METHOD AND SYSTEM FOR NON-INVASIVE VENTILATORY SUPPORT

Background of the Invention The present invention relates to respirators (also known as ventilators) of the type used to supply air to a patient in need of assisted breathing and to methods of operation of such respirators. Conventional mechanical ventilators currently in widespread use for patients in need thereof are intended to deliver a volume of oxygen enriched air into the trachio-bronchial tree, to inflate the alveoli of the lungs, where they participate in gas exchange or respiration. Various modalities have been developed to generate the correct volume and oxygen concentration at the correct pressure, to optimize gas exchange and minimize patient trauma, for a given set of patient circumstances involving chronic obstructive pulmonary disease (COPD). In a large number of such cases, the patient ventilation procedure involves an endotracheal intubation, in which a tube is inserted in the patient's airway, in order to deliver ventilation support to the patient's lungs. Since endotracheal intubation in life support ventilators is both invasive and potentially traumatic, the procedure is usually limited to treatment in intensive care units of hospitals. It is considered unwise to assist marginally ventilating patients by intubation at home or in post ICU hospital COPD beds because of the need for close observation of intubated patients.

In recent years bi-level positive airway pressure (Bi-Level PAP) ventilators have been developed to provide non-invasive ventilatory support. These known systems are, however, prohibitively expensive, and at times are not compliant to the needs of the patient. Among other things, Bi-Level PAP systems currently in use do not optimally augment the patient's tidal volume in conjunction with his or her spontaneous breathing. This can result in a potential build-up of CO ₂ in the blood which, in turn, can result in respiratory acidosis, and possible respiratory failure.

Heretofore, respirators of the type described above have typically subjected ambient intake air to a succession of compressor stages, each stage consisting of a rotor and compressor blades, before delivery to the patient. Such means of compression is expensive, produces considerable noise and results in a turbulent, non-laminar flow of air to the patient without the extensive use of turbulence baffling. Furthermore, compressor rotors require lubrication, which tends to contaminate the air passing through it despite attempts to filter and clean the air. Since respirators are generally needed by the very ill or those suffering from sleep apnea, a low noise level becomes very important. With the present state of the art respirators, the noise level is so great that the patients normally do not sleep well or deeply.

Presently known equipment, and the precision of design and manufacture it requires, also results in high cost to hospitals, home care providers and patients. It is thus a purpose and objective of the present invention to obviate the above described deficiencies by providing a radically different approach to supplying respiratory/ventilator-assisted air to a patient which has a low noise decibel level, is uncontaminated, is compliant with a wide variety of patient circumstances, is easily used, and can be provided at a greatly reduced cost in comparison to previously known systems.

Summary of the Invention To provide air to the patient requiring assisted breathing, the respirator system and method of the present invention uses a remote source of pressurized air and avoids the use of successive rotor-driven compressor stages. The invention instead uses a special air amplification means which includes a primary source of laminar (non-turbulent), low pressure compressed air, in combination with an adjustable, passive amplifier which includes a venturi chamber through

which such laminar air flows. The amplifier is arranged to admit the compressed air upstream of the venturi chamber, through a precisely controllable variable annular opening provided to control the flow of such primary compressed air into and through the venturi chamber. The venturi chamber utilizes the suction effect of the primary laminar flow of compressed air to entrain and induce ambient air to flow into and through the venturi chamber as required by the patient. The primary flow of compressed air through the venturi chamber is laminar and the induced ambient air is therefore also laminar (i.e., non-turbulent) with an extremely low level of noise. This greatly benefits the patient and makes possible for the first time a very precise regulation of the volume of airflow, and the pressure thereof, to the patient. Furthermore, the resultant laminar flow is produced with the highest degree of efficiency and controllability and with the minimal expenditure of power.

