Temporal asymmetries of short-term heart period variability are linked to autonomic regulation

1 Department of Technologies for Health, Galeazzi Orthopaedic Institute, University of Milan, Milan, Italy; 2 Department of Clinical Sciences, Internal Medicine II, L. Sacco Hospital, University of Milan, Milan, Italy; 3 Departamento de Fisiologia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil; 4 Department of Cardiology, S. L. Mandic Hospital, Merate, Italy; 5 Department of Biomagnetism, Gronemeyer Institute for Microtherapy, Bochum, Germany; and 6 Department of Medical Theory and Complementary Medicine, University of Witten/Herdecke, Herdecke, Germany Submitted 22 February 2008 ; accepted in final form 16 May 2008 We exploit time reversibility analysis, checking the invariance of statistical features of a series after time reversal, to detect temporal asymmetries of short-term heart period variability series. Reversibility indexes were extracted from 22 healthy fetuses between 16th to 40th wk of gestation and from 17 healthy humans (aged 21 to 54, median = 28) during graded head-up tilt with table inclination angles randomly selected inside the set {15, 30, 45, 60, 75, 90}. Irreversibility analysis showed that nonlinear dynamics observed in short-term heart period variability are mostly due to asymmetric patterns characterized by bradycardic runs shorter than tachycardic ones. These temporal asymmetries were 1 ) more likely over short temporal scales than over longer, dominant ones; 2 ) more frequent during the late period of pregnancy (from 25th to 40th week of gestation); 3 ) significantly present in healthy humans at rest in supine position; 4 ) more numerous during 75 and 90° head-up tilt. Results suggest that asymmetric patterns observable in short-term heart period variability might be the result of a fully developed autonomic regulation and that an important shift of the sympathovagal balance toward sympathetic predominance (and vagal withdrawal) can increase their presence. heart rate variability; autonomic nervous system; head-up tilt; fetal maturation; nonlinear dynamics Address for reprint requests and other correspondence: A. Porta, Università degli Studi di Milano, Dipartimento di Tecnologie per la Salute, Istituto Ortopedico Galeazzi, Laboratorio di Modellistica di Sistemi Complessi, Via R. Galeazzi 4, 20161 Milan, Italy (e-mail: alberto.porta{at}unimi.it )