Question 36#

Regarding preexcitation syndromes, which ONE of the following statements is TRUE? 

A. Tachycardia associated with Wolff-ParkinsonWhite syndrome (WPW) is always wide complex

Answer: D: Tachycardia associated with Wolff-Parkinson-White syndrome (WPW) can be:

• Reentrant tachycardia with orthodromic/antegrade conduction down the AV node, resulting in a concealed pathway and narrow complex tachycardia – the most common reentrant tachycardia of WPW
• Reentrant tachycardia with antidromic conduction down the accessory pathway, resulting in a wide complex QRS
• Rapid conduction of atrial tachycardia through the accessory pathway

Verapamil, digoxin, adenosine and beta-blockers are only absolutely contraindicated in AF with preexcitation as they enhance conduction through the accessory pathway. The may also unmask a concealed pathway when given in SVT and WPW. Amiodarone and flecainide are the drugs of choice in AF with preexcitation, and cardioversion is required in unstable patients.

References:

1. Ong M, Lim S, Teo W. Arrhythmias. In: Cameron P, Jelinek G, Kelly A, et al, editors. Textbook of adult emergency medicine. 3rd ed. Edinburgh: Churchill Livingston Elsevier; 2009. p. 219–31.
2. Lindberg J, Brady W. Preexcitation syndromes. In: Chan C, Brady W, Harrigan R, et al, editors. ECG in emergency medicine and acute care. Philadelphia: Elsevier Mosby; 2005. p. 112–17.

Question 37#

Regarding reentrant SVT, which ONE of the following is TRUE?

A. Most patients have reentry involving a bypass tract

Answer: D: Most (about 60%) of reentrant SVT have their reentry within the AV node as described in the previous question. About 20% have reentry involving a bypass tract (accessory pathway) and the remainder have reentry elsewhere

In WPW syndrome, 85% of reentrant SVT are orthodromic (pass through the AV node in the usual direction) and hence have a narrow QRS on ECG. Antidromic conduction will produce a broad complex QRS on the ECG.

Adenosine will revert about 90% of reentrant SVTs.

Reference:

1. Piktel J. Cardiac rhythm disturbances. In: Tintinalli J, Stapczynski J, Ma O, et al, editors. Tintinalli’s emergency medicine: a comprehensive study guide. 7th ed. NewYork: McGraw Hill Medical; 2011. p. 129–54. 

Question 38#

Regarding WPW, all of the following statements are correct EXCEPT: 

A. 15–30% of arrhythmias are atrial fibrillation

Answer: D: Only 1–2% of patients with WPW present with arrhythmias. The approximate breakdown of different types of arrhythmias in WPW are:

• 80%: AV reentrant tachycardia
• 15–30%: AF
• 5%: atrial flutter

Characteristic ECG features of WPW are a triad of:

• Short PR interval (<0.12 seconds)
• Slightly prolonged QRS complex (>0.1 seconds)
• Slurring of the upstroke of the R wave (delta wave)

Additionally, WPW may also produce a tall R wave in V1, which may be misdiagnosed as a posterior myocardial infarction.

References:

1. Chew HC, Lim SH. Broad complex atrial fibrillation. Am J Emerg Med 2007;25:459–63.
2. Mattu A, Brady W. Part 2: 12-lead ECG (intermediate level). In: Mattu A, Brady W, editors. ECGs for the emergency physician 2. Malden: Blackwell Publishing; 2008. p. 29–115.

Question 39#

According to the Australian Resuscitation Council recommendations for management of AF, which ONE of the following is TRUE?

A. Amiodarone is recommended for rate control of patients with AF and heart failure who do not have an accessory pathway

Answer: A:  Amiodarone or digoxin are recommended as IV agents to control heart rate in the acute care setting in patients with AF and heart failure, who do not have an accessory pathway.

Dihydropyridine calcium channel blockers do not reduce heart rate (indeed, they may result in reflex tachycardia). Nondihydropyridine calcium channel blockers and beta-blockers are recommended in the acute care setting for rate control in AF, taking care in patients with hypotension and heart failure. Cardioversion is not recommended after 48 hours due to the increased risk of thromboembolism. Digoxin and nondihydropyridine sodium-channel blockers are contraindicated in preexcitation syndromes, where ventricular rate may be paradoxically accelerated.

Reference:

1. Australian Resuscitation Council. Guideline 11.9: Managing acute dysrhythmias. November 2009. Online. Available: www.resus.org.au.

Question 40#

With respect to AF in the ED, which ONE of the following is TRUE?

A. Electrical cardioversion usually requires more than 100 J of energy

Answer. D: About 90% of ‘lone fibrillators’ will spontaneously revert with no treatment in the subsequent 48 hours. More than 60% of patents with AF will cardiovert with 100 J DC shock. When the patient is unstable due to rapid ventricular rate, immediate cardioversion should be attempted. It is important to exclude underlying illness, such as haemorrhage or sepsis, as the cause of the rapid ventricular rate prior to attempting cardioversion.

In the patient with chronic AF, rate control is the priority. Indeed, these patients carry a significant risk of embolic events if cardioverted without the necessary period of anticoagulation before and after cardioversion.

References:

1. Ong M, Lim S, Teo W. Arrhythmias. In: Cameron P, Jelinek G, Kelly A, et al, editors. Textbook of adult emergency medicine. 3rd ed. Edinburgh: Churchill Livingston Elsevier; 2009. p. 219–31.
2. Piktel J. Cardiac rhythm disturbances. In: Tintinalli J, Stapczynski J, Ma O, et al, editors. Tintinalli’s emergency medicine: a comprehensive study guide. 7th ed. NewYork: McGraw Hill Medical; 2011. p. 129–54.