Heart rate variability in myocardial ischemia during daily life.

The aim of this investigation was to evaluate the role of autonomic nervous activity before and during transient ischemic events. Forty-one ischemic episodes detected on Holter recordings were analyzed for heart rate and heart rate variability (HRV). A time-dependent index of HRV the windowed median successive difference, which is a continuous measure of respiratory sinus arrhythmia and therefore a marker of vagal efferent activity was used. A small window consisting of five beats, which represented one respiratory cycle, was chosen. This method permitted continuous assessment of short-term alterations of vagal modulation. With two exceptions, all ischemic episodes were preceded by an acute almost complete suppression of respiratory sinus arrhythmia. During the entire ischemic episode, HRV stayed at this reduced level, and preceding the end of the ischemia, it increased again. This suppression of intrinsic heart period variations reflects an almost complete withdrawal of modulated vagal outflow immediately before and during ischemic episodes. In 26 cases (63%), Fast Fourier Transformations were carried out when the heart rate was almost constant in two segments around the onset of ischemia. In the other 15 cases we did not perform Fast Fourier Transformations because there was no stationary stage in the data. High-frequency power always decreased drastically at the onset of ischemia, confirming a significant loss of modulated vagal activity (P < .01). The low frequency/high frequency ratio did not increase, indicating that sympathetic activity did not increase significantly at the onset of ischemia in about two-thirds of our cases. The extent to which this suppression of modulated vagal activity reflects a similar suppression of vagal efferent activity is discussed, as well as whether this withdrawal of vagal outflow is cause or consequence of the ischemic event. The results suggest that a vagal depression may influence the onset of myocardial ischemia during daily life.