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GLOSSARY OF TERMS USED IN TIME SERIES ANALYSIS
OF CARDIOVASCULAR DATA
 ALPHA INDEX

Estimate of the sensitivity of the baroreceptor-heart rate reflex based on frequency-domain analysis of spontaneous variability of systolic blood pressure and R-R interval.

The method is based on the fact that systolic blood pressure and heart rate show a high degree of linear correlation at the respiratory frequency and at 0.1 Hz in normal subjects, and on the hypothesis that the correlation at these two frequencies is due to the baroreflex coupling. First the spectra of systolic blood pressure and RR-interval series, and their squared-coherence modulus, are computed. If the coherence at 0.1 Hz and at the respiratory frequency is sufficiently high (i.e., greater than a threshold usually set at 0.5), the spectra of systolic blood pressure and RR-interval are integrated over the 0.1 Hz peak, giving the SBPLF and RRLF powers; and over the respiratory peak, giving the SBPHF and RRHF powers. Then the square-root ratio of RR and SBP powers at these two low and high frequencies is computed:


These ratios are called alpha-indexes and are taken as an estimate of  the baroreflex sensitivity.
SBP Power Spectral Density
RR Power Spectral Density
SBP-RR Squared Coherence 
Spectra and coherence in a young healthy subject supine. LF and HF powers of SBP and RR (coloured areas) are obtained by integrating the spectra over the 0.1 Hz peak and over the respiratory peak being the coherence at these two frequencies greater than 0.5. The square-root ratio of RR and SBP powers (alpha index) is taken as an estimate of the baroreflex sensitivity.

Reference:
Pagani M et al (1988) Changes in autonomic modulations induced by physical training in mild hypertension, Hypertension
Harrington F et al (2000)Relationship of baroreflex sensitivity and blood pressure in an older population.J Hypertens.

For a comparison with other methods see:
Rudiger H, Bald M.(2001)Spontaneous baroreflex sensitivity in children and young adults calculated in the time and frequency domain. Auton Neurosci.
Nollo G, et al (2000) Causal linear parametric model for baroreflex gain assessment in patients with recent myocardial infarction. Am J Physiol Heart Circ Physiol.
Tank J et al (2000)Reference values of indices of spontaneous baroreceptor reflex sensitivity. Am J Hypertens.
Gerritsen J, et al (2000) Baroreflex sensitivity in the elderly: influence of age, breathing and spectral methods. Clin Sci
Davies LC ei al (1999)Reproducibility of methods for assessing baroreflex sensitivity in normal controls and in patients with chronic heart failure.Clin Sci
Di Rienzo M et al (1997) Critical appraisal of indices for the assessment of baroreflex sensitivity. Methods Inf Med.
Maestri R et al (1998) Assessing baroreflex sensitivity in post-myocardial infarction patients: comparison of spectral and phenylephrine techniques. J Am Coll Cardiol.
Panerai RB et al (1997)Impulse response analysis of baroreceptor sensitivity. Am J Physiol.

(PC 03-12-2001)

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