**2.4 Supervisory control of pacing rate limits**

For the benefit of the patient, it is reasonable not to rigidly fix the upper and lower pacing rate limits, but to leave them sliding within certain limits depending on the patient's current medical condition. At the same time, however, both underpacing and overpacing must be strictly avoided. Both are dangerous for life, especially overpacing that can cause myocardial infarction, and must be strictly avoided [13, 14]. The principle is that the body demand for oxygen-rich blood must not exceed the ability of patient's injured heart. Energy balance between the energy supply and energy consumption of myocardium must be fulfilled in every moment of heart work. The balance conditions and overpacing risk were derived from the measurement of myocardial impedance ZM [12]. Underpacing risk was derived from the bioimpedance ZV measurement of ventricular volume—too large volumes indicate underpacing danger for the myocardium [12, 13].
