Using an ECG-based heart rate monitor typically increases the quality of detecting your heartbeats (compared to measuring them with the iPhone camera). We support the following (Bluetooth) models: Polar H7 and H10 Garmin HRM-Dual and HRM-Pro Wahoo TICKR Series(TICKR and TICKRx but not TICKRfit) – these models have limited accuracy in synchronization with the breath signal.