Unidirectional conduction block

A unidirectional switch is one that can conduct in only one direction and blocks in the reverse direction. 

In the presence of myocardial ischemia, propagation of cardiac impulses may be interfered with and a functional unidirectional block may occur. Impulses may fail to conduct longer in the anterograde direction to excite the more distal ventricular myocardium. Thus,... 

Schematic diagram of a reentry circuit that might occur in small bifurcating branches of the Purkinje system where they enter the ventricular wall. A Normally, electrical excitation branches around the circuit, is transmitted to the ventricular branches, and becomes extinguished at the other end of the circuit due to collision of impulses. B An area of unidirectional block develops in one of the branches, preventing anterograde impulse transmission at the site of block, but the retrograde impulse may be propagated through the site of block if the impulse finds excitable tissue that is, the refractory period is shorter than the conduction time. This impulse then reexcites tissue it had previously passed through, and a reentry arrhythmia is established.

Figure 19-11 Reentry mechanism of Purkinje fibers, a. Normal conduction of Impulses through triangular arrangement of cardiac fibers, b. Unidirectional block on left arm of triangular section allows impulse to reenter the regional conducting system and recycle...

The role of nonuniformities in the onset of pulse circulation in a ring of excitable tissue was shown in experiments by Mayer et al. [27]. Circulation can emerge in a ring after a single external stimulus if a local nonsymmetric gradient of a parameter results in a permanent unidirectional block of conduction. If there is a region with enhanced duration of the refractory phase within the ring, a pair of properly timed stimuli is needed to start the circulation [28]. 

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