Inappropriate sinus tachycardia (IST)

The sinus node is a small group of cells at the top of the heart. It sends out the electrical signal that starts each heartbeat. Your autonomic nervous system, the "automatic" system that runs heart rate, blood pressure, and digestion, speeds it up when you need more blood flow, like during exercise, and slows it down when you rest.

In IST, the sinus node fires too quickly, or reacts too strongly to small triggers. Unlike a normal fast heartbeat during exercise or stress, the rate in IST is out of proportion to what you are doing. Your heart may race when you are sitting still, and jump sharply with small efforts like walking up a few stairs, getting dressed, or talking.

Symptoms vary a lot. Some people with a fast heart rate feel very little. Others have many symptoms. Common ones include:

• Palpitations (a racing or pounding heart)
• Fatigue and weakness
• Lightheadedness or feeling close to faint
• Chest discomfort
• Trouble exercising, because the heart rate climbs very fast
• Shortness of breath
• Feeling shaky or on edge, a bit like anxiety

Symptoms and heart rate do not always match: some people feel very unwell even when the rate is only modestly raised.

Many people notice their symptoms are worse:

• With small physical efforts
• With caffeine or alcohol
• During stress
• In heat
• When they are ill with something else

The exact cause is not fully understood. Researchers believe several things may play a role:

• An overactive sinus node: the heart's pacemaker cells may be set to fire faster than normal.
• An imbalance in the autonomic nervous system: too much "accelerator" from the stress system, and too little "brake" from the calming system.
• Immune system activity: in some people, the immune system may affect the heart's pacemaker or its nerves, particularly after an infection.

IST can start after a viral infection, including COVID. It has also been described after other stresses on the body.

It is important to know that IST is a real, physical condition. A racing heart can feel like anxiety, and anxiety can exist alongside IST, but anxiety does not explain a heart rate that stays high day and night.

IST is a diagnosis of exclusion, which means other causes of a fast heart rate must be ruled out first. Diagnosis generally requires:

• A resting daytime sinus heart rate above 100 beats per minute
• An average 24-hour heart rate above 90 beats per minute
• Distressing symptoms
• No other condition that explains the fast rate

To reach this, your doctor will usually arrange:

• An ECG to confirm the rhythm is sinus (coming from the normal pacemaker)
• Heart monitoring over 24 hours or longer, to record your heart rate through the day and link it to your symptoms
• Heart rate and blood pressure lying and standing, to tell IST apart from POTS and from a blood pressure drop on standing
• Blood tests and a review to exclude other causes, such as an overactive thyroid, anaemia or low iron, infection, dehydration, pregnancy, and certain medicines or stimulants
• Sometimes an echocardiogram, an ultrasound of the heart, to check its structure

How IST differs from POTS. In POTS, the heart rate rises mainly when you stand up and settles when you lie down. In IST, the heart rate is high most of the time, lying down too. The two can overlap, and telling them apart matters for choosing treatment. Read the POTS guide.

There is no single cure for IST, but treatment can help many people feel better. It usually starts with everyday measures and adds medicine if needed. Heart rate control and symptom relief do not always go hand in hand, so the goal is to help you feel better, not only to lower a number.

Everyday measures:

• Cut back on stimulants: caffeine, energy drinks, nicotine, alcohol, and recreational stimulants
• Stay well hydrated, and consider more salt and fluid if tolerated
• Manage stress where you can. It will not cure IST, but it can reduce the extra triggers.
• Build up activity gradually, if you tolerate it. But if you have post exertional malaise (PEM), where you crash after effort, exercise training can make you worse. In that case, pacing comes first.
• Treat anything that adds to it, like low iron.

Medicine. If everyday measures are not enough, medicines that slow the heart rate can help. Which medicine suits you depends on your symptoms, your blood pressure, and whether you also have POTS, so it needs to be decided together with a doctor.

Procedures. Catheter based treatment of the sinus node is reserved for severe cases that do not respond to anything else. It has limited success, may need repeating, and carries risks, so it is considered carefully and only after other options fail.

IST is generally not dangerous to the heart, but it can be exhausting and frightening. These things help many people:

• Keep a diary of your heart rate, from a monitor or wearable, alongside your activity and symptoms. It helps you and your doctor see patterns and judge whether treatment is working.
• Plan your day around your better hours.
• Take small efforts slowly, like climbing stairs or carrying shopping.
• Rest before symptoms build up, rather than after.

Many people improve over time, especially when IST starts after an infection. Follow up still matters, so that treatment can be adjusted.

Call or go to the emergency department if you have:

• Chest pain or pressure
• Fainting, especially during exertion or if it causes injury
• Severe shortness of breath
• A very fast or irregular heartbeat that does not settle when you rest

Contact your clinic if:

• Your symptoms are new, worsening, or not controlled by your current plan
• You develop new fainting spells
• A family member has died suddenly at a young age or has a known heart rhythm problem