1. A system for monitoring a person's heart rate comprising: a sensor disposable over said person's heart and configured to detect QRS segments from an electrocardiogram (ECG) signal corresponding to said person's heart rate using an amplitude threshold and a search window, wherein said sensor is configured to automatically adjust said amplitude threshold based on previous QRS segment measurements and transmit a wireless signal to a monitor, said wireless signal comprising information about said detected QRS segments and corresponding heart rate; said sensor further configured to automatically adjust the temporal location and width of said search window by: set the width of said search window to a maximum value; detect a predetermined number of QRS segments using said search window; store information about said detected predetermined number of QRS segments in memory; automatically adjust the width of said search window based on said stored information; and detect subsequent QRS segments using said adjusted search window; said monitor configured to receive said wireless signal from said sensor, process said signal from said sensor, and produce an alert indication responsive to said person's heart rate falling below a predetermined level based on said information in said wireless signal.