Impact of Variable Autonomous Neural Gain to Cardiovascular System Control

Time responses of some equine ECG signal parameters after stimulation appear rather non-standard when compared with the responses known in human electrocardiology. Heart rate of the stimulated equine cardiovascular system accelerates; it means the RR intervals describing lengths of cardiac cycles shorten, as is the case in human ECG. However, there exist a variety of different responses in equine QT intervals. We distinguish three different responses of QT intervals representing electrical activity of myocardial ventricles. First, shortening of the QT intervals (the same as in human population, 30% of records); second, prolonging of QT intervals (inverse relationship, 33% of records) and third, prolonging QT consequently followed by its shortening (complex relationship, 22%); 15% of responses could not be classified. Several mathematical models were designed in the past to explain the phenomenon based on an open loop control by means of sympathetic and vagal branches of the autonomous neural system. The models were not very specific in explaining the background of the complex relationship between RR and QT intervals in equine ECG. That is why another parameters defining variable gain of both the autonomous neural branches were used in the developed models. Simulation results with the modified model have shown that it is possible to explain all the above mentioned QT responses with mutual balance between the gain functions. The slope of the function describing dependency of a total neural sensitivity on the heart rate seems to be the most significant parameter to characterize the QT response: negative slope being a sign of prolonging QT sequences and vice versa.