2 Atrial Rhythms and Supraventricular Tachycardias

Learn a systematic approach to ECG rhythm recognition, distinguish common atrial rhythms and conduction abnormalities, and apply safety-first treatment principles based on patient stability.

Read the systematically

Interpret the tracing before naming the rhythm. Confirm patient identity, calibration, and recording quality, then use a repeatable sequence to assess the rhythm and the patient.

Measure rate, rhythm, and conduction

Rate: For a regular rhythm, divide $300$ by the number of large boxes between R waves. Alternatively, count QRS complexes in a $10$ -second strip and multiply by $6$ . For an irregular rhythm, count QRS complexes over $10$ seconds and multiply by $6$ .
Regularity: Compare R–R intervals; if atrial activity is visible, inspect P–P intervals separately.
P waves: Determine whether P waves are present, consistent in shape, and related to QRS complexes. Atrial activity is often easier to see in leads II and V1.
Intervals and conduction: Measure PR and QRS duration. Assess whether every P wave conducts and whether each QRS is preceded by a P wave.
12-lead context: Compare P-wave and QRS morphology across leads, inspect axis and ST–T changes, and compare with prior ECGs. A rhythm label does not replace evaluation for ischemia or other acute causes.
Patient condition: Check symptoms, blood pressure, perfusion, oxygenation, and clinical context. The same rate can be tolerated by one patient and dangerous in another.

At a standard paper speed of $25\ \text{mm/s}$ , one small box represents $0.04\ \text{s}$ and one large box represents $0.20\ \text{s}$ . A typical adult PR interval is $0.12$ – $0.20\ \text{s}$ ; a QRS duration under $0.12\ \text{s}$ is conventionally narrow. These cutoffs guide interpretation, but clinical context matters.

Recognize sinus rhythms and early atrial beats

has a consistent sinus P wave before each QRS, a stable PR interval, and usually a regular rhythm. A normal adult resting rate is conventionally $60$ – $100$ beats per minute.

is a below $60$ beats per minute. It may be normal, for example during sleep or in a trained athlete. Treat the patient, not the number: assess for poor perfusion, ischemia, hypotension, medication effects, hypoxia, and other reversible causes.

is a above $100$ beats per minute, usually with a gradual rate change and a P wave before each QRS. It commonly reflects a cause such as pain, fever, hypovolemia, hypoxia, anemia, or anxiety. Address the cause rather than trying to suppress an appropriate compensatory rate.

In respiratory sinus arrhythmia, the sinus rate varies with breathing while P-wave morphology and P-to-QRS conduction remain consistent; it is often benign. A premature atrial complex is an early beat with a P wave that differs from sinus morphology. Its QRS is usually narrow unless conduction is aberrant, and it may reset the sinus node or be nonconducted.

Distinguish atrial rhythms and SVTs

First classify a tachycardia as regular or irregular and the QRS as narrow or wide. A wide QRS may reflect bundle branch block or aberrancy, but an undifferentiated wide-complex tachycardia should be treated as potentially ventricular until expert assessment establishes otherwise. The label supraventricular tachycardia describes a tachycardia arising above the ventricles but does not, by itself, identify the mechanism.

Atrial rhythms

Atrial fibrillation has no consistent P waves and, when AV conduction is intact, irregularly irregular R–R intervals. A regular ventricular response can occur with complete AV block or pacing.

has organized flutter waves, often saw-toothed in inferior leads, and an atrial rate commonly around $250$ – $350$ beats per minute. A regular ventricular rate near $150$ beats per minute should prompt a search for $2{:}1$ flutter; flutter can also have variable AV conduction. Flutter waves may be subtle or obscured by QRS and T waves, so inspect more than one lead and use the full 12-lead rather than assuming a regular narrow tachycardia is AVNRT.

Focal atrial tachycardia is usually regular. Its discrete P waves differ from sinus P waves and may be hidden in preceding T waves. The atrial rhythm continues even if AV conduction varies, helping distinguish it from AV-node-dependent re-entry.

Multifocal atrial tachycardia is irregular, with at least three distinct P-wave shapes, variable PR intervals, and an isoelectric baseline between P waves. Unlike AF, discrete P waves are visible. Consider and treat associated illness, often pulmonary or metabolic.

Re-entrant tachycardias

AVNRT is usually an abrupt-onset, regular, narrow-complex tachycardia; P waves may be hidden in or just after the QRS. It is a common cause of paroxysmal regular narrow-complex SVT.

