Normal sinus rhythm: ECG diagnostic criteria
Normal sinus rhythm (NSR) has five defining ECG characteristics that must ALL be present:
1. Rate 60–100 bpm: the SA node fires within the physiologic normal range. Rates outside this range (sinus bradycardia < 60, sinus tachycardia > 100) are named variants, not NSR.
2. Upright P wave before every QRS in lead II: the impulse originates at the SA node and depolarizes the atria in the normal superior-to-inferior and right-to-left direction, producing an upright P wave in the inferior leads. A different P-wave axis (inverted, biphasic, variable) indicates an ectopic atrial focus, not sinus origin.
3. Consistent P-wave morphology: all P waves look identical — same height, duration, and shape. Variable P-wave morphology indicates wandering atrial pacemaker or multiple ectopic foci.
4. PR interval 120–200 ms: normal AV conduction time. PR > 200 ms = first-degree AV block. PR < 120 ms = pre-excitation (WPW) or accelerated AV conduction.
5. Narrow QRS < 120 ms: normal ventricular conduction via the His-Purkinje network. Wide QRS indicates bundle branch block or ventricular origin.
One common addition: regular R-R intervals. Note that respiratory sinus arrhythmia — physiologic cyclic rate variation with breathing — can be considered a normal variant and does not disqualify sinus rhythm.
Normal sinus rhythm variants: bradycardia, tachycardia, and RSA
Sinus bradycardia: all NSR criteria met, rate < 60 bpm. Common causes: high vagal tone (athletes, sleep), inferior MI, medications (beta-blockers, calcium channel blockers, digoxin), hypothyroidism. Clinically benign when asymptomatic. Requires treatment only when symptomatic (hypotension, syncope, hemodynamic compromise).
Sinus tachycardia: all NSR criteria met, rate > 100 bpm. Always physiologically driven — fever, pain, anxiety, dehydration, hypoxia, anemia, PE, sepsis, thyrotoxicosis. The rate responds to the clinical state and decreases when the cause is addressed. Treatment targets the cause, not the rate.
Respiratory sinus arrhythmia (RSA): cyclic rate variation synchronized with breathing — faster during inspiration, slower during expiration. All P-wave morphology is consistently sinus. No dropped beats. Most prominent in children and athletes (high vagal tone). Not a pathologic finding.