Measuring the organization of cardiac rhythms using the magnitude-squared coherence function

The application of the magnitude-squared coherence (MSC) spectrum as a measure of the degree of organization of the cardiac electrical activity is explored. The MSC spectrum is a frequency-domain measure of the linear relationship between two signals. In the work described the two signals are two bipolar electrograms from either acutely placed catheter(s) or automatic implantable cardioverter/defibrillator electrodes. It is shown that the MSC is a dimensionless (no units), real-valued spectrum that is always in the range of zero to unity. The case of zero is found at frequencies where there is no linear relationship between the signals, and the case of unity implies a linear, noise-free relationship. The way the MSC spectrum is normalized makes it insensitive to gain or gain differences between the two signals. Example MSC spectra are presented and discussed. Striking differences in the spectra for fibrillatory and nonfibrillatory rhythms are seen.