This invention relates to mapping apparatus, and in particular to a device for charting left ventricular endocardial activation in surgery for ventricular tachycardia .

Ventricular tachycardia is characterised by abnor¬ mal and rapid contraction of the heart muscle. This most commonly occurs following a heart attack, where the site of origin is an area of scar tissue on the left ventricle of the heart. The condition may be treated surgically by locating the origin of the tachycardia followed by local ablation. The mapping procedure may be carried out using an electrode to record electrograms at a number of points on the left ventricle, and comparing the timing of activation at each successive point with a reference ventricular electrogram or surface electrocardiogram. The relative timings of the electrograms are used to identify the area of interest, early activation relative to the QRS complex of the electrocardiogram indicating the origin of ventricular tachycardia. Location of the origin is thus time consuming, being largely down to trial and error, and so requires undesirably long operating times.

Mapping of the kind described necessitates the

surgical opening of the left ventricle in order to introduce the electrode, and thus can have severe and undesirable effects on the function of the already-damaged left ventricle.

Alternatively, an "array mapping" procedure may be used, in which a spaced array of electrodes positioned on the surface of a suitable vehicle is used to record the electrograms. Mapping techniques of this kind offer no means of determining the position of any particular electrode with respect to the ventricular anatomy, and accordingly the best estimate of the origin of ventri¬ cular tachycardia is the fixed but unknown position of that electrode on the array for which the electrogram indicates earliest activation. High energy ablation may accordingly be carried out using that electrode. The number of electrodes, and thus the accuracy of the technique, is limited by the space available.

According to the invention there is provided endo¬ cardial mapping apparatus comprising releasably mounted electrode probe means for producing electrograms of endocardial activation, movable support means on which the probe means may be mounted for use, measuring means by which the relative position of the probe means may be determined, means for recording the electrograms and display means for visually mapping the instant position

of the probe means and the positions of recorded

^• * electrograms relative to previously selected reference points on the anatomy. * ^"

The support means may comprise a jointed arm assembly attachable to the operating table. Preferably the arm has three joint assemblies, each of which has two axes of rotation. The relative position of the probe may be determined by measuring the rotation of the joints about each axis.

Preferably the apparatus includes means for determining the relative timings of the recorded electrograms. The timings of the respective left ventricular electrograms relative to a reference right ventricular electrogram or surface electrocardiogram may be indicated by the display means.

Preferably also the apparatus provides cryosur- gical probe means selectively mountable on the support means to permit cryosurgical ablation at any required position.

» The invention therefore seeks to mitigate or obviate the aforementioned difficulties by providing a system which permits mapping to take place through the aortic valve (or the mitral valve) of the

heart, and thus with minimum detrimental effect on the heart itself, and in which a single electrode probe is used to record a number of successive electrograms, the positions of which can be determined relative to reference sites on particular anatomical structures. The position of each electrogram is also recorded and continuously displayed together with the current probe position during surgery, such that the electrode may be returned to the site of any particular electrogram as required. The area of interest may thus be located more accurately and more quickly. The system also permits the electrode to be substituted by a cryosurgical probe so that ablation may be carried out cryogenically, again by returning to the position of any previously recorded electrogra .

An embodiment of the invention will now be described for the purposes of illustration only with reference to the accompanying drawings in which:-

Fig. 1 is a simplified perspective view of part of an apparatus according to the invention; and

Fig. 2 is a map obtained using apparatus according to the invention.

Referring to Fig. 1, there is shown a mounting arm which forms part of an endocardial mapping apparatus. An upper part 12 of the arm is indirectly mounted on an operating table (not shown) so as to position the arm for use in an appropriate position above the table. A

lower part of the arm comprises two rigid sections 14, 16. The section 14 is joined at its upper end to the upper part 12 of the mounting arm at a first joint assembly 18. At its other end, the section 14 is joined to an end of the section 16 by way of a second joint assembly 20. The other end of the section 16 terminates at a third joint assembly 22 which connects in turn to a releasably mounted sterile electrode probe 24. Each joint assembly has two axes of rotation, the six axes of the arm being designated A to F in Fig. 1. The arm therefore has similar mobility to the human arm and wrist. The lower sections of the arm are wrapped in a sterile plastic sleeve 26. A probe mount 28 is also sterile, and permits the probe 24 to be exchanged during the mapping procedure such that an appropriate instrument may be used for each part of a map.

