Rhythm Foundations
Eleven common rhythms, each shown on twelve-lead ECGs. Wherever possible, the examples below are real patient recordings; the few that are not are clearly labelled as synthetic. For each rhythm you get a citable definition, a step-by-step way to recognize it, and several examples to scroll through.
Read each strip with the systematic method from EKG Foundations: rate, regularity, P waves, the PR interval, the P-to-QRS relationship, and QRS width. Lead II is shown by default; switch to V1 where it reads better, and turn on the calipers to measure any interval.
Normal Sinus Rhythm
SinusNormal sinus rhythm is the heart's physiologic baseline, in which each beat is initiated by the sinoatrial node and conducts normally through the AV node and His-Purkinje system. Its defining hallmark is a P wave of normal sinus axis (upright in lead II, inverted in aVR) preceding every QRS, with a constant PR interval, at a rate of 60 to 100 bpm. 1
How to identify
Rate
60 to 100 bpm (under 60 = sinus bradycardia, over 100 = sinus tachycardia); some current guidance uses 50 as the lower limit
Regularity
essentially regular; mild beat-to-beat R-R variation with the respiratory cycle (respiratory sinus arrhythmia) is normal and is expected in young, healthy patients
P waves
one P of normal sinus axis (upright in I, II, and aVF; inverted in aVR) and uniform morphology before every QRS (the V1 P is commonly upright or biphasic and is not itself a defining feature)
PR interval
constant and normal at 120 to 200 ms
P:QRS relationship
1:1, every P followed by a QRS and every QRS preceded by a P
QRS width
narrow, under 120 ms (a normal adult QRS is about 80 to 100 ms; a wide QRS implies a conduction delay or aberrancy, not the rhythm origin)
Read sinus rhythm in lead II, where the normal P-wave vector points toward the positive electrode and is most reliably upright; lead V1 helps confirm the normal biphasic P morphology.
The single discriminating feature is a consistent sinus P-wave axis: upright in lead II and inverted in aVR before every QRS, with a constant PR. Lose the upright lead II P (or find an inverted lead II P or a varying P axis) and the rhythm is no longer sinus, separating NSR from ectopic atrial, junctional, and low-atrial rhythms that can mimic a normal rate. A changing PR with normal sinus P waves is still a sinus rhythm, but with an AV conduction problem (for example Wenckebach), so it is not normal sinus rhythm.
Sinus Tachycardia
SinusSinus tachycardia is a sinus rhythm (impulse arising from the SA node) at a rate above 100 bpm. The defining hallmark is a regular rhythm with a normal, upright sinus P wave preceding every QRS, similar in morphology to the resting sinus P wave. 2
How to identify
Rate
atrial and ventricular rate > 100 bpm (typically 100 to 180 bpm in adults; rarely exceeds ~220 minus age)
Regularity
regular, with only gradual onset and offset (not abrupt like reentrant tachycardias)
P waves
one upright sinus P wave before every QRS, upright in II and inverted in aVR, with a morphology similar to the resting sinus P (it often grows taller in the inferior leads at faster rates)
PR interval
normal and constant, 120 to 200 ms (may shorten slightly at fast rates)
P:QRS relationship
1:1 conduction
QRS width
narrow (< 120 ms) unless a pre-existing bundle branch block or aberrancy is present
Lead II reads it best because the sinus P wave is reliably upright and well formed there; cross-check aVR (P should be inverted) and lead V1 to find P waves hidden in the preceding T wave at fast rates.
The most discriminating feature is a normal upright sinus P wave (upright in II, inverted in aVR) before every QRS with gradual rate changes; this separates it from atrial tachycardia (abnormal P axis) and from AVNRT/atrial flutter (abrupt onset, hidden or flutter waves), and at very fast rates the P may bury in the preceding T wave; compare with an earlier strip at a slower rate, or look in V1 and aVR for the hidden P.
