Sensing is used to inhibiting or triggering pacing pulses.

T-waves reflect repolarization and should not be sensed by the pacemaker. Depolarizations are represented by the P-wave (atrial lead) and QRS complex (ventricular lead). Specifically, the pacemakers sense intrinsic depolarizations. The pacemaker can record intrinsic cardiac activity and response appropriately. Dual-chamber systems offer the possibility of sensing and pacing in both the atria and ventricles. Nowadays, most implanted pacemakers are dual-chamber systems, meaning that two leads are used: one in the atrium and one in the ventricle. Such systems are referred to as single-chamber systems. The simplest pacemaker systems consist of a pulse generator and one lead, which is located either in the right atrium or the right ventricle. The functions of a pacemaker depend on the software, hardware, and programming. For example, there are functions that continuously reassess stimulation threshold in order to calibrate stimulations according to the excitability of the myocardium. These settings include the base rate of the pacemaker (the lowest heart rate allowed, which triggers the pacemaker), the behavior of the pacemaker at low and high heart rate, the behavior of the pacemaker in the presence and absence of intrinsic cardiac activity, Modern pacemakers are packed with algorithms that optimize the function. A wide range of settings can be adjusted. Programming is done through an external device that communicates wirelessly with the pacemaker. The pacemaker software includes pre-programmed algorithms and settings, which can be tailored to the patient’s needs. The higher the resistance in the lead tip, the smaller the current used. Given that voltage (V) is constant and battery depletion must be minimized, most pacemakers use lead tips with high resistance (400 to 1200 Ω). The current that is delivered by the pulse generator can be calculated as follows: Pacemakers generate a constant voltage (V). The relationship between current (I), voltage (V) and resistance (R) is described by Ohm’s law:

Amplitudes are normally below 1.5 V and pulse duration (width) is usually set to 0.5 ms.

The amplitude and duration of the pulse must be optimized to ensure depolarization while minimizing battery consumption. The intensity of the electrical stimulus is described by its amplitude (measured in volts) and duration (measured in milliseconds). Stimulation threshold is the minimum amount of energy required to reach threshold and evoke an action potential. As previously discussed, myocardium may be excited by external electrical stimuli that drive the cells to threshold.

Speed setting on winamp pacemaker