One of the problems associated with the turbulent air produced by the current state of the art respirators is that sensors needed to regulate the pressure and flow of air to the patient are rendered imprecise as a result of the characteristic turbulence of the air flow. In some cases, as much as 80% of the air generated by current respirators is wasted, resulting in excessive noise as well as extra expense. By providing non-turbulent laminar flow to the patient, the system of the present invention makes it possible to more precisely measure and control air flow and pressure to the patient, as may be prescribed for the patient's well being, with an absolute minimum of wasted air flow. Finally, the overall cost of respirators manufactured in accordance with the present invention is greatly reduced (in some cases by approximately fifty percent compared to the cost of conventional respirators available to the market at the present time).

The ventilator system of the invention has various ventilatory modes, among which is a variable pressure support mode that is especially suitable for the non-invasive aid to marginally breathing COPD patients. It provides for a

variable pressure support during the inspiratory cycle, to augment the patient's spontaneous breathing, so as to achieve sufficient tidal volume to avoid tachypnea, or insufficient, rapid, shallow breathing. Tachypnea is the result of the patient's spontaneous breathing not being adequate to sufficiently evacuate the lungs to prevent the build-up of CO ₂ gas in the lungs. Since the amount of pressure support required to achieve optimal tidal volume will vary with each patient's spontaneous efforts, the system of the invention has a regulated feedback loop using special flow sensors. The flow sensors monitor the average tidal volume of all the spontaneous breathing during the preceding minute. This enables continual adjustment, based on a "rolling minute ventilation" protocol. It is therefore possible for COPD patients with respiratory insufficiency to avoid the progression to respiratory failure with the use of the new ventilator system and method. The new system and method are especially valuable while the patient is sleeping, when hypoventilation is most likely to occur. Even in COPD patients with established hyperapnea, or lowered CO ₂ blood gas levels, the ventilator of the invention can "reset" or lower SaCO ₂ , or lower blood saturation of CO ₂ gas, even when the patient is not using the ventilator.

More specifically, the respirator system of the invention, in a preferred embodiment thereof, uses an air amplifier which has the ability to transform compressed air from a moderate pressure (6 to 10 psia), low volume (10 to 15 Ipm) air source into a high volume, low pressure output (20 to 200 lpm at 1 psia). The air supply is free of contamination as well as the usual high turbulence and noise.

The specific air amplification means disclosed herein comprises a pair of telescopically inter-related tubular elements defining a flow path for air. The downstream tubular element defines a venturi chamber and has output and input ends. A controlled flow of compressed (primary) air into the throat of the venturi causes ambient air to be drawn into the input side of the amplifier in considerably

larger volumes than the volume of the higher pressure primary air. The combined airflow is substantially laminar and non-turbulent, and is easily controllable as to pressure and total volume of flow, within the limits desired for the intended purposes. By accurately adjusting the telescopically movable elements of the amplifier, very precise control of the introduction of pressurized air upstream of the venturi is made possible. A digitally controlled stepper motor serves to adjustably position the telescoping elements of the amplifier in real time, such that the flow of air to the patient can be made to correspond in an ideal manner to the breathing rhythms and/or requirements of the patient.

In accordance with one aspect of the invention, both the volume and the pressure of air supplied to the patient are measured on a continuing basis and the results thereof utilized by way of continuous feedback controls. Thus, as the patient completes an inspiratory cycle, for example, and the intake of air by the patient decelerates and ends, the declining flow is sensed and the amplifier can be correspondingly adjusted to reduce the amount of air supplied to the patient as well as the pressure thereof. The process inputs are supplied to a digital microcontroller element, which can be externally controlled by medical staff. Among other things, the control system may be adjusted to impose a timed breathing cycle on a patient who is having difficulty in maintaining a suitable rhythm of inhale and exhale actions.

It has been established that digital control is more reliable and error free than analog control and furthermore that a ventilator basically requires control of time based on/off functions. Consequently, in the respirator of the present invention, the following features can be provided primarily by software:

1. A pass code is required for operating modes and changing the mode parameters.

2. Operating parameters are displayed visually (liquid crystal display (LCD) display module).

3. Remote operating control can be utilized, via standard serial interface so that operating parameters can be changed via remote serial interface.

4. Accurate and stable pressure/volume control can be provided.

For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention and to the accompanying drawings.

Description of the Drawings

Fig. 1 is a schematic diagram illustrating the functional procedures involved in the system and method of the invention.

