The importance of heart rhythm
Heart rhythm is vital to the function of the heart. If the heart fails to be excited, then no beating occurs. Within 3 to 5 seconds, the person passes out, and if rhythm is not restored within 10 minutes, brain death occurs. Therefore, it is vital that heart rhythm be reliable.
In the heart, whatever part gets excited first causes the rest of the heart to beat. In normal rhythm, the first part to get excited is usually the sinoatrial or sinus node, a pea-sized structure high in the top right chamber, the right atrium. Therefore, normal heart rhythm is called “sinus rhythm.” The sinus node responds to the needs of the body for oxygen-containing blood. While the person is asleep, the needs of the body for blood flow are low, and the sinus node excites the heart usually at less than 60 beats per minute: this is called sinus bradycardia. When the person is exercising, the body needs more blood flow, and the sinus node excites the heart usually at more than 100 beats per minute: this is called sinus tachycardia. The sinus node sets the heart rate to match the body’s need for blood-containing oxygen.
Electrical conduction in the heart
After the sinus node fires, the top right chamber, the right atrium, contracts first. The heartbeat quickly spreads along special pathways to the top left chamber, the left atrium, and it contracts as well. Thus both top chambers contract propelling blood into the bottom chambers. The blood must flow freely between the top and bottom chambers of the heart. However, it is a different story for the spark of electricity that excites the heart to beat. It must be tightly controlled.
The atrioventricular (AV) node
In the normal heart, there is only one pathway between the top and bottom of the heart. This is called the atrioventricular or AV node. The AV node acts as a filter and protects the bottom part of the heart against too fast or slow heart rhythms. When the heart rate from the top part of the heart is reasonable (50 to 200 depending on the conditions), the AV node allows each beat to make it to the bottom chambers and the pulse is produced so that the top and bottom parts of the heart are synchronized at the same rate. If the top part of the heart fails to start a heartbeat, the AV node produces an escape beat or rhythm at 30 to 50 beats to minute to sustain life. When the top part of the heart races too fast (over 200 beats per minute), the AV node filters out and blocks some of the beats from making it to the bottom part of the heart. For example, in atrial flutter, the atria beat at 300 beats per minute. If the AV node passed each of these beats to the bottom part of the heart, the pulse would be 300 beats per minute, which is too fast and could cause a cardiac arrest. As a result, the AV node filters out every other beat and the pulse is 150 beats per minute. Every other beat from the top is synchronized with the pulse and the other ignored.
The His bundle and bundle branches
The spark of electricity that has excited the top of the heart and conducted through the AV node now arrives at the bundle of His. This bundle of heart muscle fibers is capable of the most rapid conduction of any heart muscle. The bundle of His divides into the right and left bundle branches, which in turn continue to divide, producing a net of fibers coating the inside of both the right and left ventricles. Thus, the entire inner surface of both bottom chambers is excited at the same time, and the right and left ventricles contract in a synchronous fashion.
n the normal heart, with no way for the spark of electricity to get back to the top of the heart, that beat ends, and the heart relaxes to get ready for the next beat.