| Summary: | This thesis discusses the history of measurement of cardiac repolarisation (the QT interval), and subsequently the various methods to correct QT for heart rate. After highlighting the limitations of traditional QT correction formulae, I go on to describe a novel method for continuous automated measurement the relationship between QT and heart rate from ambulatory ECG recordings. As cardiac repolarisation and the QT interval is to be abnormally prolonged in certain cardiac diseases, and that QT prolongation can precipitate malignant arrhythmias, the second section of the thesis assesses the repolarisation characteristics of patient groups with heart failure and hypertrophic cardiomyopathy. The repolarisation properties are then compared to measures of disease severity, and traditional markers for sudden death in these patients. The third section of the thesis presents data gathered from patients with implantable cardiac defibrillators (ICDs), with three main goals: firstly to establish whether there is a particular time of day that these patients are prone to arrhythmias; secondly to establish whether there are particular abnormalities of the repolarisation characteristics in these patients at the time of day; and thirdly to establish whether overall 24h repolarisation properties convey any prognostic information with regards to the risk of further malignant ventricular arrhythmias. I show that in patients with heart failure and hypertrophic cardiomyopathy there are marked abnormalities in several parameters of cardiac repolarisation, and that these abnormalities are progressive with the severity of the disease. These abnormalities might be expected to confer an increased risk of sudden arrhythmic death. I also show that sustained ventricular arrhythmias are far more common in the early hours of the morning and that an increase in sympathetic activity at that time of day appears responsible. There are concomitant changes in cardiac repolarisation, which are less marked in subjects on beta blockade. Previous authors have suggested that an abnormally increased ‘rate corrected QT’ may be important in the early morning ‘high risk’ period. I have demonstrated that the repolarisation characteristic during the high risk period is that of a <i>shortened </i>QT.
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