2.4. Measurement of HRV

Four HRV examinations were performed throughout the study: before discharge (baseline), and one month, six months, and 12 months after discharge, respectively. The time for the first HRV examination was performed within one week after stabilization, and participants were informed to return to the hospital on schedule for the next 3 HRV examinations. HRV was measured by 24 h ambulatory electrocardiography beginning between 08:00–09:00 in the morning. A 12-channel Holter system, provided by Biomedical Systems in the United States, was used to record the continuous electrocardiograph (ECG) signal for 24 h for long-term measurements. HRV analysis was performed based on the Guidelines for Reporting Articles on Psychiatry and Heart rate variability (GRAPH): Recommendations to advance research communications [36]. The sampling rate of the hardware utilized was 500 Hz. The BMS Century 3000 HRV analysis software package (Version 2.0) was used to convert the analog ECG signal to a digital signal, which was analyzed after the recording. The standard deviation of all normal RR intervals (SDNN), which is one of the most common time-domain indices of HRV, was measured. Fast Fourier Transform was used to convert the normal 24-h RR interval into the heart rate power spectrum, and five frequency-domain measures were calculated [36]: (1) Total power (TP) had a frequency range of 0 to 0.4 Hz, which was the total power of the power spectral density, reflecting overall autonomic activity. (2) Ultra low frequency (ULF < 0.0033 Hz) power; (3) Very low frequency (VLF 0.0033 to 0.04 Hz) power; (4) Low frequency (LF), in a frequency range of 0.04 to 0.15 Hz, reflected the sympathetic activity that was modified by parasympathetic activity; (5) High frequency (HF), in the frequency range of 0.15 to 0.40 Hz reflected parasympathetic activity.