Thanks everyone for yet another high yield report on ECGs with Dr. Zhao!  Here are the main pearls from today:

• Remember that a negative p wave amplitude in lead I is seen in two diagnoses only: dextrocardia and limb lead reversal.  To distinguish between the two, look at the amplitude of the QRS complexes as you advance through the precordial leads.  In dextrocardia, you should see a loss of amplitude as you go from V1 to V6, because you are getting further away from the heart.  In limb lead reversal, this is not the case.
• Remember that ST depressions in anterior leads V2 and V3 should raise your suspicion for a posterior MI and prompt further evaluation with a posterior ECG!
• When dealing with an irregularly irregular wide complex tachycardia, think of these three differential diagnoses:
  • Atrial fibrillation with aberrancy (i.e. with a bundle branch block)
    • QRS waves should largely look similar in morphology
    • Rates should not exceed 170 bpm because all conduction is still going down the AV node
    • Treatment: shock if unstable, AV nodal blocking agents or amiodarone
  • Atrial fibrillation with an accessory pathway (WPW, also known as a preexcitation pathway)
    • QRS waves have varying shapes because they are conducted down the accessory pathway and the AV node
    • Because the accessory pathway has a much shorter refractory period than the AV node, heart rate can be very high and >200 bpm.
    • Treatment: shock if unstable.  Do NOT give AV nodal blocking agents (including amiodarone) because blocking the AV node can force all conduction down the much faster accessory pathway and lead to VF arrest.  The agent of choice is IV procainamide.
  • Polymorphic VT
    • QRS morphology varies (Torsades)
    • Rates should not exceed 170 bpm
    • Treatment: shock if unstable, otherwise amiodarone