The primary result of this study was that perioperative use of Dex had a significant therapeutic effect for SVT in adult patients and the adjusting effect of Dex on the function of cardiac autonomic nervous might be one of the antiarrhythmic mechanisms of action of Dex.
Perioperative use of Dex in pediatric patients undergoing cardiothoracic operations can decrease incidence of ventricular and supraventricular tachyarrhythmias , suggesting that its antiarrhythmic effect in adult patients undergoing non-cardiac surgery needs to be further explored. Our results had demonstrated that Dex had an effective therapeutic effect and few adverse effects for the treatment of patients with SVT, who simultaneously suffered from hypokalemia, and/or diabetes, and/or hypertension, and/or coronary heart disease. Thirteen patients with SVT were remarkably improved only 5 minutes after infusion of Dex (1.0 µg/kg), and SVT of twenty-one patients without recurrence of SVT in Dex group were eventually controlled and returned to NSR by continuous intravenous infusion of Dex for 10 minutes. These evidences demonstrated that Dex had therapeutic effects on SVT patients coupling with different diseases.
The HRV results had shown that HFnorm was elevated, while LFnorm and LFnorm/HFnorm were decreased after twenty-three patients infused Dex in Dex group, which revealed that Dex changed the balance between the sympathetic and parasympathetic tone, and regulated cardiac autonomic nervous system, and powerfully enhanced vagal neural activity, thereby, overall rhythm or heart rate control was achieved. These data suggested that Dex may mainly reduce sympathetic tension to decrease the ability of inducing the arrhythmia.
There are many types of SVT, which are associated with disorders of autonomic nervous regulation of cardiac activity, reentrant excitation and enhancement of autorhythmicity. Some studies have shown that paroxysmal SVT have an imbalance of autonomic nervous system, for example, injury to cardiac parasympathetic nerves, which can result in predominance of sympathetic activity during thoracic surgery that is the primary autonomic mechanism triggering postoperative SVT, thus, tachyarrhythmia is treated by stimulating the vagus nerve and regulating the activity of autonomic nerve [22, 23]. Using the clinical recommended dosage, Dex can act on the medullary vasomotor center leading to decrease of catecholamine released from the nerve terminal , which results in bradycardia and the negative chronotropic effects of the heart (e.g. treat tachyarrhythmia) . Dex containing an imidazole ring can activate imidazoline receptors in the central nervous system, which can prevent adrenaline-induced ventricular tachycardia . It is reported that Dex had therapeutic effect on atrial and junctional tachyarrhythmias , and can significantly decrease heart rate via regulating autonomic nerve homeostasis . Kamibayashi T et al.  by animal-dogs study demonstrated that the antiarrhythmic effects of Dex are mediated through enhancement of the vagal neural activity. In our study, we also demonstrate that Dex treats SVT related to changing the function of cardiac autonomic nervous system. In a word, Dex with multiple administrations is appropriate and effective for treating SVT, and may be an ideal agent for SVT.
Few patients developed clinical bradycardia, hypotension, respiratory depression, hyoxemia and hemodynamic disorder in Dex group during infusion of Dex, which is perhaps due to using Dex alone, and thus a synergistic or additive effect with other sedative and analgesic drugs is impossible. Furthermore, Dex simultaneously maintains a sufficient level of sedation with a unique property of easy arousal. Although Dex causes mild hypotension, because of its unique minimal effect on respiration compared with most other sedative drugs, Dex is widespreadly used in clinical practice. In addition, Dex does not cause any significant sinus pause or asystole. It has no negative inotropic effector proarrhythmic effect, in contrast with many other agents such as β-adrenergic antagonists and amiodarone. Moreover, Dex has bronchodilatory properties, therefore, it can still be used during asthma attacks.
SVT is associated with adverse stimulus such as emotional tension and anxiety, cardiovascular risk factors and hypoxia. Emotional tension and anxiety are the most common cause of perioperative SVT. It is also worth considering that sedation treatment should be taken in patients with SVT pre-anesthesia. Therefore, midazolam as a sedative was selected as the control group to compare the therapeutic effect of Dex on SVT in this study. Although the sedation degree in midazolam group was deeper than Dex group, only two patients are eventually improved, and other patients failed to respond in midazolam group. Earlier studies have shown that intravenous infusion of midazolam 5.0 mg did not alter the reentrant tachycardia . These data indicated that the sedation may not be the mechanism of Dex for the treatment of SVT.
Dex may be used during general anaesthesia as an adjuvant . General anesthesia comprises of unconsciousness, analgesia, anti-stress, and immobility along with maintenance of physiological stability . Dex, which combines sedative, anti-stress and enhancing analgesic effects, may be more applicable than other antiarrhythmic drugs to treat SVT in patients received general anesthesia.
It is reported that large doses or rapid infusion of Dex can potentially have an adverse impact on patients with underlying heart disease, which may cause left ventricular dysfunction and hemodynamic instability . Flores-González et al.  reported that cessation of Dex for 12 hours after continuously infusing it for 10 days resulted in occurrence of paroxysmal SVT for a 4-years-old girl. However, our observation period was the time when patients was in the operating room, and we might have missed arrhythmias that occurred afterwards. Moreover, it was impressive with a significant (91.3%) termination of SVT using Dex, which may be associated with chosen types of SVT. Therefore, there are more clinical research needed to explore the treatment efficiency of various types of SVT using Dex. Further clinical study is also needed to clarify the mechanism of effects, feasibility of wide clinical application, the dosage and rate of Dex for the treatment of SVT. Notably, this was a prospective and randomised study that confirmed the mechanism and effectiveness of Dex in the treatment of SVT. The conclusion of this study can provide a new therapeuticregimen for treating SVT using Dex.