We do not recommend the use of TENS if you have a heart condition for the following reasons:
1) If electrodes are placed on the front of the neck they can stimulate the nerve which controls your heart rate. Your heart could try to beat at 100 times a second instead of 100 times a minute. If you have an unstable heart condition this could be extremely dangerous.
2) If you place the electrodes across the chest, the maximum output is much lower than the theoretical level that could cause an additional heartbeat. However, in very exceptional circumstances an unstable heart condition might be aggravated.
Before purchasing and using any TENS device, we recommend that you seek advice from your medical practitioner.