ECG analysis

The ECG signal was processed using the free software Kubios HRV . Kubios software was used to visually inspect the raw ECG recordings and clean artefacts using the provided filtering functions. Identifying the instant of each heart beat on the time line, allows to obtain two continuous functions (one the inverse of the other) in the domain of time:

 The heart rate function, which reports the number of beats per minute at any instant in time;

 The Inter beat Interval (IBI) function, which reports the period (in milliseconds) recorded between each two consecutive heart beats, at any instant in time.

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Heart Rate

A continuous function reporting the heart rate (as beats per minute) in the domain of time was obtained for each manoeuvre performed in the simulator. Each function was divided into 5 sections or phases, as depicted in Table 8. Phase 0 and 5 were recorded before and after the execution of the manoeuvres, phases 1 to 3 were recorded during the execution of the manoeuvres.

Heart Rate Variability

The IBI signal was the starting point for the Heart Rate Variability (HRV) analysis. As described in section 2.3.3, HRV is known as a non invasive technique to measure cardiovascular autonomic regulation (Hansson & Jönsson, 2006). It expresses the balance of the regulation of the sympathetic and parasympathetic nervous systems. HRV has been extensively exploited to study the association between psychological processes and physiological reactions (Berntson, 1997). The LF/HF ratio is an important parameter in the study of the power spectral density (PSD) of the inter beat intervals function (IBI). The IBI function has to be transformed from the domain of time into the domain of frequency using a Fourier Transform. Low Frequencies (LF – from 0.04 to 0.15 Hertz) and in the High Frequencies (HF – from 0.15 to 0.40 Hertz) are the adopted in the calculation of the LF/HF index (Malik et al., 1996). LF power component is connected with the sympathetic activities of the nervous system while the HF power component is more connected with the parasympathetic activities (Lord et al., 2001). Elevated values in the LF are associated with high stress (Van Amelsvoort, Schouten, Maan, Swenne, & Kok, 2000), resulting in higher scores in the LF/HF ratio. The strong correlation between Heart Rate Variability and Stress has been extensively documented in the literature (Thayer, Åhs, Fredrikson, Sollers Iii, & Wager, 2012). For the purposes of this paper, “stress” is defined as the transition from a calm state into an excited state, through the activation of the sympathetic system (Selye, 1980), considering as “stressors”, excessive intellectual, emotional and perceptual stimuli (Skinner & Simpson, 2002). The measurements of LF/HF ratio and the Heart Rate were specifically chosen, due to their sensitivity to work related stressors (Ritvanen, Louhevaara, Helin, Väisänen, & Hänninen, 2006; Van Amelsvoort et al., 2000).

Figure 14. ECG data processing to obtain LF/HF index

Figure 14 reports the process through which it is possible to obtain the LF/HF index. In the top left of the figure there is a representation of the IBI function in the domain of time (expressed in milliseconds), as obtained from the ECG signal. Initially a specific time interval included between the instants t0 and t1. can be considered. Using different

mathematical techniques or algorithms, such as the Fast Fourier Transform, it is possible to obtain the power spectral density (PSD) function of the IBI signal, as recorded in the chosen interval (t0 – t1). The PSD function is a function in the domain of frequency and

highlights the spectral components of the original IBI signal captured in the domain of time within the interval (t0 – t1). Obtaining the amount of “power” of the original signal

in the desired bands, equals to calculate the integral of PSD function in those band intervals (expressed in Hz).

The index LF/HF at the instant t1 is obtained as the ratio between the scalar obtained

calculating the integral function of the PSD in the low frequencies (LF band – from 0.04 to 0.15 Hertz) over the scalar obtained calculating the integral function of the PSD in the high frequencies (HF band – from 0.15 to 0.40 Hertz). Choosing sequential intervals (also partially overlapping) of the IBI function, the LF/HF function can be plotted at different instants in time (t1, t3… tn). In other words, to obtain a continuous function reporting the

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interval (t0 – t1) on sequential intervals (t2 – t3, t4 – t5, t6 – t7, …, tn-1 - tn), covering the

whole duration of the chosen IBI function.

In this research, the spectral analysis (in the domain of frequency) was obtained using the algorithms provided in the open source software HRVAS (Ramshur, 2010). HRVAS software was a graphical interface developed to implement functions for ECG analysis, built in Matlab scripting code (MathWorks, 2013). Those Matlab functions were extracted and re-integrated in newly developed scripts able to batch process all the ECG recordings collected during the manoeuvres.

An ectopic detection algorithm was used at the beginning of the analysis to exclude those values in the IBI functions that were exceeding 20% of previous sample value, or exceeding 3 times the standard deviation of the signal, or using a median filter where the tau value was set to 4. Those values were substituted using a “spline” interpolation between adjacent values. The cleaned IBI signals were de-trended, meaning that their average was reduced to 0. The spectral analysis of a signal, in fact, focuses on the variation of that signal in time, not on its absolute values.

For the spectral analysis (to calculate the LF/HF values) it was chosen a shifting window of 128 seconds. To obtain the continuous LF/HF function in time, all the calculations were reiterated shifting the 128 seconds window, of 1 second at a time. This process implied that 2 consecutive windows would have overlapped for 127 seconds. The spectral analysis of the 128 seconds IBI signal intervals was obtained using the using Welch's algorithm (Welch, 1967).

Reiterating the spectral analysis for each sequential 128 seconds interval, allowed to obtain, for each IBI recording, the LF/HF function in the domain of time. Similarly to what obtained with the heart rate functions, each LF/HF function could be divided into 5 sections or phases, as reported in Table 8. Phase 0 and 5 were recorded before and after the execution of the manoeuvres. Phases 1 to 3 were recorded during the execution of the manoeuvres.