Galvanic Skin Response (GSR)

The Galvanic Skin Response (GSR) is defined as a change in the electrical properties of the skin. The signal can be used for capturing the autonomic nerve responses as a parameter of the sweat gland function. The measurement is relatively simple, and has a good repeatability. Therefore the GSR measurement can be considered to be a simple and useful tool for examination of the autonomous nervous system function, and especially the peripheral sympathetic system.

Several terms are used for this phenomena, such as EDA (electrodermal activity), EDR (electrodermal response), EDL (electrodermal level), SCA (skin conductance activity), SCR (skin conductance response), and a lot more.

Out of the number of terms used for this phenomenon, it is clear that GSR has more than one property. It can be described in terms of conductance, resistance and electro¬physiological potential. The electro-physiological signal is generated by the sweat glands, and the sweat is probably the origin of the variation in resistance and conductivity, although the vaso-dilatation and -constriction may also play an important role.

In most cases, the GSR is measured using a part of the skin having a lot of sweat glands. As a reference you can use a part of the skin with less or no sweat glands, or measure in the same area as the active electrode.

Heart rate variability (HRV)

The phenomenon that is the focus of this report is the oscillation in the interval between consecutive heart beats as well as the oscillations between consecutive instantaneous heart rates. �Heart Rate Variability’ has become the conventionally accepted term to describe variations of both instantaneous heart rate and RR intervals. In order to describe oscillation in consecutive cardiac cycles, other terms have been used in the literature, for example cycle length variability, heart period variability, RR variability and RR interval tachogram, and they more appropriately emphasize the fact that it is the interval between consecutive beats that is being analysed rather than the heart rate per sec.

During the 1970s, Ewing et al. devised a number of simple bedside tests of short-term RR diVerences to detect autonomic neuropathy in diabetic patients. The association of higher risk of post-infarction mortality with reduced HRV was first shown by Wolf et al. In 1977. In 1981, Akselrod et al. introduced power spectral analysis of heart rate fluctuations to quantitatively evaluate beat-to-beat cardiovascular control.

These frequency–domain analyses contributed to the understanding of the autonomic background of RR interval fluctuations in the heart rate record. The clinical importance of HRV became apparent in the late 1980s when it was confirmed that HRV was a strong and independent predictor of mortality following an acute myocardial infarction. With the availability of new, digital, high frequency, 24-h multi-channel electrocardiographic recorders, HRV has the potential to provide additional valuable insight into physiological and pathological conditions and to enhance risk stratification.

Variations in heart rate may be evaluated by a number of methods. Perhaps the simplest to perform are the time domain measures. With these methods either the heart rate at any point in time or the intervals between successive normal complexes are determined. In a continuous electrocardiographic (ECG) record, each QRS complex is detected, and the so-called normal-to-normal (NN) intervals (that is all intervals between adjacent QRS complexes resulting from sinus node depolarizations), or the instantaneous heart rate is determined. Simple time–domain variables that can be calculated include the mean NN interval, the mean heart rate, the diVerence between the longest and shortest NN interval, the diVerence between night and day heart rate, etc. Other time–domain measurements that can be used are variations in instantaneous heart rate secondary to respiration, tilt, Valsalva manoeuvre, or secondary to phenylephrine infusion. These diVerences can be described as either diVerences in heart rate or cycle length.

The autonomic nervous system response to athletic training and rehabilitative exercise programmes after various disease states is thought to be a conditioning phenomenon. HRV data should be useful in understanding the chronological aspects of training and the time to optimal conditioning as it relates to the autonomic influences on the heart.

Pulse waveform analysis

Analysis of the contour of the peripheral pulse to assess arterial properties was first described in the nineteenth century. With the recognition of the importance of arterial stiffness there has been a resurgence of interest in pulse wave analysis, particularly the analysis of the radial pressure pulse acquired using a tonometer. An alternative technique utilizes a volume pulse. This may conveniently be acquired optically from a finger (digital volume pulse). Although less widely used, this technique deserves further consideration because of its simplicity and ease of use.

As with the pressure pulse, the contour of the digital volume pulse is sensitive to changes in arterial tone induced by vasoactive drugs and is influenced by ageing and large artery stiffness. Measurements taken directly from the digital volume pulse or from its second derivative can be used to assess these properties. The arterial pulse waveform is a contour wave generated by the heart when it contracts, and it travels along the arterial walls of the arterial tree. Generally, there are 2 main components of this wave: forward moving wave and a reflected wave.

The forward wave is generated when the heart (ventricles) contracts during systole. This wave travels down the large aorta from the heart and gets reflected at the bifurcation or the across-road of the aorta into 2 iliac vessels. In a normal healthy person, the reflected wave usually returns in the diastolic phase, after the closure of the aorta valves. The returned wave give a notch and it also helps in the perfusion of the heart through the coronary vessels as it pushes the blood through the coronaries.

Therefore the velocity at which the reflected returns becomes very important : the stiffer the arteries are, the faster it returns. This may then enter into the systolic phase and augment final blood pressure reading.

Bio impedance and Body composition

Bioelectrical impedance analysis (BIA) is a widely used technique for estimating body composition and it is particularly useful in large, population-based studies because it is quick, portable, inexpensive and noninvasive.  Using the resistance and reactance measurements of the human body in tetrapolar mode and in multi frequency from 1 to 500 KHz, the appropriate algorithms issue from the peer reviews will allow the estimation of the body composition.

In the human body, low resistance is associated with large amounts of fat-free mass and low fat mass . High resistance is associated with smaller amounts of fat-free mass and high fat mass

In the human body, high reactance is associated with large amounts of body cell mass (intracellular mass) and low ECW (Extra Cellular Water) . Low reactance is associated with smaller amounts of body cell mass and high ECW.