Sinus Bradycardia
SinusSinus bradycardia is a sinus rhythm in which the sinoatrial node depolarizes at a rate below 60 bpm, the conventional teaching threshold (the 2018 ACC/AHA/HRS guideline and current ACLS guidance use below 50 bpm for clinically meaningful bradycardia). The hallmark is a regular rhythm with a normal, upright P wave before every QRS, identical in morphology to normal sinus, simply at a slow rate. 3
How to identify
Rate
atrial and ventricular rate less than 60 bpm (often 40 to 59)
Regularity
regular, with each P wave conducted (sinus arrhythmia may add mild beat-to-beat variation)
P waves
one uniform sinus P before every QRS, upright in II and inverted in aVR
PR interval
normal and constant, 120 to 200 ms
P:QRS relationship
1:1, every P followed by a QRS
QRS width
typically narrow (less than 120 ms) unless a coexisting bundle branch block widens it
Read lead II to confirm the upright sinus P morphology and the 1:1 P:QRS relationship; a long rhythm strip helps gauge rate and reveals associated pauses or sinus node dysfunction.
The discriminating feature is a normal upright sinus P wave (II positive, aVR negative) before every QRS at a slow rate; if P waves are absent or the rhythm is irregularly irregular, suspect a junctional escape rhythm or slow atrial fibrillation instead.
First-Degree AV Block
AV blockFirst-degree AV block is delayed conduction through the AV node (or, less often, the His-Purkinje system) that slows but never interrupts atrial-to-ventricular transmission. The defining hallmark is a fixed, uniformly prolonged PR interval over 200 ms in which every P wave still conducts to a QRS (no dropped beats). 3
How to identify
Rate
usually normal (often sinus, 60 to 100 bpm); the block itself does not change rate
Regularity
regular, because every P conducts and no beats are dropped
P waves
one normal, upright P (in lead II) precedes every QRS
PR interval
prolonged and CONSTANT, > 200 ms (greater than one large box), beat to beat
P:QRS relationship
strict 1:1 (every P followed by a QRS)
QRS width
typically narrow (< 120 ms) when delay is nodal; may be wide if a coexisting bundle branch block points to infranodal delay
Lead II reads it best: the P wave is clearly upright and the long P-to-QRS gap is easy to measure against the gridlines; confirm the P is not buried in the preceding T wave when the PR is very long.
The PR is long but FIXED with no dropped beats: a constant prolonged PR separates it from Mobitz I (progressively lengthening PR before a dropped beat) and from higher-grade block where some P waves fail to conduct.
Second-Degree AV Block, Mobitz Type I (Wenckebach)
AV blockSecond-degree AV block Mobitz type I (Wenckebach) is a conduction disorder, usually within the AV node, in which the PR interval lengthens progressively from beat to beat (the absolute PR grows while the amount it grows by gets smaller each beat) until one P wave is blocked and its QRS is dropped, after which the cycle resets and repeats. The defining hallmark is progressive PR prolongation ending in a single non-conducted P wave (a dropped QRS). 3
How to identify
Rate
atrial rate is set by the sinus node and is normal or near normal; ventricular rate is slightly slower because of the periodically dropped beats
Regularity
irregular in a repeating pattern (group beating, e.g. a 4:3 or 3:2 cycle). In the typical case the R-R intervals progressively shorten before the dropped QRS (because each PR increment is smaller than the last), and the pause containing the dropped beat is less than twice the shortest R-R interval
P waves
normal, upright, uniform sinus P waves, and the P-P interval stays regular throughout (more P waves than QRS complexes)
PR interval
progressively lengthens beat to beat until one P wave fails to conduct; the first conducted PR after the pause is the SHORTEST of the cycle (it may be normal or already prolonged, not necessarily ~200 ms)
P:QRS relationship
more P waves than QRS complexes, with a single dropped QRS per cycle (e.g. 3:2 or 4:3 conduction)
QRS width
usually narrow (< 120 ms), which supports a block at the AV node (above the His bundle); a narrow QRS favors but does not prove a nodal location, and Mobitz I is occasionally infra-nodal (more likely if the QRS is wide)
Read it on the lead with the clearest P waves and longest visible rhythm strip, typically lead II (or V1), so you can track the progressive PR change across several beats and spot the dropped QRS.
