NEONATAL VENTILATION APPARATUS

Abstract: Ventilation apparatus for aiding the breathing of an infant, comprises a ventilator (10) for supplying breathing gas to the infant in a ventilation cycle through a valve (19), the parameters of the ventilation cycle being adjustable by developing a first control signal voltage for controlling the operation of the valve which controls the flow of breathing gas for the infant. The natural breathing cycle of an infant to whom the ventilator is connected is monitored and a spontaneous breathing signal generated. The ventilation cycle of the ventilator in use is also monitored and a signal representative of the period of the ventilation cycle is generated. A control means (12, 111) receives the spontaneous breathing signal, repeatedly sensing or deducing a first selected point in the natural breathing cycle of the infant and determining the time relationship of the first selected point relative to a second selected point in the breathing cycle, selecting an average or medium value of the time relationship over a selected number of cycles of breathing, and generating a second control signal voltage in response. A relay (110) connected to the ventilator and to the control means receives the second control signal voltage and, selectively, passes the second control voltage to the ventilator to control the operation of the valve. The control means (111) also receives the signal representative of the period of the ventilation cycle, repeatedly determining whether or not the period of the ventilation cycle is within preselected limits, and, if it is not, operating the relay means to cause the first control signal voltage to control the operation of the valve in place of the second control signal voltage.

Description: NEONATAL VENTILATION APPARATUS The present invention relates to ventilation apparatus for aiding the breathing of an infant and will herein be referred to as neonatal ventilation apparatus as it is primarily (possibly exclusively) used for aiding the breathing of newly born babies, particularly premature babies. A relatively small number of babies born each year require artificial ventilation because their lungs are immature and therefore a breathing gas is supplied to them under controlled conditions by means of a ventilator. Most existing ventilators supply the breathing gas to the baby at a preset rate and with preset timing and pressure, regardless of the baby's own respiratory pattern, the rate being set by the supervising doctor. Babies who are breathing spontaneously will often breath at a different rate from the ventilator at least some of the time and often therefore "fight" the ventilator. Unfortunately, about a quarter of ventilated premature babies develop either a brain haemorrhage or a pneumothorax (lung damage) . Previous investigative work by the inventors has shown that the increased incidences of pneumothorax in ventilated babies may be due to active expiration of the baby against ventilator inflation. As a result of the problems with existing types of ventilator so-called "triggered" ventilators have been developed in order to try to match the timing of the ventilator to the baby's spontaneous respiratory timing, ventilator inflation being triggered by spontaneous inspiration. In order to achieve ventilation, sensors are used to detect the start of a baby's inspiration, detection resulting in a signal being sent to the ventilator to start a mechanical breath by means of the ventilator. Adults and older children's ventilators have used the principle of triggering for many years, but problems with using the technique with small babies mean that it cannot produce thorough synchronous ventilation, particularly in very premature babies. The reason for this is that there are inherent delays in the system which are caused by the response time of the sensor, signal processing, attainment of the required signal threshold level, and electronic and mechanical response times of the ventilator (including the patient circuit) . Different systems and types of sensor are subject to various degrees of delay, but these mean that it is not possible for the start of the ventilator inflation to coincide precisely with start of the spontaneous breath. Furthermore, the length of time of ventilation is set by the clinician so that, once the ventilator has been triggered, the mechanical inflation lasts for that set length of time. Since, in general, the clinician does not have any means of knowing what time ought to be used for an individual baby at any given time, the end of the mechanical and spontaneous inflations (i.e. at the start of expiration) are unlikely to coincide. This could lead to the situation where the baby breaths out while the ventilator is still inflating the baby and this is an extremely dangerous situation. A further problem is that if the baby stops breathing, cries, wriggles or coughs or performs any non-breathing movement a trigger ventilator may well not respond properly. The present invention is intended to overcome the problems with