Skin sympathetic nerve activity as a biomarker for syncopal episodes during a tilt table test

Background Autonomic imbalance is the proposed mechanism of syncope during tilt table test (TTT). We have recently demonstrated that skin sympathetic nerve activity (SKNA) can be noninvasively recorded using ECG electrodes. Objective To test the hypothesis that increased SKNA activation precedes tilt-induced syncope. Methods We studied 50 patients with a history of neurocardiogenic syncope undergoing a TTT. Recorded signals were band-pass filtered at 500–1000 Hz to analyze nerve activity. Results The average SKNA (aSKNA) at baseline was 1.38±0.38 μV in patients without syncope and 1.42±0.52 μV in patients with syncope (p=0.77). On upright tilt, aSKNA was 1.34±0.40 μV in patients who did not have syncope and 1.39±0.43 μV in patients who had syncope (p=0.65). In all 14 patients with syncope, there was a surge of SKNA prior to an initial increase in heart rate followed by bradycardia, hypotension, and syncope. The peak aSKNA immediately (<1min) prior to syncope was significantly higher than baseline aSKNA (2.63±1.22 vs 1.39±0.43, p=0.0005). After syncope, the patients were immediately placed in a supine position and the aSKNA dropped significantly to 1.26±0.43 (p=0.0004). The heart rate variability (HRV) during TTT shows significant increase in parasympathetic tone during syncope (LF/HF ratio: 7.15 vs 2.21, p=0.04). Conclusions Patients with syncope do not have elevated sympathetic tone at baseline or during TTT except immediately prior to syncope, when a transient surge of SKNA followed by sympathetic withdrawal along with parasympathetic surge.