Consultant cardiologist Dr Maite Tome discusses biventricular pacemakers and their benefits
One of the most fascinating organs in the human body is the heart. The heart works with the precision of a watch. The electrical power travels through specialised tissue and gives the go command to different parts of the human heart. This precision allows the heart to be connected to a closed system, the vessels, and to pump and collect blood continuously in the heart’s different chambers.
The heart’s structure
The heart has four chambers, two atrium and two ventricles. The heart is also divided internally into two sides, right and left. Each part manages different circulations
The atria are smaller and less muscular than the ventricles. The right ventricle (RV) receives the blood from the right atrium (RA) and pumps it toward the lungs so they can get oxygen. After passing through the lungs, the blood comes back to the left atrium (LA) and the left ventricle (LV). The LV is the more powerful pumping chamber of the heart and from here the blood is circulated to everywhere in the body. This life-long journey of the blood through the heart and body is designed to always go forwards. Cardiac valves open and close ensuring that there is no blood return.
The conductor of the orchestra is the electrical impulse; the atria will be relaxing when the ventricles pump and conversely. The LV in normal circumstances contracts around 40 milliseconds earlier than the RV. Every contraction is followed by a relaxation. This is then a synchronous process.
Pacing the heart
A pacemaker is a mechanical device consisting of a battery and a wire connected to the heart. The pacemaker has a small computer and can analyse the heart rhythm. The pacemaker can offer back up pacing, active pacing and other therapies depending on the program required. The main pacing wire is usually inserted in the RV and placed in the apex (the bottom part). Additionally another wire might be required in the atria. This is then called a dual chamber pacemaker.
Patients with advanced heart disease, in particular dilated cardiomyopathy with advanced heart failure and on optimal therapy, may have what we call dyssynchrony (the heart is desynchronised). Different tests, such as echocardiogram and ECG, are used to assess the presence of dyssynchrony . With biventricular pacing, a third pacing wire is inserted through a cardiac vein that starts in the RA runs through the back of the heart and has some branches reaching the LV. With three wires the pacemaker takes control of the electrical impulse of the heart and improves the timing of both ventricular filling and ventricular relaxation.
This therapy is called cardiac resynchronisation therapy (CRT). Not all the mechanisms involved in why CRT works are known. In some patients the ventricular function improves dramatically. In other patients the functional capacity improves before the ventricular function does. Other patients do not seem to benefit from it. Studies at centres around the world are now helping to define the groups of patients who benefit most.
The National Institute for Clinical Excellence (NICE) published the recommendations for implantation of these devices in the last appraisal published in 2007. These are:
Cardiac resynchronisation therapy using a pacing device is recommended as a possible treatment for people with heart failure where all of the following circumstances apply.
♥ They have moderate to severe symptoms of heart failure that are affecting their daily life (measuring Class 3 or 4 in the New York Heart Association classification system)
♥ Their heart is beating regularly but an electrocardiogram (ECG) shows that the electrical system of the heart is not working properly
♥ The left ventricle of their heart has an ejection fraction of 35 per cent or less (the ejection fraction is the percentage of blood pumped out of the heart with each contraction)
♥ They are taking the drug treatment that is most effective for them
Internal defibrillators (ICDs)
NICE has said that a different type of pacing device that contains a defibrillator (biventricular implantable cardioverter defibrillator) may be considered for people with heart failure if:
♥ Their heart failure is suitable for treatment with a pacing device – in other words, all the circumstances in the list above apply.
These patients may also be suitable for an implantable cardioverter defibrillator (ICD). As you can see from the guidelines, CRT can be used to pace the heart or to pace and defibrillate in cases where there is an increased risk of a patient developing a dangerous heart rhythm. In patients with cardiomyopathy there are two emerging groups that in some cases benefit from CRT - those with restrictive cardiomyopathy and those with hypertrophic cardiomyopathy with end stage or restrictive physiology. The use of devices in these types of conditions requires an expert team to decide whether implantation is appropriate and close monitoring.
Fitting a biventricular pacemaker is similar to that of a standard pacemaker. The pacemaker is fitted under the skin and muscle in the chest wall just below the collar bone. Access to the heart is gained through a major vein and the battery is connected to the wires.The difference is in the implantation of a third wire. As described above, a major intracardiac vein is used to take the extra wire. This placement is often difficult and might require several attempts. Sometimes it is not technically possible because of the patient’s vein anatomy. The implantation time is slightly longer for a biventricular pacemaker than a standard device.
Getting the most from your pacemaker
Heart performance after CRT implantation is different from patient to patient. Therefore a detailed adjustment of the pacemaker, guided by echocardiography, might be needed. These adjustments are normally done a few months after the device is fitted.
As with all such devices, the biventricular pacemaker needs to be checked from time to time. Your pacing clinic will ensure information about how the device has been performing is downloaded from the device’s memory. Staff will also check that the wires, the battery and the parameters are all good. Sometimes during a check up it might be noticed that lead measurements are incorrect suggesting lead displacement. This type of problem requires attention and the patient would need to be readmitted to hospital to have the wires corrected.