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ST-segment depression in atrioventricular nodal reentrant tachycardia: Preliminary results.

BACKGROUND
The ST-segment is part of the electrocardiogram and physiologically, it forms an isoelectric line. The ST-segment depression is often observed in young, healthy people with paroxysmal tachycardia with narrow QRS complexes. In this group of patients, the 'mysterious tachycardia-induced ST-segment depression', 'subendocardial myocardial ischemia' and other not fully understood terms are used to explain this phenomenon.


OBJECTIVES
To assess the presence and possible mechanisms of ST-segment depression during atrioventricular nodal reentrant tachycardia (AVNRT) in patients undergoing radiofrequency (RF) ablation of the underlying arrhythmia.


MATERIAL AND METHODS
The study included 50 patients (35 women and 15 men) aged about 49 years with clinically relevant paroxysmal narrow QRS complex tachycardia. During electrophysiological study (EPS), all patients had measured QRS components - QR, RS and RJ during the tachycardia and during the sinus rhythm. All of the measurements were done in lead V5.


RESULTS
There was a statistically significant difference in cycle length during sinus rhythm and tachycardia (707.0 ±137.8 ms compared to 327.5 ±29.1 ms, p = 0.000), the RJ component (0.819 ±0.381 mV compared to 0.878 ±0.376 mV, p = 0.003) and the difference RJ-QR (0.081 ±0.083 mV compared to 0.163 ±0.108 mV, p = 0.000). The differences in RS and QR components during sinus rhythm and tachycardia did not reach the statistical significance. The difference RJ-QR during tachycardia correlated negatively with tachycardia cycle length (R = -0.39, p = 0.0049). The tachycardia cycle length correlated positively with the age of the studied patients (R = 0.28, p = 0.043).


CONCLUSION
In patients with AVNRT, there is a ST-segment depression during the episodes of tachycardia and the degree of this change is related to tachycardia cycle length. The most probable explanation of the ST-segment depression is the overlap of the QRS complex on the preceded T wave. Some intrinsic properties of individual electrocardiogram (ECG) also influence this phenomenon. The ischemic origin of the presented ST-segment change can be excluded.