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Note: this applet doesn't seem to work properly with the Windows version of Netscape. Use Internet Explorer, or a different platform (Mac or Unix) for correct behavior.
What this is a model of
Action potentials are electrical waves which travel through the heart, causing the heart to contract. Normally, an action potential wave is initiated at a special location on the heart, called the sinus node. The wave then propagates away from the sinus node, covers the atria, then enters the ventricles through the so-called AV node. You can think of the first circular wave you see in this Java applet as the wave which propagates away from the AV node, out into the ventricles, which correspond roughly to the simulated region. Occasionally, something very terrible happens. A second, abnormal firing can occur, as depicted by the second excitation in the applet. I've designed the applet so that this second firing occurs at a very bad time and place. The result is a self-sustaining pattern called spiral wave reentry. This wave rotates forever, spewing out action potentials at a furious pace, showing up on EKGs as tachycardia (abnormally rapid heart rate). This type of pattern is typically significantly less efficient in pumping blood, which leads to less oxygen for all parts of the body including the heart, which can then end in ventricular fibrillation and almost immediate death.
Your first task, should you choose to accept it, is to stop this nasty spiral wave pattern and save the patient. Your weapon is your mouse. By dragging it across regions of the simulation, you can define rectangular regions of applied stimulus. The point at which you put the mouse button down defines one corner of the rectangle, while the point at which you let the mouse button up defines the diagonally opposite corner. Try making little stimulus regions by dragging your mouse diagonally across small regions.
If you define your stimulus regions at the proper times and places, you can stop these waves. Of course, one trivial way of stopping the wave is to apply a stimulus to the entire region, thus effectively resetting the entire heart. By using this method, you're doing pretty much the same thing they do in the emergency room when they attach the paddles and deliver the big shocks. Both the patient and I are expecting you to find a nicer, kinder solution, though. Once you have gotten the entire region to turn blue or purple (by whatever method), you have succeeded in saving the patient from the spiral waves.
Well, now that you have saved the patient, it's time to put him back on the ropes again. One way to start up a spiral wave is to excite the entire left edge of the system, which will produce a rightward propagating action potential, and then when that has traveled about halfway across the system, stimulate the top edge of the system. This is called cross-stimulation and should produce a single spiral wave. You should be able to see how the spiral wave gets its name from this simulation. Now try to stop this spiral wave.
Other things to look for and try:
Some technical information
This is a real simulation of excitable tissue. It runs on your computer as a so-called "Java applet." It uses a simple but fairly realistic set of equations called the Fitzhugh-Nagumo equations. These equations include a rest state, a firing threshold, and a refractory state--all of which are defining properties of an excitable medium.
The following subject areas are involved in designing this simulation and relating its behavior to the functioning of the heart: (1) cardiac electrophysiology, (2) theory of dynamical systems, (3) theory of partial differential equations, (4) numerical methods for initial value problems, and (5) object-oriented, threaded computer programming (in Java).
If all you see is a picture of spiral waves which doesn't move, it either means that your browser is not capable of displaying Java applets or has its Java capability turned off. In the former case, upgrade your browser. (Internet Explorer 4 and Netscape Navigator 4 both have Java capability.) In the latter case, go to your browser Preference dialog box and turn Java capability on.
Note: this applet is at present identical to the one on the home page (but it may not stay that way).
http://reentry.cwru.edu/otani