Frequently asked questions
concerning Heart Rate Variability and the ChronoCord:
The following answers to questions of users have been collected by our work group in 23 years of actively measuring heart rate variability.
Q: Is there any advantage of sampling an ECG with 8 000 samples per sec, if an ordinary Holter monitoring device gets along with 200 or 600 s/s?
Ordinary Holter ECG monitoring is used to assess and cathegorize the shape of the ECG wave. A heart rate variability monitor has to be optimized to measure the distances between consecutive R peaks of the ECG. These distances should be measured as accurate as possible as they are the relevant information to compute heart rate variability (HRV). Failing to do so inevitably results in loss of valuable information about the subjects HRV. There are several reasons for this loss: temporal and amplitude quantization error due to slow sampling and weak A/D circuits, electrical noise due to unshielded cables and poor preamplifier concepts, noise from movements inducing currents due to piezoelectric-properties of the cables, just to mention a few. During further processing, a disturbing “roughness” in the data and artificially generated HRV components not coming from the subjects heart can be observed. Expanding the distance between two heartbeats to one meter, 100 samples/sec correspond to a measuring accuracy no better than 1 cm, 1000 samples/sec, no better than one mm, 8000 samples/sec means that each heartbeat is determined exactly to 1/8th of a mm. To observe changes e.g. through interventions, the accuracy should be at least this good. Especially for scientific research there is no doubt you should use the best equipment available!
Q: The ECG signal is usually filtered in the recording device so that only information up to a maximum of 40 Hertz is contained in the signal. What sense does it make therefore, to sample more accurate than up to 256 Samples/sec?
The sampling theorem, stating that the sampling frequency has to be faster than twice the highest frequency in the signal is valid only for strictly periodic signals, for which is has been developed originally. This is not the case in the ECG R-peak, which resembles a Dirac impulse containing high frequencies, even if some of the high frequency content is filtered. We have found that the measured heart rate variability for the same subject decreases with increasing sampling frequency. Since the total variability consists of a physiological variability and a sampling error and the physiological variability is independent of the sampling rate, this is an indicator for a smaller sampling error at higher sampling rates. According to our investigations, a clearly measurable difference in the sampling error was even between a sampling rate of 4000 Hertz and one of 8 000 hertz. Research of our institute has additionally shown that a more precise definition of the R peak timing is possible through the information contained in the R-wave, if a high sampling rate (8000 samples/sec) as well as high resolution (16 bit) is chosen.
Q: There are now plenty of pulse watches of varying quality. Why is not possible to use such a pulse belt monitor to measure heart rate variability?
Our research group has tested pulse watches (claiming eg. ECG accuracy) and the accompanying belts and found an accuracy which corresponds to a sampling rate of 200 Hertz, 12 Bit. This is far below that of the ChronoCord by a factor of 40 concerning sampling rate and 12 concerning bit resolution. A recent paper of Wallen et. Al. (Europ.J.Appl Physiology, 2012,112:1153-1165,) concluded from a 800 Polar™ watch study: For a scientific determination of HRV, or any procedure which requires more complex analysis of the frequency, pulse watches are not suitable due to their beat to beat inaccuracy.
Q: Are 24 hour recordings with pulse belts and watches reliable and useful?
Especially if someone sleeps in lateral position, the belt electrode can be lifted from the body due the belt’s stiffness. This makes a continuous recording of heart beat intervals impossible. The accuracy of the clock also is not sufficient for an exact calculation of the HRV parameters as explained in the previous question.
Q: Why do you use electrodes for the measurement of the HRV? Wouldn’t a pulse belt be more convenient?
For trouble-free determination of heart beat intervals a low and uniform impedance between skin and electrodes is of crucial importance. According to our experience this currently can be achieved only using the best available adhesive electrodes. Any relative movements between electrode and skin raises voltage fluctuations, which might be interpreted as artifacts or extrasystolic ECG spikes by the recorder. These relative movements are much lower in flexible adhesive electrodes compared to the relatively rigid pulse belts. A sufficient pressure must be applied to record a reasonably usable ECG from a pulse belt. Additionally at night, when external influences are smaller, the belt is particularly unpleasant for the user. Subjects who alternately used belts and electrodes found the belt pressure uncomfortable. Therefore, the belts are ok for averaged heart rate determination during exercises, but unreliable to measure circadian heart rate variability.
Q: Why does the ChronoCord use expensive and elaborate ECG cables?-there are also quite cheap cables on the market.
According to our experience in numerous projects with medium-quality cables, the cable has to meet the following requirements:
1 Mechanical strength even at low temperatures and tolerance to strong bending and tension of the cable.
2 Shielding of the inside to exclude electrical hum.
3 Mechanical movement of the cable should not induce electric currents.
4 Small dimensions and high flexibility to prevent a shearing of the electrodes by mechanical forces and warrant a comfortable feeling during work and during the night as well.
The quality of available standard cables in the terms mentioned above is in no way according to the requirements. A specially designed cable is essential if one wants to avoid fast cable rupture and electronic noise. The price of the cable is according to the high quality materials used (sturdy Kevlar soul anchored at the base, cold bending resistant, flexible insulation materials, high-quality materials with neglectable piezo-electric properties).
Q: Why do we need so elaborate technique in such a small device?
In 1991, a device developed by our research group was sent to Space Station MIR for the first time, optimized for the recording of sequences of long term heart rate variability. It has been used for more than 10 years by Russian cosmonauts to determine their health status. Whereas Holter ECG devices on the market are designed for optimal recording of the shape of the ECG, our device always aimed for the best available recording of Heart Rate Variability. Meanwhile, our device is in its 7th generation so that we can present an unprecedented technical standard on precision, application experience, miniaturization and handling.