Abstract: A wearable medical system detects cardiac arrythmia condition of a patient. The wearable medical system comprises a support structure, a plurality of ECG electrodes to sense an ECG signal, and an energy output device to store an electrical charge. The wearable medical system further comprises an output circuit coupled to the energy output device, a plurality of therapy electrodes, and a processor. The plurality of therapy electrodes is coupled to the support structure and the output circuit and delivers therapy to the patient. The processor is coupled to the plurality of ECG electrodes and the output circuit. The processor comprises a QRS detector module that has a first threshold and a second threshold. The QRS detector module comprises first and second QRS detectors that analyze the ECG signals for the first and second thresholds, respectively. The first and second thresholds are configured to detect tachyarrhythmia and bradyarrhythmia, respectively.