existing ventilators by providing a spontaneous respiratory pattern-matched method of ventilation which does not rely on triggering the ventilator as each spontaneous breath occurs, but on analysis of the baby's overall respiratory pattern to run the ventilator continuously at a variable rate with which the baby synchronises. According to the present invention there is provided ventilation apparatus for aiding the breathing of an infant, the apparatus comprising a ventilator for supplying breathing gas to the infant in a ventilation cycle through a valve, the parameters of the ventilation cycle being adjustable by means for developing a first control signal voltage for controlling the operation of the valve which controls the flow of breathing gas for the infant in use; means for monitoring or deducing the natural breathing cycle of an infant to whom the ventilator is connected in use and generating a spontaneous breathing signal; means for monitoring or deducing the ventilation cycle of the ventilator in use and generating a signal representative of the period of the ventilation cycle; control means receiving the spontaneous breathing signal, repeatedly sensing or deducing a first selected point in the natural breathing cycle of the infant and determining the time relationship of the first selected point relative to a second selected point in the breathing cycle, selecting an average or median value of the time relationship over a selected number of cycles of breathing, and generating a second control signal voltage in response thereto; relay means connected to the ventilator and to the control means and arranged to receive the second control signal voltage and, selectively, pass the second control voltage to the ventilator to control the operation of the valve; the control means also receiving the signal representative of the period of the ventilation cycle, repeatedly determining whether or not the period of the ventilation cycle is within preselected limits, and, if it is not, operating the relay means to cause the first control signal voltage to control the operation of the valve in place of the second control signal voltage. The invention also includes ventilator control apparatus for controlling a ventilator for aiding the breathing of an infant, wherein the ventilator supplies breathing gas to the infant in a ventilation cycle through a valve, the parameters of the ventilation cycle being adjustable by means for developing a first control signal voltage for controlling the operation of the valve which controls the flow of breathing gas for the infant in use; the control apparatus comprising means for monitoring or deducing the natural breathing cycle of an infant to whom the ventilator is connected in use and generating a spontaneous breathing signal; means for monitoring or deducing the ventilation cycle of the ventilator in use and generating a signal representative of the period of the ventilation cycle; control means receiving the spontaneous breathing signal, repeatedly sensing or deducing a first selected point in the natural breathing cycle of the infant and determining the time relationship of the first selected point relative to a second selected point in the breathing cycle, selecting an average or median value of the time relationship over a selected number of cycles of breathing, and generating a second control signal voltage in response thereto; relay means connected to the ventilator in use and to the control means and arranged to receive the second control signal voltage and, selectively, pass the second control voltage to the ventilator to control the operation of the valve; the control means also receiving the signal representative of the period of the ventilation cycle, repeatedly determining whether or not the period of the ventilation cycle is within preselected limits, and, if it is not, operating the relay means to cause the first control signal voltage to control the operation of the valve in place of the second control signal voltage. Preferably, the apparatus of the invention includes or is used with a ventilator in which the timing and length of inspiration and expiration of the ventilation cycle are independently controllable by respective control signal voltages, the control means developing plural second control signal voltages to control a number of these parameters. The second selected point in the breathing cycle may repeatedly be sensed or deduced by the same means as is used within the control means to sense the first selected point. The means for monitoring the natural breathing cycle of the baby may comprise a non-invasive respiratory sensor of conventional type placed on the baby's abdomen (more specifically on the baby's xiphisternum) and arranged thereby to sense movement due to breathing only and pass an appropriate signal to the control means. The means for monitoring the ventilation cycle of the ventilator may be included in any case within normal ventilator functions in order to trigger an alarm if the period of the ventilation cycle falls