The heart rate is typically expressed in the number of beats per minute at which a heart is beating. While this may sound like a simple matter, there are various definitions of heart rate that differ slightly but significantly. The various terms used include: average heart rate, instantaneous heart rate, true heart rate.
When a doctor measures heart rate by looking at a watch and counting the beats during a 15-, 30- or 60-second interval, an average heart rate is obtained. Since only whole beats are counted in this method, the method is not very accurate, and the rate obtained does not correspond to the true heart rate. The error is larger with shorter counting intervals.
The true heart rate is the most accurate indication of heart rate. It is obtained by counting both whole and fractional beats during a fixed time interval. Since the counting period is usually not determined by the heartbeats themselves, more than one whole beat is typically counted, and this true heart rate is therefore also an average rate.
The instantaneous heart rate is determined by measuring the inter-beat-interval between two successive heart rates. This rate is based on a single interval. The rate is still converted to beats per minute, and such instantaneous heart rates can therefore differ significantly between successive beats (intervals). The instantaneous rate can also be converted to an average value over several successive beats. Such an average heart rate is as accurate as the true heart rate, since only whole beats are counted, but the counting period is adjusted to exclude the possibility of partial beats.
Chabot, D., Bayer, M., and De Roos, A. 1991. "Instantaneous heart rates and other techniques introducing errors in the calculation of heart rate." Canadian Journal of Zoology 69: 1117-1120.