Fig 1.
Heart Rate and Blood Pressure.
The central mark within each box is the median, the edges of the box are the 25th and 75th percentiles, the whiskers are the standard deviations of the data at each stage and the outliers are plotted in red ‘+’. (a): Box plot depicting the interpersonal variability of HR in each stage across all subject; (b): Box plot of overall mean and standard deviation of arterial BP across each stage of the LBNP experiment.
Fig 2.
Outline of the various discrete steps involved in the designed methodology.
Fig 3.
X axis is the time in seconds (example shown is 60 seconds), Y axis is epsilon width from the origin for piecewise linear reconstruction. (a)The black and red waveforms are the taut-string margins, z − ε and z + ε, and the dotted blue lines is the taut-string estimation; (b) Shows the reconstructed signal based on the taut-string estimation.
Table 1.
Table of selected features.
Fig 4.
the HRV signals and their taut string estimates for a window at baseline, middle stage and final stage. Bottom row: the differences between the HRV waveforms and their taut string estimates. The figure also shows the value of the Taut String 32 feature at each stage.
Fig 5.
Predicted versus actual severity levels.
The predicted severity levels are computed using multiple linear regression of the 10 selected features versus the typical HRV features. The bars indicate 95% confidence intervals.
Table 2.
Prediction Model Comparison Table—The table compares the AUC, accuracy, sensitivity and specificity of the typical HRV features and the presented features for different window sizes as well as the 95% confidence intervals for the AUC and accuracy values.
Asterisks indicate statistical significance at the α = 0.01 level. Bold indicates best score for that feature set. A non-parametric Mann-Whitney U test was used. The number of instances is denoted by n for each feature set.
Fig 6.
AUCs and Accuracies for Each Feature Set.
(a) Areas under the receiver operating curve (AUCs). For the presented features, pair-wise Mann-Whitney U tests were used to compare the AUC’s for different window sizes to the one with the highest AUC (210-beats). The differences that are statistically significant are denoted by asterisks. The same was done for the typical HRV features and window sizes whose AUC was significantly different from the best window size (180-beats) are denoted by plus signs; (b) Accuracies averaged across all ten validation folds for each window size. Error bars represent a 95% confidence interval.
Fig 7.
Receiver operating curves for each feature set’s highest-performing window size.
Fig 8.
Area under the curve calculated choosing the top ten, eight, six, four, and two features from the presented features’ best-performing window size (120-beats). The AUC for the Typical HRV features’ best performing window size (150-beats) is included (in blue) for comparison. The presented features continue to outperform the entire typical HRV feature set even when reduced to just four of the original ten features. The error bars indicate 95% confidence intervals.