WO1997047237A1

WO1997047237A1 - Apparatus for analyzing hrv signals (heart rate variation)

Info

Publication number: WO1997047237A1
Application number: PCT/DK1997/000257
Authority: WO
Inventors: Knud Erik BÆKGAARD
Original assignee: Bang & Olufsen Technology A/S
Priority date: 1996-06-12
Filing date: 1997-06-12
Publication date: 1997-12-18
Also published as: AU3165797A; EP0906057A1

Abstract

Basic research has shown that in connection with characteristic and critical changes in certain physiological conditions, e.g. critically decreasing blood sugar content with diabetic patients, there appear some preceding, marked changes in the pattern of the person's heart rate variations. According to the invention, advantage is taken of this in providing for an apparatus, e.g. shaped as a wrist watch, which currently detects the heartbeats of the person and forms an expression of the variation pattern of the last 10-50 heartbeats; the apparatus has a program unit coded with the expression that is premonitory of the relevant change and an alarm unit, which is activated if or when this expression happens to coincide with the corresponding detected expression. Hereby the person gets a chance to take countermeasures against the relevant change, which has thus been preadvised in real time. The apparatus is also applicable for indication of the stress condition of the body, which may provide useful information e.g. for strongly trained sportsmen for avoiding overtaxing.

Description

APPARATUS FOR ANALYZING HRV SIGNALS (HEART RATE VARIATION)

The present invention relates to an apparatus for registratmg and analysing heartbeat variations (HRV, Heart Rate Variability) .

During the recent years, different basic research projects have made it plausible that there is a connection between the notoriously existing heart rate variations and different illness or health conditions. The research is based, inter alia, on recordings of electrocardiograms over relatively long time intervals, with reference also to healthy persons having certain health characteristics. It is quite normal that the heart rate or rhythm changes m a detectable and recordable manner in different activity situations or states of mind and that it even changes with a higher frequency during the course of these situations. By a closer analysis, these changes may be given various graphical expressions, and it is then observable that there are differences m these expressions all according to the observed persons being "normal" or subject to various physiological or mental disturbances . It has also been found that based on these analyses it is possible to demonstrate that ahead of some physiological changes there may occur characteristic variation patterns of the heart rate or changes in these patterns, respectively. Thus, by computer based analyses of electrocardiograms it has been found that such operation patients, who suffer the fate of dying for inexplicable reasons during or in succession of the operation, seem to have or have had heart rate variation patterns with certain common features that are not refound with normal, surviving persons.

Many other interesting circumstances have been found, and while the said basic research has been based on an after analysing of measuring data collected over longer or shorter time, there has later on been developed different techniques for an immediate monitoring of the relevant measuring date in order to enable skilled observers to "read the message" and take action when this is found relevant. Reference can be made to the following publications: US-A-4, 832, 038, EP-A- 565,084, US-A-5,265,617 and US-A-5,437, 285, in which representative techniques are disclosed in detail. It should be added that also WO 94/27497, belonging to the applicant company, discloses a highly relevant technique for an accurate determination of the heart rate variations, in particular with respect to a precise determination of the time intervals between the important "R peaks" in a relevant cardiogram.

In connection with the invention it is a superior aspect that the apparatus is not or at least not alone used for a more or less instantaneous monitoring of the relevant data or combination of data, but is equipped in such a manner that it may be programmed to interprete the measuring results and react, e.g. by signalling a simple alarm, whenever the measurement is indicative of a changed condition that can be recognized as a preliminary stage of a forthcoming, noticeable physiological or neurological change or, respectively, a tendency for such a change to occur.

Conventional supervising measurements are based on a registration of symptoms of the commencing change itself, which in many cases will imply that while it is of course possible to bring help, it is not possible to prevent the occurrance of the symptoms. There are more types of attacks, by which the patients' ability to make decisions and take actions will be reduced right by the onset of the attack, e.g. in connection with low blood sugar with diabetics, and it may then already be too late to alert the patient.

With the invention it has been recognized that in spite of the complexity of the said HRV measurements it is nevertheless possible, in practice, to make use of them on a reference basis for providing, with some purposive aim, a direct indication of forthcoming relevant changes. By the current research there is provided for a rising number of examples of recognized correlations between relevant changes and certain forewarnings thereof in the form of reasonably well defined changes in the HRV pattern. It is a clear tendency that these changes do amount to forewarnings which, by reconstruction, are found to have occurred a certain time ahead of the moment of time at which symptoms of the relevant changes can be detected or, respectively, be perceived by the patient. In the instances mentioned as examples, therefore, it is highly relevant to make use of a portable apparatus which can provide to the user an alarm signal to the effect that an attack seems to be approaching, even if it still totally latent, such that the user can take suitable precautions while still being fully able to decide and to act . The "normal" HRV pattern is almost as individual as a finger print, and in general the apparatus should be able to be calibrated with the measuring data derived from the relevant person, this per se already being known. What is thereafter important is to detect the deviations which in special situations may occur from this basic reference, and it is necessary to hereby distinguish between different types of situations:

1) Person specific situations:

This heading typically refers to the said approching attacks, typically diabetic and epileptic attacks, but also attacks of a psychic nature such as with manic-depressive persons. Even though it is possible, based on available research results, to indicate general lines for the manner in which the HRV pattern will change by an approaching attack, it is still - so far - practically impossible to predict how the change should be with a specific person in order to qualify for an alarm function. It will be important, therefore, if in an initial phase it is possible to obtain a registration of not only the normal HRV pattern, but also of the pattern in connection with a commencing attack, as it is then possible to interpolate for determining a suitable prewarning threshold. More about this below.

