Most deterministic functions are nonperiodic. It is sometimes worthwhile to consider an “almost
periodic” type of signal. The ECG signal can sometimes be considered “almost periodic.” The ECG’s RR
interval is never constant; in addition, the PQRST complex of one heartbeat is never exactly the same as
that of another beat. The signal is definitely nonperiodic. Under certain conditions, however, the RR
interval is almost constant, and one PQRST is almost the same as the other. The ECG may thus sometimes
be modeled as “almost periodic.”