outside limits which can be set into the ventilator by the clinician. In a prototype apparatus the control means comprises a personal computer having an internally located interface card and an externally located interface box containing a relay and rate alarm switching unit (R.A.S.U.), the relay attached to a conventional ventilator, but this may be replaced by a dedicated processing circuit if desired. An advantage of using a PC is that developments and changes can be readily incorporated and the data received from the abdominal respiratory sensor can be stored (at least temporarily) for analysis purposes using other softaware, rather than having the results print out on paper automatica1ly. A particular advantage of the present invention is that existing ventilators do not require major modification, but merely the connection of the relay in order to determine whether the signals conventionally provided by ventilator or by the computer determine the ventilation cycle, this being dependent upon the sensing of the ventilation cycle which is conventionally provided in order to provide an alarm function within the ventilator. Existing ventilator equipment can therefore be used. Preferably, the start of inspiration is chosen as the selected point in the natural breathing cycle of the infant and the second selected point is the start of expiration, the interval between these two values being recorded for each breathing cycle and a median or other average value being selected after a chosen number of cycles and being used to control the ventilation cycle to be closer to the spontaneous breathing cycle. As a result, the baby's respiration becomes phase-locked, in effect, with the ventilator by means of the feedback from the sensor and the adjustment of the ventilator parameters accordingly. The relay is provided in order to ensure that a breathing pattern outside predetermined limits results in continued operation of the ventilator with the preset parameters (for example if the baby starts breathing very quickly or much too slowly) and the relay is arranged in such a way that its default and also its mode of failure result in the signals from the ventilator controlling the ventilator valve to maintain artificial ventilation of the baby. If the baby stops breathing altogether the apparatus continues, by means of the ventilator, to provide ventilation at the current rate, but an alarm is triggered. Conventionally, a sensor is provided, for example an opto-sensor located in the ventilator's airway pressure manometer, which is used for conventional alarms such as patient disconnect or ventilator malfunction. An opto- sensor in the manometer provides two pulses for each ventilator inflation (as the manometer needle passes first up and then down through it) . It has the advantage that it provides pulses only when the patient's airway pressure actually rises above a certain level and so is a real measure of actual ventilation. In the apparatus of the invention, the signal from the sensor is monitored by two circuits simultaneously, one circuit checking that the rate is not exceeding an upper limit and the other checking that the rate is not below a minimum limit. Under conventional operation the ventilator timing is controlled through potentiometers connected to inspiratory and expiratory knobs on the front panel of the ventilator, the output voltages of which are multiplexed and presented to an analog-to-digital converter which communicates with the main control board of the ventilator to determine when to open and close the ventilator valve which is usually an expiratory valve, i.e. controlling the return from the baby rather than the supply to the baby. To control the ventilator by the apparatus of the present invention a double-pole relay switch is preferably used, the relay determining which set of voltages are presented to the analog-to-digital converter, i.e. those from the ventilator front panel settings or those generated by the control apparatus (in the prototype, the computer) . A particular advantage of the present invention is that the timing of the ventilator can be controlled in a way which varies with the spontaneous breathing cycle of the baby and also without the delays inherent in triggered systems which generally control only the onset of the ventilator operation, not the actual duration of the inspiration and expiratory phases. One example of apparatus according to the present invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a block schematic diagram of ventilator apparatus according to the invention; Figure 2 is a block diagram of the RASU; Figure 3 is a flow diagram of the RASU operation; Figure 4 is a diagram showing the arrangement of the ventilation cycle period limits; Figures 5 to 8 are screen dumps of displayed data for four different breathing conditions seen in babies. The neonatal ventilation apparatus of this example (prototype) includes a conventional Sechrist Infant Ventilator (model IV-100B) 10, to which is connected an interface box 11 which contains a relay 110 and