ECG rhythms Sinus rhythms
Sinus bradycardia ECG
Sinus bradycardia. Sinus P waves with a normal PR before every QRS, regular, at a rate below 60/min.
A 30-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 40/min |
|---|---|
| PR | 158 ms |
| QRS | 84 ms |
| QT | 433 ms |
| QTc (Bazett) | 353 ms |
| QTc (Fridericia) | 378 ms |
What it is
A sinus rhythm slower than normal: every beat still starts in the sinus node, with a normal P wave and PR. The 2018 ACC/AHA/HRS guideline defines it as a sinus rate below 50/min; many texts use below 60/min.
How to read this strip
| Rate | Below 50/min — sinus bradycardia by the 2018 ACC/AHA/HRS definition (below 50/min) and by the conventional below-60 cut-off. |
|---|---|
| Rhythm | Regular. |
| P waves | Upright in II, one before every QRS. |
| PR | Normal and constant. |
| QRS | Narrow. |
| QT | Normal for the rate. |
Causes
- Physiological: athletic training, sleep, high vagal tone.
- Drugs: beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone, ivabradine.
- Sinus node dysfunction (age-related fibrosis).
- Inferior myocardial infarction (vagal reflex, or ischemia of the sinus node artery, usually from the right coronary).
- Hypoxia (especially in children and critically ill patients); sedatives such as dexmedetomidine and opioids.
- Hypothyroidism, hypothermia, hyperkalemia, obstructive sleep apnea.
- Increased intracranial pressure (with hypertension: the Cushing response).
What to consider
- Symptoms decide significance: dizziness, syncope, hypotension, confusion, chest pain or heart failure mean the rate is too slow for this patient.
- Review medications, potassium, thyroid function and temperature when relevant.
- Unstable bradycardia follows the adult bradycardia algorithm and local protocols (atropine; if ineffective, transcutaneous pacing or an epinephrine or dopamine infusion, with expert consultation). Correct hypoxia first, especially in children.
- When sinus node dysfunction is suspected, longer monitoring looks for pauses and a rate that fails to rise with exertion.
Key point
Treat the patient, not the number: a slow sinus rate in a well athlete is normal physiology; the same rate with low blood pressure and confusion is an emergency.
Not to be confused with
- Junctional escape rhythm (simulator) — A junctional rhythm has no normal P wave before the QRS. Here an upright P wave precedes every QRS.
- Second-degree AV block with 2:1 conduction — Every P wave is followed by a QRS. No P wave is blocked.
- Third-degree AV block — P waves and QRS complexes are linked by a constant PR. In complete block they run independently.