So what happens after the heart's natural pacemaker, the SA node, sits at the head of the boat and yells "Row!" to the rest of the heart's crew by transmitting an electrical signal?
The atria, the two chambers in the upper part of the heart, respond first, contracting as the signal travels through. Next, the signal arrives at the atrioventricular (AV) node. The atria contract a split second before the signal reaches the ventricles and causes them to contract. This delay creates a synchronized squeezing action, starting in the upper atria and passing through the lower ventricles. This squeezing motion forces blood into your arteries.
The signal is a stickler for routine. It follows a specific path through all four chambers, and it normally prevents any other heart cells from mutinying and trying to send out their own pulses. Interestingly, all heart cells are able to create an electrical signal. The SA node just does it faster and better. Other cells don't get the chance to fire off their own electrical cues because they're too busy responding to the prompt sent by the SA node.
This normal heartbeat -- and the sequence of electrical events that produce it -- is known as the sinus rhythm. It fluctuates between 60 and 100 beats per minute, depending on if you're daydreaming about butterflies or fielding questions from an Internal Revenue Service auditor [source: A.H.A.]. A heartbeat going faster than this for no good reason is called a tachycardia, and one that lags behind is called a bradycardia.
Both types of arrhythmia occur when your heart's normal electrical sequence gets out of whack. Sometimes the SA node misfires or establishes a bad rhythm. Other times the electrical signal takes a wrong turn and gets to different parts of the heart in the wrong order. Some arrhythmias are caused by thyroid conditions, diabetes or drug abuse. In these cases, treating the underlying cause normally fixes the problem. If you feel like your heartbeat is adding extra beats or beating out an avant-garde rhythm, get it checked out immediately.
Other problems, such as ventricular tachycardia -- an electrical problem in a damaged ventricle, causing rapid-fire contractions -- may result from a previous heart attack and may in turn cause more heart attacks. Ventricular tachycardia varies in intensity. Some episodes may require no action, and others won't stop until a new rhythm is introduced to replace the broken one. For ventricular tachycardia and similar arrhythmias, automatic implantable cardioverter defibrillators can perform cardioversion, or deliver a series of shocks that returns the heart to a normal rhythm.
So that device implanted underneath your collarbone can be pretty handy. Next, we'll learn how the AICD can stop your heart and leave you feeling grateful for it the next morning.