3 Arrhythmias and Conduction Disorders

Learn a systematic approach to ECG rhythm recognition, common arrhythmias and conduction blocks, and how clinical stability and pulse status shape urgency.

Interpreting a rhythm systematically

Arrhythmias are abnormalities of heart rate, rhythm, or electrical impulse conduction. ECG interpretation identifies the electrical pattern, while clinical assessment determines whether it is dangerous. Always assess the patient and check for a pulse: an ECG rhythm alone does not establish effective circulation.

Rate, regularity, and ECG features

Use the same sequence for each rhythm strip:

Rate: Estimate the atrial and ventricular rates. For a regular rhythm, estimate the rate by dividing $300$ by the number of large boxes between R waves:
$\text{Rate} = \frac{300}{\text{large boxes between R waves}}$
For an irregular rhythm, count the QRS complexes in a $6$ -second strip and multiply by $10$ .
Regularity: Compare R–R intervals, then P–P intervals. A rhythm may have regular atrial activity but irregular ventricular responses.
QRS width: A narrow QRS is less than $120\ \mathrm{ms}$ ; a wide QRS is at least $120\ \mathrm{ms}$ . Narrow complexes usually indicate ventricular activation through the usual conduction system. Wide complexes may reflect , ventricular origin, pacing, or another conduction delay.
P waves and PR interval: Check whether P waves have a consistent shape, whether each P wave is followed by a QRS, and whether each QRS has a preceding P wave. The normal PR interval is $120\text{–}200\ \mathrm{ms}$ .
Relationship and context: Look for dropped beats, AV dissociation, pauses, or changing QRS morphology. Relate the rhythm to symptoms, blood pressure, perfusion, and pulse status.

This pattern-based approach helps classify tachyarrhythmias as regular or irregular and narrow- or wide-complex.

Bradyarrhythmias and escape rhythms

is conventionally a heart rate below $60$ beats/min in adults. A slow rate is not automatically an emergency: fitness, sleep, medications, and other conditions affect its significance. In the AHA adult algorithm, a rate typically below $50$ /min prompts consideration of bradyarrhythmia in context. Urgency depends especially on hypotension, altered mental status, shock, ischemic chest discomfort, or acute heart failure.

Common slow rhythms include:

Sinus : Normal sinus P waves precede each QRS with a consistent PR interval, but the rate is slow.
Sinus pause or arrest: Expected sinus P waves temporarily disappear, creating a pause. A junctional or ventricular escape beat may follow. Sinus-node dysfunction can also cause alternating slow rhythms and atrial tachyarrhythmias.
Junctional escape rhythm: A slower backup rhythm arising near the AV junction. P waves may be absent, inverted, or just before or after the QRS. QRS complexes are often narrow unless another conduction abnormality is present.
Ventricular escape rhythm: A slower backup rhythm arising in the ventricles, usually with wide QRS complexes. It may occur when higher pacemakers fail to produce or conduct impulses.

Atrioventricular blocks

AV block is delayed or interrupted conduction from the atria to the ventricles. Classify it by examining how P waves relate to QRS complexes:

First-degree AV block: Every P wave conducts, but the PR interval is prolonged, greater than $200\ \mathrm{ms}$ , and remains constant. This is delayed conduction, not a dropped beat.
Second-degree AV block, Mobitz I (Wenckebach): The PR interval progressively lengthens until a P wave is not followed by a QRS; the cycle then repeats.
Second-degree AV block, Mobitz II: Conducted beats have a stable PR interval, but some P waves are suddenly not followed by QRS complexes. This pattern can indicate more serious conduction-system disease.
Third-degree (complete) AV block: P waves and QRS complexes occur independently, with no consistent PR relationship. A slower junctional or ventricular escape rhythm may maintain the ventricular rate.
2:1 AV block: One QRS conducts for every two P waves. Because there are too few consecutive conducted beats to identify the PR pattern, the ECG alone may not classify the block as Mobitz I or Mobitz II.

