title: Decrease in cardiac output and muscle sympathetic activity during vasovagal syncope.
abstract: The importance of cardiac output (CO) to blood pressure level during vasovagal syncope is unknown. We measured thermodilution CO, mean blood pressure (MBP), and leg muscle mean sympathetic nerve activity (MSNA) each minute during 60 degrees head-up tilt in 26 patients with recurrent syncope. Eight patients tolerated tilt (TT) for 45 min (mean age 60 +/- 5 yr) and 15 patients developed syncope during tilt (TS) (mean age 58 +/- 4 yr, mean tilt time 15.4 +/- 2 min). In TT patients, CO decreased during the first minute of tilt (from 3.2 +/- 0.2 to 2.5 +/- 0.3 l x min(-1) x m(-2), P = 0.001) and thereafter remained stable between 2.5 +/- 0.3 (P = 0.001) and 2.4 +/- 0.2 l x min(-1) x m(-2) (P = 0.004) at 5 and 45 min, respectively. In TS patients, CO decreased during the first minute (from 3.3 +/- 0.2 to 2.7 +/- 0.1 l x min(-1) x m(-2), P = 0.02) and was stable until 7 min before syncope, falling to 2.0 +/- 0.2 at syncope (P = 0.001). Regression slopes for CO versus time during tilt were -0.01 min(-1) in TT versus -0.1 l x min(-1) x m(-2) x min(-1) in TS (P = 0.001). However, MBP was more closely correlated to total peripheral resistance (R = 0.56, P = 0.001) and MSNA (R = 0.58, P = 0.001) than CO (R = 0.32, P = 0.001). In vasovagal reactions, a progressive decline in CO may contribute to hypotension some minutes before syncope occurs.
authors.affiliation: Department of General Medicine, Christchurch Hospital, Christchurch, New Zealand. David.Jardine@cdhb.govt.nz
authors.forename: D L
authors.initials: DL
authors.lastname: Jardine
authors.forename: I C
authors.initials: IC
authors.lastname: Melton
authors.forename: I G
authors.initials: IG
authors.lastname: Crozier
authors.forename: S
authors.initials: S
authors.lastname: English
authors.forename: S I
authors.initials: SI
authors.lastname: Bennett
authors.forename: C M
authors.initials: CM
authors.lastname: Frampton
authors.forename: H
authors.initials: H
authors.lastname: Ikram
mesh_terms: D000328:Adult; D000368:Aged; D001794:Blood Pressure; D002302:Cardiac Output; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate; D006801:Humans; D007866:Leg; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000294:innervation*; D010100:Oxygen / Q000097:blood; D010101:Oxygen Consumption; D011187:Posture; D012727:Sex Characteristics; D013564:Sympathetic Nervous System / Q000503:physiopathology*; D019462:Syncope, Vasovagal / Q000503:physiopathology*
chemical_list: D010100:Oxygen