The episode starts with Sherlock trying to imitate death and we here a flat-line-sound from the heart-monitor. A common mistake in various movies and TV-series. A cardiac arrest that is reversible will not produce a flat-line but instead a very spiky heart-rate consistent with ventricular fibrillation, indicating there is still electrical activity in the heart which, with the right defibrillation or chest-compressions (cpr), can result in a resumed normal heart-rhythm. In case of a flat-line, the heart is void of electricity and the person is irreversibly dead.