The Diagnosis of Arrhythmias

1.  Diagnosis of Arrhythmias

a. ECG

History and physical examination may detect an arrhythmia and suggest possible causes, but diagnosis requires a 12-lead ECG or, less reliably, a rhythm strip, preferably obtained during symptoms to establish the relationship between symptoms and rhythm.

The ECG is approached systematically; calipers measure intervals and identify subtle irregularities. The key diagnostic features are

b. Rate and regularity of atrial activation

c. Rate and regularity of ventricular activation

d. The relationship between the two

Irregular activation signals are classified as regularly irregular or irregularly irregular (no detectable pattern). Regular irregularity is intermittent irregularity in an otherwise regular rhythm (eg, premature beats) or a predictable pattern of irregularity (eg, recurrent relationships between groups of beats).

A narrow QRS complex (< 0.12 seconds) indicates a supraventricular origin (above the His bundle bifurcation).

A wide QRS complex (≥ 0.12 seconds) indicates a ventricular origin (below the His bundle bifurcation) or a supraventricular rhythm conducted with an intraventricular conduction defect or with ventricular preexcitation in the Wolff-Parkinson-White syndrome .

1.1 Bradyarrhythmias

Bradyarrhythmias have a slow ventricular rate (< 60 beats/minute in adults). ECG diagnosis of bradyarrhythmias depends on the presence or absence of P waves, morphology of the P waves, and the relationship between P waves and QRS complexes.

Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles. There are 3 degrees of AV block: first, second, and third.

In first-degree AV block, each P wave is followed by a QRS complex but the PR interval is > 0.2 seconds. First-degree AV block does not itself cause bradycardia but often coexists with other conditions that do.

In second-degree AV block, some normal P waves are followed by QRS complexes, but some are not. Bradycardia may or may not be present.

Third-degree AV block is indicated by a bradyarrhythmia with no relationship between P waves and QRS complexes and more P waves than QRS complexes; the escape rhythm can be

a. Junctional with normal AV conduction (narrow QRS complex)

b. Junctional with aberrant AV conduction (wide QRS complex)

c. Ventricular (wide QRS complex)

Absence of second- or third-degree AV block is indicated by a regular QRS bradyarrhythmia with a 1:1 relationship between P waves and QRS complexes. P waves preceding QRS complexes indicate sinus bradycardia (if P waves are normal, even if first-degree AV block is present) or sinus arrest with an escape atrial bradycardia (if P waves are abnormal).

P waves after QRS complexes indicate sinus arrest with a junctional or ventricular escape rhythm and retrograde atrial activation. A ventricular escape rhythm results in a wide QRS complex; a junctional escape rhythm usually has a narrow QRS (or a wide QRS with bundle branch block or pre-excitation).

When the QRS rhythm is irregular, P waves usually outnumber QRS complexes; some P waves produce QRS complexes, but some do not (indicating second-degree AV block). An irregular QRS rhythm with a 1:1 relationship between P waves and the following QRS complexes usually indicates sinus arrhythmia with gradual acceleration and deceleration of the sinus rate (if P waves are normal).

Pauses in an otherwise regular QRS rhythm may be caused by blocked P waves (an abnormal P wave can usually be discerned just after the preceding T wave or distorting the morphology of the preceding T wave), sinus arrest or sinus exit block, as well as by second-degree AV block.

1.2 Tachyarrhythmias

Tachyarrhythmias have a rapid ventricular rate (> 100 beats/minute in resting adults); tachyarrhythmias may be divided into 4 groups, defined by the QRS complexes:

a. Visibly regular vs irregular QRS complexes

b. Narrow vs wide QRS complexes

Irregular, narrow QRS complex tachyarrhythmias include the following 4 rhythms. Differentiation is based on atrial ECG signals, which are best seen in the longer pauses between QRS complexes.

A. Atrial fibrillation (AF): Atrial ECG signals (usually best seen in lead V1) that are continuous, irregular in timing and morphology, and very rapid (> 300 beats/minute) without discrete P waves

B. Atrial flutter with variable AV conduction: Regular, discrete, uniform atrial signals (usually best seen in leads II, III, and aVF) without intervening isoelectric periods, usually at rates > 250 beats/minute

C. True atrial tachycardia with variable AV conduction: Regular, discrete, uniform, abnormal atrial signals with intervening isoelectric periods (usually at rates < 250 beats/minute)

D. Multifocal atrial tachycardia: Discrete P waves that vary from beat to beat with at least 3 different morphologies

Irregular, wide QRS complex tachyarrhythmias include

A. The above 4 irregular, narrow atrial tachyarrhythmias conducted with either bundle branch block or ventricular preexcitation

B Polymorphic ventricular tachycardia (VT)

Differentiation is based on atrial ECG signals and the presence in polymorphic VT of a very rapid ventricular rate (> 250 beats/minute).

Regular, narrow QRS complex tachyarrhythmias include

A. Sinus tachycardia

B. Atrial flutter with a consistent AV conduction ratio

C. True atrial tachycardia with a consistent AV conduction ratio

D. Paroxysmal supraventricular tachycardias([SVT] such as AV nodal reentrant SVT, orthodromic reciprocating AV tachycardia in the presence of an accessory AV connection, and SA nodal reentrant SVT)

Vagal maneuvers or pharmacologic AV nodal blockade can help distinguish among these tachycardias. With these maneuvers, sinus tachycardia is not terminated, but it slows or AV block develops, disclosing normal P waves. Similarly, atrial flutter and true atrial tachycardia are usually not terminated, but AV block discloses flutter waves or abnormal P waves. The most common forms of paroxysmal SVT (AV nodal reentry and orthodromic reciprocating tachycardia) must terminate if AV block occurs.

Regular, wide QRS complex tachyarrhythmias include

A. The above 4 regular, narrow QRS complex tachyarrhythmias conducted with bundle branch block or ventricular preexcitation

B. Monomorphic VT

Vagal maneuvers can help distinguish among them. ECG criteria to distinguish between VT and SVT with an intraventricular conduction defect are often used (see figure Modified Brugada criteria for ventricular tachycardia). When in doubt, the rhythm is assumed to be VT because some drugs for SVTs can worsen the clinical state if the rhythm is VT; however, the reverse is not true.

                                     Modified Brugada criteria for ventricular tachycardia