Postural Response of Low-Frequency Component of Heart Rate Variability: HUTT and Analysis of HRV

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.