The single most discriminating feature is that the PR interval lengthens progressively before the dropped beat and then resets to its shortest after the pause; if the PR stays constant before each dropped QRS it is Mobitz type II, not type I. Because each PR increment shrinks, the R-R intervals typically shorten just before the dropped beat, and the pause is less than two of the shortest cycles.
Second-Degree AV Block, Mobitz Type II
AV blockMobitz Type II second-degree AV block is an intermittent, all-or-none conduction failure in the infranodal conduction system (the His bundle or, more often, the bundle branches), so some sinus P waves abruptly fail to conduct to the ventricles. The hallmark is a CONSTANT PR interval on all conducted beats followed by a sudden non-conducted P wave (dropped QRS) with no preceding PR lengthening and no change in PR after the pause. 3
How to identify
Rate
the atrial rate is set by the sinus node (often normal); the ventricular rate is lower because of the dropped beats. The sinus P-P marches through undisturbed, so the pause containing the blocked P is about two P-P intervals for a single dropped beat.
Regularity
the P-P interval is regular. The R-R is regular until a QRS is dropped, producing a sudden pause; a fixed conduction ratio (such as 3:2) gives a regularly irregular ventricular rhythm, while a variable ratio gives an irregularly irregular one.
P waves
normal, uniform, upright sinus P waves at a constant P-P interval, with more P waves than QRS complexes.
PR interval
CONSTANT and identical on every conducted beat, both before and after the dropped beat, with no progressive lengthening. This fixed PR is the defining contrast with Mobitz I (Wenckebach).
P:QRS relationship
intermittently greater than 1:1 (for example 3:2 or 4:3) with one non-conducted P wave and no PR change. Note the 2:1 trap: with 2:1 conduction there is only one conducted PR before each dropped P, so Mobitz I and II cannot be distinguished and the block is called indeterminate.
QRS width
often wide (greater than 120 ms) when the block is in the bundle branches, but it can be narrow when the block is intra-Hisian. QRS width is supportive only, not diagnostic; the diagnosis rests on the fixed PR interval, and a narrow QRS with a dropped beat should also prompt consideration of AV-nodal Wenckebach.
Read it in a long rhythm strip (lead II or V1) where consecutive P waves and PR intervals are clearly visible, so you can confirm the PR stays constant and lay calipers on the P-P interval to show the blocked P marches through the pause on time.
The discriminating feature is a fixed PR interval across consecutive conducted beats: if the PR is identical before and after the dropped QRS it is Mobitz II, whereas progressive PR lengthening that resets after the drop is Mobitz I (Wenckebach). This comparison requires at least two conducted beats in a row, so 2:1 block is indeterminate. Because the lesion is infranodal, Mobitz II carries a much higher risk of sudden progression to complete heart block, so it is treated as a high-risk rhythm that needs prompt clinical review.
Third-Degree (Complete) AV Block
AV blockThird-degree (complete) AV block is a total failure of conduction across the AV node or His-Purkinje system, so no atrial impulse reaches the ventricles. The hallmark is complete AV dissociation: P waves and QRS complexes march out at their own independent rates, with the atrial rate faster than a slower, escape-driven ventricular rate. 3
How to identify
Rate
two independent rates, with the atrial (P) rate faster than the ventricular (QRS) rate; the escape rate is typically 40 to 60 bpm for a junctional escape and 20 to 40 bpm for a ventricular escape.
Regularity
P-P intervals are regular and R-R intervals are regular, but the two rhythms are unrelated to each other (regular but dissociated).
P waves
present and regular at the underlying sinus rate; some P waves fall on, before, or after QRS complexes with no fixed link.
