ECG Case 130

• Regular narrow complex tachycardia, rate 132 bpm
• Slight right axis deviation
• Dominant R wave in V1 meets voltage criteria for right ventricular hypertrophy (RVH)
• Pseudo R’ waves in the terminal portion of the QRS in V1, an isoelectric baseline, and lack of flutter waves in V1 point towards this rhythm being an AV nodal re-entry tachycardia (AVNRT)
• No rate-related ischaemia

There is one other feature on this ECG that suggests this patient has a congenital structural abnormality, can you spot it?

Reveal answer

• Notching near the apex of the R wave in inferior leads is known as Crochetage sign
• The presence of this sign in all three inferior leads has a 92-100% specificity for the diagnosis of a secundum atrial septal defect (ASD)

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Shortly after the above ECG was taken, the patient spontaneously reverted into sinus rhythm and a repeat ECG was taken:

What is your interpretation?

Reveal answer

• Normal sinus rhythm, rate 84 bpm
• Multiple premature atrial complexes (PACs) — 2nd and 10th beat
• Features of secundum ASD are again seen — Crochetage sign in all three inferior leads, slight RAD and dominant R wave in V1

This patient self-reverted in and out of AVNRT multiple times during her stay in the emergency department. When in sinus rhythm she had frequent PACs, which reduced following the administration of metoprolol. She subsequently remained in sinus rhythm and was referred for formal TTE and evaluation for definitive repair of secundum ASD.

Secundum ASD is a common sporadic congenital heart defect with a prevalence of around 1 in 800 in the general population. Patients with an isolated defect usually remain asymptomatic until the 3^rd decade of life, with the most common adult presentation being supraventricular tachycardia (SVT), followed by exercise intolerance and/or fatigue. If left untreated, long term morbidity relates to the development of pulmonary hypertension.

ECG changes likely relate to functional RV volume overload due to left-to-right shunting, with resultant raised pulmonary pressures. The characteristic Crochetage sign often disappears following defect repair, which is usually via transcatheter device closure.

ECG features of secundum ASD

Differentials for “SVT” in this patient

The term “SVT” is a misnomer commonly used synonymously with AVNRT, which is one of many forms of SVT. SVT refers to any tachyarrhythmia arising from above the level of the Bundle of His, and encompasses regular atrial, irregular atrial, and regular atrioventricular tachycardias.

Differentials for a regular narrow complex tachycardia include:

• AVNRT
• Atrial flutter
• Focal atrial tachycardia

The most common form of AVNRT (slow-fast) is precipitated by the arrival of a PAC whilst the AV node is still refractory. This leads to the formation of a re-entry circuit, which causes simultaneous anterograde activation of the Bundle of His and retrograde activation of the atria. Resultant HR will depend on the length of the re-entry circuit in the AV node which is variable between patients.

The presence of retrograde P waves on the ECG is variable – they may be buried within or after the QRS complex. The term pseudo R’ wave refers to the presence of a retrograde P wave visible in the terminal portion of the QRS complex, usually best seen in V1 or V2, which may mimic an RSR’ complex appearance. These can be very subtle.

Differentiation of AVNRT from atrial flutter on the ECG can be difficult and may not always be possible. Atrial flutter is caused by a similar re-entry circuit occurring within the right atrium. The length of this circuit corresponds to the size of the right atrium, usually resulting in flutter waves at a rate of ~300bpm. The ventricular rate depends on the AV conduction ratio. This ratio is often referred to as the “AV block”, however this is again a misnomer. Such AV bock is a physiological response to rapid atrial rates and implies a normally functioning AV node.

High grade AV conduction ratios in atrial flutter (4:1, 5:1), particularly with variable block, are often seen in the presence of AV nodal blocking drugs in the elderly population.