Temporal Artery Doppler Spectrum Morphology Responses to Tilt and LBNP as an Early Indicator of Syncope

Abstract

In a study to identify an early hemodynamic predictor of syncope, 12 men (25-40 yr) underwent 30 min of 80 degrees head-up tilt, followed by progressive lower body negative pressure (LBNP) until presyncope. Temporal (supplying extracranial tissues: TEMP), middle cerebral (MCA), and superficial femoral (FEM) arterial flow velocity (V) and vascular resistance indices (VR) were evaluated continuously using Doppler ultrasound. Ratios of the Doppler V(MEAN) (V(MCA)/V(FEM) or V(MCA)/ V(TEMP)) were used to assess flow redistribution between these areas. The progression of the testing protocol showed increases in vascular resistance in all territories. At presyncope, both MCA(VR) and FEM(VR) were reduced while there was a large increase in TEMP(VR). Vasoconstriction of the vascular bed supplied by the temporal artery occurred early during central hypovolemia resulting in the appearance of negative velocity deflections, which could be used for the early detection of impending syncope. Analysis of the velocity ratios showed little change until the onset of presyncope where there was an increase in V(MCA)/V(TEMP) which confirmed that vasoconstriction of the vascular bed supplied by the TEMP artery contributed to cardiac output redistribution in favor of the brain, and a reduction in V(MCA)/V(FEM) suggesting a redistribution of cardiac output toward the legs. In 67% of the tests, the appearance of the negative component of V(TEMP) was an early sign of increasing TEMP(VR) that occurred before visually detectable changes in VE(FEM) or V(MCA) and within 5 min before presyncope. Such easily identifiable in real time Doppler signs allowed experimenters to anticipate test termination.