Differential Diagnosis of Narrow and Wide Complex Tachycardias

Introduction

The differential diagnosis of tachycardias using the electrocardiogram (ECG) remains among the most difficult problems faced by cardiologists in daily practice. Distinguishing supraventricular tachycardias (SVTs) from ventricular tachycardias (VTs) has obvious importance because treatments are very different. This chapter explores the tools available to address the diagnosis; after this has been obtained, further evaluation and treatment strategies generally flow naturally. An important distinguishing feature for clinical implications of a tachycardia episode is whether structural heart disease (SHD; e.g., prior infarction, cardiomyopathy, prior surgery) is present. In most cases of SVT, SHD is either absent or unrelated to the tachycardia episode, whereas in most VT patients SHD is the basis of, or substrate for, the arrhythmia.

The first major differentiator in correctly diagnosing tachycardias is the width of the QRS complex: narrow (<20 ms) QRS complex tachycardias (NCTs) in adults are almost always supraventricular in origin (involving tissue at or above the His bundle), whereas wide (≥120 ms) QRS complex tachycardias (WCTs) are often, but not always, ventricular in origin.

Narrow Qrs Tachycardias

Diagnostic Possibilities

The major categories of NCTs include those that are primarily atrial in origin (atrial tachycardia, flutter, and fibrillation), those that are based in the atrioventricular (AV) junction, and those that incorporate the atrium and the ventricle in a large circuit (accessory pathway–mediated AV reentry). Because of the typical obvious irregularity of ventricular response, atrial fibrillation (AF) is not discussed in this chapter, but flutter and atrial tachycardias (ATs) deserve consideration. Classic electrocardiographic atrial flutter is now understood to be a continuous wavefront propagating either “clockwise” or “counterclockwise” around the tricuspid annulus when viewed from a left anterior oblique perspective (see Chapter 71, Chapter 78 ). Other atrial arrhythmias are termed “flutter” on ECG but are mechanistically distinct; these can be either focal in origin or reentrant (usually large circuits bounded by natural barriers, such as valves or scar tissue). ATs can be focal (a true focus or a micro-reentry that appears focal in its propagation pattern) or macro-reentrant, incorporating significant amounts of atrial tissue in the circuit. The latter are noteworthy in that the P wave includes a relatively large portion of the tachycardia cycle, as opposed to focal ATs (and all other types of SVTs, during which atrial activation begins at a discrete point as though it were a focus). A major limitation to discerning P wave morphology is distinguishing between a P wave and ST segment, T wave, and QRS complex. Helpful aids include finding periods of NCT with 2:1 AV conduction or a spontaneous ventricular premature complex, comparing the complex in question with a sinus rhythm P-QRS-T cycle, and increasing ECG gain ( Fig. 61.1 ).

Fig. 61.1, Determination of P waves during narrow complex tachycardia using a variety of methods.

History and Physical Examination

Patients with NCTs usually have recurrent episodes of arrhythmia. The age at which episodes begin often suggests a diagnosis: episodes from birth onward are likely to be AV reentrant tachycardia (AVRT) using an accessory AV pathway present from birth (see Chapter 74 ) or, less commonly, AT (see Chapter 71 ). Onset of symptoms during or after puberty is common in AV nodal reentrant tachycardia (AVNRT; see Chapter 72 ). Nevertheless, although these scenarios are generally true, any type of NCT can occur at any age. Symptoms related to tachycardia include palpitations, lightheadedness, dyspnea, chest pain, and throat fullness. In many, episodes are facilitated by exercise and/or emotional upset. Physical maneuvers that increase vagal tone, such as the Valsalva maneuver or breath holding, can often terminate episodes. Episodes tend to become more common and longer lasting with aging. Physical examination during NCT episodes shows tachycardia in a conscious, often anxious patient. Blood pressure is usually preserved. Bulging of neck veins can sometimes be perceived. In patients with repaired congenital heart disease, scar-based atrial macro-reentry should be suspected (see Chapter 71, Chapter 78 ).

Electrocardiographic Differential Diagnosis

Among NCTs, the differential diagnosis is based on three factors: (1) the atrial/ventricular (A:V) ratio; (2) among those with 1:1 AV ratio, the timing of the P wave relative to a QRS complex; and (3) P wave morphology ( Table 61.1 ). Although there is some individual variability, some patterns are relatively constant.

