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Automatic mode switching, enabling the pacemaker to pace at an independent ventricular rate when atrial fibrillation occurs, was introduced to provide protection against rapid ventricular pacing during atrial arrhythmia. This study proposes a technique to test automatic mode switching performance. It is based on a programmable system (Arrhythmia Simulator) generating pulse trains that, when applied to the patient's skin, may interfere with the implanted device simulating supraventricular arrhythmias. The amplitude of the output signals is 5 V and they are delivered through an output resistance of 50 kΩ limiting the current to 100 μA that guarantees no risk of muscle stimulation during the test and meanwhile low noise signals on the surface ECG. The duration of pulses is 20 ms. Pulses delivered by the Arrhythmia Simulator were correctly sensed by the pacemaker's atrial channel, and the detected amplitude was 2 ± 0.8 mV, ranging from 1 to 3 mV. The performance of the system was reliable and safe in every patient despite the use of different pacemaker leads. Automatic mode switching was induced in every patient at every attempt. The pulses delivered by Arrhythmia Simulator didn't alter the quality of ECG tracings making easy and reliable the analysis.