Interactive article about heart arrhythmias

Summary

I wrote this post to answer some of the questions, I had as a medical student, when I first learned about cardiac arrhythmias. Mainly, what are the conditions required to initiate and maintain a reentrant arrhythmia? That is, a state where the heart muscles are continuously activated by a self-sustaining loop, independent of the normal pace-making system. These arrhythmias can be benign, as with AV-nodal reentry tachycardia (AVNRT); a common, and usually harmless, cause of palpitations in young people. Or they can be deadly, as with ventricular fibrillation. Heart muscle cells (cardiomyocytes) have some interesting properties. When they are activated, they depolarize. Depolarization causes the cell to contract, but also activates neighboring cells, setting of a chain reaction. For the following demonstrations, we use i highly simplified model of heart. Below is a grid of simulated cardiomyocytes, each ready to activate their neighboring cells. They all start in a resting state, ready to be activated. Activated cells are black. After a short moment, they become inactive and refractory. Refractory cells cannot be activated. In the simulation, refractory cells go from dark gray to light gray, until they have regained their resting state, and are ready for a new activation. 👆️ Click/tap anywhere in the box below to activate a cell. Notice, that the activation is not entirely symmetric and predictable. To give the simulation a more organic feel, I added a bit of randomness to the activation of cells. Each cell is activated with a higher probability the more of its four neighbors are active (33% chance if one neighbor is active, 100% chance when 3 or 4 neighbors are active). Pacemaker cells Another interesting property of cardiomyocytes is their automaticity; cardiomyocytes spontaneously activate if they are not stimulated by a neighboring cells for a period. Cells in the sinoatrial node (sinus node) in the right atrium have the fastest rate of spontaneous activation, and...