This invention relates to an apparatus for body surface mapping, for use in diagnosing different conditions of the human myocardium.
There are various methods which present the information contained in an ECG to the cardiologist, the most successful of which has been the standard twelve lead ECG. Unfortunately the standard ECG, in many instances, fails to provide an unequivocal diagnosis.
Even before the conventional twelve lead ECG became a medical standard, electrocardiographic body surface mapping (BSM) was being investigated as a method which increased spatial resolution and thereby increased diagnostic capability. These xe2x80x9cmapsxe2x80x9d were viewed as pictures presenting lines joining areas of the same electrical potential (isopotentials) at a specified instant of time. This is shown in FIG. 1, which is an isopotential map of a normal healthy subject mid-way through the heart""s QRS (depolarization) period. In FIG. 1 the map has been superimposed onto the outline of a human torso and is obtained from multiple ECG electrodes located substantially fully around the torso from the anterior (left hand side of FIG. 1) to the posterior (right hand side of FIG. 1).
Since each map shows the isopotentials at only a single point in time, to see the whole electrical xe2x80x9cpicturexe2x80x9d requires the viewing of successive maps at successive time instants throughout at least part of the cardiac cycle. Furthermore viewing these maps their pattern and morphology to be considered from one instantaneous map to the next. Thus, for example, FIG. 2 illustrates six body surface isopotential maps showing how map morphology changes with time during the QRS period, the ECGs on the left of FIG. 2 showing the successive points in time during the QRS period that the maps represent. Since the body is cylindrical in shape the maps actually represent the body xe2x80x9cunwrappedxe2x80x9d onto a two dimensional plane (see FIG. 1). It is conceivable, however, that such maps could be produced which do not xe2x80x9cunwrapxe2x80x9d the torso.
However, BSM is difficult to use and as a result much research has been carried out which attempts to bring the benefits of BSM in a fast easy to use form. Methods were introduced of portraying BSM information as mathematical integrations of the individual ECG waveforms. Isointegral maps present the body surface as lines which join areas possessing the same integral values of pre-defined sections of the ECG. Such isointegral maps have shown that there is more information outside the spatial scope of the standard twelve lead which could be used by a clinician to improve patient management, and have proved their ability to provide an accurate diagnosis in instances where the standard twelve lead was equivocal.
The use of isointegral maps and discriminant function analysis rose from a need to speed the processing of body surface map information, reduce the data storage requirements of recordings and provide ultimately some form of automated diagnostic system. The mathematical nature of these isointegrals provides a platform which easily lends itself to the use of computers thereby allowing a diagnostic evaluation in minimal time. This diagnosis takes the form of a statistical discriminant function. Statisticians using discriminant function analysis to analyze isointegral maps are presented with the problem of firstly establishing just how many dimensions are involved with solving the problem. This is necessary in order to choose an appropriate discriminant algorithm. However, due both to the integration with respect to time of the ECG signals and the use of a statistical analysis which ignores the nature of the heart""s electrical field, the isointegral maps still fail to provide a diagnosis in many instances.
It is an object of the present invention to provide an apparatus for body surface mapping which can provide an improved diagnostic capability and is relatively easy to use.
Accordingly, the present invention provides an apparatus for body surface mapping, comprising a plurality of electrodes for attachment to spatially separate locations on a human torso, each electrode being capable of detecting the electrical activity associated with a heartbeat and producing a corresponding voltage, means for sampling the voltages on the electrodes a plurality of times during at least part of a cardiac cycle to produce a like plurality of sets of sampled values, and means for calculating and displaying in graphical form the variation with respect to time of at least one characteristic of the sampled values.
In a preferred embodiment the characteristic, for example the instantaneous overall maximum or minimum of the sampled values, varies in position with respect to time and the calculating and displaying means calculates and displays the said variation in position. Preferably the characteristic is displayed as projections of the trajectory onto two planes perpendicular to each other and to the plane containing the electrodes.
The invention also provides a method of body surface mapping, comprising locating a plurality of electrodes at spatially separate locations on a human torso, each electrode being capable of detecting the electrical activity associated with a heartbeat and producing a corresponding voltage, sampling the voltages on the electrodes a plurality of times during at least part of a cardiac cycle to produce a like plurality of sets of sampled values, and calculating and displaying in graphical form the variation with respect to time of at least one characteristic of the sampled values.
In the context of this specification the xe2x80x9ctrajectoryxe2x80x9d of a characteristic means the notional path traced out by that characteristic in a three dimensional space of which two dimensions correspond to mutually perpendicular directions in the plane containing the electrodes and the third dimension is time represented as a third spatial direction perpendicular to the plane containing the electrodes.