2) Momentary situations:

Even though the invention primarily relates to "prewarning", it is nevertheless also relevant in certain situations in which the warning should amount to such a short notice that in practice it should involve a momentary reaction, decidedly as an alarm. A relevant example pertains to the finding that in connection with surgical operations situations may occur in which the patient has not been sufficiently anaesthetized to be fully insensible, though sufficiently to be unable to react towards the surroundings in case of a sensed pain. This creates stress with the patient, and it should of course be avoided. It has been found that such a latently perceived pain expresses itself clearly in a marked change of the HRV pattern in a well defined direction from the normal pattern, and since there are no other sources for causing such a change (looking apart from external noise) , it will be possible to program the apparatus with an alarm threshold which, without requiring any kind of current visual supervision of the measuring result, will induce an alarm by so to speak any kind of marked deviation from the normal pattern, here again without using any kind of "symptom sensors" capable of registering the perceived pain. If an alarm signal occurs, the surgeon will stop cutting until the anaesthesia has been suitably intensified. 3) Emperical situations:

In the example already mentioned relating to operation patients dying for inexplicable reasons, it will of course not be relevant to wait for an initial appearance of the symptom itself. It is necessary - and possible - to react to the circumstance that the HRV pattern itself indicates that the patient in question is in the relevant risk group, respectively changes to provide such an indication, and of course this indication should not be produced by any arbitrary change of the HRV pattern, but only by such a change which, with reference to the normal pattern, is indicative of this particular risk.

4) General tendency situations;

Through the HRV research it has been found that sportsmen exaggerating their training are exposed to certain risks later on, and that this potential possibility reveals itself by their HRV patterns. In connection with the invention this finding can be used to the effect of providing a supervisory apparatus which should not produce any real alarm, but only provide for an indication whether the user is changing towards (or away from) a critical condition, whereafter the person will have ample time to adjust his or her behaviour according to the circumstances. Also such an indication will largely be based on general empirical data, as it is impossible to predict how or when the condition of the specific person will worsen noticeably.

As mentioned, it is already known to measure or determine the variance of the heartbeats by means of more or less refined techniques. For relevant estimations, use is made of analyses of the successive occurrance of the R peaks of the heartbeats based on one or more of the following detections :

A) Variation in time: The R-R intervals are registered as a function of the time over a period of time, e.g. during the course of a sleep or another "normal period", e.g. just over a few minutes.

B) Frequency variation: For a detection of brief time critical changes it is possible, by means of an equipment with high measuring accuracy and a high signal processing capacity, to carry out a real time frequency analysis of the R peaks, this providing for a qualified basis for observation of a characteristic change that may require a rapid reaction. Hereby it is even possible to provide an expression for the activity in both the sympathetic nervous system (about frequency 0.1 Hz) and the parasympathetic system (about frequency 0.25 Hz) , in order to produce a reliable picture of the physiological condition and a rapid signalling of changes therein. C) Scattergram: This refers to a graphical illustration of the variation of the R-R intervals between the single heartbeats. In normal cases the variations are located in a rather sharply limited "lump area", but in connection with e.g. manic-depressive attacks (and even ahead of these) the intervals appear with a much larger spreading. The invention is based on the consideration that in using any of these kinds of registration it is possible to determine certain threshold values that may be read into a memory of the signal processing system so as to give rise to a relevant utterance, whether a harsh alarm or a discrete indication, when one or more of the actual registrations coincide with the relevant threshold, which, in the signal processing system, will be present as a more or less complex algorithm. As far as the detailed technique is concerned, already well known principles may be used, so in the present connection it is deemed unnecessary to examplify this in more detail . The determination of the threshold complexes may present difficulties inasfar as there is aimed at a reaction to a change of condition already prior to the change occurring in a directly detectable manner. In the person specific cases it may be a relevant procedure to effect, during a course of supervising of a patient, a reading in of a command signal to the signal processing unit of the apparatus as soon as it is experienced, from outside or by the patient, that a change of condition is initiated, given by initial symptoms of the change. Based thereon, it will be possible to interpolate to a point at which it is relevant to make the HRV analysis provide for a prewarning of the forthcoming change, and the relevant threshold combination may then be adjusted correspondingly.

In the other discussed situations the alarm function or signalling will have to be based on the fact that, empirically or in general, a connection has been found between the normal HRV pattern and such deviations therefrom which will justify an intervention or at least an increased attention from the surroundings. It will be a further possibility that the total parameter picture can be completed with further measuring data such as expressions for the person's blood pressure, respiration, etc.

