Qualitative Spatial and Temporal Reasoning in Cardiac Electrophysiology

We have space here to give only the briefest outline of the processes we are trying to model; the interested reader should consult one of the standard textS2. The heart consists of four chambers: two atria and two ventricles . Electrically, however, the two atria can be considered as one, and we shall refer only to the atrium ; likewise for the two ventricles. In essence the function of the cardiac electrical system is to generate impulses at the correct rate, and to deliver them to other parts of the heart so that they arrive at the correct times . Impulse conduction can be likened to a row of falling dominoes . Cells that are polarised have a negative potential across their walls (the upright dominoes) . If they remain in this state for long enough, they will spontaneously depolarise with the wall potential becoming positive (the domino will fall over) . However cells in different parts of the heart have different spontaneous depolarisation rates . Under normal circumstances the cells in the Sino-Atrial (S/A) node have the shortest spontaneous depolarisation time ; this region therefore serves as the primary generator of impulses . Any polarised cell next to a cell which depolarises will itself depolarise (one domino knocks over the adjacent one) . In this way, the impulse is propagated throughout the conducting system and hence to the heart muscle . After some time in the depolarised state, a cell will repolarise (the domino picks itself up) ready to repeat the cycle .