(Circulation. 1998;98:1587-1590.)
© 1998 American Heart Association, Inc.
Heart Rate Variability Standards
Cheng-Deng Kuo, MD, PhD;
; Gau-Yang Chen, MD, MS
Cardiopulmonary Laboratory,
Respiratory Therapy Department,
Veterans General Hospital-Taipei and Institute of Clinical
Medicine,
National Yang-Ming University School of Medicine,
Taipei, Taiwan,
Republic of China
To the Editor:
The special report on heart rate variability by the European
Society of Cardiology and North American Society of
Pacing and Electrophysiology1 presented
important standards of measurement for heart rate variability
analysis. In the report, the frequency range of the total power
was defined as 0.04 to 0.4 Hz, the low-frequency (LF) component as 0.04
to 0.15 Hz, and the high-frequency (HF) component as 0.15 to 0.4 Hz.
There are several problems in the calculation of specific spectral
powers using these standards of measurement in heart rate variability
analysis.
As stated in the report, the HF component is respiration related, and
the distribution of the power and the central frequency of LF and HF
components are not fixed but vary in relation to changes in autonomic
modulations of heart period in the short-term recordings.
Therefore, integrating the HF power within the all-frequency range of
0.15 to 0.4 Hz might have inherent error, especially when the
respiration rate does not fall within this range for patients who have
tachypnea or are under controlled respiration. Because the maximum
frequency in the spectrum is the Nyquist frequency (half the sampling
frequency),2 3 it might be better to use the
Nyquist frequency as the upper limit of both HF power and total
power.
When the direct current is excluded by baseline or trend removal in the
calculation of spectral powers, the nonharmonic components in the
very-low-frequency (VLF) region (<0.04 Hz) can be removed. In this
case, it is not necessary to set a cutoff limit (0.04 Hz in most
instances) for LF power or total power. In addition, the purpose of
normalization of the LF and HF powers by the total power is to minimize
the effect of the changes in total power on the values of LF and HF
components.1 The placement of a cutoff at 0.04 Hz
to the lower limit of total power will result in incomplete
normalization of the LF and HF components. Finally, if a lower limit
(0.04 Hz in most instances) was set to the LF power, the VLF power
(
0.04 Hz in most instances) must be dealt with for the sake of
completeness. However, the physiological
explanation of the VLF component is much less defined than other
components in the spectrum. The existence of a specific
physiological process attributable to these heart
period changes might even be questioned. The VLF is then a dubious
measure and should be avoided.1
In spectral analysis used in sciences such as physics or
chemistry, integration of the area under the peak rather than
fixed-range integration is usually suggested to evaluate the relative
contribution of a specific frequency to total
power.4 5 Thus, a peak-related integration
according to the respiration rate might be a better method of
representing HF power. Fixed-range integration is justified
only if no apparent peak can be identified in the HF range.
Because of the above considerations, it seems that the area of
spectral peaks within the entire range of 0 Hz to the upper limit of HF
peak or to the Nyquist frequency can be used as the total power, the
area of spectral peaks within the range of 0 to the lower limit of HF
peak as the LF power, and the area under the HF peak as the HF power.
If no apparent peak related to respiration can be identified, the total
power can be defined as the area of spectral peaks within the entire
range of 0 Hz to the Nyquist frequency, the LF power as the area of
spectral peaks within the range of 0 to 0.15 Hz, and the HF power as
the area of spectral peaks within the range of 0.15 Hz to the Nyquist
frequency.
References
1.
Task Force of the European Society of
Cardiology and the North American Society of Pacing and
Electrophysiology. Heart rate variability: standards of measurement,
physiological interpretation, and clinical use.
Circulation. 1996;93:1043–1065.[Free Full Text]
2.
DeBoer RW, Karemaker JM, Strackee J. Comparing
spectra of a series of point events particularly for heart rate
variability data. IEEE Trans Biomed Eng.
198431:384–387.
3.
Krauss TP, Shure L, Little JN. Signal
Processing Toolbox for Use With MATLAB. Natick,
Massachusetts: MathWorks Inc; 1992.
4.
Braithwaite A, Smith FJ.
Chromatographic Methods. 4th ed. New York,
NY: Chapman & Hall; 1985:329.
5.
Skoog DA. Principles of Instrumental
Analysis. 3rd ed. New York, NY: Saunders College
Publishing; 1985:749.
Response
Marek Malik, MD, PhD
Department of Cardiological Sciences,
St. George's Hospital Medical School,
London, UK on behalf of the ESC/NASPE Task Force
Drs Kuo and Chen suggest a substantial modification to the
standards for computing low-frequency and high-frequency spectral
components of heart rate variability. While I appreciate their
theoretical and technical reasoning, I am afraid that Drs Kuo and Chen
ignore many practical and physiological issues that
make their proposal inappropriate. Actually, the discussion about
proper definitions of spectral components of heart rate variability is
not new. Similar proposals as the one by Drs Kuo and Chen were made and
dismissed in the past.
There is very little experience with physiological
interpretation of heart rate variability components if the respiration
is forced, by a pathological process or by instruction, to an extreme
frequency that falls outside the limits of 0.15 to 0.4 Hz. Extending
the upper limit of the high-frequency component beyond 0.4 Hz would
only be applicable to extreme tachypnea of >24 respiratory cycles per
minute. This is linked to extreme sympathetic overdrive under which it
is rather difficult to interpret the high-frequency component.
Moreover, because the cardiac period signal is discrete rather than
continuous, it is difficult to properly estimate respiratory
arrhythmia under such conditions of very fast tachypnea.
Regular periodic bradypnea of <8 respiratory cycles per minute
may appear with forced metronome breathing. In such a case, the
recommendations made by the Task Force cannot be applied blindly. More
importantly, however, forcing a subject into an extremely slow
respiration rate is again sympathetically stimulating, which makes it
difficult to compare the heart rate variability components with those
obtained under different circumstances.
The very-low-frequency component seems indeed to be a dubious
measure because its physiological background is not
known. It is likely that even with short-term recordings, these
components reflect nonstationarity of heart rate modulations.
Consequently, extending the limits of low-frequency components below
0.04 Hz would pollute the measurement and make the
physiological interpretation even more difficult,
especially when the dominant spectral peak belongs to the
very-low-frequency component.
It is important to understand that the proposals of frequency
components made in our report are based on experience with existing
physiological models that allow interpretation of
individual components. This is quite different from making a proposal
based merely on hypothetical speculations.
Our report clearly suggested that if parametric methods are
used for the spectral analysis, the integration of the area
under distinct peaks should be used, whereas fixed-range integration
was proposed for a nonparametric spectral analysis,
which is, in practice, much more frequently used. Drs Kuo and Chen seem
to forget that even when parametric methods are used, total
power is not a simple sum of high-, low-, and very-low-frequency
components. Frequently, other components are present that cause the
sum of normalized high-frequency and normalized low-frequency
components not to be constant.
Finally, Drs Kuo and Chen forget that for practical reasons, the
definitions of high- and low-frequency components must not depend on
the duration of the analyzed RR-interval series. For instance,
if their proposal were applied to 24-hour recording, the
results would be completely meaningless.