AVRT is often regular, and orthodromic AVRT is usually narrow-complex. A delta wave and short PR interval in can indicate pre-excitation. A wide, irregular, very rapid rhythm in a patient with pre-excitation may be dangerous .

An may suggest a mechanism without establishing it with certainty.

Identify AV blocks and bundle branch block

First-degree AV block means every P wave conducts but the PR interval is prolonged, over $0.20\ \text{s}$ . It is a delay, not a dropped beat.

In , PR intervals lengthen progressively until a P wave is not followed by a QRS; the sequence then repeats. In , conducted beats have a fairly constant PR interval, with intermittent nonconducted P waves. Mobitz II can progress to complete block and warrants urgent clinical assessment. A $2{:}1$ pattern alone cannot reliably be classified as Mobitz I or Mobitz II.

In , P waves and QRS complexes occur independently, with no consistent PR relationship. A slow escape rhythm may maintain ventricular activity; assess urgently, especially if the patient is symptomatic or unstable.

Bundle branch block produces a widened QRS, typically at least $0.12\ \text{s}$ , with characteristic morphology. RBBB commonly has an rSR′ pattern in V1 and a broad terminal S wave in lateral leads. LBBB commonly has a predominantly negative QRS in V1 and broad or notched R waves in lateral leads. Compare with prior ECGs and assess symptoms and ST–T changes; a bundle branch block pattern alone does not establish an acute infarction.

Let stability guide treatment

These are recognition principles, not a substitute for emergency protocols, trained clinical judgment, or specialist care.

Tachycardia with a pulse

First determine whether tachycardia is causing cardiopulmonary compromise, such as hypotension, shock, acutely altered mental status, ischemic chest discomfort, or acute heart failure. If it is, follow the current resuscitation protocol for prompt synchronized cardioversion; do not delay emergency treatment for a detailed rhythm diagnosis.

If the patient is stable, continue monitoring, obtain a 12-lead when feasible, and select treatment based on QRS width and regularity. For a stable, regular, narrow-complex tachycardia, vagal maneuvers and adenosine may be considered under protocol. The 2025 AHA algorithm permits adenosine for wide-complex tachycardia only when it is regular and monomorphic; seek expert help for wide-complex rhythms.

AF and flutter

For a stable patient, assess rate control, the trigger or underlying illness, and stroke risk. Decisions about anticoagulation and elective cardioversion require clinical assessment, including rhythm duration and thromboembolic risk. Hemodynamic instability calls for emergency care; stroke-prevention questions still matter but should not delay lifesaving treatment.

warning

A very rapid, irregular, wide-complex rhythm in a patient with known or suspected pre-excitation is an emergency. Avoid AV-nodal-blocking drugs in suspected , follow the emergency protocol, and obtain expert support. Do not mistake it for ordinary AF with a bundle branch block.

Bradycardia and AV block

If bradycardia is causing cardiopulmonary compromise, support airway and circulation, identify reversible causes, and follow the current bradycardia algorithm. The 2025 AHA algorithm recommends atropine; if it is ineffective, transcutaneous pacing and/or dopamine or epinephrine infusion may be used, with expert consultation and consideration of transvenous pacing. Mobitz II, complete AV block, syncope, or poor perfusion requires urgent evaluation.

Apply the rhythm patterns

Use the distinctions together rather than relying on rate alone:

A regular narrow tachycardia at $150$ beats per minute: consider $2{:}1$ and inspect inferior leads and V1 for flutter waves.
An irregularly irregular rhythm without consistent P waves: AF is likely. If the QRS is wide and the rate very fast, consider pre-excitation or another dangerous wide-complex rhythm.
An irregular rhythm with three distinct P-wave shapes: consider MAT rather than AF, and search for and address the underlying cause.
Progressively longer PR intervals followed by a dropped QRS: Mobitz I.
Fixed PR intervals in conducted beats with intermittent dropped QRS complexes: Mobitz II; escalate clinical assessment.

The overall sequence is rate, regularity, P waves, PR and QRS intervals, AV relationship, 12-lead morphology, and patient stability. Sinus rhythms retain consistent sinus P waves; AF lacks organized P waves, flutter has organized rapid atrial activity, and MAT has multiple P-wave morphologies. Distinguish regular narrow tachycardias from irregular or wide rhythms before considering treatment, and let clinical stability and the underlying cause guide decisions.

Read the ECG systematically