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Normal Heart Function

The heart is a pump responsible for maintaining blood supply to the body. It has four chambers. The two upper chambers (the right atrium and left atrium) are the chambers which receive blood as it returns from the body via the veins. The lower chambers (the right and left ventricle) are the chambers responsible for pumping the blood out to the body via the arteries. Like any pump, the heart has an electrical system that controls how it functions.

Normal heart rhythm

In order for the heart to do its work (pumping blood throughout the body), it needs a sort of spark plug or electrical impulse to generate a heartbeat.  Normally this electrical impulse begins in the upper right chamber of the heart (in the right atrium) in a place called the sino-atrial (SA) node. The SA node is the natural pacemaker of  the heart.  The SA node gives off electrical impulses to generate a heartbeat in the range of 60 to 100 times per minute.  If you are exercising, doing strenuous work or you are under a lot of stress, your heart rate may be faster. When you rest or sleep your heart rate will slow down. If  you take certain medications, your heart rate may be slower. All of this is appropriate.

From the SA node, the electrical impulse is relayed along the heart’s conduction system. It spreads throughout both the right and left atria causing them to contract evenly. 

When the impulse spreads over the right atrium it reaches the atrio-ventricular (AV) node.   This is a very important structure in the heart because it is the only electrical connection between the top chambers and the bottom chambers. It is therefore the only way in which an electrical impulse can reach the pumping chambers (the ventricles). The impulse spreads through the AV node and down into the lower chambers or ventricles of the heart. This causes them to contract and pump blood to the lungs and body.



Abnormal Conduction Develops in Cardiomyopathy Patients

Some patients with cardiomyopathy develop an abnormality in their conduction system. The most common abnormality is a delay in the normal electrical conduction through one of the normal wires – the left bundle branch. This is known as left bundle branch block (LBBB) and is seen as a widening of the QRS (>120ms) on the 12 lead ECG. This is seen in approximately 40% of patients with cardiomyopathy. Because of the delay in conduction down the left bundle branch, the right ventricle is activated a fraction of a second earlier than the left ventricle.

Additionally the LBBB causes the lateral wall of the left ventricle to be activated much later than normal.  This dyssynchrony leads causes an overall decrease in the pump function of the an already weakened heart in heart failure patients Patients with LBBB on ECG and heart failure symptoms may benefit from Cardiac Resynchronisation Therapy (CRT) , this is also called a Bi-Ventricular Pacemaker.

What is Cardiac Resynchronization Therapy (CRT)?

The aim of CRT is to restore the normal coordinated pumping action of the ventricles by overcoming the delay in electrical conduction delay caused by the left bundle branch block. This is achieved by implantation of a specialized pacemaker called a CRT device or Bi-Ventricular pacemaker. In addition to the standard wires (leads) that are implanted in the right atrium and right ventricle present in a normal dual chamber pacemaker, a third wire (lead) is implanted to stimulate the lateral wall of the left ventricle. We can pace the lateral wall of the left ventricle via the coronary sinus which is a vein which wraps around the back of the heart, around the left ventricle. This vein can be accessed via the right atrium.

Once the LV lead is positioned, the CRT device will be programmed to pace the right ventricle and left ventricle simultaneously. This leads to improved co-ordination of the heart’s pump function and can be seen as a narrowing of the QRS in the ECG. Studies have shown CRT leads to improved exercise tolerance and improved quality of life. 

Who is a candidate for CRT?

Patients who will benefit from a CRT device include those with:

  • Heart failure and moderate to severe heart failure symptoms (decreased exercise tolerance and shortness of breath)

  • Cardiomyopathy (weakened and enlarged heart muscle)

  • Significant left bundle branch block.

Some patients with cardiomyopathy who are candidates for CRT are at also high risk of sudden cardiac death from dangerous heart rhythms originating from the ventricles (Ventricular Tachycardia and Ventricular Fibrillation). For these patients a special CRT device is available that can stop these dangerous heart rhtyhms dy delivering a shock known as a defibrillator. These devices incorporate a standard implantable Cardioverter defibrillator (ICD) with a CRT pacemaker and are called CRT-D devices. The choice of a CRT versus and CRT-D is dependent on many clinical factors and will be discussed with you.

What are the benefits of CRT?

The response to CRT can vary between patients. Published studies involving several thousand patients worldwide have demonstrated improvements in exercise tolerance, heart failure symptoms and quality of life. It often takes weeks to months for these improvements to be noticed. Unfortunately a small number of heart failure patients do not benefit from the CRT therapy. 


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About Us


Melbourne Heart Rhythm is the arrhythmia service at the Department of Cardiology at the Royal Melbourne Hospital. We are comprised of a team of highly trained and dedicated health care professionals specializing in heart rhythm management. We have a strong focus on patient education and patient centered care. Our arrhythmia specialists have an international reputation of excellence in arrhythmia research and clinical arrhythmia management.