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M. Koeny
The sympathetic nervous system increases vital functions of the body, like heart rate
and blood pressure during episodes of stress. The parasympathetic system brings
vital functions back to a normal level. Though most functions are regulated automatically, some functions can be influenced such as breathing. This fact has a significant
influence on pain or stress assessment, which is discussed later in detail.
In fact, the ANS is influenced by stress, where pain is one of the most important
stressors that influence the ANS. Different indices have been developed to measure
the level of stress and pain. The indices that can be computed from ECG and PPG
are introduced in the following section.
9.3 Fundamentals
Stress influences ANS, which in turn influences measurable parameter like heart rate
and blood pressure. Hence, most algorithms for pain and stress assessment are based
on evaluation of these parameters. Pain may be the most significant stressor for the
body, but other stressors too influence the ANS. In fact, the stress measurement leads
to pain measurement, when the influence of other stressors are reduced.
9.3.1 Heart Rate Variability
Heart rate is controlled by ANS and the variation in heart rate (heart rate variability) enables us to quantify the function of ANS [1]. Time domain analysis of
heart rate variability (HRV) is often used as a parameter to assess the health of
heart [2]. However, the frequency domain analysis is important for stress assessment. Frequency distribution is computed using Fourier transformation or wavelet
transformation.
Figure 9.1 shows a frequency spectrum with three significant peaks. The frequency
spectrum is divided into four main frequency bands (Table 9.1).
The first peak at 0.1 Hz (a) is caused by the baroreflex, the second peak (b) is caused
by respiratory sinus arrhythmia (RSA), and the third peak (c) is the first harmonic
of the RSA. The baroreflex is associated with internal blood pressure regulatory
mechanisms [1]. The RSA peak moves according to the breathing frequency and
amplitude. Further, breathing amplitude varies depending on the stress level of the
patient [3]. Therefore, the RSA forms the basis of computation of the analgesia
nociception index.
M. Koeny
The sympathetic nervous system increases vital functions of the body, like heart rate
and blood pressure during episodes of stress. The parasympathetic system brings
vital functions back to a normal level. Though most functions are regulated automatically, some functions can be influenced such as breathing. This fact has a significant
influence on pain or stress assessment, which is discussed later in detail.
In fact, the ANS is influenced by stress, where pain is one of the most important
stressors that influence the ANS. Different indices have been developed to measure
the level of stress and pain. The indices that can be computed from ECG and PPG
are introduced in the following section.
9.3 Fundamentals
Stress influences ANS, which in turn influences measurable parameter like heart rate
and blood pressure. Hence, most algorithms for pain and stress assessment are based
on evaluation of these parameters. Pain may be the most significant stressor for the
body, but other stressors too influence the ANS. In fact, the stress measurement leads
to pain measurement, when the influence of other stressors are reduced.
9.3.1 Heart Rate Variability
Heart rate is controlled by ANS and the variation in heart rate (heart rate variability) enables us to quantify the function of ANS [1]. Time domain analysis of
heart rate variability (HRV) is often used as a parameter to assess the health of
heart [2]. However, the frequency domain analysis is important for stress assessment. Frequency distribution is computed using Fourier transformation or wavelet
transformation.
Figure 9.1 shows a frequency spectrum with three significant peaks. The frequency
spectrum is divided into four main frequency bands (Table 9.1).
The first peak at 0.1 Hz (a) is caused by the baroreflex, the second peak (b) is caused
by respiratory sinus arrhythmia (RSA), and the third peak (c) is the first harmonic
of the RSA. The baroreflex is associated with internal blood pressure regulatory
mechanisms [1]. The RSA peak moves according to the breathing frequency and
amplitude. Further, breathing amplitude varies depending on the stress level of the
patient [3]. Therefore, the RSA forms the basis of computation of the analgesia
nociception index.