The assessment of cardiac dyssynchrony is another interest of the University of Queensland Cardiovascular Imaging Research Group. This area of cardiac imaging has developed rapidly over the past 5 years in parallel with the increasing use of cardiac resynchronisation therapy for patients with advanced chronic heart failure. Cardiac resynchronisation therapy is a novel application of cardiac pacing that corrects incoordinate cardiac contraction thereby improving the clinical outcome of these patients who otherwise have a poor prognosis. Suitable patients for this treatment are those with advanced cardiac conduction system disease. This has conventionally been evaluated by an abnormal appearance on the surface electrocardiogram. More recently, however, it has been observed that the electrocardiogram is an insensitive means of detecting cardiac dyssynchrony. It is now accepted that cardiac imaging offers a more reliable means of assessment of these patients and predicting response to resynchronisation therapy. Our group and others have been focusing on the use of Tissue Doppler techniques.

Tissue Doppler echocardiography (Figure 1) provides a means of accurately determining the sequence of activation of different segments of the left ventricle. Increased variation between segments in their timing of activation is a reflection of worsening dyssynchrony. Patients with a significant amount of dyssynchrony have been shown to benefit most from cardiac resynchronisation therapy.

Figure 1: Analysis of the tissue Doppler image is performed by placing a region of interest in the wall of the left ventricle. The graphical representation displays the velocity of myocardial motion of this region throughout the cardiac cycle. The timing of myocardial events in different regions can be compared and the variation in event timing used as an index of dyssynchrony. In this example motion in the septal (yellow) and lateral (green) is sampled. There is delay in the systolic (positive signal) activation of the lateral wall relative to the septal wall.

The number of patients in previously published studies is relatively small and this is a very active area of cardiac imaging research. Despite significant advances there remains a lack of consensus in how to use tools such as tissue Doppler to optimally predict response to resynchronisation therapy. Also, other imaging technologies are currently developing rapidly and a considerable amount of work will be required to determine the relative merits of each of these.

At the University of Queensland our work is being directed at specific questions:

• What is the importance of dyssynchrony for long-term outcome in patients with chronic heart failure who are not referred for resynchronisation therapy?
• How stable are measures of dyssynchrony and what is the variability and reproducibility of recordings?
• What is the relationship between tissue Doppler indices of dyssynchrony and other novel techniques such as three-dimensional echocardiography?
• Dyssynchrony can also be identified in many patients who have undergone cardiac pacing for standard indications such as heart block but who do not have heart failure. How can cardiac imaging techniques be best used to more comprehensively evaluate these patients and reduce the risk of long-term complications of such pacing therapy?