Inappropriate sinus tachycardia (IST) is defined as sinus tachycardia that is not caused by identifiable medical ailments, a physiological reaction, or pharmaceuticals (a diagnosis of exclusion) and is accompanied by symptoms, frequently invalidating and affecting quality of life. IST symptoms include palpitations, chest discomfort, exhaustion, shortness of breath, presyncope, and syncope. While sinus tachycardia is very common and is the most common type of tachycardia, it is rare to be diagnosed with inappropriate sinus tachycardia as an independent symptom that is not part of a larger condition. Although somewhat rarely diagnosed, 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.
Signs and symptoms Palpitations are the most common symptom in roughly 90% of patients. Other symptoms include chest pain, fatigue, shortness of breath, presyncope, syncope, reduced exercise tolerance, anxiety, panic attacks, and headaches. These symptoms are usually associated with an elevated heart rate. While some patients have persistently elevated sinus rates, others have paroxysmal episodes with normal heart rates in between.
Causes The exact cause of Inappropriate sinus tachycardia is still being debated and remains unknown. Several mechanisms have been suggested, including increased sympathetic or decreased parasympathetic drive, increased intrinsic heart rate, dysfunctional neurohormonal modulation, ectopic sinus node activity, and beta-adrenergic receptor autoantibodies. Some data show an abnormal response to autonomic stimulation as a result of tissue/cell level changes (intrinsic mechanism), whereas others show a disruption of the autonomic stimulation itself with normal tissues/cell level findings (extrinsic mechanism). It is possible that both mechanistic theories are correct because, despite sharing a single common pathway of sinus tachycardia, individual patients' underlying mechanistic etiologies may differ.
Mechanism Over 15 electrical currents tightly control the sinus node. The calcium clock and the funny current appear to be the most important currents in regulating sinus node rate. Sinus activation and thus heart rate is regulated further through the autonomic nervous system. At rest, the sinus node is primarily regulated by tonic and phasic parasympathetic activation in normal, healthy individuals. Exercise causes vagal activation, sympathetic activation, and increases in catecholamine levels, which raises sinus rates. Any aspect of regular regulatory processes influencing sinus rate may be compromised in patients with IST.
Diagnosis Inappropriate sinus tachycardia is diagnosed when there is persistent or recurrent sinus tachycardia on a 12-lead electrogram or long-term monitoring that is not explained by other means. Invasive testing, such as electrophysiology studies, are not helpful in making the diagnosis, but they may be useful in ruling out a concomitant supraventricular tachycardia mechanism. Inappropriate sinus tachycardia is a diagnosis of exclusion that is rarely made in an asymptomatic patient. The following criteria are commonly used to define inappropriate sinus tachycardia:
The axis and morphology of the P wave during tachycardia similar to or identical to that experienced during sinus rhythm A resting heart rate of 100 beats per minute or an increase in heart rate of 100 beats per minute with minimal exertion Excluding any potential secondary causes of sinus tachycardia Ruling out atrial tachycardias Palpitations or presyncope (or both) symptoms that have been clearly linked to resting or easily induced sinus tachycardia. Secondary causes of sinus tachycardia must be ruled out and corrected if present. A full endocrinology evaluation for disease entities such as hyperthyroidism, pheochromocytoma, and diabetes mellitus with evidence of autonomic dysfunction should be included in the evaluation for inappropriate sinus tachycardia.
Differential diagnosis Inappropriate sinus tachycardia 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) and hereditary myopathy with lactic acidosis (Larsson–Linderholm syndrome). Sinus tachycardia is a feature of both postural orthostatic tachycardia syndrome and Inappropriate sinus tachycardia. In POTS, there's an abnormal response by the autonomic nervous system when standing up. POTS symptoms are most common when the patient is upright. POTS syndromes and inappropriate sinus tachycardia may overlap, raising the possibility of shared mechanisms. The most common symptoms of POTS are dizziness and, on occasion, syncope, which are also common in IST. Sinus node reentry is another differential diagnosis for Inappropriate sinus tachycardia. An ectopic atrial rhythm occurring near the sinus node may also mimic Inappropriate sinus tachycardia. Syncope or pre-syncope may occur in IST patients and be the dominant symptom, with associated prodromal symptoms such as diaphoresis and visual blurring, leading to the diagnosis of vasovagal syncope and the diagnosis of IST being overlooked.
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