Biomedical Engineering Reference
In-Depth Information
gradely back to the atria via an abnormal accessory pathway between the ventri-
cles and the atria. In all these reentrant arrhythmias, a part of the circuit is be-
lieved to be a comparatively thin strand of tissue. Cardiologists often conceive
of the reentrant rhythm as traveling in a one-dimensional ring (18). This has an
important implication for therapy: "if you cut the ring, you can cure the rhythm."
By inserting catheters directly into a patient's heart and delivering radio-
frequency radiation to precisely identified loci, the cardiologist destroys heart
tissue and can often cure these serious arrhythmias. In these cases, the cardiolo-
gist is thinking like a topologist since changing the topology of the heart cures
the arrhythmia.
Although the conceptualization of a wave traveling on a one-dimensional
ring seems overly simplistic, from perspectives of both mathematics and medi-
cine there are several interesting consequences (19). Experimental systems,
simulations, and theoretical analyses have demonstrated that waves circulating
on one-dimensional rings may experience an instability such that the circulation
is not constant but there can be a complex fluctuating propagation velocity that
arises as a consequence of the interaction with the wavefront with its own re-
fractory tail (20-23). In addition, if a single stimulus is delivered to the medium
during the course of the reentrant propagation, the propagating wave will either
be reset or annihilated (21,24-26). Further, periodic stimulation can lead to the
entrainment or annihilation of the propagating wave (25-27). Finally, a se-
quence of premature stimuli delivered to the heart during normal sinus rhythm
can often lead to the initiation of tachycardia.
In some clinical settings, analysis of the resetting, entrainment, and initia-
tion of tachycardias offers clinicians important clues about the mechanism, and
consequently can help the cardiologist choose an appropriate therapy (18,28).
The ability to induce monomorphic ventricular tachycardia using a sequence of
up to three premature stimuli is often taken as an indication of a reentrant
mechanism for the tachycardia. In such cases, resetting and entrainment of the
induced tachycardia can help the cardiologist localize a site for ablation. For
example, using intracardiac catheters cardiologists might attempt to identify an
anatomical locus that has the following characteristics during a monomorphic
ventricular tachycardia: (i) activation of the site occurs at a fixed time interval
before the appearance of a deflection on the surface electrocardiogram; (ii) dur-
ing periodic pacing at a rate slightly faster than the tachycardia from that site,
the time interval from the stimulus to the surface deflection on the electrocar-
diogram is the same as during the spontaneous rhythm; and (iii) during periodic
pacing, the morphology of the tachycardia on a 12-lead electrocardiogram is
identical to what is observed during the tachycardia. If these three criteria are
satisfied, the cardiologist would identify the site as part of the reentry circuit and
would target it for ablation. Recent advances in mapping of arrhythmias and in
the ability to identify suitable anatomical sites for ablation based on an under-
standing of the anatomical substrate of arrhythmias have further advanced the
