Vignette: This is a 70 yo patient who is admitted due to shortness of breath and managed as CHF. While on telemetry several tachycardic events were noted. Patient was just on bed sleeping and was even upset when checked. What is this rhythm?
Figure 1 - This is the full disclosure view at 2x magnification showing regular wide QRS tachycardia (~100 bpm) with P waves that are difficult to appreciate.
In telemetry settings, it is a good practice to document the initiation (beginning), middle and termination (end) of an arrhythmia. This will aid in arrhythmia interpretation.
Figure 2 - On the right side is the start of the tachycardia and on the left is the termination of the tachycardic event.
This is a regular wide complex tachycardia (right bundle branch block morphology) with sudden onset and termination. A tachycardia with sudden onset and termination practically eliminates sinus tachycardia (ST) . Sinus tachycardia presents with gradual increase and decrease and rate.
Figure 1 can be interpreted as supraventricular tachycardia. This arrhythmia was also one time interpreted as junctional tachycardia and machine read it as atrial fibrillation. However, close inspection will reveal distortions of the T waves. Those distortions (marked with red arrows in Figure 3) are actually P waves fusing with the terminal portion of the T waves. The difference in the contour or difference in the shapes of the T waves are not obvious in all leads. It is good practice in arrhythmia interpretation to inspect PQRST in simultaneous leads and it takes a lot of practice to see those tiny P waves . The 2-lead strip will always fall short in arrhythmia diagnosis. In Figure 3, P waves are best seen in leads I, II and aVR. Most of the P waves are fused with the T waves and are "hidden from view". If you recognize that there are P wave, then you are tempted to call this (Figure 1) as sinus tachycardia. However, you might be wrong.
Figure 3 - ECG strip in Figure 1 with P waves marked with red arrows.
During the initiation of the arrhythmia, sinus P waves are appreciated (blue arrow in Figure 4). After that, an early P wave initiates the tachycardia. It is upright (positive) in II, III and aVF and predominantly upright (positive) in aVL. It is hard to rely on V1 because it could be in the wrong location because the P wave morphology in the rhythm strip is very different in the 12 lead. As the tachycardia progresses, the P waves are hidden from view because it is fused with the T waves. The tachycardia terminated with a P wave. The P wave is difficult to appreciate because it is fused with the T wave of last conducted QRS. We know that there is a P wave buried there because of the difference of morphology of the T wave compared to during sinus rhythm. So, there is a blocked P wave at the end. The PRI of the first conducted beat is shorter than the last conducted beat. This is a feature of a Wenckebach block. A tachycardia with sudden onset and visible P waves but of different morphology during sinus rhythm and terminates with a Wenckebach block is atrial tachycardia (AT) or AT with a block.
Figure 4 - P waves marked. Sinus P wave marked with blue arrow and the ectopic P waves are marked with red arrows.
Interpretation: Paroxysmal atrial tachycardia with a block, right bundle branch block (fixed)
Supraventricular tachycardia (SVT) is the term often given for a narrow complex tachycardia (NCT) with no identifiable P waves. SVT can be a wide QRS complex tachycardia in patients with fixed bundle branch block or during conduction with aberrancy. SVT is a general term for a group of arrhythmia with the impulse originating above (supra - Latin for above) the ventricles which could either be the sinus node, atria, AV node or the bundle of His.
SVT diagnosis can be made easier by classifying it based on regularity and the relationship of the R wave to the P wave (long RP or short RP) as you can see in the table.
Table 1 - Supraventricular tachycardia classification based on regularity and RP-PR relationship (from: Kumar UN et al. 2006. The 12L Electrocardiogram in Supraventricular Tachycardia. Cardiology Clinics ;24: 427-437)
The case presented is paroxysmal atrial tachycardia (with a block). Atrial tachycardia (AT) is a regular atrial rhythm originating from the atrium with rates ranging from 100-240 bpm. AT is one of the supraventricular tachycardia (SVT).
The contour of the P wave depends on the site of origin. The P wave is different during sinus rhythm but may look like the appearance during sinus rhythm if the origin is near the SA node. It may also be low amplitude or negative in II, III and aVF. The PRI may be normal or prolonged.
The AV conduction ratio may be 1:1 at rates about 240 bpm. At rapid rates, there may be an AV block (Atrial tachycardia with a block) because the impulse will encounter the AV node in the absolute refractory period. The AV block can be 2:1 or the ratio may be higher. A Wenckebach block is common.
The QRS complex usually resemble that of patient's sinus complex. A wide QRS complex may be due to aberrant ventricular conduction. In most cases, the aberrant conduction is a right bundle branch block (RBBB) morphology but left bundle branch block (LBBB) can be seen. Occasionally the aberrant beats are seen at the start but normal QRS configuration returns as the tachycardia continues. The QRS complex may be wide because of the existing ventricular conduction defect. In these cases, differentiating it from ventricular tachycardia becomes a challenge.
AT commonly occurs in patients with significant structural heart disease but can occur in those without structural heart disease. It can occur in paroxysms (recurrent burst) or as an incessant tachycardia. Patients can tolerate the arrhythmia but it depends on the rate and underlying heart disease. Incessant tachycardia can result to tachycardia-induced cardiomyopathy and present as congestive heart failure. Incessant tachycardia tend to have lower rates and goes unnoticed for years until symptoms of cardiomyopathy develops.
Depending on the clinical situation, a beta-blocker or calcium blocker is used to slow down the ventricular rate. If the tachycardia is still present, other drug classes are used. Some AT's can be terminated by adenosine but persistence of the tachycardia with AV block is also a common response to adenosine. It is important to document the moment adenosine takes effect to see those P waves. Ablation is considered for those who fail drug therapy and those without underlying heart disease.
Bayes de Luna A. 2011. Clinical Arrhythmology. UK John Wiley and Sons.
Bonnow et al. 2014. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 10th Edition. PA Saunders
Das and Zipes. 2012. Electrocardiography of arrhythmias : a comprehensive review. PA Elsevier
Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.
Issa Z, Miller J and Zipes D. 2012. Clinical Arrhythmology and Electrophysiology: A Comprehensive Review - A Companion to Braunwald’s Heart Disease 2nd Ed. PA Saunders
Kumar UN et al. 2006. The 12L Electrocardiogram in Supraventricular Tachycardia. Cardiology Clinics ;24: 427-437