Fig. 2 is a simplified elevational view, partly in section, of a controllable air amplifier unit incorporated in the system of the invention.

Fig. 3 is a cross sectional view illustrating the telescoping elements of the air amplifier unit adjusted to a position for minimum airflow through the amplifier unit.

Fig. 4 is a cross sectional view similar to Fig. 3, showing the telescoping elements adjusted to a position for maximum air flow through the amplifier unit.

Fig. 5 is a fragmentary, cross sectional view as taken generally on line 5-5 of Fig. 2, illustrating a drive cam arrangement for adjusting the telescoping elements of the air amplifier unit.

Fig. 6 is a simplified flow diagram illustrating control steps involved in operating the system of the invention in a spontaneous breathing mode, in which

the control of the system follows the spontaneous breathing activity of the patient.

Fig. 7 is a simplified flow diagram illustrating a modified form of control for the system of the invention, in which a controlled tidal volume can be imposed upon the patient, where the patient's spontaneous activity is deemed to be insufficient to prevent CO ₂ build-up.

Description of Preferred Embodiments of the Invention Referring now to the drawing, and initially to Figs. 2-4 thereof, there is shown a novel and highly advantageous form of air amplifier that is utilized in the respiratory system and method of the invention. The amplifier unit, which is generally designated by the reference numeral 10, includes a pair of close fitting, telescopically adjustable tubular elements 11 , 12 forming respectively upstream and downstream elements of the amplifier. The upstream element 11 is formed with a cylindrical body 14, which closely receives the downstream element 12, while allowing for limited telescopic adjustment of the two elements, as will be further described.

Internally, the downstream portion 12 of the air amplifier is formed with a gradually converging passage 15 at its upstream end, which merges with a throat portion 16. The downstream portion 17 of the flow passage gradually diverges from the throat 16. The flow passage 16-17 through the downstream element thus forms a venturi chamber, as will be understood.

At its upstream extremity, the tubular element 12 is formed with an annular end surface of arcuate cross sectional configuration, as indicated at 18. Immediately adjacent to the annular surface 18, on the downstream side thereof, is an outwardly facing annular groove 19.

The upstream amplifier element 11 telescopically overlaps with the downstream element 12 and is formed with an inlet opening 20 in the side wall 14 thereof. The inlet opening 20 communicates with the annular recess 19 in any of the telescopically adjustable positions of the elements 11 , 12 within their operative limits. Slightly pressurized air (e.g., 6 to 10 psig) is supplied to the inlet opening 20 and flows circumferentially around the recess 19.

In the illustrated amplifier device, the upstream tubular element 11 thereof is formed with an annular arcuate recess 21 facing in a generally downstream direction and directly opposing the annular arcuate surface 18 of the downstream element to define an air gap G. Air at elevated pressure which flows into the annular recess 19 is able to flow through the gap G defined by the arcuate surfaces 18, 21 and is injected into the upstream end of the amplifier element 12. The radially innermost portions of the arcuately recessed surface 21 extend around far enough to project somewhat in a downstream direction. As a result, the air discharged through the gap G flows in a downstream direction toward the throat 16 of the venturi chamber. The effect of this flow of somewhat pressurized air is to reduce the pressure in the upstream end 15 of the venturi chamber and induce the flow of ambient air through an inlet opening 22 defined by the upstream element 11.

In the illustrated form of the invention, the upstream amplifier element 11 can be telescopically adjusted relative to the downstream element 12. Desirably, an O-ring or similar sealing element 23 is provided adjacent the downstream end of the amplifier element 11 , held in place by a sealing ring 24, so that the compressed air delivered to the inlet opening 20 is discharged exclusively through the annular air gap G. The combined airflow through the amplifier unit 10 (compressed air combined with entrained ambient air) can be accurately controlled over a significant range by varying the width of the air gap G defined by the surfaces 18, 21. When the air gap G is large, increased volumes of

pressurized air are discharged through the gap and into the venturi chamber, inducing the flow of greater amounts of ambient air through the amplifier inlet 22. Likewise, when the air gap is reduced, the flow of pressurized air is correspondingly reduced and less ambient is induced to flow through the amplifier. A typical range of adjustment of flow of the pressurized air (e.g., 6 to 10 psig) in a volume ranging for example 10-15 Ipm, results in an overall airflow through the amplifier unit ranging from around 20 to around 200 Ipm at 1 psig. The amplified flow is smooth, laminar and non-turbulent and is accompanied by a minimum noise in comparison to known systems.