During surgery, as described in more detail hereinafter, the probe 24 is inserted into the left ventricle to produce the required electrograms. The rotation of the arm sections about each of the six axes A to F is measured by coaxially mounted servopotentio- meters. The rotation data of each joint, together with the dimensions of the arm may be used to define the position of the electrode on the tip of the probe 24. Voltage signals from the potentiometers are processed and analysed by computer, as are electrograms recorded

by the probe, together with reference electrograms of the right ventricle.

The instant position of the probe 24, together with the positions of recorded electrograms, is displa¬ yed continously during surgery on a map on a computer screen, relative to fixed reference points on the anatomy of the heart which are defined at the beginning of the operation as hereinafter described. Fig. 2 illustrates a map so obtained. The display depicts the geometry of the left ventricular endocardial surface in a simplified manner to ensure unambiguous representation in two dimensions of mapped points. In this particular case, this is achieved by constructing a parabolic surface around the left ventricle and projecting all positions on the endocardium onto this surface. The resulting map is further projected onto a plane surface, to give the displayed two-dimensional representation.

The patient is prepared for operation by being placed on cardiopulmonary bypass according to establis¬ hed techniques. The aorta is cross-clamped, the aortic root opened and the coronary arteries are cannulated and perfused with blood. The left ventricular endocardium is thus exposed for mapping across the aortic valve. The probe 24 is first used to mark and record the reference points on the anatomy. These are the

positions of the anterior, left and right aortic valve commissures, designated AC, LC and RC respectively in Fig. 2, an upright reference position and the left ventricular endocardial apex. These are used to form a reference coordinate system which is in turn used to produce the two dimensional display surface on which all subsequent electrograms may be charted. Interactive software is used to assist the surgeon, who then proceeds to record electrograms at selected positions within the left ventricle. In each case, the position of the electrogram is measured and recorded by following the rotation of each of the joints 18, 20, 22. The position of each electrogram is shown by a diamond shape 50 on the map of Fig. 2. The instant position of the probe is also continuously displayed, in Fig. 2 this is represented by a cross within a square.

When several electrograms have been recorded the relative timings between the reference and the probe electrograms, determined automatically, are compared. In this way, the region sought may be roughly identified, after which it may be explored and mapped in greater detail. The relative timings of the electrograms may be indicated on the map, for example by colour-coding the various mapped points.

On completion of the map, the electrode 24 is removed from the mount 28, and replaced by the chosen

ablating system, which in this case is a cryoprobe. Any other known or proposed ablating system could be substi¬ tuted. The cryoprobe may then be inserted into the left ventricle and accurately located for surgery at the origin of the ventricular tachycardia previously identified by the mapping procedure.

There is thus described a mapping system allowing a transannular approach, and in which the probe position and the positions of sampled electrograms may be recor¬ ded and displayed on a map relative to known reference points on the left ventricular endocardial surface. The position of the probe is displayed continuously to the surgeon during the operation enabling him to return to the position of a previously recorded electrogram. The system is also arranged such that on completion of the mapping procedure the mapping probe may be removed and replaced by a cryoprobe which may be accurately directed to the previously-determined origin of the ventricular tachycardia. Thus, more effective cryosurgical techniques may be employed for the ablation instead of high energy or radio frequency electrical methods.

It will be appreciated that modifications may be made without departing from the scope of the invention. The particular mechanical arrangement of the arm may be

different from that described and shown, and the rota¬ tion of the joints and thus the position of the probe may be determined in any appropriate manner. It will also be appreciated that the apparatus may be used to refine maps recorded via other techniques, such as the array mapping as hereinbefore described, and to introduce ablating equipment.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.