title: Assessment of cardiac autonomic modulation during graded head-up tilt by symbolic analysis of heart rate variability.
abstract: Two symbolic indexes, the percentage of sequences characterized by three heart periods with no significant variations (0V%) and that with two significant unlike variations (2UV%), have been found to reflect changes in sympathetic and vagal modulations, respectively. We tested the hypothesis that symbolic indexes may track the gradual shift of the cardiac autonomic modulation during an incremental head-up tilt test. Symbolic analysis was carried out over heart period variability series (250 cardiac beats) derived from ECG recordings during a graded head-up tilt test (0, 15, 30, 45, 60, 75, and 90 degrees ) in 17 healthy subjects. The percentage of subjects showing a significant linear correlation (Spearman rank-order correlation) with tilt angles was utilized to evaluate the performance of symbolic analysis. Spectral analysis was carried out for comparison over the same series. 0V% progressively increased with tilt angles, whereas 2UV% gradually decreased. The decline of 2UV% was greater than the increase of 0V% at low tilt angles. Linear correlation with tilt angles was exhibited in a greater percentage of subjects for 0V% and 2UV% than for any spectral index. Our findings suggest that symbolic analysis performed better than spectral analysis and, thus, is a suitable methodology for assessment of the subtle changes of cardiac autonomic modulation induced by a graded head-up tilt test. Moreover, symbolic analysis indicates that the changes of cardiac sympathetic and vagal modulations observed during this protocol were reciprocal but characterized by different absolute magnitudes.
authors.affiliation: Dipartimento di Tecnologie per la Salute, Istituto Ortopedico Galeazzi, Laboratorio di Modellistica di Sistemi Complessi, Universita' degli Studi di Milano, Via R. Galeazzi 4, 20161 Milan, Italy. alberto.porta@unimi.it
authors.forename: Alberto
authors.initials: A
authors.lastname: Porta
authors.forename: Eleonora
authors.initials: E
authors.lastname: Tobaldini
authors.forename: Stefano
authors.initials: S
authors.lastname: Guzzetti
authors.forename: Raffaello
authors.initials: R
authors.lastname: Furlan
authors.forename: Nicola
authors.initials: N
authors.lastname: Montano
authors.forename: Tomaso
authors.initials: T
authors.lastname: Gnecchi-Ruscone
mesh_terms: D000328:Adult; D000465:Algorithms; D001341:Autonomic Nervous System / Q000502:physiology*; D003198:Computer Simulation; D005260:Female; D006321:Heart / Q000294:innervation* / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008955:Models, Cardiovascular; D018667:Tilt-Table Test / Q000379:methods*