The Heart

Heart Anatomy

The heart has two parts:
  1. A mechanical pump.
  2. An electrical conduction system.

The body requires both parts working efficiently to effectively circulate the blood throughout the cardiovascular system.


The heart is a four chambered muscular organ which circulates blood throughout the body. It is located in the chest behind the lower part of the breastbone. Blood returns to the heart on the right side and is pumped to the lungs where it receives oxygen and releases carbon dioxide. The blood then returns to the left side of the heart where it is pumped out to the body. The heart receives its own supply of blood via the coronary arteries.

If these arteries become blocked, cardiac arrest can occur.

The flow of blood through the arteries creates a pulse. There are points on the body where a pulse can be felt. The most common and easily accessible pulse on an adult is the Cartoid pulse. It is located in the neck.


The heart has a conduction system, which moves electrical impulses along pathways in the heart. These impulses stimulate the heart muscle to contract. The pumping activity of the heart cannot occur without organized electrical activity moving through these pathways. We refer to this organized electrical activity as a rhythm. The heart's rhythm can be captured on paper using a device known as an electrocardiogram.

Defibrillation & Heart Rhythms

Defibrillation is the delivery of an electrical current or shock, through the heart to stop ventricular fibrillation. The intent of the shock is to allow the heart to restore electrical activity through it's normal pathways. Normal electrical activity causes the heart to contract and pump blood throughout the body. We will then be able to feel a pulse.

The act of defibrillation, or shocking, is more successful when it is delivered rapidly. Cardiac arrest victims who are shocked early are more likely to survive.

The heart's electrical activity is represented and measured by the electrocardiogram. This representation is called a rhythm. Rhythms that do not follow the normal pathway through the heart are called "dysrhythmias".

During a cardiac arrest, the AED will analyze the heart rhythm for you and tell you what to do. There are two main kinds of heart rhythms. A "shock advised" rhythm and a "no shock advised" rhythm. Examples of these are shown on the next page. These are shown for your interest. In fact, the AED does not even show you the heart rhythm. It does everything internally, decides what to do and then tells you to do it. It's that easy!

Before we take a closer look at the two main heart rhythms that can occur during a cardiac arrest, we'll show you what a normal heart rhythm looks like.

Normal Sinus Rhythm- This rhythm represents normal electrical conduction throughout the heart. There will be a carotid pulse that can be felt in the neck. This electrical activity is organized and the pulse rate is within a normal range of 60 - 100 beats per minute.

As we discussed earlier in the chain of survival, when an adult collapses, they are most often in cardiac arrest and in ventricular fibrillation. This rhythm must be shocked. As a PAD provider, your AED will analyze the rhythm for you and advise you when a shock should be given. We have given an example of ventricular fibrillation below. Ventricular fibrillation is a "shock advised" rhythm.

Ventricular Fibrillation- This "dysrhythmia" is unorganized chaotic electrical activity. There is no signal received by the heart muscle cells to contract. The heart will not pump and there will be no pulse. If this rhythm is shocked early, there is a good chance the victim's pulse will be restored.

There will be circumstances where the victim will not be in ventricular fibrillation when they go into cardiac arrest. They will be in a rhythm that does not require a shock. As a PAD provider, your AED will analyze the rhythm for you and advise you that no shock is required. The AED will then advise you to begin CPR. We have given an example of asystole ("flatline"). Asystole is a "no shock advised" rhythm.

Asystole - This represents complete heart standstill and there is no electrical activity. There will be no pulse. Defibrillation does not work in this situation to restore electrical activity.

When a victim is in cardiac arrest, it is possible for their heart rhythm to change from a "shock advised" to a "no shock advised" rhythm. The AED will automatically detect these rhythm changes and tell you what to do.

This website is operated by the City of Toronto - ( Toronto Paramedic Services ) ©Toronto Paramedic Services 1999 - 2014
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