Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating a depolarization wave front. The impulse causes adjacent myocardial tissue cells in the atria to depolarize, which in turn causes adjacent myocardial tissue cells to depolarize. The depolarization propagates across the atria, causing the atria to contract and empty blood from the atria into the ventricles. The impulse is next delivered via the atrioventricular node (or "AV node") and the bundle of HIS (or "HIS bundle") to myocardial tissue cells of the ventricles. The depolarization of these cells propagates across the ventricles, causing the ventricles to contract.
This conduction system results in the described, organized sequence of myocardial contraction leading to a normal heartbeat.
Sometimes aberrant conductive pathways develop in heart tissue, which disrupt the normal path of depolarization events. For example, anatomical obstacles in the atria or ventricles can disrupt the normal propagation of electrical impulses. These anatomical obstacles (called "conduction blocks") can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called "reentry circuits," disrupt the normal activation of the atria or ventricles. As a further example, localized regions of ischemic myocardial tissue may propagate depolarization events slower than normal myocardial tissue. The ischemic region, also called a "slow conduction zone," creates errant, circular propagation patterns, called "circus motion." The circus motion also disrupts the normal depolarization patterns, thereby disrupting the normal contraction of heart tissue.
The aberrant conductive pathways create abnormal, irregular, and sometimes life-threatening heart rhythms, called arrhythmias. An arrhythmia can take place in the atria, for example, as in atrial tachycardia (AT) or atrial flutter (AF). The arrhythmia can also take place in the ventricle, for example, as in ventricular tachycardia (VT).
In treating arrhythmias, it is essential that the location of the sources of the aberrant pathways (call foci) be located. Once located, the tissue in the foci can be destroyed, or ablated, by heat, chemicals, or other means. Ablation can remove the aberrant conductive pathway, restoring normal myocardial contraction.
Today, physicians examine the propagation of electrical impulses in heart tissue to locate aberrant conductive pathways. The techniques used to analyze these pathways, commonly called "mapping," identify regions in the heart tissue, called foci, which can be ablated to treat the arrhythmia.
One form of conventional cardiac tissue mapping techniques uses multiple electrodes positioned in contact with epicardial heart tissue to obtain multiple electrograms. The physician stimulates myocardial tissue by introducing pacing signals and visually observes the morphologies of the electrograms recorded during pacing, which this Specification will refer to as "paced electrograms." The physician visually compares the patterns of paced electrograms to those previously recorded during an arrhythmia episode to locate tissue regions appropriate for ablation. These conventional mapping techniques require invasive open heart surgical techniques to position the electrodes on the epicardial surface of the heart.
Conventional epicardial electrogram processing techniques used for detecting local electrical events in heart tissue are often unable to interpret electrograms with multiple morphologies. Such electrograms are encountered, for example, when mapping a heart undergoing ventricular tachycardia (VT). For this and other reasons, consistently high correct foci identification rates (CIR) cannot be achieved with current multi-electrode mapping technologies.
Another form of conventional cardiac tissue mapping technique, called pace mapping, uses a roving electrode in a heart chamber for pacing the heart at various endocardial locations. In searching for the VT foci, the physician must visually compare all paced electrocardiograms (recorded by twelve lead body surface electrocardiograms (ECG's)) to those previously recorded during an induced VT. The physician must constantly relocate the roving electrode to a new location to systematically map the endocardium.
These techniques are complicated and time consuming. They require repeated manipulation and movement of the pacing electrodes. At the same time, they require the physician to visually assimilate and interpret the electrocardiograms.
Furthermore, artifacts caused by the pacing signals can distort the electrocardiograms. The pacing artifacts can mask the beginning of the Q-wave in the electrocardiogram. In body surface mapping, the morphology of the pacing artifact visually differs from the morphology of the electrocardiogram. A trained physician is therefore able to visually differentiate between a pacing artifact and the electrocardiogram morphology. This is not always the case in endocardial or epicardial mapping, in which there can be a very close similarity between the morphology of the pacing artifact and the bipolar electrogram morphology. Under the best conditions, the pacing artifact and electrogram complex are separated in time, and therefore can be distinguished from one another by a trained physician. Under other conditions, however, the presence of the pacing artifact can sometimes mask the entire bipolar electrogram. In addition, its likeness to the bipolar electrogram often makes it difficult or impossible for even a trained physician to detect the beginning of depolarization with accuracy.
There thus remains a real need for cardiac mapping and ablation systems and procedures that simplify the analysis of electrograms and the use of electrograms to locate appropriate arrhythmogenic foci.