Heart rate turbulence (HRT) is the return to equilibrium of heart rate after a premature ventricular contraction (PVC). It consists of a brief speed-up in heart rate, followed by a slow decrease back to the baseline rate. An important feature of HRT is that PVCs occur naturally in most adults, so measuring the characteristics of a given person's HRT offers a non-invasive way to evaluate his or her cardiac function without applying artificial external stimuli. The measured values of HRT parameters have been shown to be a statistically significant predictor of the probability of dying from cardiac disease after a patient suffers a myocardial infarction. HRT can also be used to predict death in patients with congestive heart failure from a lethal arrhythmia. HRT has nothing to do with turbulence in fluid dynamics. History The concept of HRT was introduced to the medical community by Georg Schmidt and colleagues from the Technical University of Munich in 1999 in the British medical journal the Lancet. While studying PVC characteristics, Schmidt and his colleagues noticed that heart rate seemed to speed up after a PVC. To clarify, they listed the time from one heartbeat's R-wave to the next R-wave (called RR intervals) and synchronized these lists to the time of the PVC beat and averaged the values in the list. A plot of this averaged RR interval list (called a PVC tachogram) not only confirmed their observation that heart rate sped up for a few beats after a PVC, but highlighted another less obvious feature, that heart rate then slows down beyond what it was before the PVC, before returning to the original heart rate. Schmidt reasoned that just as loss of variability in heart rate indicated patients more likely to be at high risk of dying after a heart attack, this phenomenon might also be an indicator of a healthy control of heart rate in such patients. They proceeded to test this hypothesis using 24-hour electrocardiogram (Holter monitor) recordings from one hundred survivors of heart attacks with frequent PVCs. Greater turbulence seemed correlated with better prognosis. They then used this data to determine the optimal discriminating threshold between normal and abnormal HRT values, and came up with the values TS=2.5, TO=0%. Now came the test. These thresholds were applied to Holter records from a total of 1191 patients who had experienced a heart attack. There were 162 deaths (13.6%) during the