Frequency Domain HRV

  • Can be obtained from long-term (e.g. 24-hour) or short-term recordings
  • Calculated using autoregressive techniques or fast Fourier transforms (FFTs)
  • Requires at least 80% of beats in analyzed segments to be present and normal
  • Partitions the total variance in heart rate into underlying rhythms that occur at different frequencies
  • Frequencies can be associated with different intrinsic, autonomically-modulated periodic rhythms

Standard frequency domain HRV

Total power (TP)

  • Reflects total variance in heart rate pattern over length of recording
  • In long-term recordings, influenced predominantly by circadian changes
  • Although measured from entire recording period, sometimes reported as average of every 5 min

Ultra low frequency power band (ULF)

  • At >every 5 min to once in 24 hours
  • Reflects circadian, neuroendocrine, activity, other unknown rhythms
  • Not meaningful in short-term recordings

Very low frequency band (VLF)

  • At ~every 20 sec-every 5 min underlying frequency
  • Reflects vasomotor changes, thermoregulatory, possibly parasympathetic influences on heart rate
  • Exaggerated by sleep-disordered breathing
  • Can be estimated from 10-15 min recordings

Low frequency band (LF)

  • At Mayer wave frequencies ~every 8-Â 10 sec frequency
  • Reflects baroreceptor-mediated sympathetic and parasympathetic influences on heart rate
  • Usually measured for every 5-min and then averaged
  • Can be estimated from a minimum of one 2-min recording

High frequency band (HF)

  • At respiratory frequencies (usually 9-24 cycles/minute)
  • When rhythm is normal reflects parasympathetic influences on heart rate
  • Exaggerated by abnormal heart rate patterns
  • Usually measured every 5-min and then averaged
  • Very slow breathing exaggerates HF because includes both parasympathetic and sympathetic inputs

Ratio measures of heart rate variability

  • Proposed as measures of “sympathovagal balance”
  • Reliable under very limited circumstances
  • Very much affected by abnormal heart rate patterns

Normalized low frequency power (NLF)

  • Proportion of total HRV that occurs in the low frequency band
  • Usually calculated over 5 min and then averaged

Normalized high frequency power (NHF)

  • Proportion of total HRV that occurs in the high frequency band
  • Usually calculated over each 5 min and then averaged

Low-to-high frequency power ratio (LF/HF ratio)

  • Ratio of low to high frequency power calculated over each 5 min and then averaged
  • Most frequently report and misinterpreted ratio measure of HRV
  • Can be obtained over a minimum of 2 mins