Cardiac Modeling: Dynamic Imaging

While there is a lot that can be learned from looking at single-cell models, there are dynamics that can only be seen at higher dimensions. One example is the development of spiral wave reentrant arrhythmias (more at http://www.scholarpedia.org/article/Cardiac_arrhythmia).

Before proceeding with a discussion of spiral waves and how they develop, I need to introduce a concept called the refractory period. In excitable cells such as those found in the heart and brain, there is a period during which the cell cannot be excited again.

That is, near the end of an action potential, if you try to stimulate the cell again, it’s impossible to generate another action potential (or extremely difficult to do so) until the cell has had a certain amount of time to recover, known as the refractory period.

As an electrical wave propagates through a region of tissue from left to right, the cells on the left will recover from their refractory periods first. This means that the cells on the right are harder to excite than those on the left, as they are still in their refractory periods.

Now, if a stimulus is delivered to the same region of tissue, the stimulus should generate an activation wave that spreads in both directions. But if the stimulus is delivered prematurely, the cells to the right will still be refractory and the stimulus will be blocked in one direction, only spreading to the left.

This unidirectional block results in a backwards-traveling wave and eventually the development of a spiral wave of electrical activation (images of which can be seen at http://www.scholarpedia.org/article/Cardiac_arrhythmia).

We are interested in spiral waves in cardiac electrophysiology because some severe (and potentially lethal) arrhythmias actually consist of spiral waves of electrical activity that spin rapidly and repeatedly in the heart. In addition, these arrhythmias (called reentrant arrhythmias) are often initiated by some combination of conditions that create a unidirectional block and a premature stimulus.

I should point out that I’ve maintained a somewhat narrow focus here. In addition to electrical activity, the heart exhibits mechanical properties (after all, the heart’s ultimate function is to serve as a pump). There are many models that simulate the development of tension in cardiac tissue, as well as those that provide realistic descriptions of how different regions of the heart differ from one another.

Either way, computer models can supplement experiments performed with real tissue by allowing us to pose questions and gain big-picture views that are not possible with state-of-the-art experimental techniques.