The protocol of HUTT and the method of HRV analysis at baseline have been reported previously. Briefly, patients were asked to avoid cigarette smoking and beverages containing caffeine after 8:00 pm the day preceding the HUTT. The test was performed in an air-conditioned room (temperature range, 23°C to 24°C) between 2:30 and 3:30 pm and at least 2 h after consuming a meal. After a 30-min rest in the sitting position, patients were placed on a tilt table. ECG electrodes (CM5 lead) and a respiration sensor (node-tip thermistor) were attached and connected to a polygraph system and a frequency modulation tape recorder (model MR30; Teac; Tokyo, Japan).
To control the effects of respiration on HRV measures, the HUTT was performed under paced breathing. After a 10-min stabilization period, patients were instructed to breathe quietly in synchrony with a metronome signal (for 15 breaths/min; 0.25 Hz) until the end of the HUTT. Data were collected with the patient in the supine position for 5 to 10 min and in the 70° head-up tilt position for 6 to 10 min after the table was moved to the head-up tilt position for > 15 s. After the end of the data collection in each position, the cuff BP was measured on the right upper arm with a sphygmomanometer. buy claritin online
HRV was analyzed off-line on a personal computer. The ECGs were played back from the frequency modulation tape and were digitized to 12-bit data at a sampling frequency of 1 KHz. All R-R intervals were measured with a fast-peak detection algorithm, and all errors in the detection of R waves were edited manually. R-R interval time series of 256 s in a stationary state and including < 1% of ectopic beats were selected for both supine and head-up tilt periods (excluding the first minute after tilting).
The power spectral density of the R-R interval time series was estimated by fast-Fourier transformation. Normal-to-normal R-R intervals were interpolated by cubic spline function, were resampled at 1 Hz, were detrended with linear regression, and were filtered with a Hanning window.