Feelings from the Heart Part II: Simulation and Validation of Static and Dynamic HRV Decrease-Trigger Algorithms to Detect Stress in Firefighters

Figures

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  Figure 1. An adapted schematic representation of a dynamic AddHRVr algorithm from Schwerdtfeger and Rominger [5].

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  Figure 2. Decision tree of the dynamic intercept implemented in the dynamic AddHRVr algorithm.

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  Figure 3. Example of a simulation for two firefighters. Both algorithms were run with a window threshold of 4 and a window length of 6 (i.e., 4 out of 6) with a silent period of 20 min in between. The figure illustrates an observation time of 3 h. The x-axis depicts minutes and the y-axis RMSSD. The red line represents the amount of AEE (kcal), the blue line is the actual RMSSD, and the bold blue line represents the estimated threshold (predicted RMSSD—0.5 × SD RMSSDcalibration). Green asterisks indicate AddHRVr triggers, and the black asterisks indicate a 1-min segment with the actual HRV being lower than the predicted threshold. Panel (A) represents a static and Panel (B) a dynamic AddHRVr algorithm.

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  Figure 4. Mean number of AddHRVr triggers for 6 different silent settings of the algorithm (from 10 to 60 min) for the static and the dynamic algorithm, separately.

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  Figure 5. Left panel (A) illustrates the power for each of the 435 bootstrapped multi-level analyses using the dynamic algorithm settings of RMSSD window length (x-axis) and window threshold (y-axis; i.e., y out of x to be a trigger; see [29] for an interactive 3D illustration). Panel (B) illustrates the power for each of the 435 bootstrapped multi-level analyses using the static algorithm (500 samples with n = 38; for an interactive 3D illustration see [30]). The silent setting of both figures was 20 min.

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  Figure 6. Illustration of bootstrap simulations (500 samples each) of the association between power (y-axis) of the 7 out of 10 setting (20 min silent setting) and the sample size (x-axis). Red dashed horizontal lines represent the power thresholds of 0.70, 0.80, and 0.90. The grey solid vertical lines represent the number of firefighters at least needed to reach a robust power of 0.70, 0.80, and 0.90.