A bradycardic rhythm at 36/minute with P’s before each QRS and another P wave following the QRS that does not conduct. There is also a wide QRS complex consistent with a LBBB.

This rhythm is either 2:1 AV block (a 2° AV block of undetermined type) or atrial bigeminy with blocked PACs. If this is 2:1 AV block, the P waves should be regular…and they are not in this tracing! This is a notorious and well-described masquerader of high-grade AV block. Remember Dr. Marriott’s recommendation “cherchez la P.”

Compare this week’s EKG rhythm strip

…with a similar appearing rhythm that does have regular P waves, and therefore is likely 2:1 AV block

Sinus at 84/minute
Left axis (-50°)
Intervals: PR and QT normal; QRS appears prolonged at 120ms

The interesting feature of this EKG is that it nearly meets the criteria and appearance for LBBB. There is a wide QRS, a tall R wave in V₆, and some secondary repolarization changes like poor R wave progression and the appearance of ST-segment elevation. What isn’t consistent with LBBB is the presence of small Q waves in V_5-6, the “septal Q’s” which indicate early depolarization of the septum by way of the LBBB. This initial, and largely unopposed depolarization wave is left to right, thus small negative waves (Q waves) routinely appear in lateral chest leads, and their appearance assures that the left bundle is present and functioning.

This EKG is severe LVH with voltage criteria and all the secondary changes that begin to accrue – left axis, prolongation of the QRS, repolarization changes in the anterior chest leads. This is one of the most difficult EKG patterns of ST-elevation to differentiate from a STEMI – comparison with an old is helpful and then following the apparent ST-elevation in the anterior leads closely as these repolarization changes are not dynamic and should not evolve as will happen with an acute MI. This pattern is one of the most common causes of “inappropriate” calls to interventional cardiology – often only made clear by repeat EKGs and clinical course. If in doubt, an interventional consult is always the correct answer.

Sinus at 100/minute
Axis: right axis deviation
Intervals: QRS prolonged at 160ms with a tall R in V₁ – RBBB

Now, chamber enlargement?
With a RBBB the usual criteria for both right and left ventricular chamber enlargement are somewhat altered.

R wave in lead V₁ > 15mm suggests RVH, especially with rightward deviation of the axis

LVH criteria become less sensitive (eg not seen as reliably), but if voltage criteria for LVH are met with a RBBB they are quite specific.

In this week’s tracing, the very tall T in V₁ is 18mm, and with the right axis deviation is diagnostic of right ventricular hypertrophy.