Inappropriate sinus tachycardia (IST) is a condition where your heart beats faster than it should, even when you’re resting, relaxed, or doing very little physical activity. A resting heart rate above 100 beats per minute, or an average rate above 90 over a 24-hour period, with no other medical explanation points toward IST. Unlike a fast heart rate from exercise, fever, or stress, this elevated rate has no proportional trigger, and it often comes with symptoms that can significantly disrupt daily life.
Who Gets IST
IST is not especially rare. A population study of 604 middle-aged adults found that about 1.2% met the diagnostic criteria. It disproportionately affects women. In clinical settings, roughly 90% of patients diagnosed with IST are female, and the average age at diagnosis tends to fall in the mid-40s. That said, it can appear in younger adults and occasionally in men. Many people live with the symptoms for years before receiving a formal diagnosis, partly because the condition is one of exclusion: other causes of a fast heart rate have to be ruled out first.
What It Feels Like
The most common symptom is palpitations, that unmistakable awareness of your own heartbeat pounding or racing. But IST rarely stops there. Dizziness, lightheadedness, near-fainting episodes, and anxiety are all frequently reported. Some people feel short of breath or fatigued with minimal exertion. The symptoms can be persistent or come and go in episodes, and for many patients they are genuinely debilitating, limiting the ability to work, exercise, or handle routine tasks without distress.
Quality of life takes a real hit. Because the heart rate stays elevated throughout the day, even mundane activities like standing up from a chair, climbing a few stairs, or walking across a parking lot can trigger a wave of symptoms. The chronic nature of IST sets it apart from occasional episodes of a racing heart that most people experience now and then.
What Causes the Fast Rate
Your heart’s natural pacemaker, a small cluster of cells in the upper right chamber, sets the rhythm for every beat. In IST, this pacemaker fires too quickly on its own. The exact reason is not fully understood, but the leading theories involve two overlapping problems. First, the pacemaker cells themselves may be hypersensitive, responding to normal levels of adrenaline as though the body were under stress. Second, the balance between the “gas pedal” and “brake pedal” branches of the nervous system that regulate heart rate may be tilted toward acceleration, with too much stimulation or too little calming input.
Neither of these mechanisms involves structural damage to the heart. The electrical signal still originates in the correct place and travels through the heart normally. It just fires more often than it should.
How IST Differs From POTS
Postural orthostatic tachycardia syndrome (POTS) is the condition most commonly confused with IST, and the two can look similar on the surface. The key difference lies in what happens when you lie down. A person with POTS will typically have a normal heart rate while flat on their back. Their heart rate spikes dramatically when they stand up, often jumping 30 or more beats per minute. A person with IST, by contrast, still has an elevated heart rate even while lying flat, often in the 90s or higher.
A 24-hour heart monitor makes the distinction clearer. Because POTS patients spend most of their day and night with a normal rate (the spikes happen mainly with position changes), their average heart rate over 24 hours comes back normal. An IST patient’s average will be elevated, classically above 100 but often at least in the high 90s, because the rate stays up regardless of position or activity level.
How IST Is Diagnosed
There is no single test that confirms IST. Instead, doctors work through a checklist of other explanations for a fast heart rate: thyroid disorders, anemia, dehydration, medication side effects, anxiety disorders, and other heart rhythm problems. If none of those account for the elevated rate and you have distressing symptoms that seem tied to the tachycardia, IST becomes the working diagnosis.
The diagnostic workup typically includes blood tests, an electrocardiogram to confirm the rhythm is originating from the normal pacemaker site, and often a 24-hour Holter monitor to capture the average heart rate over a full day. A tilt-table test may be used to help separate IST from POTS.
Treatment Options
Beta-blockers are the most commonly prescribed first-line treatment. These medications slow the heart rate by blunting the effect of adrenaline on the heart. Metoprolol is one of the most frequently used. Calcium channel blockers like verapamil are another option. Both can reduce resting heart rate, but side effects like low blood pressure and excessive slowing of the heart sometimes force dose reductions. In one study, half the patients on metoprolol needed their dose lowered because of blood pressure drops or an overly slow heart rate.
Ivabradine works through a different mechanism. Rather than blocking adrenaline, it directly slows the firing rate of the heart’s pacemaker cells. In a head-to-head comparison with metoprolol, both drugs lowered resting heart rate by a similar amount, but ivabradine performed notably better during daily activity and exercise. Seventy percent of patients on ivabradine became completely free of IST-related symptoms, and the drug was well tolerated. Every patient in the study was able to reach the full prescribed dose, compared to only half on metoprolol. For people whose symptoms flare most during movement and exertion, ivabradine can be a meaningful step up.
When Medication Is Not Enough
For patients who do not respond to any combination of medications, catheter ablation of the sinus node is sometimes considered. The goal is to modify the pacemaker tissue so it fires at a more appropriate rate. A hybrid surgical approach has shown the ability to restore a normal heart rate in about 84% of patients, but the procedure carries serious risks. In one registry, half of the patients who underwent ablation of the sinus node eventually needed a permanent pacemaker, nearly half developed inflammation around the heart, and about 14% experienced injury to a nerve that controls the diaphragm. Because of these complication rates, ablation is generally reserved for severe cases that have exhausted other options.
Long-Term Outlook
IST is not considered dangerous to the heart in most cases. It does not typically cause structural heart damage the way some other sustained fast rhythms can. The real burden is on quality of life. Many patients describe years of frustration, both from the symptoms themselves and from the difficulty of getting a clear diagnosis. The condition can wax and wane over time, with some people experiencing improvement as they age and others dealing with persistent symptoms that require ongoing management. Finding the right medication or combination often takes trial and adjustment, but most people do achieve meaningful symptom relief with treatment.