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# Sinus, junctional and escape rhythms

- Regulation
- Exam priority: medium

The sinus node sets the pace and speeds up or slows down with need. If it or the wiring fails, slower backup pacemakers take over after a pause.

Updated Nov 13, 2026

Also called: Sinus tachycardia, Respiratory sinus arrhythmia, Junctional premature beats, Overdrive suppression, Stokes–Adams attacks

**See it in 3D:** [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/) · [Atrioventricular (AV) node](https://durgical.com/en/anatomy/atrioventricular-node/) · [Bundle of His](https://durgical.com/en/anatomy/atrioventricular-bundle/) · [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/)

## Key points

- Sinus rhythm: upright P in I, II and aVF before every QRS, constant PR; tachycardia > 100/min, bradycardia

## What it is

The sinus node is the fastest natural pacemaker, so it normally drives the heart and keeps the slower sites below it silent. Its rate changes with autonomic tone, temperature and hormones, which gives sinus tachycardia, sinus bradycardia and the normal breath-by-breath variation. When the sinus impulse fails or is blocked, the AV junction or the ventricles take over as escape pacemakers, but only after a delay.

## How it works, step by step

- Sinus node cells depolarize slowly on their own (funny current If, then Ca²⁺ currents) until they reach threshold.
- Their intrinsic rate (≈ 100/min) is higher than that of the AV junction or Purkinje fibres, so they fire first and reset the others.
- At rest vagal tone dominates, which is why resting sinus rate (≈ 60–80/min) is below the intrinsic rate.
- If the sinus node fails, a junctional cell can fire: the atria are then activated upward, so P is inverted in the inferior leads.
- Whether the inverted P falls before, inside or after the QRS depends on which arrives first: the impulse to the atria or to the ventricles.
- After sudden complete AV block the lower pacemakers are still suppressed by the earlier fast drive, so no beat comes for ≈ 5–30 s.
- The brain tolerates only a few seconds without flow: the person faints until a ventricular escape rhythm starts and restores some output.

### Electrocardiogram: a normal beat in lead II

**HR** 100 /min **RR** 0.60 s **PR** 0.16 s **QRS** 0.09 s **QT** 0.33 s **QTc** 0.43 s

Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.

- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part

Sinus rhythm at 100/min, the upper limit of normal: upright P before every QRS.

### Action potential

**Maximum diastolic potential** −60 mV **Threshold** −40 mV **Peak** +10 mV **Cycle** ≈ 700 ms (≈ 85 bpm)

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

- Mainly Na⁺
- Mainly K⁺
- Mainly Ca²⁺
- Maximum diastolic potential
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period

The slow diastolic depolarization that the autonomic nerves speed up or slow down.

## Pacemaker rates (adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Resting sinus rate | ≈ 60–100 /min | |
| Intrinsic sinus rate (no autonomic input) | ≈ 100 /min | falls with age |
| Junctional escape | ≈ 40–60 /min | narrow QRS |
| Ventricular escape | ≈ 15–40 /min | wide QRS |
| PR in a junctional beat with P first |
- *+* **Sympathetic (noradrenaline on β1)**: Raises If and Ca²⁺ current, steepens the pacemaker slope: faster sinus rate.
- *+* **Fever, thyroid hormone**: Speed up pacemaker currents: rate rises roughly 10/min per °C of fever.
- *+* **Inspiration**: Lowers vagal tone to the sinus node (lung stretch, brainstem coupling): rate rises.
- *−* **Vagus (acetylcholine on M2)**: Opens K⁺ channels and slows If: more negative start, flatter slope, slower rate.
- *−* **Baroreceptor reflex**: High pressure → more vagal, less sympathetic drive → bradycardia.
- *−* **Faster upstream pacemaker**: Overdrive suppression: repeated firing loads cells with Na⁺, the Na⁺/K⁺ pump hyperpolarizes them and they stay silent.

