Hi when my 21 month old is asleep it sounds like her heart beats irregular from fast to slow and sounds like it misses a beat.

May be normal: It is normal for hr to slow down during sleep. Also normal for hr to varry between inspiration (faster hr) and expiration (slower hr), this is called sinus arrhythmia and is perfectly normal. This may be what you are hearing. If worried, however, best to check with the pediatrician. But, why are you listening to your 21m old's hr?If it is a healthy kid then no need to check the hr during sleep.

What prompted listen: Why were you listening? Was something else going on? Heart rate variability is common, & more pronounced, in children. It usually slows during sleep. Skipping beats can be due to premature contractions of upper (pacs) or lower (pvcs) heart chambers. They are also more common at slower rates and not infrequent in babies. Often or many in succession could make an evaluation necessary. Call your dr.