With respect to heart block, which ONE of the following is TRUE?
Answer: D: Third-degree heart block complicating AMI confers an increased mortality, even when rate is controlled with pacing. In particular, anterior myocardial infarction with third-degree heart block implies extensive anterior infarction and poor prognosis. First-degree heart block will display a PR interval of >200 ms.
Mobitz type I second-degree heart block is where the PR interval progressively lengthens, eventually resulting in a non-conducted or ‘missed’ atrial complex. Mobitz type II second-degree heart block displays a constant PR interval with intermittent failure of P wave conduction. Often the ratio of conducted to non-conducted beats is constant.
Third-degree heart block is a complete failure of conduction between atria and ventricles. The escape rhythm is generated either at the AV node (which will be evident by a narrow QRS complex) or at an infranodal level (where the QRS will be prolonged).
References:
Regarding atrioventricular block, which ONE of the following statements is TRUE?
Answer: C: The location of the block in Mobitz type I is at or above the AV node. Mobitz type II is characterized by conduction abnormality below the AV node in the bundle of His, hence the QRS complex is usually widened. The block in third-degree heart block may be at or below the AV node. If the block is at the AV node then the escape rhythm may be narrow complex, otherwise the escape rhythm will be broad (ventricular). The most frequent block in AMI is Wenckebach (Mobitz type I) at 15%. Complete heart block is the most common unstable rhythm complicating AMI at 8–10%, particularly inferior AMI.
Regarding VT, which ONE of the following statements is TRUE?
Answer: B: AV dissociation is the hallmark of VT, but occurs infrequently in up to 25% of cases. A north-west QRS axis (–90 to +180) is the only QRS axis that is specific for VT.
Other ECG findings suggestive of VT include:
RBBB morphology is more likely to be SVT with aberrancy, although an rSR morphology suggests VT. VT may be slower than 140 beats per minute in patients taking cardioactive medications (e.g. amiodarone).
Regarding broad-complex tachycardia, which ONE of the following is TRUE?
Answer: A: Cannon A waves result from dissociation between atrial and ventricular activity and in this setting indicate that VT is the likely diagnosis. In the ED, VT is responsible for about 80% of broad-complex tachycardias. Retrograde P waves on ECG can be found with either VT or SVT. Cardiac ischaemia is occasionally the cause of torsades de pointes. More common causes are drug toxicity, electrolyte abnormalities and congenital prolonged QT syndrome.
Reference:
Regarding broad-complex tachycardia, which ONE of the following characteristics would favour a diagnosis of VT rather than SVT with aberrancy?
Answer: C: Age >35 years makes VT the more likely diagnosis. A preceding P wave indicates a supraventricular origin to the tachycardia. Care must be taken, however, to make sure it is not mistaken for a retrograde P wave, which can occur in both VT and SVT. Carotid sinus massage will have no effect on VT but may slow the AV nodal rate of an SVT. An rSR pattern in lead V1 is typical of an RBBB pattern, and therefore would be more common in SVT with aberrancy.
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