PR interval
no consistent PR interval; P waves and QRS complexes are dissociated. In second-degree and high-grade block some P waves still conduct with a reproducible PR (fixed in Mobitz II and high-grade block, lengthening then resetting in Mobitz I).
P:QRS relationship
complete AV dissociation with more P waves than QRS complexes and NO P wave conducting to the ventricles.
QRS width
narrow (under 120 ms) suggests a junctional/AV-nodal escape and a higher block; wide (120 ms or more) suggests an infranodal ventricular escape and a slower, less reliable rhythm.
Use a long lead II rhythm strip, where upright P waves are clearest, to march out the P-P and R-R intervals separately and prove the P waves bear no fixed relationship to the QRS complexes.
The discriminating feature of complete (third-degree) AV block is that the atrial rate clearly exceeds the ventricular rate, the PR intervals vary randomly, and NO P wave ever conducts despite ample opportunity, which proves true conduction block. This separates it from interference (usurpation) AV dissociation, where the ventricular pacemaker (an accelerated junctional rhythm or AIVR) is equal to or faster than the atrial rate, so impulses fail to conduct only because the junction is physiologically refractory rather than blocked, and from isorhythmic dissociation, where the atrial and ventricular rates are nearly identical. It also differs from second-degree and high-grade block, where some P waves still conduct with a consistent, reproducible PR relationship.
Atrial Fibrillation
AtrialAtrial fibrillation is a supraventricular arrhythmia driven by chaotic, disorganized atrial electrical activity at roughly 300 to 500 impulses per minute, so the atria quiver rather than contract. The AV node passes these impulses irregularly, so the defining ECG hallmark is an irregularly irregular ventricular rhythm with no discrete P waves, replaced by low-amplitude fibrillation waves on an undulating baseline. 4
How to identify
Rate
the atrial rate is roughly 300 to 500 per minute; the ventricular response is variable and untreated is often 100 to 160 bpm (rapid ventricular response); a ventricular rate of 60 to 100 is described as a controlled response, and under 60 as a slow ventricular response
Regularity
irregularly irregular, with no repeating R-R pattern (the single most important clue). Exception: if the ventricular rhythm becomes REGULAR, suspect complete (third-degree) AV block with a junctional or ventricular escape, or an accelerated junctional rhythm (classically digoxin toxicity), not a benign finding
P waves
absent; replaced by chaotic fibrillation waves, often with an undulating or flat baseline (their size varies, and they may be hard to see)
PR interval
not measurable (no organized atrial activity to anchor it)
QRS width
usually narrow (<120 ms); wide only with bundle branch block, aberrant conduction, or pre-excitation
Read V1 and lead II: V1 best exposes coarse fibrillatory waves that can mimic flutter, while lead II is the standard rhythm strip for confirming the absence of organized P waves and the irregular R-R pattern.
The combination of irregularly irregular R-R intervals plus absent P waves is what separates AFib from atrial flutter (organized atrial activity, a regular sawtooth at about 250 to 350 per minute, giving a regular ventricular rate when AV conduction is fixed) and from multifocal atrial tachycardia (irregular, but with at least 3 distinct P-wave morphologies). One key exception to remember: fibrillation waves with a REGULAR ventricular rhythm are not a return to sinus rhythm. They mean the ventricles are being driven by something other than conducted AF: most often complete AV block with a regular junctional or ventricular escape, an accelerated junctional rhythm (classically from digoxin toxicity), or a paced rhythm.
