Cardiac resynchronisation therapy (CRT) improves symptoms and quality of life and improves the prognosis of patients with chronic heart failure. CRT works by improving co-ordination of cardiac contraction, and is indicated in people with heart failure and left bundle branch block (LBBB) on an ECG. The importance of LBBB is that it indicates that the electricity is spreading only very slowly over the surface of the heart with each heart beat, thereby making contraction very "dyssynchronous"; that is, instead of all the heart muscle contracting simultaneously, parts contract and are relaxing before other parts even start to contract.
With a standard pacemaker, a pacing lead is implanted in the right ventricle. In those patients with a normal heart rhythm ("sinus rhythm"), a second lead is usually placed in the right atrium close to the heart's natural pacemaker. The lead in the ventricle can then track the heart's natural heart rate as detected by the lead in the atrium, or, if the natural rate is too slow, the pacemaker can sequentially pace the atrium and then the ventricle.
A CRT system is similar, but with the addition of an extra lead positioned to pace the left ventricle. Now, the pacemaker is able to stimulate both left and right ventricles simultaneously, restoring the normal co-ordination of ventricular contraction.
Approximately 25% of patients do not achieve significant clinical benefit with CRT. Such patients are termed "non-responders", and lack of response is typically measured as a failure to improve exercise capacity with CRT, or a failure of the heart to improve on echocardiography. One option to reduce the number of non-responders may be to optimise the CRT device by adjusting its settings based on clinical variables (such as ECG and echocardiogram findings). Both ECG and echocardiogram optimisation give similar results in terms of clinical response to CRT, but patients who had their CRT optimised using ECG variables had a significantly greater impact on echocardiographic response, that is a greater proportion of that group had a LV end-systolic volume reduction \>10%) 3. Another possibility for optimising CRT is cardiogoniometry (CGM), which is what the study aims to investigate. CGM is form of 3D vector electrocardiography which can provide quantitative analysis of myocardial depolarisation and repolarisation. Like standard 12 lead electrocardiogram (ECG), CGM uses different electrodes to identify electrical potential gradients produced by cardiac electrical activity. The ECG can only visually represent this information in a two dimensional way, whereas CGM can create a three dimensional display. Electrode placement is important: and complex mathematical modelling is used to generate the displays. CGM gives the same output as a standard ECG. One additional output is vector loop graphs. These are sequentially plotted values of electrical activity of the heart in the x, y and z axis, in three orthogonal planes. When the vector loops follow the same pathway it means that the electrical activity of the heart is following the same pathway with each ventricular depolarisation and repolarisation. By contrast, when there is abnormal electrical conduction, the vector loop pathways can vary. CGM is useful for identifying stable coronary artery disease and recognising the acute coronary syndromes, but its clinical value outside patients with acute ischaemic heart disease is unclear. This feasibility study aims to see if CGM can detect the different settings of a CRT device, by assessing the CGM vector loops with different device settings.