The threshold keeper function of the invention will be perfectly applicable in clinical practice, where it is even possible to operate with more different threshold definitions at the same time, e.g. both for the anaesthesia depth and for the said death risk, whereby the requirements as to the attention of the supervising personel can be considerably decreased. Likewise it will be possible, e.g. in connection with mentally afflicted persons, to observe changes in the course of the suffering and, based thereon, draw conclusions with respect to a reasonable thresholding. These experiences, in turn, might then be usable for the threshold programming of a person related, portable apparatus .

With the use of an integrated signal processing it has been found possible to minimize a relevant supervising apparatus e.g. to the shape and size of a bracelet, with an associated possibility of a detection of the R peaks of the heartbeats directly on the arm arteria, and it is also possible to program the apparatus with a threshold setting accurately adapted to the need of the particular user, such that a relevant reaction, e.g. an alarm function, can be instituted when the HRV signal is interpreted to have changed in a critical direction. A further possibility is that the apparatus, e.g. with the use of a discrete display unit, can produce an informative showing referring e.g. to the stress condition of the body without any associated alarm effect. As mentioned, this may incur a valuable information e.g. to sportsmen.

The apparatus may be extended with an integrated or separately added equipment for recording of alarms or other threshold surpassings or registered tendencies, aiming at a later analysis of the supervised course of events, optionally also for recordal of the ECG signal about an alarm moment of time. Moreover, in relevant cases, the apparatus may be connected to a supervising network (telephone, radio, mobile phone, internet etc.) for simplex or duplex communication with a supervising central .

In the accompanying drawing,

Fig. 1 is an elementary picture of an ECG signal, Fig. 2 a) -c) is a graphical representation of typical courses of measurements, and

Fig. 3 is a block diagram for further illustration of the invention. The signal shown in Fig. 1 is the well known ECG curve with the characteristic peaks Q, R and S. It is in particularly the R peaks, which in the present connection are detected and analysed with respect to the variations in their temporal spacings.

Fig 2 shows three different, but interrelated expressions for the relevant signal analysis. Fig. 2a shows the variation of the R-R intervals over a time period of any suitable duration. Fig. 2b shows a so-called scattergram, in which each interval is shown as a function of the previous interval, providing for an easy survey over the regulating ability of the heart as related to the autonome balance. Fig. 2c expresses a completed frequency analysis, preferably an FFT analysis, of the R peaks, where the two maxima A and B are representative for the condition in the sympathetic and the parasympathetic nervous system, respectively.

Fig. 3 shows the principal design of an apparatus according to the invention. The detected ECG signal is cleansed for various noise in a unit 2, and non-acceptable information signals are suppressed in a filter 4. Thereafter, a unit 6 operates to effect an adaptive estimation of the quality and characteristic features of the signal relative to a reference signal model, and in a unit 8 the signal expression is formed for comparison with a programmed threshold expression, such that an output signal can be produced based on a pattern recognition principle. In a last stage 10 it is re-examined whether the estimation result can be correct, based on general rules and basic physiological conditions, and if the result is accepted in this stage the signal which actuates the associated alarm or display will be generated.

In principle, the apparatus may be adapted as an interpreter of different signal expressions, e.g. such that no expert knowledge will be required for interpreting an information indicative of the body being approaching a higher or a lower degree of stress. As already indicated, such an interpretation ability my well be used in clinical equipment, while in a person specific apparatus it may be still more relevant in that the apparatus may already have useful previous informations in its memory.

Claims

C L A I M S :

1. An apparatus for analysing HRV signals (Heart Rate Variation) , comprising a sensor for detecting heartbeats and a unit for current registration of the variations in the regularity of the successive heartbeats and current analysing of these variations, characterized in that the apparatus also comprises a memory unit operable to be programmed with expressions for one or more such variation patterns which are relevant for the person concerned, and means for actuating an alarm or another reaction in response to at least approximate coincidence between the actual registration of the variation or pattern of variation and, respectively, the corresponding expression as read into the memory unit.

2. An apparatus according to claim 1, characterized in that the memory unit is at least partly programmed in a standard manner for producing a reaction in response to a detected variation pattern which has been empirically found to be generally indicative of a condition or development demanding increased attention from the surroundings.

3. An apparatus according to claim 1, characterized in that the memory unit is of such a type which is operable to be caused to automatically adopt an actually occurring measuring result expression.

4. An apparatus according to claim 1, characterized in that it compries or is conneted with relevant additional apparatus units such a display unit, a registration unit for alarm conditions and for the heartbeats in connection with an alarm situation, a transmission unit for remote indication of measuring results, or corresponding auxiliary units.

5. An apparatus according to claim 1, characterized in that the memory unit is adapted so as to enable an interpretation of succssive changes in a "good" or "bad" direction, and that the apparatus is provided with indicator means for monitoring the interpretation.

6. An apparatus according to claim 5, characterized in that without relying on an alarm function it is provided with indicator means referring to the general stress condition of the body.

7. An apparatus according to claim 1, characterized in that in connection with an associated memory unit it appears as a portable unit, e.g. as a bracelet or wristwatch.

8. An apparatus according to claim 7, characterized in that it is provided with an integrated alarm unit.