ANATOMICAL ARCHITECTURE AND ELECTRICAL-ACTIVITY OF THE HEART

In most early studies of cardiac electrophysiology, the correlation be tween-propagation of excitation and the architecture of cardiac fibers was not addressed. More recently, it has become apparent that the spr ead of excitation, the sequence of recovery, the associated time-varyi ng potential distributions and the intra- and extracardiac electrocard iograms are strongly affected by the complex orientation of myocardial fibers. This article is a review of older and very recent, partly unp ublished, mathematical simulations and experimental findings that docu ment the relationships between cardiac electropizysiology and fiber st ructure. Important anatomical factors that affect propagation and reco very ar-e : the elongated shape of myocardial fibers which is the basi s for electrical anisotropy; the epi-endocardial rotation of fiber dir ection in the ventricular walls, the epi-endocardial obliqueness of th e fibers (''imbrication angle''), and the conduction system. Dire to t he complex architecture of the fibers, many different pathways are ava ilable to an excitation wavefront as it spreads from a pacing site: th e straight line; the multiple, bent pathways resulting from the epi-en docardial rotation of fiber direction; the coiling intramural pathways associated with the ''imbrication'' angles (Streeter) and rite pathwa ys involving the Purkinje network. Only in a few cases is the straight line the fastest pathway. The shape of an excitation wavefront at a g iven time instant results from the competition between all possible pa thways. To compute the potential distributions and ECG waveforms gener ated by a spreading excitation wave we must know the successive shapes and positions of the wavefront, the architecture of the fibers throug h which it propagates and the spatial distribution of their anisotropi c electrical properties.