Inappropriate sinus tachycardia (IST) is a condition in which the sinus node (the heart’s natural pacemaker) fires at an abnormally elevated rate without a physiological reason to explain it. It is defined as a persistent resting heart rate greater than 90 to 100 beats per minute, with an exaggerated and disproportionate increase in heart rate with minimal physical activity. Unlike sinus tachycardia caused by fever, anaemia, dehydration or exercise, IST has no identifiable trigger.
Symptoms include palpitations, breathlessness, chest discomfort, fatigue, lightheadedness and, in some cases, near-fainting. The condition predominantly affects women and often begins in young adulthood. IST can be debilitating and has a significant impact on quality of life, though it is not itself directly life-threatening. The mechanism is thought to involve abnormal autonomic nervous system regulation of the sinus node, with an excess of sympathetic (adrenaline-like) drive or reduced vagal (parasympathetic) tone.
Diagnosis requires a 24-hour Holter monitor or event recorder to document the heart rate pattern over time, combined with investigation to exclude secondary causes of tachycardia including atrial fibrillation, thyroid disease, anaemia and pulmonary embolism. IST should also be distinguished from postural orthostatic tachycardia syndrome (POTS), in which the tachycardia is triggered specifically by standing rather than being persistently elevated at rest. The two conditions can coexist.
Management is challenging. Beta-blockers are first-line treatment and reduce symptoms for many patients, though they do not suit everyone. Ivabradine, a drug that selectively slows the sinus node without affecting blood pressure, is an alternative or add-on therapy. Lifestyle measures including regular gentle exercise, adequate hydration and caffeine reduction are also recommended. For cardiac arrest survivors who develop IST as part of autonomic dysfunction during recovery, symptom management and gradual return to activity form the basis of care.
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