Pediatric Resynchronization Therapy for Cardiomyopathy

A cardiomyopathy signifies a poorly functioning and frequently dilated heart. Severe cardiomyopathy results in heart failure because the heart is unable to pump enough blood to meet the body's needs.

The causes of cardiomyopathy in children include:

• Heart muscle disease
• Heart valve disease
• Congenital heart disease, especially if the right ventricle serves as the pumping ventricle to the body
• Having a pacemaker pace the ventricle for a prolonged period of time
• Infection of the heart
• Past heart attack
• High blood pressure

When a heart is dilated from a cardiomyopathy there may be alterations to the electrical conduction through the ventricles, resulting in a conduction delay or bundle branch block. Thus, it takes longer for the electrical signal to spread throughout the ventricles. When a conduction delay is present, the right and left ventricles do not contract at the same time. Their contractions are not synchronized. Dyssynchronous contraction of the ventricles leads to further worsening of heart function and increased dilation of the ventricle. A vicious cycle may ensue.

Dyssynchronous contraction of the ventricles may be due to causes other than a dilated cardiomyopathy. Specifically, pacing the right ventricle results in delayed spread of conduction from the right ventricle to the left ventricle. This dyssynchrony due to pacing, over a long time, can result in worsening of heart function and ventricular dilation.

A potential therapy for the condition of cardiomyopathy associated with dyssynchronous contraction is cardiac resynchronization therapy (CRT). Resynchronization therapy involves a device, a type of pacemaker, in which electrical impulses are delivered to the heart to help correctly time or "resynchronize" the heart's contractions. This is done by using the device to stimulate the left and right ventricles at nearly the same time to restore the coordinated pumping action of the heart.

Successful CRT may help, along with medications, to alleviate the symptoms of heart failure. These symptoms include difficulty breathing, especially when lying flat or when active, rapid breathing, swelling in feet and legs, and decreased energy. Over a long period of time, CRT may even help the heart return to its normal shape and size.

Multiple clinical studies have demonstrated that CRT helps decrease symptoms of heart failure, improve functional status, and reduce mortality in adult patients with heart failure. The causes of heart failure in children and young adults tend to be different from adults. Nevertheless, some limited studies have demonstrated that CRT helps to improve heart function and improve symptoms in pediatric patients with various diagnoses.

A CRT device consists of two parts, much like a pacemaker. The pulse generator and the leads. The pulse generator contains the battery and computer circuitry of the device. The battery supplies the electrical impulses that stimulate the heart while the circuitry controls the amount and timing of these impulses. Leads are insulated, flexible wires that attach to the heart and carry the electrical impulses from the pulse generator to the heart muscle. Typically, a CRT device has three leads: one in the atrium, one in the right ventricle, and one on the left ventricle.

Implanting a CRT device is very similar to the implantation procedure of a pacemaker. Typically, a small incision is made in the left upper chest under the collar bone and a "pocket" is created under the skin. The leads are then inserted through the incision into the vein under the collarbone. With the help of fluoroscopy or live action X-ray, the leads are then positioned in the heart. One lead is passed into the right ventricle, one lead is placed in the right atrium, and one leads is guided into the coronary sinus. This vein is on the back surface of the left ventricle. The lead in the coronary sinus contacts the surface of the left ventricle through this vein.

Once in place, each of the leads is tested to make sure they sense and pace the heart appropriately. The leads are then attached to the pulse generator which is then placed into the pocket in the chest. The incision is closed and a sterile dressing or glue is applied to the scar. The procedure is typically done under general anesthesia so the patient has no discomfort during the procedure.

For patients that are too small to have leads in their vessels or in certain patients with congenital heart disease (specifically, those with unrepaired holes in their heart or those with single ventricle defects), the leads are placed on the outside of the heart (epicardial approach). This is done by a cardiac surgeon in the operating room. Three leads are still involved: one on the right atrium, one on the right ventricle, and one on the left ventricle. For epicardial devices, the leads are attached to the pulse generator which is then placed below the muscle in the upper abdomen. This procedure is also performed under general anesthesia.

A CRT device that is only a pacemaker is sometimes called a biventricular pacemaker. Occasionally a patient with heart failure is also at risk for having life threatening arrhythmias. In such cases, a CRT defibrillator or biventricular ICD (implantable cardioverter defibrillator) may be indicated. Such a device is exactly the same as a biventricular pacemaker except that the lead placed in the right ventricle has an added defibrillation coil and the pulse generator is slightly larger. Thus, a biventricular ICD will serve both as a biventricular pacemaker and as an ICD.