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
• Chest pain
• Dyspnea (shortness of breath) and palpitations on exertion
• Tachypnea (rapid breathing)
• Pre-syncope (feeling as if about to faint)
• Fatigue
• Dizziness
• Exercise intolerance
• Headaches
• 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
• Phaeochromocytoma 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 I_f 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.

Differential diagnoses

IST is primarily a diagnosis of exclusion. Upon exertion, an inappropriate heart rate response of sinus tachycardia can be seen in some Inborn Errors of Metabolism that result in metabolic myopathies, such as McArdle Disease (GSD-V). Rare diseases are more likely to be misdiagnosed, indeed, "Ninety percent of people with GSD V received a misdiagnosis before a corrected diagnosis (GSD VII unknown), resulting in a median diagnostic delay of 29 years, which can seriously affect QoL [Quality of Life]… Of those who are misdiagnosed, 62% report being misdiagnosed more than once."

See also

• Supraventricular tachycardia
• Sinus tachycardia
• Postural orthostatic tachycardia syndrome
• Dysautonomia
• Metabolic myopathies

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.