respiratory sinus arrhythmia, Agitation, HF, Coma
COMA
The stored data were extracted to and analyzed in Kubios HRV Premium version 3.4.3 (Kubios Oy). All recordings were pre‐processed to remove artifacts. The software uses an An explorative approach to HRV‐based assessments of autonomic function was pursued, as there is currently no established gold standard (Shaffer & Ginsberg, Baseline characteristics.Abbreviations: GCS, Glasgow Coma Scale; IQR, interquartile range; RASS, Richmond Agitation and Sedation Scale.The time domain measures comprised the standard deviation (SD) of successive RR intervals (SD of normal‐to‐normal intervals [SDNN]) and root mean square of successive RR intervals differences (RMSSD). These measures principally reflect the degree of respiratory sinus arrhythmia, which is regulated by parasympathetic activity.Frequency domain calculations were based on the fast Fourier transform with a 50% Welch window overlap and included total power, LF power (LF: 0.04–0.15 Hz), HF power (HF: 0.15–0.40 Hz) and the ratio between LF and HF power (LF/HF). Total power expresses the signal energy and can be used after normalization to compare the frequency domain between different patients or even between patients and healthy, age‐matched individuals. LF depends mainly on sympathetic, baroreceptor drive, but also on parasympathetic regulation, and to a lesser extent on unspecific factors. HF reflects mainly parasympathetically‐mediated respiratory sinus arrhythmia, while LF/HF is classically considered an index of “sympathetic‐vagal balance” of heart rate regulation, particularly during long‐term recordings.The nonlinear analysis consisted of sample entropy as a quantification of total irregularity and detrended fluctuations analysis (DFA1). This analysis results in slope α1 and α2, which describes long‐term fluctuations, of which α1 reflects the complex interactions between various heart rate control mechanisms in the arterial baroreflex, while α2 reflects the complex interactions between the regulatory mechanisms that limit fluctuations in heart rate.