a rate alarm switching unit (R.A.S.U.) Ill which, as will be described later, control the signals which are applied to operate the ventilator. The interface box 11 is connected, in this prototype, to a personal computer 12 in which appropriate processing of signals received from the baby is carried out. In use air and oxygen supplies 13,14 are supplied through a mixing and pressure regulating valve 15, which can be adjusted as required, via a flowmeter 16 and humidifier 17, to a manifold 18 (usually referred to as the "patient connector") which is connected to an expiratory valve 19, the opening and closing of which is controlled by the ventilator 10. From the manifold 18 a ventilator pipe 20 leads to the baby's lungs. A pressure tapping is provided in the manifold 18 and a line 21 leads from this to the manometer 22 in which is provided an opto-sensor 23 as will be described below. The manifold 18 and its associated pipework form part of a conventional occluded T- piece ventilation circuit. Attached to the baby's xiphisternum is a conventional respiration sensor 24 which is used to provide signals to the computer 12 in order to enable the apparatus to determine the timing of the baby's natural or spontaneous breathing cycle. The manometer 22 in the ventilator 10 provides a visual indication of the ventilation cycle that is actually occurring, the dial on the manometer 22, being provided with a moving needle (not shown) indicative of the pressure within the manifold 18, and the pressure being supplied to the ventilator manometer 22 by the pressure line 21 from the manifold 18. The opto-sensor 23 in the manometer 22, which monitors the movement of the manometer needle, also provides a signal to circuitry which includes a timer to determine the time between consecutive breaths being provided in the ventilator 10 and compare it with a preset upper maximum. Visual and audible alarms (not shown) are triggered appropriately in the conventional fashion. The ventilator 10 also includes controls 101, 102 to adjust the timing (length) of the inspiration and expiration phases of the ventilator cycle respectively, the controls 101 and 102 operating potentiometers which supply voltages to a multiplexer connecting to a central processing unit (CPU) which in turn controls the opening and closing of the expiration valve 19 through a digital-to-analog converter, closure of the valve 19 causing breathing gas to be fed, under pressure, to the baby and opening of the expiration valve allowing expiration of the air from the baby's lungs. As well as controlling the timing of the inspiration and expiration phases of respiration, the ventilator 10 includes similar controls 103,104 to determine the extremes of pressure during the inspiration and expiration phases. The ventilator as described above is conventional and the apparatus of the present invention is designed so that the ventilator 10 can, in certain circumstances, continue to function as normal. However, since the conventional ventilator does not adapt to changing conditions of the baby's respiration, the relay 110 in the interface box 11 is provided to control the supply of the voltages set by the potentiometers of the timer controls 101,102 to the central processor of the ventilator, the relay 110 passing either the signals developed by the potentiometers or else signals developed within the computer 12. The signals (voltages) provided by the computer 12 are developed within the computer as a result of signals provided by the abdominal respiration sensor 24 attached to the baby's xiphisternum, and provide an indication (see figures 5 to 8) of the spontaneous breathing cycle of the baby. By means of appropriate software, the timing of the inspiratory and expiratory phases of the spontaneous breathing cycle can be determined and over a period of time (or a number of breaths - 100 in the prototype) , the times of the two phases can be stored and a running median value of each determined. Upon this determination, the computer 12 provides appropriate voltages to the relay 110 in turn to control, through the ventilator's central processor, the opening and closing of the expiratory valve 19, in order to produce a very close match between the cycle of the ventilator and the baby's spontaneous respiration. Synchrony is thus induced between the ventilator and the baby. It will be appreciated that, in an alternative apparatus, the personal computer 12 could be replaced by a dedicated microprocessor circuit. An integral part of the apparatus is the appropriate hardware (and/or software) to monitor the ventilation provided by the ventilator, i.e. the rate of breaths delivered by the ventilator, the opto-sensor 23 providing the required signal for such determination. It has already been described how this signal is used for conventional alarms such as patient disconnect or ventilator malfunction, but since the sensor provides pulses only when the baby's airway pressure actually rises above a certain level, it can be used to monitor the ventilation of the baby accurately. The