WO1987000415A1

WO1987000415A1 - A device for supervision of the respiration of infants

Info

Publication number: WO1987000415A1
Application number: PCT/SE1985/000288
Authority: WO
Inventors: Dag Linnarsson; Bo Tedner
Original assignee: Karolinska Institutet
Priority date: 1984-01-30
Filing date: 1985-07-19
Publication date: 1987-01-29
Also published as: SE8400449D0; EP0230407A1; SE452946B; SE8400449L

Abstract

A device for supervision of the respiration of infants is based on the principle that the breathing sounds are detected by means of a signal microphone (10) adapted on the infant's throat. In order to eliminate the influence of disturbing surrounding sounds a reference microphone (12) is positioned on the infant so that it picks up substantially the same disturbing sounds as the signal microphone (10) but substantially no breathing sounds. By subtraction of the reference microphone signal from the output signal or the signal microphone, e.g. in a differential amplifier (30), an output signal indicating the respiration and substantially independent of disturbing sounds is formed which can be made to sound the alarm via suitable signal treatment circuits (32, 34) when the breathing sounds have ceased for a predetermined time.

Description

A Device for Supervision of the Respiration of Infants

This invention relates to a device for supervision of the respiration of infants by determination of the breathing sounds in the respiratory system. Irregular breathing and breaks of respiration (apnoeas) are usual with infants, both infants born too early and older infants. It is calculated that each fourth child with a birth weight of below 1750 g often has apnoeas. Almost all children with a birth weight of below 1000 g have apnoeas. The apnoeas can be dangerous to life and lead to grave handicaps. As moreand more infants born too early now survive (more than 50 % of those having a birth weight of below 1000 g) and will be quite sound the need of an adequate supervision of apnoea on departments for new- born babies will increase.

Also older babies may have apnoeas, especially at the age of 1-4 months. There is strong proof that there is a connection between these apnoeas and a sudden unexpected infant death (SIDS). In the USA about 7000 infants are calculated to die yearly from SIDS, in Sweden about 50. Considerably more have apnoeas and are considered as so-called near SIDS or abortive sudden unexpected infant death. It is desirable to supervise the respiration of these infants. lounger brothers and sisters of children deceased from SIDS are also regarded as risk cases.

Apnoeas or stopping of breathing can be either central (no breathing movements are initiated from the centrum of respiration in the brain) or obstructive (the upper respiratory system will collapse or contract). In the latter case fruitless breathing movements can be carried out by abdomen and thorax. In order to forestall SIDS it is thus desirable that supervision of breathing of infants born too early and infants having an irregular respiration pattern is provided. There are a number of known methods for such a supervision, viz. a) detection of breathing movements, where the alarm is sounded if no such movements are detected within a certain time, e.g. 15 or 30 seconds. Among existing detection principles the following should be noted: -Thorax impedance measurement. This impedance varies with breathing and can be measured by passing a weak high-frequency current through thorax by means of electrodes. The method is simple but has the disadvantage that simultaneously occurring heart synchronous impedance variations cannot always be distinguished from the respiration synchronous impedance variations.

-Adaptation of belts around thorax or abdomen, the extension of which is detected by a strain gauge or by inductive means. The method is relatively specifie to respiratory movements but has the disadvantage that the belt is difficult to apply and that loading of the breathing movements may result.

-Detection of diameter variations of thorax and abdomen by means of magnetometers. This method is technically complicated and most suited for research laboratories.

-Placement of the infant on a segmented air mattress so that the breathing movements provoke motion of air between the segments. No connection to the infant is required, and therefore the method is very simple. The disadvantage is that breathing movements and other movements cannot be distinguished and that a false alarm is often obtained.