Figs. 2 and 5 of the drawing illustrate an advantageous form of precision control for the telescopically adjustable amplifier 10 of Figs. 3 and 4. In Fig. 2, the reference numeral 30 designates a housing of a rotary stepper motor whose shaft 31 (Fig. 5) extends vertically upward from the housing. A fixed, L-shaped bracket 32 is secured by its base 33 to the motor housing and has an opening in a vertical portion 34 thereof for receiving the downstream end of the amplifier element 12. A flange 35, formed on the outer portion of the amplifier element 12 seats against the bracket portion 34 and is suitably fixed thereto. A coil spring 36 extends between the sealing ring 24 and the flange 35, urging the upstream amplifier element 11 in a direction (upstream) tending to enlarge the air gap G between the arcuate surfaces 18, 21.

A movable L-shaped bracket 38 has a horizontal portion 39 (Fig. 5) which is slideably guided in the fixed bracket portion 33 for limited sliding movement in an axial direction relative to the air amplifier unit 10. A vertical portion 40 of the bracket 38 is provided with an opening for the reception of the upstream end 41 of the amplifier element 11 , as shown in Fig. 2. The lower or base portion 39 of the movable bracket is formed with a transversely elongated opening 42 therein which receives a cam 43 mounted eccentrically on the stepper motor shaft 31.

As will be evident in Fig. 2, while the spring 36 tends to urge the telescoping amplifier elements 11 , 12 in a separating direction, they are confined against such movement by the brackets 32, 38. Thus, by controllably moving the movable bracket 38 toward and away from the fixed bracket 32, the width of the air gap between the arcuate surfaces 18, 21 can be controllably varied in the operating limits of the mechanism.

In a preferred form of the invention, the cam 43 has an eccentricity of approximately 0.030 inch, such that a full excursion of the movable bracket 38 of 0.060 inch can be achieved in 180 degree rotation of the cam 43. A suitable stepper motor for the purposes of this invention is arranged to rotate in increments of 1.8 degrees per step, to achieve a full 180 degrees of rotation in one hundred steps. Thus, the full adjustment of the amplifier air gap, from 0 to 0.060, can be divided into increments of one hundred, providing a highly precise control of airflow through the amplifier unit, for delivery to the patient. As will be described hereinafter, the unique ability to control the flow of air to the patient, provided by the system of the invention, enables respiratory assistance to be effectively tailored to specific needs of the patient and, where appropriate, to be modified and adjusted on a continuing basis during periods of respiratory assistance.

With reference now to Fig. 1 , the system of the invention includes a primary source 50 of pressurized air, typically at a pressure in the range of 6 - 10 psig. The specific source of the pressurized air is not significant, as long as it is clean, filtered, free of oil, etc., and non-turbulent. In a preferred embodiment of the invention, the compressed air source may comprise a fractional hosepower rotary compressor. From the primary source 50, the pressurized air is delivered to an accumulator 51 which retains the air in a quiescent state, for delivery to the inlet opening 20 per demand, through a suitable flow connection 52. The main body of air passing through the amplifier unit 10 is simply filtered, ambient air

which is provided to the opening 22 at the upstream end 41 of the amplifier element 11. The discharge airflow from the amplifier, indicated at 54 in Fig. 1 , flows through a suitable delivery hose 55 (Fig. 2) where both the pressure and velocity of flow are detected by a pressure sensor and flow sensor 56, 57 respectively. The outputs of the pressure and flow sensors are directed to a microcontroller 58 as feedback data. After passing through the pressure flow sensors, the controlled air is delivered to the patient, typically using an appropriate mask (not shown) that accommodates the comfortable delivery of the air as well as enabling the patient to exhale through the mask.

