Mechanisms of tilt‐induced vasovagal syncope in healthy volunteers and postural tachycardia syndrome patients without past history of syncope

Abstract

Upright tilt table testing has been used to test for vasovagal syncope (VVS) but can result in “false positives” in which tilt‐induced fainting (tilt+) occurs in the absence of real‐world fainting. Tilt+ occurs in healthy volunteers and in patients with postural tachycardia syndrome (POTS) and show enhanced susceptibility to orthostatic hypotension. We hypothesized that the mechanisms for hypotensive susceptibility differs between tilt+ healthy volunteers (Control‐Faint ( N = 12)), tilt+ POTS patients (POTS‐Faint ( N = 12)) and a non‐fainter control group of (Control‐noFaint) ( N = 10). Subjects were studied supine and during 70° upright tilt while blood pressure (BP), cardiac output (CO), and systemic vascular resistance (SVR), were measured continuously. Impedance plethysmography estimated regional blood volumes, flows, and vascular resistance. Heart rate was increased while central blood volume was decreased in both Faint groups. CO increased in Control‐Faint because of reduced splanchnic vascular resistance; splanchnic pooling was similar to Control‐noFaint. Splanchnic blood flow in POTS‐Faint decreased and resistance increased similar to Control‐noFaint but splanchnic blood volume was markedly increased. Decreased SVR and splanchnic arterial vasoconstriction is the mechanism for faint in Control‐Faint. Decreased CO caused by enhanced splanchnic pooling is the mechanism for faint in POTS‐Faint. We propose that intrahepatic resistance is increased in POTS‐Faint resulting in pooling and that both intrahepatic resistance and splanchnic arterial vasoconstriction are reduced in Control‐Faint resulting in increased splanchnic blood flow and reduced splanchnic resistance.

Evaluation of Orthostatic Hypotension by Using Hutt

“…Only 0.6% of the patients were diagnosed with orthostatic hypotension. At initial perusal this may seem surprising given the suspected prevalence of OH in the communitythe ARIC (Atherosclerosis Risk In Communities) study, a community-based prospective cohort of 15,792 middle-aged adults 45-64 years of age, showed that OH was present in 5% of individuals [7][8][9][10][11] .…”

Timing of Circulatory and Neurological Events in Syncope

“…Instruments that would allow direct measurements of venous filling or the distribution of blood across body regions might yield as much new information regarding the venous system as continuous blood pressure measurements did for the arterial system. Impedance measurements should help fill his gap, and have shown important results, but are not used widely (29,83).…”