Atrial Flutter
AtrialAtrial flutter is a macro-reentrant atrial tachycardia, classically using a fixed circuit around the tricuspid annulus (cavotricuspid isthmus-dependent), that discharges the atria at about 250 to 350 per minute (typically near 300, though antiarrhythmic drugs or prior ablation can slow it). The defining hallmark is a continuous sawtooth of uniform flutter waves, best seen in the inferior leads (II, III, aVF) with no isoelectric interval between them, conducted to the ventricles at a fixed or variable AV ratio (most often 2:1). 2
How to identify
Atrial (flutter) rate
regular flutter waves at about 250 to 350/min, classically ~300/min
Ventricular rate
depends on the AV conduction ratio. Assuming an atrial rate of 300/min, 2:1 block gives ~150 bpm (the classic clue), 3:1 gives ~100 bpm, and 4:1 gives ~75 bpm
Regularity
regular when the AV ratio is fixed (e.g. 2:1 or 4:1). A variable ratio makes the ventricular response irregular, often irregularly irregular and able to mimic atrial fibrillation; the giveaway is the regular sawtooth atrial waves at ~300/min, not the ventricular regularity.
P waves
no discrete P waves; replaced by continuous, uniform sawtooth flutter waves, classically with no isoelectric baseline in the inferior leads (II, III, aVF). In lead V1 the flutter waves are often discrete and upright with a short isoelectric segment
Flutter waves per QRS
more flutter waves than QRS complexes (e.g. 2:1, 3:1, 4:1) because the AV node blocks most atrial impulses
QRS width
usually narrow (< 120 ms) because conduction is supraventricular, unless there is aberrancy or pre-existing bundle branch block
Inferior leads II, III, and aVF best show the classic negative sawtooth of typical (counterclockwise isthmus-dependent) flutter, and lead V1 shows discrete upright flutter deflections that help count the atrial rate and confirm the ratio.
A regular narrow-complex tachycardia at almost exactly 150 bpm is flutter with 2:1 block until proven otherwise. When AV conduction slows transiently (for example during a vagal maneuver or after adenosine given by the treating team), the hidden sawtooth becomes visible and the flutter continues, whereas AVNRT and AVRT, which depend on the AV node, usually stop.
Supraventricular Tachycardia (SVT)
ReentrantSupraventricular tachycardia (SVT) is a broad term for any tachycardia that needs atrial or AV-junctional tissue to be maintained, which is what separates it from ventricular tachycardia. In everyday teaching SVT usually refers to paroxysmal SVT (PSVT): an abrupt regular narrow-complex tachycardia, most often AVNRT, then AVRT, then atrial tachycardia. AVNRT and AVRT are reentrant; focal atrial tachycardia is usually automatic or triggered. The ECG hallmark of PSVT is a sudden regular narrow-complex tachycardia at roughly 150 to 250 bpm with P waves usually absent, buried in the QRS, or distorting its terminal portion. (Note: the wider SVT family also includes irregular rhythms such as atrial fibrillation and multifocal atrial tachycardia, so SVT is not always regular.) 2
How to identify
Rate
roughly 150 to 250 bpm (commonly ~180 to 200 in AVNRT), with characteristically abrupt onset and offset; atrial tachycardia can run slower (atrial rate usually 100 to 250 bpm) and AVRT can run faster
Regularity
regular, with essentially constant R-R intervals (true of paroxysmal SVT; remember the broader SVT family includes irregular rhythms like atrial fibrillation and MAT)
P waves
usually not visible, buried in the QRS, or distorting its terminal portion. In typical (slow-fast) AVNRT a retrograde P sits in or just at the end of the QRS, giving a pseudo-R' in V1 and a pseudo-S in II/III/aVF (very short RP). A clearly separate retrograde P later after the QRS (longer RP) suggests AVRT
P:QRS relationship
typically 1:1 when atrial activity is discernible
QRS width
narrow (< 120 ms) unless there is preexisting bundle branch block or rate-related aberrancy
Lead II and V1 read this best: V1 reveals a pseudo-R' and II/III/aVF a pseudo-S from retrograde P waves, and both leads show flutter waves best when the ventricular rate slows.
The most useful bedside discriminator is regularity together with the absent or retrograde P wave. A perfectly regular narrow tachycardia at almost exactly 150 bpm should make you exclude atrial flutter with 2:1 block before calling it PSVT: look for sawtooth flutter waves. If AV conduction slows transiently (for example during a vagal maneuver or after adenosine given by the treating team), flutter or atrial tachycardia continues with fewer QRS complexes, whereas AVNRT and AVRT usually stop.