signal from the opto-sensor 23 is provided to a pair of circuits 112,113 within the R.A.S.U. 111, through a monostable flip-flop 114. The circuits 112 and 113 provide respectively maximum rate detection and minimum rate detection in order to check, respectively, that the rate is not exceeding an upper limit and that it is not below a minimum limit. However, in an alternative, it may be desirable to detect the inspiratory and expiratory durations independently instead of the maximum and minimum rates. The arrangement of the rate limits is illustrated in figure 4. Figure 3 illustrates the operation of the R.A.S.U. ill by way of a flow diagram. It will be seen that if either the maximum or minimum limits are exceeded then control of the ventilator passes back to the ventilator's central processor and appropriate alarms are initiated, but if neither limit is exceeded then the ventilator continues to operate under computer control, the relay 110 being actuated appropriately by outputs from a flip-flop 115 which receives signals from the maximum and minimum rate detection circuits 112,113 in the case that either of the rates is exceeded. A manual reset is provided so that computer control can be re-established at an appropriate time after control has been switched to the ventilator. The relay and R.A.S.U. are designed so as to be fail-safe, in that any failure causes the ventilator 10 to continue to ventilate the baby at its preset rate. Figures 5 to 8 illustrate both the ventilator signal and the spontaneous breathing cycle 120,121 respectively, over a period of time, inspiration being indicated by negative gradients on the graph and expiration by positive gradients. The onsets of inspiration and expiration are indicated by short vertical intersectors 122,123 and these points are determined in accordance with a predetermined algorithm provided within the software running on the computer 12. Figure 5 indicates a regular ventilator pressure signal which is asynchronous with the baby's respiration, and figure 6 illustrates 1:1 synchrony between the ventilator signal and the signal indicative of the spontaneous breathing of the baby. As well as providing 1:1 synchrony, the computer can be used to set other synchronous periods for example 1:2 or 1:3 as indicated by the traces shown in figures 7 and 8. In order to determine the points of onset of expiration and inspiration on the trace of spontaneous respiration 121, the computer utilises a finite state machine within the software logic in order to recognise the up and down thresholds of the data being received from the sensor 24. Certain conditions such as closely spaced double maxima within a single breath, or equally false minima, artefacts caused by external factors etc., need to be taken account of, all of which is achieved within software. The electrical power to operate the interface box 11 is preferably provided, for safety, by the ventilator in order to avoid the need for an extra power supply.

Claims: CLAIMS 1. Ventilation apparatus for aiding the breathing of an infant, the apparatus comprising a ventilator (10) for supplying breathing gas to the infant in a ventilation cycle through a valve (19) , the parameters of the ventilation cycle being adjustable by means for developing a first control signal voltage for controlling the operation of the valve which controls the flow of breathing gas for the infant in use; means (12,24) for monitoring or deducing the natural breathing cycle of an infant to whom the ventilator is connected in use and generating a spontaneous breathing signal; means (11) for monitoring or deducing the ventilation cycle of the ventilator in use and generating a signal representative of the period of the ventilation cycle; control means (12,111) receiving the spontaneous breathing signal, repeatedly sensing or deducing a first selected point in the natural breathing cycle of the infant and determining the time relationship of the first selected point relative to a second selected point in the breathing cycle, selecting an average or median value of the time relationship over a selected number of cycles of breathing, and generating a second control signal voltage in response thereto; relay means (110) connected to the ventilator and to the control means and arranged to receive the second control signal voltage and, selectively, pass the second control voltage to the ventilator to control the operation of the valve; the control means (111) also receiving the signal representative of the period of the ventilation cycle, repeatedly determining whether or not the period of the ventilation cycle is within preselected limits, and, if it is not, operating the relay means to cause the first control signal voltage to control the operation of the valve in place of the second control signal voltage. 