QT Syndrome. The QT interval is a time interval between the start of the Q wave and the end of the T wave in a heart’s electrical cycle (Fig XXXXXXXXXXIt measures the time required for depolarization...

QT Syndrome. The QT interval is a time interval between the start of the Q wave and the end of the T wave in a heart’s electrical cycle (Fig. 2.19). It measures the time required for depolarization and repolarization to occur. In long QT syndrome, the duration of repolarization is longer than normal, which results in an extended QT interval. An interval above 440 ms is considered prolonged. Although the mechanical function of the heart could be normal, the electrical defects predispose affected subjects to arrhythmia, which may lead to sudden loss of consciousness (syncope) and, in some cases, to a sudden cardiac death.

The data set QT.dat|mat was compiled by Christov et al. (2006) and is described in http://www.biomedical-engineering-online.com/content/ 5/1/31. It provides 548 QT times taken from 293 subjects. The subjects include healthy controls (about 20%) and patients with various diagnoses, such as myocardial infarction, cardiomyopathy/heart failure, bundle branch block, dysrhythmia, myocardial hypertrophy, etc. The Q-onsets and T-wave ends are evaluated by five independent experts, and medians of their estimates are used in calculations of the QT for a subject. Plot the histogram of this data set and argue that the data are reasonably “bell-shaped.” Find the location and spread measures of the sample. What proportion of this sample has prolonged QT?