Rationale: Assessing the relative contributions of cardioinhibition and vasodepression to the blood pressure (BP) decrease in tilt-induced vasovagal syncope (T-VVS) requires methods that reflect BP physiology accurately. Objective: To assess the relative contributions of cardioinhibition and vasodepression to T-VVS using novel methods. Methods and Results: We studied the parameters determining BP, i.e. stroke volume (SV), heart rate (HR) and total peripheral resistance (TPR), in 163 patients with T-VVS documented by continuous ECG and video EEG monitoring. We defined the beginning of cardioinhibition as the start of a heart rate decrease (HR) before syncope, and used logarithms of SV-, HR- and TPR-ratios to quantify the multiplicative relation BP=SV·HR·TPR. We defined three stages before syncope and two after it based on direction changes of these parameters. The earliest BP decrease occurred nine minutes before syncope. Cardioinhibition was observed in 91% of patients at a median time of 58 s. before syncope. At that time SV had a strong negative effect on BP, TPR a lesser negative effect, while HR had increased (all p<0.001). At the onset of cardioinhibition, median HR was at 98 bpm higher than baseline. Cardioinhibition thus initially only represented a reduction of the corrective HR increase, but was nonetheless accompanied by an immediate acceleration of the ongoing BP decrease. At syncope, SV and HR contributed similarly to the BP decrease (p<0.001), while TPR did not affect BP. Conclusions: The novel methods allowed the relative effects of SV, HR and TPR on BP to be assessed separately, even though all act together. The two major factors lowering BP in T-VVS were reduced SV and cardioinhibition. We suggest that the term 'vasodepression' in reflex syncope should not be limited to reduced arterial vasoconstriction, reflected in TPR, but should also encompass venous pooling, reflected in SV.

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Objective To define and evaluate hemodynamic criteria to distinguish between classical orthostatic hypotension (cOH) and vasovagal syncope (VVS) in tilt table testing (TTT). Methods Inclusion criteria for VVS were a history of VVS and tilt‐induced syncope defined as a blood pressure (BP) decrease and electroencephalographic changes during syncope with complaint recognition. Criteria for cOH were a history of cOH and a BP decrease meeting published criteria. Clinical diagnoses were established prior to TTT. We assessed (1) whether the decrease of systolic BP accelerated, “convex,” or decelerated, “concave”; (2) the time from head‐up tilt to when BP reached one‐half its maximal decrease; (3) the difference between baseline heart rate (HR) and HR at BP nadir. We calculated the diagnostic yield of optimized thresholds of these features and their combinations. Results We included 82 VVS cases (40% men, median age 44 years) and 65 cOH cases (66% men, median age 70 years). BP decrease was concave in cOH in 79% and convex in VVS in 94% (p < 0.001). The time to reach half the BP decrease was shorter in cOH (median 34 sec, interquartile range (IQR) 19–98 sec) than in VVS (median 1571 sec, IQR 1381–1775 sec, p < 0.001). Mean HR increased by 11 ± 11 bpm in cOH and decreased by 20 ± 19 bpm in VVS (p < 0.001). When all three features pointed to VVS, sensitivity for VVS was 82% and specificity was 100%. When all three pointed to cOH, sensitivity for cOH was 71% and specificity was 100%. Interpretation These new hemodynamic criteria reliably differentiate cOH from VVS.

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