2. Ventilator control apparatus for controlling a ventilator for aiding the breathing of an infant, wherein the ventilator supplies breathing gas to the infant in a ventilation cycle through a valve, the parameters of the ventilation cycle being adjustable by means for developing a first control signal voltage for controlling the operation of the valve which controls the flow of breathing gas for the infant in use; the control apparatus comprising means (12,24) for monitoring or deducing the natural breathing cycle of an infant to whom the ventilator is connected in use and generating a spontaneous breathing signal; means (11) for monitoring or deducing the ventilation cycle of the ventilator in use and generating a signal representative of the period of the ventilation cycle; control means (12,111) receiving the spontaneous breathing signal, repeatedly sensing or deducing a first selected point in the natural breathing cycle of the infant and determining the time relationship of the first selected point relative to a second selected point in the breathing cycle, selecting an average or median value of the time relationship over a selected number of cycles of breathing, and generating a second control signal voltage in response thereto; relay means (110) connected to the ventilator in use and to the control means and arranged to receive the second control signal voltage and, selectively, pass the second control voltage to the ventilator to control the operation of the valve; the control means (111) also receiving the signal representative of the period of the ventilation cycle, repeatedly determining whether or not the period of the ventilation cycle is within preselected limits, and, if it is not, operating the relay means to cause the first control signal voltage to control the operation of the valve in place of the second control signal voltage. 3. Apparatus according to claim 1 or claim 2, which further includes a ventilator (10) in which the timing and length of inspiration and expiration of the ventilation cycle are independently controllable by respective control signal voltages, the control means developing plural second control signal voltages to control a number of these parameters. 4. Apparatus according to any of claims .1 to 3, wherein the second selected point in the breathing cycle is repeatedly sensed or deduced by the same means as is used within the control means to sense the first selected point. 5. Apparatus according to any of claims 1 to 4, wherein the means (24) for monitoring the natural breathing cycle of the baby comprises a non-invasive respiratory sensor, placed on the baby's abdomen in use, and arranged thereby to sense movement due to breathing only and pass an appropriate signal to the control means. 6. Apparatus according to any of claims 1 to 5, wherein the means (11) for monitoring the ventilation cycle of the ventilator functions to trigger an alarm if the period of the ventilation cycle falls outside limits which can be set into the ventilator by the clinician. 7. Apparatus according to any of claims 1 to 6, wherein the control means (12) comprises a personal computer having an internally located interface card and an externally located interface box (11) containing a relay (110) and rate alarm switching unit (R.A.S.U.), the relay being attached to a conventional ventilator in use. 8. Apparatus according to any of claims 1 to 6, wherein a dedicated processing circuit is provided to provide alarm and relay functions. 9. Apparatus according to any of claims 1 to 8, including means (12) for selecting the start of inspiration as the selected point in the natural breathing cycle of the infant, means for selecting the start of expiration as the second selected point, the interval between these two values being recorded for each breathing cycle and a median value being selected by the apparatus after a chosen number of cycles and being used to control the ventilation cycle to be closer to the spontaneous breathing cycle. 10. Apparatus according to any of claims 1 to 9, wherein the relay (110) is arranged in such a way that its default and also its mode of failure result in the signals from the ventilator controlling the ventilator valve to maintain artificial ventilation of the baby. 11. Apparatus according to any of claims 1 to 10, including an opto-sensor (23) located in the ventilator's airway pressure manometer, the opto-sensor providing two pulses for each ventilator inflation, and wherein the signal from the sensor is monitored by two circuits simultaneously, one circuit (112) checking that the rate is not exceeding an upper limit and the other (113) checking that the rate is not below a minimum limit. 12. Apparatus according to any of claims 1 to 11, wherein, to control the ventilator, a double-pole relay switch (110) is used, the relay determining which set of voltages are presented to an analog-to-digital converter within the ventilator, i.e. those from the ventilator front panel settings or those generated by the control apparatus.

Inventors: MOCKRIDGE JEFFREY NICHOLAS ANT, MORLEY COLIN JOHN, MOCKRIDGE, JEFFREY, NICHOLAS, ANTHONY, MORLEY, COLIN, JOHN
Applicants: CAMBRIDGE LIFE SYSTEMS LTD, CAMBRIDGE LIFE SYSTEMS LIMITED
Application year: 1992
Main classification: A61M16
IPC codes: A61M16