Junctional Rhythm
JunctionalA subsidiary pacemaker in the AV junction drives the ventricles when it outpaces or replaces the sinus node, producing a regular narrow-QRS rhythm. The defining hallmark is the absence of normally conducted (upright) sinus P waves: P waves are either missing or retrograde (inverted in the inferior leads) and may precede, hide within, or follow the QRS. 5
How to identify
Rate
junctional escape ~40-60 bpm; accelerated junctional 60-100 bpm; junctional tachycardia exceeds 100 (in adults commonly about 110 to 130, overlapping sinus and atrial tachycardia rates, where the P-wave relationship is the separator)
Regularity
regular (the junctional focus fires at a steady cycle length)
P waves
absent, or retrograde and inverted in II, III, and aVF; may sit just before, on, or after the QRS
PR interval
short (under ~120 ms) when a retrograde P precedes the QRS, since atrial and ventricular activation are near-simultaneous; not measurable when P is buried or follows
P:QRS relationship
Often 1:1, with a single retrograde P (inverted in lead II) just before, within, or after each QRS, or no visible P at all when the retrograde P is buried in the QRS. Independent sinus P waves (AV dissociation) appear only when the junctional rate is at least as fast as the sinus rate. If the atrial rate is FASTER and the P waves never conduct, the diagnosis is third-degree AV block; the regular narrow-QRS rhythm is then a junctional escape rhythm, and the block is the primary diagnosis.
QRS width
narrow (under 120 ms) because the impulse uses the normal His-Purkinje system (may be wide if a bundle branch block or aberrancy coexists)
Read lead II best: it is the most sensitive for detecting the retrograde inverted P wave and its timing relative to the QRS, with aVF and III confirming the inferior-lead inversion.
The single most discriminating feature is the P wave: an inverted P in leads II/III/aVF (retrograde atrial activation) with a short PR, or no visible P at all, separates junctional rhythm from sinus rhythm and from a low atrial rhythm; do not call it junctional if normal upright sinus P waves with a normal PR are present.
References
Identification criteria follow the relevant ACC/AHA/HRS practice guidelines; the foundational sinus-rhythm definitions follow a standard peer-reviewed ECG reference.
- Hafeez Y, Grossman SA. Sinus Bradycardia. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023 edition (later editions give 50 to 100 bpm for normal sinus rhythm). Bookshelf ID NBK493201.
- Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia. Circulation. 2016;133(14):e506-e574.
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140(8):e382-e482.
- Joglar JA, Chung MK, Armbruster AL, et al. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation. Circulation. 2024;149(1):e1-e156.
- Hafeez Y, Wollard K, Grossman SA. Junctional Rhythm. In: StatPearls. Treasure Island (FL): StatPearls Publishing. Bookshelf ID NBK507715.
- Wagner P, Strodthoff N, Bousseljot RD, et al. PTB-XL, a large publicly available electrocardiography dataset. Sci Data. 2020;7:154. The source of most example tracings (CC BY 4.0).
- Reyna M, Sadr N, Gu A, et al. Will Two Do? Varying Dimensions in Electrocardiography: The PhysioNet/Computing in Cardiology Challenge 2021 (version 1.0.3). PhysioNet; 2022. doi:10.13026/34va-7q14. Source of the Chapman-Ningbo Mobitz II tracing (Ningbo First Hospital; Zheng J, et al. Sci Data. 2020;7:48 and Sci Rep. 2020;10:2898; CC BY 4.0).
Rhythm Foundations is an educational reference for learning ECG interpretation. Example tracings are de-identified records from the PTB-XL and Chapman-Ningbo databases (references 6 and 7), apart from the two synthetic Mobitz II strips, which are labelled as such. This is not medical advice and is not a substitute for clinical judgement or formal training.