Apple Watch - Heart rate and Health App

In Health app, i can see low heart rates are recorded frequently and the "Heart Rate context" was mentioned as "ECG". I see this low hear rate behavior only when the context was "ECG". This happens only when open my ECG app is opened and keep running in the background mode. Is ECG app designed to calculate the heart rate in the background?. Whenever i close the ECG app forcefully, this is not happening and i don't see any low heart rate recorded. Kindly suggest.