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
    1. AVNN
      1. Average interval between normal heart beats in milliseconds (ms)
      2. Equivalent to heart rate of normal-to-normal beats. (HR=60,000/AVNN)
    2. SDNN
      1. Standard deviation of N-N intervals in ms (total HRV)
      2. Reflects total HRV for period of interest
    3. SDANN
      1. Standard deviation of 5- min mean values of N-Ns for each 5 minute interval in ms (circadian HRV)
      2. Reflects longer-term trends HRV and would not be meaningful in a short recording
    4. SDNNIDX
  2. Short-term measures calculated from differences between intervals of successive normal heart beats.
    1. rMSSD
      1. Root mean square of successive differences of normal-to-normal intervals in ms or the average change in interval between any two normal beats
      2. When rhythm is normal reflects parasympathetic control of heart rate
      3. Exaggerated by abnormal, highly irregular heart rate patterns
    2. pNN50
      1. Percent of successive differences of intervals between normal heart beats >50 ms
      2. When rhythm is normal, reflects parasympathetic control of heart rate
      3. Exaggerated by abnormal, highly irregular heart rate patterns