## Clinical notes

- Ask why a sinus tachycardia is present: treating the rate without the cause (bleeding, sepsis, pulmonary embolism) is dangerous.
- Loss of respiratory sinus arrhythmia (fixed heart rate) is an early sign of autonomic neuropathy, as in diabetes.
- Postural tachycardia syndrome: an excessive sinus rate rise on standing without a fall in blood pressure.
- Sick sinus syndrome alternates slow sinus rates and pauses with bursts of tachycardia; long pauses can cause syncope.

## Memory aid

The fastest drummer leads the band; when he stops, the backup drummer needs a few seconds to wake up.

## Test yourself

Think of the answer, then tap the question to see it.

## A healthy 19-year-old's heart rate varies from 58 to 76/min in time with breathing. What is this, and what drives it?

Respiratory sinus arrhythmia, a normal finding. Inspiration lowers vagal tone to the sinus node and the rate rises; expiration restores it.

## A premature narrow QRS is preceded by an inverted P in II, III and aVF with a PR of 0.08 s. Where did the beat start?

In the AV junction: the atria are activated upward (retrograde), so P is inverted inferiorly, and the PR is short because the impulse starts near the ventricles.

## Why can a patient with sudden complete AV block faint even though the ventricles have their own pacemakers?

Overdrive suppression: lower pacemakers were held hyperpolarized by the fast sinus drive and need ≈ 5–30 s to start. Without flow the brain fails within seconds (Stokes–Adams attack).

## Which drug lowers sinus rate by blocking the funny current, and why does it not affect contractility?

Ivabradine. It acts on the If channel of pacemaker cells, which is not involved in the excitation–contraction coupling of working ventricular muscle.

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## Connections

## See it in 3D 4

The structures of Durgical's 3D anatomy where it happens.

- [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/)
- [Atrioventricular (AV) node](https://durgical.com/en/anatomy/atrioventricular-node/)
- [Bundle of His](https://durgical.com/en/anatomy/atrioventricular-bundle/)
- [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/)

## Related topics 5

- [The SA node pacemaker potential](https://durgical.com/en/physiology/the-sa-node-pacemaker-potential/)
- [The cardiac conduction system](https://durgical.com/en/physiology/the-cardiac-conduction-system/)
- [How arrhythmias arise](https://durgical.com/en/physiology/how-arrhythmias-arise/)
- [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/)
- [Nervous control of the circulation and the CNS ischemic response](https://durgical.com/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/)

## Related conditions 5

- [Sinus node dysfunction (sick sinus syndrome) and symptomatic bradycardia](https://durgical.com/en/conditions/sinus-node-dysfunction-and-symptomatic-bradycardia/)
- [Atrioventricular (heart) block](https://durgical.com/en/conditions/atrioventricular-block/)
- [Postural orthostatic tachycardia syndrome (POTS) and inappropriate sinus tachycardia](https://durgical.com/en/conditions/postural-orthostatic-tachycardia-syndrome-and-inappropriate-sinus-tachycardia/)
- [Premature atrial and ventricular contractions](https://durgical.com/en/conditions/premature-atrial-and-ventricular-contractions/)
- [Syncope (vasovagal, orthostatic hypotension, cardiac)](https://durgical.com/en/conditions/syncope/)

## In Pharmacology 3

- [Atropine](https://durgical.com/en/pharmacology/atropine/)
- [Ivabradine](https://durgical.com/en/pharmacology/ivabradine/)
- [Metoprolol](https://durgical.com/en/pharmacology/metoprolol/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Arrhythmia | Irregular Heartbeat | MedlinePlus](https://medlineplus.gov/arrhythmia.html) (MedlinePlus, US National Library of Medicine, 2025); [Physiology, AV Junction](https://www.ncbi.nlm.nih.gov/books/NBK546663/) (StatPearls (NCBI Bookshelf), 2025); [Atrioventricular Block](https://www.ncbi.nlm.nih.gov/books/NBK459147/) (StatPearls (NCBI Bookshelf), 2025).

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