Common to these principles based on detection of breathing movements is the serious deficiency that they are not capable of detecting obstructive apnoea as certain breathing movements - even if fruitless - are present. b) detection of respiration through the nose (oral respiration merely occurs at cries - not at calm respiration) by means of thermistor, by means of carbon dioxide analysis etc. The disadvantage is that infants are sensitive in the mouth-nose region and that it is not suitable, therefore, that a catheter, a thermistor or the like is in contact with the infants here for a long time, c) transcutaneous measurement of blood gas, which has become widely spread despite certain restrictions. A normal reading e.g. of transcutaneous P_O2 means with a great certainty normal respiration conditions, while a low value need not necessarily be caused by a bad respiration but may also be due ^to bad bleed-through of the skin or even an electrode error. in addition, the electrode may be difficult to apply and may In certain cases cause local injuries.

It is the object of this invention to enable supervision of respiration in a more reliable as well as easier way than what is possible by methods known so far. According to the invention this is achieved in that the supervisory device has been given the characteristic features defined in claim 1. Special favourable embodiments are defined in the other claims.

Thus, the invention is based on the fact that the breathing sound is supervised which is a specific magnitude of the respiratory flow in the resplratory system. As the respiratory system has its smallest cross-sectional area in larynx the throat has been selected as a suitable place for detection of the breathing sound. By using two microphones and a suitable combination of the output signals thereof it is achieved that the influence of disturbing surrounding sounds can be eliminated. One microphone (the signal microphone) is then preferably placed straight at the front of the throat to receive the sound generated when breathing air streams through trachea and larynx. On the other hand, the other microphone (the reference microphone) is placed so that substantially no breathing sounds are picked up e.g. on the side of the throat, in the neck or, still better, behind one ear where it is least in the way. This reference microphone will then register the other sounds from the body whereas the sound from the flow of the respiratory air has essentially lower strength or can be neglected. Both the microphones register outer sounds from the surroundings with about the same signal strength, both direct sounds and outer sounds transmitted via the body. These sounds can then be eliminated by subtraction of the microphone signals, which will minimize the risk of outer sounds with the same frequency range as the breathing sounds being interpreted as respiration sounds.

The invention will be described more in detail below in the form of an illustrative example and with reference to the enclosed drawing, the only figure of which shows a device in the form of a block scheme made according to the invention for supervision of respiration.

As is apparent from the figure two microphones are used for absorbing sounds, viz. a signal microphone 10, which is preferably adapted straight at the front of the throat to receive the sounds in trachea and larynx caused by the respiration air and a reference microphone 12 which is applied, as mentioned above, for example at the side of the throat, in the neck or behind the ear and which picks up substantially the same disturbing sounds as the signal microphone. The two microphones consist preferably of miniature microphones of electret type and are preferably attached by means of an adhesive plaster or a thin elastic band around the throat.

On its input side the supervisory device has two identically built channels 14 and 16, i.e. one for each microphone. Each channel contains an amplifier 18 and 20, respectively, a band-pass filter 22 and 24, respectively, connected after these and a circuit 26 and 28, respectively, for rectification and mean value formation of the output signal of the filter. The amplification factor of the amplifiers 18 and 20 is of the order of 1000 times and can be adjusted with a ganged potentiometer 21 indicated in the figure for optimal signal Level. For infants the band-pass filters 22, 24 should have the approximate pass band 300-1000 Hz and can for example consist of the filter of the fourth order with the damping 24 dB/octave outside the pass band. The filters are matched to each other to give an identical signal actuation in the two channels. The sound from the activity of the heart has been found to have a frequency content similar to that of the breathing sounds. However, the energy content of the heart sounds decreases with the frequency above about 200 Hz which means that the two sound types can be separated by means of the filters. At very small children (e.g. premature babies) the heart sounds are strong if the microphone is placed in the vicinity of the heart. The heart sounds are normally weaker than the breathing sounds when the microphone is placed on the throat. The strength of the heart sounds, when the microphone is placed on the throat, decreases rapidly with increasing age of the infants.

