Respiratory System

Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)

Heart muscle

Muscles cells are told to contract by electrical impulses. In addition to the ability to contract, muscle cells can conduct these electrical impulses to neighboring cells. In fact, some specialized muscle cells in the heart do nothing except conduction.

There are three types of muscle tissue in your body : skeletal (a.k.a. voluntary) muscle, smooth muscle, and cardiac muscle. Your biceps are an example of skeletal muscle. Another skeletal muscle is the diaphragm. The diaphragm is the big muscle that separates your chest cavity from your abdominal cavity. When it is time to breathe in, your brain sends a signal down a nerve which tells the diaphragm to contract. This in turn causes the chest to expand and air to rush in the lungs. The brain thus controls every breath you take (as well as every move you make). People who receiveinjury to their spinal cord above a certain level might damage this nerve and consequently lose the ability to breathe on their own.

Your heart is different than the diaphragm. It tells itself to beat; it is the master of its own destiny. It does not require the brain. That's not to say the brain cannot influence the heart; indeed, your brain gives your heart advice on when to speedup and when to slow down. In one of the Indiana Jones movies, the evil cult leader punches through a man's chest and rips out his still beating heart. While I imagine the punching and ripping-the-heart-out aspects of the film are impossible, the fact that the heart still beats is not pure Hollywood fiction.

What tells the heart to beat? Pacemakers do. (When I use the word pacemaker, you should assume that I am referring to the heart's natural pacemakers. I will call those man-made devices artificial pacemakers.) What are pacemakers exactly? They are special muscle cells in the heart that fire themselves after a certain amount of time has elapsed. This property is called automaticity. The normal pacemaker is the sinus pacemaker, which usually fires between 60 and 100 times a minute.

The pacemaker generates the electrical impulse and sends it to its neighboring heart muscle cells. These cells then, in turn, conduct to neighboring cells.

The normal contractile cells of the heart will do two things :

1. Conduct. This means that it can spread the signal to the neighbors. I will generally use the term electrical to refer to this aspect of the heart cells.
2. Contract. Contraction is what muscles do, and in the heart, the contraction is what makes the heart beat. When it beats, it is actually moving in an organized pattern that pumps (squeezes) the blood in a certain direction. I will refer to this as the heart's mechanical ability.

Other heart muscle cells are specialized for conduction only.

While all these conduction cells are muscle, it might be helpful to break them down according to function and location :

1. Sinus (SA) node - conduction only
2. Atria muscle - contraction and conduction
3. Atrioventricular (AV) node - conduction only
4. Bundle of His - conduction only
5. Bundle branches - conduction only
6. Purkinje fibers - conduction only
7. Ventricular muscle - contraction and conduction

When talking about the general region of the AV node and the bundle of His, we refer to it as the AV junction.

An ECG says nothing about the contraction of the heart cells. In fact, it is possible for heart cells to lose their contraction ability while maintaining the conduction. Thus, a person may show a perfectly normal ECG without having a heart beat. Even if the heart is beating, there is no guarantee that there is any blood to pump. You can only measure the pulse by mechanical means (e.g. pressing your finger against the artery in the patient's neck or wrist). Don't ever forget this.

Once upon a time (1800's and before), scientist were not that familiar with the underlying electrical system that control the heart. The main thing that was measured was the pulse. The terms bradycardia and tachycardia were given to signify a slow and fast pulse, respectively. There is somewhat of a danger in using these words with ECG interpretation in that we are seeming to imply the presence of a pulse when we describe an electrical pattern. If I use the term "beats per minute", be sure to adapt it to whatever context. It would probably be more accurate to say "ventricular depolarizations per minute," but I will not.

When a heart cell contracts, it does not occur the same instance it is signaled to do so. While a fraction of a second may seem "instantaneous" to us, it is not to the heart.