## 3 Results
### Physiological Assessment of Mental Workload
#### 3.6.2 Cardiac Measures
With long-period rhythms, the physiological significance of the VLF-related features has been determined [62]. It has been demonstrated that the LF/HF ratio reflects the balance between the parasympathetic nervous system and the sympathetic nervous system, also known as the \"fight or flight\" system. Because of this, when determining how sympathetic the response is in relation to parasympathetic, the LF/HF ratio is frequently used. Moreover, frequency-domain HRV data have been employed owing to their documented association with mental workload, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Features** & **Description** & **Employed in** \\ \hline NN & \begin{tabular}{c} Normal-to-normal interval. Also called the R-R interval or the \\ interbeat interval (IBI). Measures the time between QRS peaks. \\ \end{tabular} & [8] \\ \hline SDNN & \begin{tabular}{c} The standard deviation of all NN intervals. \\ \end{tabular} & [43] \\ \hline SDANN & \begin{tabular}{c} The standard deviation of the averages of NN intervals in all 5 min \\ segments of the entire recording. \\ \end{tabular} & \(-\) \\ \hline RMSSD & \
\begin{tabular}{c} The square root of the mean of the sum of the squares of the \\ differences between adjacent NN intervals. \\ \end{tabular} & [4,8,43] \\ \hline pNN50 & Proportion of differences in consecutive NN intervals that are longer \\ \multicolumn{2}{c}{than 50 ms.} & \(-\) \\ \hline HRVTi &
\begin{tabular}{c} The sum of all R-R intervals divided by the maximum density \\ distribution. \\ \end{tabular} & \(-\) \\ \hline \hline \end{tabular} Note. – not employed in any of the reviewed studies.  
\end{table} 
Table 3: Common HRV features in the time domain. characterized notably by a decrease in parasympathetic activity or an increase in LF/HF ratio [34]. Lastly, the unpredictability of a time series can be measured using nonlinear features, as the time-domain and frequency-domain characteristics of HRV signals fall short of fully capturing the nonlinear aspects of HRV signals. Some authors are currently using nonlinear analysis methods to analyze HRV signals due to mental workload, including sample entropy (SaEn) and detrended fluctuation analysis (DFA) [62]. Blood pressure measurements, such as systolic, diastolic, and mean arterial pressure, were frequently employed to measure MWL in addition to ECG measurements [34; 48]. Blood pressure (BP) is a measurement of the force that blood flowing through the body exerts on the blood vessel walls. The pressure that is applied as the heart muscle contracts (the systole) and relaxes (the diastole) is the standard way that blood pressure is expressed. Usually, an increase in systolic BP with workload and stress is observed. Peripheral oxygen saturation (SPO2) is a measure that indicates the percentage of hemoglobin in the blood that is saturated with oxygen. Hemoglobin is a protein found in red blood cells that transports oxygen from the lungs to the body's tissues. SPO2 is measured non-invasively, typically using a device called a pulse oximeter, which is placed on the finger, earlobe, or other parts of the body. SPO2 was used by [34] to discriminate different cognitive tasks. Regarding its limitations, cardiac measurements rely on wearable devices, such as chest straps and smartwatches, for the collection of physiological data. While these devices are non-invasive, their accuracy is contingent upon the quality of the equipment and its application. Additionally, cardiac measures are highly susceptible to external factors, including workplace conditions such as noise and temperature, as well as the operator's emotional state, which may introduce inaccuracies--a challenge inherent to other physiological signals. Moreover, depending on the intervals used for data collection, the system may fail to capture rapid fluctuations in the operator's mental state, thereby limiting its ability to promptly detect and respond to sudden variations in workload levels.