level is reached would not affect the physiologically important net inspired concentration of G.sub.1 in the alveolar compartment of the lung.

(44) Furthermore, with respect to the finite response time of any flow controller, the shorter the breath, and the greater the inspiratory flow, the more significant the delay period of the flow controller becomes with respect to providing the overall volumetric concentration of additive gas. Therefore for rapid breathing, where inspiratory flows are high and inspiratory times short, errors of DA.sub.n volumetric concentrations in the alveoli are magnified by response delays of flow controllers. Furthermore, even flow controllers with very rapid response times may still accumulate physiologically important errors of concentrations of DA.sub.n, particularly where DA.sub.n is a potent physiologic molecule (defined as having a large physiologic effect for a small change in concentration), in the overall volumetric concentration of DA.sub.n in the accumulated inspired volume.

(45) The system according to the invention operates a closed control loop in order to maintain the concentration of additive, by volume, in the accumulated inspired volume at a target value throughout the breath. At each point in time, the total inspired volume and the total volume of inspired additive is assessed. An error signal is generated equal to the volume of additive that must be added to the inspiratory gas stream so that the overall concentration of additive, by volume, in the gas accumulated in the lung during inspiration will be equal to the target value throughout the breath. The error signal is provided to a volume controller which provides a signal to a gas delivery means (alternatively called a gas delivery device). This maintains the concentration of additive, by volume, in the accumulated inspired volume at the target value throughout the breath.

(46) As described below, the invention contemplates that multiple gases can be combined according to a method of the invention or using a device, computer program product (including any known format in which the requisite program code can be recorded or hard-wired), processor or system according to the invention based on adaptations described herein and evident to those skilled in the art. A reference to blending or delivering a G.sub.n in tandem with a G.sub.0 that does not explicitly specify a single G.sub.n and that can be understood to be a general case in which there is more than one added gas having the same or a different component gas of interest are meant to disclose this general case of the invention in which permutations and adaptations to accommodate more than one added component are understood to be related. Furthermore each of the general classes and specific embodiments of the invention are meant to refer back to the variety of aspects of the invention described herein and any logistical permutation of these various classes of and specific embodiments are understood to be described within the general concepts for implementing the invention elaborated above. Therefore, while a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the foregoing description and following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. Alternative terms for any features, elements, components etc of the invention as defined herein are not meant to be differentiated by virtue of the use of alternative language and each term is intended to be given its broadest meaning consistent with the context and the function it serves according the description of the invention as a whole. The scope of the claims should not be limited by the preferred embodiments, but should be given the broadest interpretation consistent with the description of the invention as a whole.

REFERENCE LIST

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