Time Domain Heart Rate Variability

  • Time Domain Heart Rate Variability refers to numbers that can be obtained from statistical analysis of the intervals between heart beats.
  • They describe how much HRV there is on various scales.
  • These can be measured from longer-term (e.g. 24-hour) recordings or from short-term recordings of a few minutes.

Definitions for time domain heart rate variability

1. Measures calculated from the intervals between normal heart beats

AVNN

  • Average interval between normal heart beats in milliseconds (ms)
  • Equivalent to heart rate of normal-to-normal beats. (HR=60,000/AVNN)

SDNN

  • Standard deviation of N-N intervals in ms (total HRV)
  • Reflects total HRV for period of interest

SDANN

  • Standard deviation of 5- min mean values of N-Ns for each 5 minute interval in ms (circadian HRV)
  • Reflects longer-term trends HRV and would not be meaningful in a short recording

SDNNIDX

  • Average of standard deviations of N-Ns for each 5 min interval in ms (combined sympathetic and parasympathetic HRV)
  • Reflects intermediate-term HRV in a longer recording

2. Short-term measures calculated from differences between intervals of successive normal heart beats.

rMSSD

  • Root mean square of successive differences of normal-to-normal intervals in ms or the average change in interval between any two normal beats
  • When rhythm is normal reflects parasympathetic control of heart rate
  • Exaggerated by abnormal, highly irregular heart rate patterns

pNN50

  • Percent of successive differences of intervals between normal heart beats >50 ms
  • When rhythm is normal, reflects parasympathetic control of heart rate
  • Exaggerated by abnormal, highly irregular heart rate patterns