Your Heart's Electrical System

For the heart to pump oxygenated blood throughout the body, it must be electrically stimulated to beat. Each heart beat that you hear of feel as a pulse begins as an electrical impulse that is automatically released from the sinus node, which is a special group of cells that function as the heart's pacemaker (rate). Under normal conditions or at rest, the pacemaker initiates between 50 and 100 beats per minute. When you are active or experiencing emotional, psychological or physical stress, such as working in the garden on a hot day, the pacemaker speeds up the heart rate to meet the additional demands of the body and working tissues.

The electrical signal for each heartbeat starts in the upper right chamber of the heart and uses both upper chambers to contract and squeeze blood into the lower chambers.

The signal then travels along special conduction tissues to the AV node where it is delayed for a fraction of a second. This delay allows time for the lower chambers to completely fill with blood.

After the delay, the electrical signal moves down the bundle and spreads into small branches that stimulate the lower chambers to move blood from the heart to the lungs and the rest of the body. The normal rhythm of the heart occurs when the electrical signal moves through the heart in the proper sequence.

Arrhythmia

Sometimes the heart's electrical system does not function normally. It may race, become slow, irregular, or skip beats. Some of these are normal. However, if you feel your heart racing, you should stop your activity, sit down, and cough strongly and deeply.

One way to evaluate the heart's electrical signal is to record it on an EKG. The EKG shows a diagram of the signal, beat by beat.

The heart contracts rhythmically because the electrical conduction system sends impulses throughout the heart. The electrical impulse travels a set path and spreads from the pacemaker or sinus node throughout the atria, to the atrio-ventricular node and into the ventricles. The "natural pacemaker" of the heart is the sinus node and it sets the rate for the heartbeat.

Sometimes the heart's electrical signal does not move in the proper sequence. This causes the heart to beat faster or slower than normal, or erratically. These abnormal rhythms are called arrhythmias. They can cause a variety of symptoms: dizziness, fainting, fatigue, shortness of breath and chest pain or rapid palpitations that may feel like flutters or pounding of the heart. If left untreated, arrhythmia's can be life threatening.

There are four major kinds of arrhythmias:

Bradycardia: Occurs when the heart's electrical signal is delayed too long or blocked, resulting in a slower than normal heartbeat. If it happens only once in awhile, bradycardia is not a problem. However, if it continues over a long period of time, the body will not receive an adequate blood supply, which can be very serious. Heart disease and some drugs can cause bradycardia, and a physician should evaluate it to determine if treatment is required. Treatment can include discontinuing a medication and/or a pacemaker to make sure the heart beats at a normal rate.

Irregular or extra heartbeats: Even completely healthy people have irregular or extra heartbeats every once in awhile. In some cases, irregular or extra beats can lead to rapid heartbeats.

Supraventricular Tachycardia (SVT): Occurs when the heart's electrical signal begins above the ventricles (the lower chambers of the heart) causing the heart to beat very rapidly or erratically. As a result, the heart is strained, and the body receives an inadequate blood supply. There are three types of SVT:

  - Atrial flutter
  - Atrial fibrillation
  - Paroxysmal SVT

A number of underlying conditions can lead to SVT. Medication and/or electrical shock treatment (cardioversion) can restore normal heartbeat. To prevent recurrences, additional treatment and medication may be necessary.

Ventricular Tachycardia (VT): Occurs when the heart's electrical signal begins in the ventricles (lower chambers of the heart) and the heart beats too rapidly. When the ventricles pump too fast,. they cannot deliver enough blood to the body. In some cases, VT can create a very rapid, erratic heartbeat (ventricular fibrillation), or cardiac arrest. If VT lasts for only a second or two, it may not be noticed and probably will not cause any serious problems. However, if it lasts longer, it can be very serious and should be evaluated by a physician.

Poor blood supply to the heart, diseases of the heart valves or chemical imbalances in the body can cause VT. It often occurs during or after a heart attack. No matter how long it lasts, a physician must evaluate it. Normal heartbeat can be restored with electrical shock treatment. Long-term control of VT may require an implanted defibrillator and/or medication.

When an arrhythmia is suspected, your physician can order some tests to determine what is causing your symptoms. Possible tests include an electrocardiogram (ECG), Holter Monitor, a stress test, a tilt table test, and/or sometimes an electrophysiology study (EP) if necessary. Additionally, if an arrhythmia is not revealed with routine testing then an implantable loop recorder may be indicated.