One of the outputs of the microcontroller, indicated at 59 in Fig. 1 , is directed to a motor control 60, which drives the stepper motor 30 and its cam 31 to vary the air gap G in the amplifier unit 10. In one preferred embodiment of the invention, it is contemplated that the air gap can be varied from approximately zero to approximately 0.60 inch in one hundred individual steps. The position of the stepper motor is at all times fed back to the microprocessor through a position sensor 61.

The operating mode of the system is under the control of an operator by way of a bank 62 of panel switches connected to the microcontroller 58.

Likewise, control of the unit may be done remotely through an external serial interface 63. A suitable display panel 64 is associated with the microcontroller to display the operating mode and relevant parameters.

The system and method of the invention desirably has multiple operating modes, which include the following: (1) Continuous Mode, (2) Timed Mode, (3) Spontaneous Mode, (4) Spontaneous/Timed Mode, (5) Volume Pressure Regulated Support.

In the Continuous Mode, the ventilator system supplies air continuously at a constant pressure during both inspiratory and expiratory cycles of the patient. The pressure of air to be delivered to the patient is set by adjustment of the microcontroller, and feedback from the pressure sensor element 56 causes the stepper motor to be adjusted as appropriate to maintain the desired pressure in the flow tube 55.

In the Timed Mode, the ventilator provides respiratory support at a prescribed rate of breaths per minute on a preset pressure basis. In this mode, the stepper motor is repetitively cycled according to the prescribed breaths per minute to be delivered, cycling the pressure according to a prescribed cycle of increasing and decreasing pressures to achieve a desired cycle of inhalation and exhalation by the patient. The pressure delivered to the patient is measured by the pressure sensor element 56 at a rapid rate (e.g., in cycles of around 6 ms). These pressure measurements are repetitively fed to the microcontroller, which determines the response of the stepper motor to vary the air gap of the air amplifier unit 10 and increase or decrease the air gap G accordingly, in order to maintain pressures at prescribed levels.

In the Spontaneous Mode, the ventilator system auto-synchronizes to the patient's own cycle of breathing while imposing prescribed levels for inspiration positive airway pressure (IPAP) and expiration positive airway pressure (EPAP). The control procedures for the spontaneous mode are illustrated in Fig. 6. During the IPAP portion of the breathing cycle, for example, the airflow pressures at the pressure sensor 56 are measured every 0.1 second and compared with the IPAP pressure setting entered by the operator. If the pressure is insufficient, the stepper motor is adjusted to increase the air gap G and increase the rate of airflow to the patient. The feedback cycle is repeated rapidly until the delivered pressure agrees with the preset IPAP pressure levels. The control then passes

to a hysteresis time delay unit, imposing a delay of about 0.3 seconds after which the flow rate of the delivered air is measured at the sensor 57.

In order to determine the rate of change of the flow rate, two flow rate measurements are taken, separated by an interval of 6.125 milliseconds (ms). By measuring the difference in flow rate between these consecutive readings, it can be determined when the patient's normal breathing cycle is undergoing a significant change, as at the end of an inspiratory or expiratory action. Empirically, a suitable threshold value for the rate of change of flow may be on the order of 2.45 lpm change in the 6.125 ms interval. If the rate of change equals or exceeds that value, the end of a cycle is indicated and the state of the respirator system is changed from IPAP to EPAP (if ending an inspiratory cycle) or from EPAP to IPAP (if ending an expiratory cycle). Typical pressure setpoints for the IPAP and EPAP portions of the breathing cycle might be 25 - 35 cm H ₂ O (IPAP) and 5 cm H ₂ O (EPAP).

The Spontaneous/Timed Mode functions in the same manner as described with respect to the Spontaneous Mode, except that, if a breath does not occur spontaneously within a prescribed period of time, an override is effected and the system causes the patient to breaths by momentarily increasing the pressure delivered to the patient.

