Computer Analysis of the RR Interval-Contractility Relationship during Random Stimulation of the Isolated Heart

Hemodynamic variability in patients with atrial fibrillation may originate from a direct influence of the variations in RR intervals on myocardial contractility. With the aid of a computer the serial autocorrelation function and the histogram of the RR intervals of patients with atrial fibrillation receiving no medication were produced. The RR intervals were randomly distributed and the histograms rather skew. Next, random rhythms with histograms matching those of the patients were produced with a radioactive source and a Geiger-Müller counter. These rhythms were used to stimulate isolated perfused rat hearts to study the relationship between the RR interval and a number of contractile parameters. The ECG and the isotonic contractions were digitized and processed by the computer. Serial crosscorrelation coefficients were computed between the RR interval on the one hand, and contraction height, contraction area, and maximum of the first derivative of the contraction on the other hand. The zero order crosscorrelation coefficient between the RR interval and contraction height was 0.6, between the RR interval and contraction area 0.8, and between the RR interval and maximum of first derivative 0.5. Although small, the first and second to approximately the tenth order coefficients were definitely negative. It is concluded that during a random rhythm the contractile parameters of an isolated heart are strongly related to the preceding RR interval. It is conceivable that this relation contributes to the variability of hemodynamic parameters during atrial fibrillation in man.