Sinus Tachycardia
Sinus Tachycardia
Normal sinus rhythm over 100 bpm: a symptom of something else, not an arrhythmia in its own right.
On the trace
Read the strip in this order.
- Take the rate.
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120 bpm, R-R 500 ms. Over 100, narrow, and regular. That description covers several rhythms, so the next two steps are what separate them. - Find a P wave before every QRS.
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There is one, upright, 0.12 mV, the same shape every time. That single finding excludes most of the differential, because the re-entrant tachycardias either bury their P inside the QRS or put it behind. - Measure the PR.
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120 ms, at the short end of normal and shorter than the 175 ms of the same heart at 72. Sympathetic drive speeds conduction through the AV node as well as speeding the node itself. - Measure the QRS.
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74 ms, unchanged. Whatever is driving the rate, it is not changing the route. - Look at where the P is sitting.
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Still clear of the preceding T here. Push the rate higher and it climbs onto the T and becomes hard to find, which is the point at which this gets confused with the re-entrant tachycardias.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Rate | 100 to about 150 in adults | 120 here. Past 150, sinus tachycardia becomes much less likely at rest. |
| P wave | Upright, one per QRS | The finding that excludes the re-entrant tachycardias. |
| PR interval | Normal, often short | 120 ms here against 175 at rest. Sympathetic drive speeds the node. |
| QRS | Narrow, 74 ms here | Unchanged from rest. |
| Onset and offset | Gradual | It speeds up and slows down with whatever is driving it. |
| Response to vagal manoeuvres | Slows, then returns | It does not terminate. A rhythm that stops abruptly was re-entrant. |
The strip
Mechanism
Sympathetic drive and vagal withdrawal both raise the rate at which the sinus node reaches threshold. Nothing else changes: the impulse leaves the node and travels exactly as it always did.
That is why this is not really an arrhythmia. It is the normal pacemaker doing what it is designed to do, in response to something else. The useful question is never how to slow the rate. It is what the heart is compensating for.
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Background
The P wave, the QRS and the route between them are unchanged, which is why sinus tachycardia is better read as a vital sign than as a rhythm disturbance.
The list worth running through
Pain, fear and exertion. Fever, in which the rate rises roughly ten beats for each degree. Hypovolaemia from bleeding or dehydration. Anaemia, hypoxia, sepsis, pulmonary embolism, heart failure and thyrotoxicosis. Alcohol withdrawal and stimulants. Salbutamol, and the anticholinergics. Most of these are found by examining the patient rather than by looking harder at the ECG.
The arithmetic that catches people out
The maximum sinus rate falls with age, roughly as 220 minus the age in years. A resting rate of 170 is therefore plausible in a frightened child and implausible in a 70 year old, in whom it points at a re-entrant tachycardia or flutter with 2:1 conduction. Rate alone does not diagnose, but it changes the prior a lot.
Why treating the rate can be harmful
Tachycardia in hypovolaemia or in early heart failure is maintaining cardiac output. Slowing it with a beta-blocker removes the compensation and the blood pressure follows. The exceptions are the situations in which the rate is itself the problem, such as thyrotoxicosis or ongoing ischaemia, and those are diagnoses rather than reflexes.
Inappropriate sinus tachycardia
A persistently raised resting sinus rate with no identifiable cause, usually in young women, with symptoms out of proportion to the rate. It is a diagnosis of exclusion, made only after the list above has been worked through properly, and it is managed quite differently from a compensatory tachycardia.
Clinical impact
A sinus tachycardia on a monitor is a prompt to examine the patient. It is one of the earliest signs of shock and one of the least specific findings in medicine, and both of those are reasons to take it seriously rather than to treat it.
Management
Find the cause, and treat that
Fluids for hypovolaemia, oxygen for hypoxia, analgesia for pain, antibiotics for sepsis. The rate follows the cause down.
Do not rate-control a compensatory tachycardia
Slowing a heart that is defending its output lowers the blood pressure. Beta-blockade here can precipitate the collapse it was meant to prevent.
Use vagal manoeuvres to diagnose, not to treat
A sinus tachycardia slows briefly and returns. A re-entrant tachycardia stops. That difference is the answer to the question the strip poses.
Question the diagnosis above about 150 at rest
Flutter with 2:1 block sits at 150 and is the classic miss. Look for a second P wave in the T.
Differential
The one that matters. Abrupt onset and offset, no visible P, and it terminates with adenosine. This slows and comes back.
Sits at almost exactly 150 with 2:1 conduction and is read as sinus tachycardia routinely. The baseline between complexes is never flat.
Also a P before every QRS, but a P of the wrong shape, and it warms up rather than tracking a stressor.
The same trace under 100. At high rates the P climbs onto the preceding T, which is where the two become hard to tell apart.
References
- Sinus Tachycardia — Life in the Fast Lane, ECG Library, 2024
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020