TABLE 61.1

Narrow Complex Tachycardia Diagnostic Features

	Atrioventricular Ratio
	>1 (A > V)	1 (A = V)	<1 (A < V) ^a	Indeterminate (No Clear P)
Diagnostic possibilities	Atrial tachycardia Atrial flutter (AVNRT)	AVNRT AVRT Atrial tachycardia (junctional tachycardia)	Sinus rhythm with 1:2 conduction AVNRT Junctional tachycardia Nodofascicular tachycardia	AVNRT (junctional tachycardia, atrial tachycardia)
	R-P Interval
	No visible P	Short RP	Intermediate RP	Long RP
Diagnostic possibilities	AVNRT (atrial tachycardia)	AVRT AVNRT Atrial tachycardia (junctional tachycardia)	AVNRT Atrial tachycardia AVRT (junctional tachycardia)	Atrial tachycardia AVNRT AVRT (junctional tachycardia)
	P Wave Morphology
	Positive leads II, III, aVF	Negative Leads II, III, aVF	Negative Lead I	Positive all precordial leads
Diagnostic possibilities	Atrial tachycardia AVRT	AVNRT AVRT Atrial tachycardia	AVRT (left lateral pathway) Left atrial tachycardia	Pulmonary vein ostia

Diagnostic possibilities are listed in order of frequency. Terms in parentheses denote rare situations.

AVNRT, Atrioventricular nodal reentrant tachycardia; AVRT, atrioventricular reentry tachycardia.

a All four items in this group are rare.

A:V Ratio

NCTs with A:V ratios greater than 1 include AT, flutter, and rare cases of AVNRT with 2:1 block, usually in the His bundle (see Fig. 61.1 , far right ).
NCTs with an A:V ratio equal to 1 are a large and diverse group, among which are AVNRTs, AVRTs, ATs, and the uncommon automatic junctional tachycardia (see Chapter 73 ). ATs may at times have a 1:1 AV ratio, but the timing relationship between the QRS complex and the subsequent P wave is not fixed.
NCTs with an A:V ratio of less than 1 are rare and include sinus rhythm with simultaneous conduction over fast and slow AV nodal pathways, nodofascicular pathway–based reentry, and either AVNRT or junctional tachycardia with retrograde block.

R-P Interval in Cases With 1:1 A:V Ratio

“No R-P” interval NCTs, which indicate the absence of a visible P wave (because it is subsumed in the QRS complex), are common in AVNRT (anterograde slow–retrograde fast pathways) but can occur in AT with a long AV conduction time (long PR interval).
NCTs with a short R-P interval (P wave in the first one-third of the R-R interval) include AVRTs, AVNRTs (especially in patients older than 50 years) and AT, with junctional tachycardia as a rare cause.
Intermediate R-P interval NCTs (P wave in the middle one-third of the R-R interval) are made up of the same types as short R-P NCTs, but AVNRT (“slow–slow”) and AT are more common than AVRT.
Long R-P NCTs are an interesting group with the same diagnostic possibilities as the other R-P subsets, but ATs predominate; AVNRT is of the less common anterograde fast–retrograde slow pathway variety and unusual. Uncommon slowly conducting accessory pathways are also a cause.

P Wave Morphology

Atrial activation in NCTs with positive P waves in the inferior leads begins near the cephalad aspect (top) of the atria. Potential sites include the upper crista terminalis, superior vena cava and appendage in the right atrium, the pulmonary veins and appendage in the left atrium, and the cephalad portions of the tricuspid and mitral annuli. Evaluation of precordial leads (providing information as to the relative anteroposterior position) and lead I (indicating relative left-right position) further refine the site of origin of atrial activation in the other two axes. ATs account for many of these tachycardias, but AVRTs with pathway atrial insertions on the cephalad portions of mitral or tricuspid annuli are also in this group.
Negative (“inverted”) P waves in the inferior leads denote onset of atrial activation in the lower portion of the atria (low crista terminalis, coronary sinus ostium, low septum and tricuspid annulus in the right atrium, and low septum or mitral annulus in the left atrium). All varieties of AVNRT, and AVRT using posteriorly (inferiorly) located AV pathways, fall into this group, as do some ATs.
A negative (inverted) P wave in lead I is a reliable indicator of left-to-right atrial activation, either from AT arising in the left atrium or pulmonary veins, or AVRT using a left lateral pathway.
When all precordial leads show positive P waves, a left atrial or pulmonary venous source should be suspected.

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