Before the signals of the two channels are subtracted from each other the signals must be rectified. The microphones 10, 12 are very sensitive to the phase position of the absorbed sound, and therefore the signal phase can be changed as much as 180 at an extremely small change of the micro- phone position ( 5 mm). This might result in that an additional sound registered in the two channels and having frequencies within the pass band of the filters 22, 24 is summed instead of being subtracted and then interpreted as a respiration sound. As the signals are rectified and formed to mean values in the circuits 26, 28 before they are subtracted from each other in a differential amplifier 30 the influence from phase differences between the signals is quite eliminated. The circuits 26, 28 perform a full-wave rectification of the signals and form mean values of the rectified signals with a time constant of about 0.1 s. The low value of the time constant also results in a detection of relatively rapid changes of the sound. The rectified and mean value formed signals are subtracted and further amplified about 10 times in the differential amplifier 30. As the signal channel and the reference channel register outer sounds with the same sensitivity and as the two microphones are placed relatively close to each other the output signal from the differential amplifier 30 will be close to zero in the absence of breathing sounds. If there are breathing sounds the result will be a positive output signal from the differential amplifier. The output signal, from the differential amplifier 30 is fed to a threshold circuit 32 detecting if the output signal exceeds a predetermined level (e.g. 0.5 V), i.e. if there are breathing sounds. Is this the case the output of the circuit 32 lies at a high level and maintains a time circuit 34 inactive. As soon as the output of the circuit 32 changes into a low level corresponding to a break of the breathing sounds the time circuit 34 is however released. If the breathing sounds return so that the output signal of the circuit 32 returns to a high level the time circuit 32 is immediately restored. If, however, the breathing sounds do not appear the time circuit 32 will after an adjustable time interval (e.g. 1-60 seconds) activate an alarm circuit 35 which, in turn, remains activated until a restoration button 38 is depressed. The alarm circuit can be connected to a warning lamp 30, a summer 42 etc. and to another outer alarm device via a relay output 44.

The output of the time circuit 34 is also connected to an accumulating memory circuit 46 which registers past time from the activation of the alarm until respiratory signals return or until the restoration button 38 of the alarm is depressed, and corresponding alarm times accumulated for 24 hours. Moreover, the memory circuit 46 registers the total number of alarms for 24 hours. The information stored in the memory can be presented on a digital display unit 48.

A pulse counter 50 can be connected to the output of the differential amplifier 30 for registration of the respiration frequency and display of the resulting value on a digital display unit 52.

Claims

Patent Claims

1. A device for supervision of the respiration of a human being, e.g. an infant, by determination of the breathing sounds in the respiratory system by means of a first and a second microphone (10 and 12, respectively), c h a r a c t e r i z e d in that the first microphone (10) intended for adaptation to the supervised infant's throat for registration of breathing as well as disturbing sounds and the second microphone (12) intended to be positioned on the infant so that it will register the same disturbing sounds as the first microphone (10) but substantially no breathing sounds, is connected to a band-pass filter (22 and 28, respectively) intended to filter cardiac sound signals, said filter being connected in series with a corresponding circuit (26 and 28, respectively) for rectification and mean value formation of the respective microphone output signal before it is emitted to a subtraction circuit (30) common to the two microphones(10, 12) and arranged to subtract the output signals of the two microphones (10, 12) from each other to form an output signal indicating the respiration and substantially independent of disturbing sounds.

2. The device of claim 1, c h a r a c t e r i z e d in that the first microphone (10) is arranged to be placed straight at the front of the throat and the other (12) at the side of the throat, in the neck or behind one ear.

3. The device of any one of the foregoing claims, c h a r a c t e r i z e d in that the output of the subtraction circuit (30) is connected to a threshold circuit (32), the output of which, in turn, is connected to an alarm circuit (36), preferably via a time circuit (34) causing the alarm circuit to be activated only when the output signal of the threshold circuit (32) has indicated for a predetermined time that no respiration is going on.

4. The device of claim 3, c h a r a c t e r i z e d in that the output of the threshold circuit (32) Is connected, whenever applicable via the time circuit (34), to a memory circuit (46) for registration of the duration of the respiration break, the total duration of all respiration breaks and/or the number of detecting respiration breaks.