Tachyarrhythmias

Tachycardia is a heart rate above $100$ beats/min in adults. First sort an ECG tachyarrhythmia by QRS width and regularity. The AHA adult tachyarrhythmia algorithm notes that rates are typically at least $150$ /min when a tachyarrhythmia is responsible for the clinical problem; rate alone does not establish the cause or urgency.

Narrow-complex rhythms

Sinus tachycardia: A normal-shaped P wave precedes each QRS, with a consistent PR interval and usually gradual acceleration or slowing. It often reflects an underlying cause such as fever, pain, dehydration, hypoxia, or stress.
Supraventricular tachycardia (SVT): Often a regular, narrow-complex rhythm with abrupt onset and termination. P waves may be hidden in or close to the QRS. The ECG pattern suggests a rhythm originating above the ventricles, but a rhythm strip may not identify its precise mechanism.
: There are no consistent discrete P waves, and the ventricular rhythm is irregularly irregular. QRS complexes are usually narrow unless a separate conduction abnormality is present.
Atrial flutter: Repeating, uniform flutter waves, often with a sawtooth appearance, replace normal discrete P waves. The ventricular rhythm may be regular with fixed conduction or irregular with variable conduction.
Multifocal atrial tachycardia: An irregular rhythm with at least $3$ distinct P-wave shapes and varying PR intervals. It can resemble AF, but discrete P waves are present.

Wide-complex rhythms

A wide-complex tachycardia may be VT or a supraventricular rhythm conducted with a or another conduction abnormality. Monomorphic VT is regular, with similar-looking wide QRS complexes; polymorphic VT has changing QRS shape and amplitude. When a wide-complex tachycardia cannot be confidently identified, it is generally treated as VT in emergency decision-making because misclassifying VT as an ordinary SVT can be dangerous.

is a form of polymorphic VT associated with a prolonged QT interval; its QRS complexes appear to twist around the baseline. Sustained polymorphic VT is an emergency. The AHA recommends immediate unsynchronized shock for sustained polymorphic VT and notes that magnesium may be considered for recurrent polymorphic VT associated with long QT.

has chaotic electrical activity without organized, identifiable QRS complexes. It is a cardiac-arrest rhythm, not a perfusing tachycardia.

Bundle-branch and fascicular blocks

A delays activation of one ventricle and produces a QRS duration of at least $120\ \mathrm{ms}$ .

In a typical right (RBBB), the right ventricle activates late. This often creates an $rSR^\prime$ pattern in $V_1$ and a broad terminal S wave in lateral leads such as $I$ and $V_6$ .
In a typical left (LBBB), the QRS is wide, with broad or notched R waves in lateral leads and a predominantly negative QRS in $V_1$ .

Confirm suspected on a 12-lead ECG; a single rhythm strip may not show enough leads for reliable identification.

Fascicular blocks reflect delayed conduction through a division of the left bundle and are recognized mainly by the frontal-plane QRS axis on a 12-lead ECG. A combination of right and a left fascicular block is often called a bifascicular block. These findings describe conduction patterns; symptoms and the broader ECG determine their significance.

Clinical stability and immediate priorities

Interpret the tracing alongside the patient. Look for signs of poor perfusion, including hypotension, shock, altered mental status, ischemic chest discomfort, or acute heart failure, and verify whether a pulse is present.

The AHA algorithms distinguish or tachycardia with a pulse from cardiac arrest. Persistent tachyarrhythmia causing cardiopulmonary compromise may require synchronized cardioversion, while VF and pulseless VT are shockable cardiac-arrest rhythms. For unstable , the algorithm includes atropine and, if it is ineffective, transcutaneous pacing and/or dopamine or epinephrine infusion. These are emergency-care pathways for trained responders, not a substitute for local protocols or clinical judgment.

In summary, read each rhythm using rate, regularity, QRS width, P waves, PR interval, and the relationship between atrial and ventricular activity. Sinus-node disorders affect impulse formation; AV blocks delay or interrupt conduction; bundle-branch blocks widen the QRS; and tachyarrhythmias are initially sorted into regular or irregular, narrow- or wide-complex patterns. ECG recognition is only part of assessment: pulse and clinical stability determine urgency and the appropriate emergency pathway.