As a medical professional with expertise in cardiology, I can explain the concept of the R and R intervals in an ECG (electrocardiogram) reading. The R interval refers to the time between two successive R waves, which are the first and most prominent deflections in a normal QRS complex on an ECG. The R wave represents the depolarization of the ventricles, which is the electrical signal that causes the heart muscle to contract and pump blood. The R-R interval, on the other hand, is the time between two successive R-R intervals. It is essentially the same as the R interval but is often used to describe the regularity of the heartbeat. The R-R interval is a measure of the heart rate; a shorter R-R interval indicates a higher heart rate, and a longer one indicates a lower heart rate. In a normal ECG, the R-R interval is consistent, reflecting a regular heartbeat. Variations in the R-R interval can indicate various heart conditions, such as arrhythmias. To summarize: - R interval: Time between two R waves. - R-R interval: Time between two successive R-R intervals, reflecting the heart rate. read more >>

Other terms used include: "cycle length variability", "RR variability" (where R is a point corresponding to the peak of the QRS complex of the ECG wave; and RR is the interval between successive Rs), and "heart period variability".read more >>