Typically, if you have no history of heart disease, if there is no history of cardiac arrest in your family, and the episodes of palpitations are brief, the palpitations are not a life-threatening condition. If, however, you have some history of a heart problem (like a heart attack), or if someone in your family has had heart issues, or if the palpitations are continuous for minutes to hours, then further evaluation would be recommended.
ECGs done in the office can often be normal because they just happen to miss the palpitations. There are many different types of ECG monitors you can use at home to try to capture these episodes. If the episodes last long enough, there are portable hand-held monitors that you can carry around with you and quickly record your rhythm when it happens. If the episodes are too brief and are gone before you have a chance to pull out your portable monitor, you can wear a continuous monitor for 24 hours, and up to 30 days if necessary. If the palpitations are bothersome enough, you can explore these methods of capturing them so your physician can evaluate them in depth.