Inappropriate sinus tachycardia
Inappropriate sinus tachycardia (IST) is a rare type of cardiac arrhythmia within the category of supraventricular tachycardia (SVT). IST may be caused by the sinus node itself having an abnormal structure or function, or it may be part of a problem called dysautonomia, a disturbance and/or failure of the autonomic nervous system. Research into the mechanism and etiology (cause) of inappropriate sinus tachycardia is ongoing.
IST is viewed by most to be a benign condition in the long-term. Symptoms of IST, however, may be distracting and warrant treatment. The heart is a strong muscle and typically can sustain the higher-than-normal heart rhythm, though monitoring the condition is generally recommended.
The mechanism and primary etiology of inappropriate sinus tachycardia has not been fully elucidated. An autoimmune mechanism has been suggested, as several studies have detected autoantibodies that activate beta adrenoreceptors in some patients. The mechanism of the arrhythmia primarily involves the sinus node and peri-nodal tissue and does not require the AV node for maintenance. Treatments in the form of pharmacological therapy or catheter ablation are available, but the condition is currently difficult to treat successfully.

Symptoms
Symptoms reported by patients vary in frequency and severity. They may include:

Frequent or sustained palpitations
Dyspnea (shortness of breath) and palpitations on exertion
Pre-syncope (feeling as if about to faint)
Fatigue
Dizziness
Exercise intolerance
Occasional paresthesia and cramping
Symptoms associated with autonomic nervous system disturbance, including gastrointestinal disturbance

Diagnosis
No formal diagnostic criteria exist. A diagnosis of inappropriate sinus tachycardia is primarily one of exclusion, and the following may be observed:
Exclusion of all other causes of sinus tachycardia
Common forms of supraventricular tachycardia (SVT) must be excluded
Normal P wave morphology
A resting sinus tachycardia is usually (but not always) present
Nocturnal dip in heart rate
Inappropriate heart rate response on exertion
Mean heart rate in 24hrs >95 bpm
Symptoms are documented to be due to tachycardia
Hypotension is occasionally observed
Syncope (fainting) is occasionally reported

Treatment
IST has been treated both pharmacologically and invasively, with varying degrees of success. IST, in and of itself, is not indicative of higher rates of mortality, and non-treatment is an option chosen by many if they have minimal symptoms.Some types of medication tried by cardiologists and other physicians include: beta blockers, selective sinus node If channel inhibitors (such as ivabradine), calcium channel blockers, and antiarrhythmic agents. Some SSRI drugs are also occasionally tried, as are treatments more commonly used to treat postural orthostatic tachycardia syndrome, such as fludrocortisone.Invasive treatments include forms of catheter ablation such as sinus node modification (selective ablation of the sinus node), complete sinus node ablation (with associated implantation of a permanent artificial pacemaker), and AV node ablation in very resistant cases (creation of iatrogenic complete heart block, necessitating implantation of a permanent artificial pacemaker). However, invasive treatments can also make the symptoms worse.

See also
Supraventricular tachycardia
Sinus tachycardia
Postural orthostatic tachycardia syndrome
Dysautonomia

References
Further reading
Yusuf, Shamil; Camm, A. John (2005). "Deciphering the sinus tachycardias". Clinical Cardiology. 28 (6): 267–76. doi:10.1002/clc.4960280603. PMC 6654702. PMID 16028460.
Still, A; Raatikainen, P;  et al. (2005). "Prevalence, characteristics and natural course of inappropriate sinus tachycardia". Europace. 7 (2): 104–12. doi:10.1016/j.eupc.2004.12.007. PMID 15763524.
Leon, Hernando; Guzman, Juan Camilo;  et al. (2005). "Impaired Baroreflex Gain in Patients with Inappropriate Sinus Tachycardia". Journal of Cardiovascular Electrophysiology. 16 (1): 64–8. doi:10.1046/j.1540-8167.2005.04441.x. PMID 15673390.
Sanchez-Quintana, D; Cabrera, JA;  et al. (2005). "Sinus node revisited in the era of electroanatomical mapping and catheter ablation". Heart. 91 (2): 189–94. doi:10.1136/hrt.2003.031542. PMC 1768731. PMID 15657230.
Cruz Filho, Fernando E. S.; Maia, Ivan G.;  et al. (1998). "Modificação do nódulo sinusal via cateter por energia de radiofreqüência em paciente com taquicardia sinusal inapropriada. Avaliação dos resultados imediatos e tardios" [Sinus node modification by catheter using radiofrequency current in a patient with inappropriate sinus tachycardia. Evaluation of early and late results]. Arquivos Brasileiros de Cardiologia (in Portuguese). 70 (3): 173–6. doi:10.1590/S0066-782X1998000300006. PMID 9674178.
Lee, SH; Cheng, JJ;  et al. (1997). "Radiofrequency catheter modification of sinus node for inappropriate sinus tachycardia: A case report". Zhonghua Yi Xue Za Zhi. 60 (2): 117–23. PMID 9360339.