In the Volume Pressure Regulated Support Mode, the system operates basically in a form of the Spontaneous Mode, in which the system tends to augment the natural breathing rhythms of the patient, as previously described. In addition, however, the VPRS Mode includes provisions for monitoring the tidal volume of air supplied to the patient. In this respect, the maintenance of an adequate tidal volume of air delivered to the patient can be very significant in order to prevent the build-up of CO ₂ in the patient's blood. Inadequate tidal

volume, where resulting in build-up of CO2 , can result in respiratory acidosis and, in severe cases, respiratory failure.

Fig. 7 is an illustrative flow diagram of the control sequences for carrying the VPRS Mode. As in the Spontaneous Mode, the system is set to a predetermined pressure setpoint, and the pressure of the air flowing to the patient is measured repetitively by the pressure sensor 56. In addition, the rate of airflow to the patient is measured in the flow sensor 57 and the pressure and flow rate values are integrated to establish a volume rate. When the system reaches the proper pressure setpoint, the process will continue to the hysteresis time delay. Until the proper time (e.g., 0.3 seconds has elapsed), the control system will loop back. After the proper delay, the system will test for the rate of change of the airflow. If it does not exceed a predetermined threshold, the system will again loop back. If the threshold is exceeded, the system will change the state from IPAP to EPAP or vice verse, and then the control will loop back.

During each loop back, the tidal volume is calculated on the basis of a rolling minute average. In this respect, all of the measurements over the previous minute are combined and averaged. If the rolling minute average is below a desired, preset target level, the pressure setpoint is increased and the system recycles until the air flowing to the patient reaches the increased setpoint, as determined by the pressure sensor 56. If the rolling minute average exceeds the desired target level, the pressure setpoint is reduced. In this respect, it is understood that, when the measured pressure at the pressure sensor 56 is either below or above a desired setpoint, the stepper motor 30 is incrementally actuated to open or close the gap G of the air amplifier, to increase or decrease airflow to the patient until the desired setpoints are achieved.

In the VPRS system reflected in Fig. 7, the rate of change of flow to the patient is constantly measured, as in the Spontaneous Mode described with

W

15 respect to Fig. 6, such that the system changes from IPAP to EPAP and vice versa in response to the rate of change of flow increasing to a predetermined level (for example, 2.45 lpm in 6.125 ms).

In the system shown in Fig. 7, although the tidal volume is calculated multiple times each second, the individual measurements are accumulated over a period of time, preferably one minute, and the system makes corrections only in response to the average tidal volume over that period of time. Accordingly, momentary changes in the breathing action of the patient will not trigger an immediate response.

In any of its various operating modes, the system and method of the invention are capable of continuous, and where desired, substantially instantaneous response to patient activities. Where appropriate, the system of the invention can simply augment the spontaneous actions of the patient. In other cases, the system may override the patient's activities, where they are insufficient to maintain adequate circumstances, such as sufficient tidal volume, adequate breathing rate, etc.

The entire system of the invention is greatly simplified in relation to conventional systems. The air amplifier mechanism enables precise and instantaneous control over airflow conditions by means of telescopic adjustment of the amplifier components, using a simple stepper motor-operated mechanism providing for high speed adjustments in very small increments, and thus providing virtually infinite control over the airflow conditions within the maximum and minimum limits of the system.

The supply of compressed air for driving the air amplifier can be obtained from any suitable source, such as a low power rotary compressor, and can be remote in appropriate circumstances. The compressed air is delivered to the air

amplifier unit via an accumulator such that the flow of the compressed air to the amplifier is non-turbulent. Airflow exiting the amplifier includes not only the injected compressed air, but a much greater volume of ambient air which is induced to flow through the amplifier by reason of discharge of the compressed air into the venturi chamber of the amplifier. The ability to control the flow of the compressed air into the venturi chamber on a virtually instantaneous basis allows the system to be able to respond closely to patient requirements as well as to inputs by an attending physician, for example.

Among other things, the system of the invention enables bi-level positive airway pressure ventilators to be provided which are non-invasive, which are very quiet, and which enable accurate and varied controls to be applied using a simplified and economical control system.

It should be understood, of course, that the specific forms of the invention herein illustrated and described are intended to be representative only, as certain changes may be made therein without departing from the clear teachings of the disclosure. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.