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Introduction. Neurocardiogenic syncope is the most frequent transitional form of loss of  onsciousness, which is caused by inappropriate triggering of the Bezold-Jarisch reflex. Most commonly, the triggering agent is an upright posture. With the help of spectral analysis, researchers have demonstrated that there is a large increase in sympathetic activity just before syncope, while at the moment of syncope a paradox inhibition of this activity occurs with consequential vasodilatation and bradycardia, which result in loss of consciousness. The objective of our study was to discover whether there are differences in autonomic nervous system activity between patients and control subjects in their responses to head-up tilt table testing, and whether these differences are greater in a tilt table test with nitroglycerine.

Subjects and methods. A group of 30 subjects who received nitroglycerine during the test (NTG+) was compared with another group of 30 who were tested without nitroglycerine (NTG–). In half of the patients in each group, syncope occurred during the test (SIN+) while in the other half it did not (SIN–). Spectral analysis of heart rate variability was used to determine the ratio of medium- (SF) and high-frequency (VF) bands (SF/VF), which represents the balance between the sympathetic and parasympathetic nervous systems. The results were compared using t-test for dependent samples for changes within each group, and with Student’s t-test for comparison between the groups. A statistically significant difference was one with p ≤ 0.05.

Results. A significant difference was found in the SF/VF ratio and heart rate between the supine and tilted position in all groups, except for group NTG–/SIN–. The results for SF/VF were: p<0.001 (NTG+/SIN+), p<0.005 (NTG+/SIN–), p<0.05 (NTG–/SIN+), NS (NTG–/SIN–). Significant differences existed between the supine position and addition of nitroglycerine (SF/VF: NTG+/SIN+: p < 0.01, NTG+/SIN–: p < 0.001). There were no statistically significant differences in the changes of these parameters between the groups.

Conclusion. Spectral analysis of heart rate variability showed that the initial response to orthostatic stress is identical in subjects both with and without neurocardiogenic syncope. The change in position from supine to tilted at an angle of 60° increases the relationship between the ratio of the sympathetic and parasympathetic systems and the heart rate increases. With the addition of nitroglycerine, these changes increase. Nitroglycerine increases the sympathetic tone in all subjects, with no significant differences between the groups.