BIOPAC® Systems, Inc. Logo

Unexpected HRV results

There are several possible issues. Most relate to the manner in which the power spectral density is computed. Before discussing those, a brief mention of data quality is in order. To compute heart rate variability, the first step is to locate heart beats. By default, the HRV analysis in AcqKnowledge uses an algorithm similar to that used for “Detect and Classify Heartbeats” and for “Locate Human ECG Complex Boundaries” to find the times of heart beats. While the algorithm is generally robust, it is not possible to determine via inspection exactly where beats are located when the software uses the “QRS detector”. It may thus be advantageous to use “Events” instead. See “How do I compute Heart Rate?” for information about placing event marks on an ECG trace. When using event marks, it is possible to ensure that heart beats are not missed, added, or erroneously placed before the HRV analysis is run. That is, it is possible to know exactly what the input is to the HRV computations. Some believe HRV analysis should only be applied to normal beats, so any ectopic beats should be identified and removed prior to HRV computation. This is possible when event marks are used to define the locations of heart beats.

Another issue of concern is the units. AcqKnowledge presents the amount of power in units of seconds2. Results are often instead expressed in milliseconds2. Note that there are one million square milliseconds in one square second. Because 1 second = 1000 milliseconds, (1 second)2 = (1000 milliseconds)2 = 1 million milliseconds2. Therefore, to convert the numbers AcqKnowledge produces into numbers of square milliseconds, the numbers must be multiplied by 106.

There is no universally agreed upon method for computing power spectral density (PSD). Generally the PSD is estimated by computing the Fourier spectrum of overlapping segments of data and then averaging these spectra to obtain the final result. The Fourier transform is typically computed from data that have been detrended and had their mean value removed. Older versions of AcqKnowledge detrend the entire waveform at once instead of detrending each segment independently of the others. Newer versions allow choice between these alternatives. Better estimates are probably obtained by detrending independently. This issue is described in more detail in Application Note 246.

That application note also performs comparisons between results obtained by the HRV analysis in AcqKnowledge and those obtained by other software packages. The ratios of powers computed across different parts of the spectrum agree across these different packages even though the absolute power levels do not. AcqKnowledge computes FFTs using the Cooley and Tukey algorithm applied to the raw time series data. No scaling is performed on the coefficients. In addition, this spectrum-based HRV analysis routine in AcqKnowledge allows you to transparently apply various windowing functions on the segments prior to FFT computation. Algorithms utilized by other software packages may use different (or no) windowing and may scale coefficients leading to disagreement over fine details of the spectrum and absolute power levels.

Page last modified 19Jan2015

Spotlight On
distance learning device

Practical Lab Kit for Remote Learning

BSL Home enables students to conduct hands-on physiology labs in the safety of their own home, dorm room, or any remote location while social distancing and stay-at-home orders keep them out of campus labs Complete lab solution for instructors and students to preserve practical lab experience! For instructors—BSL Home makes it easy for instructors to continue […]

View All
Latest News

BIOPAC Systems Helps STARK Industries Launch VITAL Ventilator for NASA JPL

BIOPAC’s ventilator testing equipment and expertise supports STARK Industries’ effort to manufacture, validate, and test NASA JPL’s VITAL Ventilators Goleta, California – June 16, 2020 BIOPAC Systems, Inc. announced a partnership with STARK Industries, LLC to provide ventilator validation and testing equipment for the mass production of ventilators. STARK Industries was awarded a global license […]

New Citations | Kick Off Summer with New Research in Exercise Physiology!

Comparing Muscle Movement and Involvement between Boxing Punches In the fast-paced, high-intensity combat sport of boxing, the right punch can win a fight. However, boxing is also a sport where opponents can become exhausted in mere minutes of action, and prioritizing their strength is key to winning a fight. This team recorded EMG on test […]

Read All
Request a Demonstration
Request a Demonstration