title: Increased phase synchronization and decreased cerebral autoregulation during fainting in the young.
abstract: Vasovagal syncope may be due to a transient cerebral hypoperfusion that accompanies frequency entrainment between arterial pressure (AP) and cerebral blood flow velocity (CBFV). We hypothesized that cerebral autoregulation fails during fainting; a phase synchronization index (PhSI) between AP and CBFV was used as a nonlinear, nonstationary, time-dependent measurement of cerebral autoregulation. Twelve healthy control subjects and twelve subjects with a history of vasovagal syncope underwent 10-min tilt table testing with the continuous measurement of AP, CBFV, heart rate (HR), end-tidal CO2 (ETCO2), and respiratory frequency. Time intervals were defined to compare physiologically equivalent periods in fainters and control subjects. A PhSI value of 0 corresponds to an absence of phase synchronization and efficient cerebral autoregulation, whereas a PhSI value of 1 corresponds to complete phase synchronization and inefficient cerebral autoregulation. During supine baseline conditions, both control and syncope groups demonstrated similar oscillatory changes in phase, with mean PhSI values of 0.58+/-0.04 and 0.54+/-0.02, respectively. Throughout tilt, control subjects demonstrated similar PhSI values compared with supine conditions. Approximately 2 min before fainting, syncopal subjects demonstrated a sharp decrease in PhSI (0.23+/-0.06), representing efficient cerebral autoregulation. Immediately after this period, PhSI increased sharply, suggesting inefficient cerebral autoregulation, and remained elevated at the time of faint (0.92+/-0.02) and during the early recovery period (0.79+/-0.04) immediately after the return to the supine position. Our data demonstrate rapid, biphasic changes in cerebral autoregulation, which are temporally related to vasovagal syncope. Thus, a sudden period of highly efficient cerebral autoregulation precedes the virtual loss of autoregulation, which continued during and after the faint.
authors.affiliation: Department of Physiology, The Center for Hypotension, New York Medical College, 19 Bradhurst Ave., Suite 1600S, Hawthorne, NY 10532, USA.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Ocon
authors.forename: John
authors.initials: J
authors.lastname: Kulesa
authors.forename: Debbie
authors.initials: D
authors.lastname: Clarke
authors.forename: Indu
authors.initials: I
authors.lastname: Taneja
authors.forename: Marvin S
authors.initials: MS
authors.lastname: Medow
authors.forename: Julian M
authors.initials: JM
authors.lastname: Stewart
mesh_terms: D000293:Adolescent; D000367:Age Factors; D001783:Blood Flow Velocity; D001794:Blood Pressure; D002245:Carbon Dioxide / Q000378:metabolism; D016022:Case-Control Studies; D002560:Cerebrovascular Circulation; D005260:Female; D006339:Heart Rate; D006706:Homeostasis; D006801:Humans; D017078:Laser-Doppler Flowmetry; D008297:Male; D008955:Models, Cardiovascular; D017711:Nonlinear Dynamics; D011187:Posture; D015656:Respiratory Mechanics; D016683:Supine Position; D013575:Syncope / Q000000981:diagnostic imaging / Q000378:metabolism / Q000503:physiopathology*; D018667:Tilt-Table Test; D013997:Time Factors; D017585:Ultrasonography, Doppler, Transcranial; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide