Differential Diagnosis of Narrow and Wide Complex Tachycardias

59


Differential Diagnosis of Narrow and Wide Complex Tachycardias




Many patients with cardiovascular problems have sudden onset of severe symptoms; among the variety of diagnoses, rapid tachycardias perhaps are most likely to elicit symptoms in caregivers. Part of this anxiety arises from uncertainty about the specific diagnosis (i.e., supraventricular tachycardia [SVT] vs. ventricular tachycardia [VT]), part from uncertainty about how to treat, and part from uncertainty about clinical implications (e.g., Is the patient having a heart attack? Is he going to die before treatment?). In this chapter, we will explore the tools available to address the first problem (diagnosis); after getting this correct, answers to the other problems generally flow naturally. An important distinguishing feature for clinical implications of a tachycardia episode is whether or not structural heart disease (SHD; prior infarction, cardiomyopathy, prior surgery, etc.) is present: In most cases of SVT, SHD is either absent or unrelated to the episode, but in most VT patients, SHD serves as the basis of, or substrate for, the arrhythmia.


The first major differentiator in correctly diagnosing tachycardia is the width of the QRS complex: Narrow (<120 ms) QRS complex tachycardias (NCTs) in adults are almost always supraventricular in origin (involving tissue at or above the bundle of His), 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, fibrillation); those that are based in the atrioventricular (AV) junction; and those that incorporate atrium and ventricle in a large circuit (accessory pathway medicated AV reentry). In this chapter, atrial fibrillation will not be considered further, but flutter and atrial tachycardia (AT) deserve consideration. Classic electrocardiographic atrial flutter is now understood to be a continuous wave front propagating either clockwise or counterclockwise around the tricuspid annulus. Other atrial arrhythmias are termed flutter on electrocardiogram (ECG) but are mechanistically distinct; these can be focal in origin or reentrant (usually large circuits bounded by natural barriers such as valves or scar tissue). ATs can be focal (true focus or microreentry that appears focal in its propagation pattern) or macroreentrant, incorporating significant amounts of atrial tissue in the circuit. The latter are noteworthy in that the P wave makes up a relatively large portion of the tachycardia cycle, as opposed to focal ATs (and all other types of SVT, during which atrial activation begins at a discrete point as though it were a focus).1 A major limitation of discerning P-wave morphology is the need to determine what is a P wave and what are ST segment, T wave, and QRS complex. Helpful aids include finding periods of NCT with 2 : 1 AV conduction; comparing the complex in question with a sinus rhythm P-QRS-T cycle; and increasing ECG gain (Figure 59-1).




History and Physical Examination


Patients with NCTs usually have recurrent episodes of arrhythmia. The age of onset of episodes often suggests a diagnosis: Episodes from birth onward are likely to be AV reentrant tachycardia (AVRT) using an accessory AV pathway present from birth, or AT. Onset of symptoms during or after puberty is common in AV nodal reentrant tachycardia (AVNRT). Although these scenarios are generally true, any type of NCT can occur later in life. Symptoms include palpitations, light-headedness, dyspnea, chest pain, and neck fullness. In many, episodes are facilitated by exercise and emotional upset. Physical maneuvers such as Valsalva 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 sometimes can be perceived. In patients with repaired congenital heart disease, scar-based atrial macroreentry should be suspected.



Electrocardiographic Differential Diagnosis


Among NCTs, the differential diagnosis is based on the A : V ratio; among those with 1 : 1 AV ratio, the timing of the P wave relative to a QRS complex; and P-wave morphology (Table 59-1). Although individual variability is noted, some patterns are relatively constant.




A A : V ratio:



B R-P interval in cases with 1 : 1 A : V ratio



1. Absence of a visible P wave (subsumed in the QRS complex) is common in AVNRT (anterograde slow, retrograde fast pathways) but can occur in AT with a long AV conduction time.


2. NCTs with a short R-P interval (P wave in the first one-third of the R-R interval) include AVRT, AVNRT (especially in patients >50 years old), and AT, with junctional tachycardia a rare diagnosis.


3. Intermediate R-P interval NCTs (P wave in middle one-third of the R-R interval) are of the same types as short R-P NCTs, but AVNRT (“slow-slow”) and AT are more common than AVRT.


4. Long R-P NCTs are an interesting group with the same diagnostic possibilities as are seen in the other R-P subsets, but ATs predominate; AVNRT is of the less common “fast-slow” variety, and accessory pathways are of the very uncommon slowly conducting type.


C P-wave morphology2



1. Atrial activation in NCTs with positive P waves in the inferior leads begins near the top of the atria, including the upper crista terminalis, superior vena cava, and appendage in the right atrium, and the pulmonary veins and appendage in the left atrium, as well as cephalad portions of the tricuspid and mitral annuli. Evaluation of precordial leads (anteroposterior) and lead 1 (left-right) further refines the site of origin in the other two planes. As such, ATs account for many of these, but AVRT with pathway atrial insertions on the tops of mitral or tricuspid annuli are also part of this group.


2. Negative P waves in the inferior leads denote onset of atrial activation in the lower portion of the atria (low crista terminalis, coronary sinus os, low septum, and tricuspid annulus in the right atrium, and low septum or mitral annulus in the left atrium). All varieties of AVNRT as well as AVRT using posterior AV pathways fall into this group, as do some ATs.


3. An inverted P wave in lead 1 is a reliable indicator of left-to-right atrial activation, either from AT arising in the left atrium or pulmonary veins, or from AVRT using a left lateral pathway.


4. When all precordial leads show positive P waves, a left atrial or pulmonary venous source should be suspected.


Tags:
Jun 5, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Differential Diagnosis of Narrow and Wide Complex Tachycardias

Full access? Get Clinical Tree
Get Clinical Tree app for offline access