Mechanisms of Vasovagal Syncope in the Young: Reduced Systemic Vascular Resistance Versus Reduced Cardiac Output.

Background- Syncope is a sudden transient loss of consciousness and postural tone caused by cerebral hypoperfusion. The most common form is vasovagal syncope (VVS). Presyncopal progressive early hypotension in older VVS patients is caused by reduced cardiac output (CO); younger patients have reduced systemic vascular resistance (SVR). Using a priori criteria for reduced CO (down arrow CO) and SVR (down arrow SVR), we studied 48 recurrent young fainters comparing subgroups of VVS with VVS-down arrow CO, VVS-down arrow SVR, and both VVS-down arrow CO & SVR. Methods and Results- Subjects were studied supine and during 70-degrere upright tilt with a Finometer to continuously measure blood pressure, CO, and SVR and impedance plethysmography to estimate thoracic, splanchnic, pelvic, and calf blood volumes, blood flows, and vascular resistances and electrocardiogram to measure heart rate and rhythm. Central blood volume was decreased in all VVS compared to control. VVS-down arrow CO was associated with decreased splanchnic blood flow and increased splanchnic blood pooling compared to control. Seventy-five percent of VVS patients had reduced SVR, including 23% who also had reduced CO. Many VVS-down arrow SVR increased CO during tilt, with no difference in splanchnic pooling, caused by significant increases in splanchnic blood flow and reduced splanchnic resistance. VVS-down arrow CO & SVR patients had splanchnic pooling comparable to VVS-down arrow CO patients, but SVR comparable to VVS-down arrow SVR. Splanchnic vasodilation was reduced, compared to VVS-down arrow SVR, and venomotor properties were similar to control. Combined splanchnic pooling and reduced SVR produced the earliest faints among the VVS groups. Conclusions- Both down arrow CO and down arrow SVR occur in young VVS patients. down arrow SVR is predominant in VVS and is caused by impaired splanchnic vasoconstriction.