Day-by-day variability of spontaneous baroreflex sensitivity measurements: implications for their reliability in clinical and research applications.

OBJECTIVE: To assess day-by-day variability of spontaneous baroreflex sensitivity (BRS) measurements, providing implications for their reliability in clinical and research studies. METHODS: Forty-four healthy volunteers and 57 patients with previous myocardial infarction (MI) underwent an 8 + 8 min (spontaneous + paced breathing) recording of ECG and noninvasive arterial pressure on two consecutive days. BRS was computed according to the sequence method (BRS_seq), the original and modified transfer function method (BRS_TF and BRS_TF(mod)) and the alpha method (BRS_alphaLF and BRS_alphaHF). Absolute and relative reliability were assessed by the 95% limits of random variation (LoV) and by the intraclass correlation coefficient (ICC), respectively. The sample size needed to detect a clinically relevant change was also estimated. RESULTS: In healthy volunteers during spontaneous breathing, BRS_seq, BRS_TF, BRS_alphaLF and BRS_alphaHF could not be measured in 18, 3, 2 and 2% of recordings, respectively. By definition, BRS_TF(mod) could always be measured. The 95% LoV indicated that individual day-by-day changes may range from -50% to +101% for BRS_TF(mod) (best case) and from -58% to +135% for BRS_alphaHF (worst case). The ICC ranged from 0.70 (BRS_seq) to 0.76 (BRS_TF(mod)). The sample size varied from 56 (BRS_TF(mod)) to 80 (BRS_seq). In MI patients, measurability was lower whereas reliability indexes were similar. Results during paced breathing were similar. CONCLUSION: Day-by-day variability should be taken into account when using spontaneous BRS measurements to detect treatment effects in individual patients. The observed substantial to good relative reliability, as assessed by the ICC, indicates that spontaneous BRS measurements are suitable to detect differences between individuals, which is a prerequisite for proper diagnosis and prognosis.

Reliability of heart rate variability measurements in patients with a history of myocardial infarction.

Despite a well-established prognostic value in cardiac patients, HRV (heart rate variability) indexes have been used little in the clinical setting. Poor reliability of the measurements might be a possible explanation for this. In the present study, we assessed the reliability of short-term HRV indexes in post-MI (myocardial infarction) patients. We studied 61 MI patients [50 males; age, 59+/-8 years; and LVEF (left ventricular ejection fraction), 46+/-6%; values are means +/-S.D.],who underwent a 5+5 min ECG recording during spontaneous and paced breathing on two consecutive days. Standard time-domain [SDNN (S.D. of RR interval values) and RMSSD (root- mean-square of successive RR interval differences)] and frequency-domain [LF (low-frequency) and HF (high-frequency) power, and LF/HF] indexes of HRV were computed. Absolute and relative reliability were assessed by the 95% limits of random variation and by the ICC (intra-class correlation coefficient). The agreement between the two measurements in classifying patients at low or high risk, according to different cut-points, and the sample size needed to detect a clinically relevant change, were also assessed. During spontaneous breathing, individual changes in test-retest measurements ranged from -41 to + 61% (SDNN; best case) and from -76 to + 316% (LF/HF; worst case). The ICC ranged from 0.72 to 0.81. Most patients (79-90 %) were assigned to the same class by the two measurements. Paced breathing did not improve reliability. In conclusion, short-term HRV parameters in MI patients may have large day-to-day variations, making the detection of treatment effects in individual patients difficult; however, the ICC values and the analysis of the consistency of classification between repeated tests indicate that HRV measurements fulfill the criteria required to be used for diagnostic or classification purposes.

Heart rate variability measures: a fresh look at reliability.

Despite its extensive use in physiological and clinical research, the analysis of HRV (heart rate variability) is still poorly supported by rigorous reliability studies. The main aim of the present study was to perform an in-depth assessment of absolute and relative reliability of standard indexes of HRV from short-term laboratory recordings. In 39 healthy subjects [mean age (min-max): 38 (26-56) years; 18 men and 21 women] we recorded 5 min of supine ECG during spontaneous and paced (15 breaths/min) breathing. The test was repeated on the next day under the same conditions. From the RR intervals we computed standard indexes of HRV: SDNN (S.D. of RR interval values), RMSSD (root-mean-square of successive RR interval differences), LF (low frequency) and HF (high frequency) power (absolute and normalized units) and LF/HF. Absolute reliability was assessed by 95% limits of random variation; relative reliability was assessed by the ICC (intraclass correlation coefficient). The sample size needed to detect a mean difference > or =30% of the between-subject S.D. was also estimated. Although there was no significant mean change between the two tests, we found that in individual subjects the second measurement was as high/low as 1.9/0.5 times (SDNN, best case) and 3.5/0.3 times (LF/HF, worst case) the first measurement, due to pure random variation. For most parameters the ICC was >0.8 (range 0.65-0.88). The estimated sample size ranged from 24-98 subjects. Reliability indexes tended to improve during paced breathing. We conclude that short-term HRV parameters are subject to large day-to-day random variations. Random error, however, represents a limited part of the between-subject variability; therefore observed differences between individuals mostly reflect differences in the subjects' error-free value rather than random error. Overall, paced breathing improves reliability.