ECG of the month
A 10 year old boy was referred for further management. He was started on diuretics and an ACE inhibitor at a peripheral hospital. He continues to become dyspnoeic with minimal effort and has no other history of note.
Which one of the following represents the best management option for this patient?
a) Referral for cardiac transplantation
c) Cardiac resynchronization therapy by implantation of a biventricular pacemaker
d) Implantation of an Automatic Implantable Cardioverter-Defibrillator.
e) Electrophysiology study and ablation.
Please analyse the ECG and commit yourself to an answer before checking the explanation.
The ECG shows a regular broad complex rhythm at a rate of 84 beats/min.
Before deciding on the best management option, one has to come to an ECG diagnosis.
The differential diagnosis of any regular broad complex rhythm is:
1. Rhythm of ventricular origin.
2. Aberrantly conducted supraventricular rhythm
3. Supraventricular rhythm with pre-existing bundle branch block
4. Supraventricular rhythm in the setting of electrolyte disturbances / toxic effect of drugs.
5. Supraventricular rhythm with ventricular pre-excitation.
The obvious default diagnosis is ventricular tachycardia, but careful analysis of this ECG does reveal some helpful features.
The key to happiness in life is relationships, and this is true for ECGs too – specifically the relationships between atrial and ventricular electrical activity.
A closer look at V2 reveals a small p wave preceding each QRS complex. If one only used the rhythm strip to look for p waves, one might have interpreted the p wave as part of the QRS complex. This brings me to the second rule of ECG interpretation – Always use all your leads. Some readers may point out that the time interval between atrial and ventricular events appears to be very short, and one might wonder if it is possible to determine if the atrial rhythm is in fact “driving” the ventricular rhythm or if the atrial and ventricular rhythms are isorhythmicallydissociated? If one was to measure the P-P intervals in the rhythm strip, a slight irregularity in the atrial rate would be apparent. Comparing each P-P interval with the following R-R interval would confirm that beat to beat changes in the atrial rate precedes the same change in the ventricular rate, confirming that the atrial rhythm is “driving” the ventricular one. This effectively excludes a ventricular rhythm in this case.
The options of bundle branch aberrancy and pre-existing bundle branch block can be discussed together. The QRS morphology is reminiscent of Left bundle branch block (LBBB), so one has to ask, is this LBBB?
The criteria for LBBB
i. QRS duration > 95th centile for age (102 msec for 10 year old)
ii. Broad, notched monophasic R waves in I, aVL, V5 and V6.
iii. Absence of septal q waves in I, V5, V6.
iv. R Peak time (intrinsicoid time) is delayed (>0.06 seconds) in V5 and V6, but is normal in V1 and V2
v. In majority of cases there are small r waves in V1 and V2, followed by wide S waves.
Close inspection of the ECG reveals that the initial activation appears to be delayed in all the leads, including the right sided precordial leads. This is against a typical LBBB pattern. This excludes a pre-existing LBBB or a supraventricular rhythm with LBBB aberrancy from the differential diagnosis.
Considering the effects of electrolytes / drugs / toxins: Cardiac depolarization is mainly dependent on Sodium and Calcium, and repolarization on Potassium. Any disturbance in the relative concentrations of these ions is expected to influence the depolarization and repolarization phases of cardiac myocytes. These effects tend to affect the heart globally, and one would not expect to see some parts of the heart conducting slowly and other parts rapidly. The terminal parts of the QRS complexes in this patient still have very steep (almost vertical) slopes – indicating rapid activation during the terminal phases of depolarization. This would not be in keeping with electrolyte / drug / toxic effects.
The only option remaining in the differential diagnosis is that of a supraventricular rhythm with ventricular pre-excitation.
This is clearly the correct option, as the rhythm is clearly of supraventricular origin, the PR interval is short, and there are clear delta waves visible in the left sided leads.
In the diagnostic work-up of dilated cardiomyopathy the initial step should be to exclude reversible / treatable causes of cardiomyopathy, thus (a)referral for transplantation would be premature. (b) Cardioversion would be ineffective. (c) Cardiac resynchronization would be inappropriate because a potentially reversible cause for the cardiomyopathy had been identified. (d) An AICD would be inappropriate for the same reason.
Ventricular pre-excitation even in the absence of recurrent / incessant tachycardia has been associated with dilated cardiomyopathy. In a number of patients abolishment of the pre-excitation resulted in normalization of ventricular function. The best way to reliably abolish the accessory pathway function is by ablation of the pathway, and therefore en electrophysiology study with successful radiofrequency ablation of the accessory pathway was performed in this patient. The hope is that the patient will have full recovery of ventricular function in the coming months.
The correct answer is (e).
A careful search for possible reversible causes of cardiomyopathy should be performed in all patients with newly diagnosed dilated cardiomyopathy.
A subset of patients with ventricular pre-excitation develops a potentially reversible dilated cardiomyopathy.