Monday, October 3, 2011

ECG Interpretation Review #30 (Bundle Branch Block - RBBB - LAHB - LPHB - PACs - Aberrant Conduction)

Interpret the 12-lead ECG shown below in Figure 1, obtained from a 72-year-old woman as a “baseline tracing”. 
  • What type of “block” and what kind of “early” findings do you see? 
  • Is there evidence of recent infarction?
  • Clinically – What would you do?
NOTE: Parts of our answer to this very interesting tracing are advanced.  That said – I think there are lessons to be learned for all levels of interpreters.  Are you up for the challenge?

Figure 1 – 12-lead ECG obtained as a “baseline” from a 72-year-old woman. What type of "block" do you see. What to do?

INTERPRETATION:  The underlying rhythm for the 12-lead ECG shown in Figure 1 is sinus, as confirmed by the upright P waves with fixed PR interval for the 2nd and 4th beats in lead II.  There is variability in the overall ventricular response, in part due to sinus arrhythmia and in part due to the 3 PACs (Premature Atrial Contractions) that are seen on the tracing.  Lack of a lead II rhythm strip makes it more difficult to identify these rhythm characteristics.  We’ve therefore labeled the 14 beats in this tracing (Figure 2).

Figure 2 – 12-lead ECG from Figure 1 with each beat and key findings labeled. 

Note the following in Figure 2:
  • The 3rd beat (seen in simultaneously recorded leads I,II,III ) is a PAC.  The premature P wave is well seen notching the preceding T wave in leads II,III (small red arrow in lead III ) – but not seen in the small amplitude QRST complex of lead I.  This emphasizes one benefit of assessing rhythms in more than a single lead – which is that some leads are better than others for identifying certain findings.
  • Beats #7 and #11 are also PACs (Note small red arrows highlighting the premature P wave in leads aVF and V1,V2,V3).  We would need a longer rhythm strip to determine if the pattern seen here (every 4th beat being a PAC) continues – in which case the rhythm would be atrial “quadrigeminy”.
  • There is slight (subtle) alteration in QRS morphology of these PACs.  That is – the q wave of beat #3 in lead III is not as deep as it is for the 3 normally-conducted beats in this lead. The same holds true for the QRS of prematurely conducted beat #7 in lead aVF.  We suspect this same phenomenon (slight alteration in QRS morphology) also occurs for beat #11 – although it is difficult to tell because beat #11 occurs just before the lead change …  This interesting advanced concept is known as aberrant conduction – which sometimes is seen when a PAC occurs early enough in the cycle to fall within the relative refractory period (See ECG Blog #15).
Now that we’ve interpreted the rhythm for Figures 1,2 – it is time to proceed with the rest of our Systematic Approach.  We’ll avoid the QRST complexes for beats #3, 7 & 10 in our assessment – because of the above noted alteration in morphology resulting from aberrant conduction of these PACs:
  • The QRS complex is wide (more than half a large box or ~0.11 second in lead V1).  QRS morphology in the 3 key leads (I,V1,V6) is consistent with complete RBBB = Right Bundle Branch Block (See ECG Blog #3).  Thus (as shown within the RED-BLACK rectangles in leads I,V1,V6 of Figure 2) – there is an rsR’ in lead V1, and wide, terminal S waves in leads I and V6 that satisfy criteria for RBBB. 
  • There is also LPHB (Left Posterior HemiBlock), making the conduction disturbance in Figure 2 a bifascicular block (RBBB plus LPHB).  Because the posterior hemifascicle of the left bundle branch receives a dual blood supply and is much thicker than the anterior hemifascicle – LPHB is far less common than LAHB.  The diagnosis is made by the finding of a disproportionately deep S wave in lead I (Figure 3) in a patient with underlying RBBB.  Note that leads II and III show the opposite QRS pattern as lead I with LPHB (small q with tall R in leads II,III – vs small r with deep S in lead I ).  The occurrence of bifascicular block with LPHB is often associated with more extensive underlying cardiac disease – and the inferior and lateral precordial Q waves seen on this tracing may be indicative of prior infarction in these areas. 
  • There is no chamber enlargement in Figure 2.
Figure 3 – Schematic drawing of bifascicular blocks
— LEFT – RBBB/LAHB, recognized by the net negative QRS deflection in lead II. This is by far the most common form of bifascicular block. 
— RIGHT – RBBB/LPHB, recognized by the very deep straight component to the S wave in lead I with tall qR complex in lead II. 

The final part to our Systematic Approach to interpretation of the ECG shown in Figure 2 relates to assessment of Q-R-S-T Changes:
  • As noted above – there are small inferior and lateral precordial q waves of uncertain significance.
  • Transition is early (with the tall R wave in lead V1 due to the RBBB).
  • There are typical secondary ST-T wave changes of RBBB – with ST-T waves in the 3 key leads (I,V1,V6) being opposite the last QRS deflection in these leads as is expected with BBB (Secondary ST-T wave changes were explained in ECG Blog #3).
  • Perhaps the most interesting part of this tracing lies with assessment of ST-T wave morphology for beat #8 in lead V3 and beat #12 in lead V4 (within the RED ovals).  The ST segment is coved for both of these beats, with suggestion of ischemic-looking T wave inversion.  However, there is resolution of these ST-T wave abnormalities in the beats that immediately follow (beats #9 and 13, within the BLUE ovals).  At times – the normally-conducted beat following a PAC or PVC may show ST-T wave changes that are not seen on other sinus-conducted beats on the tracing.  Whether such changes reflect underlying ischemia or not is uncertain.  Thus, there is no evidence of acute ischemia or injury on this tracing with PACs and bifascicular block.

SUMMARIZING THOUGHTS:  This interesting 12-lead ECG obtained on an apparently asymptomatic 72-year-old woman shows underling sinus arrhythmia with PACs (some of which conduct with aberration). There is RBBB/LPHB (bifascicular block) — but no evidence of acute ST-T wave change (albeit with some alteration in ST-T wave morphology in the beat following PACs). This case provides an excellent illustration of the uncommonly encountered form of bifascicular block with LPHB — and — serves as a reminder of the importance of carefully scrutinizing QRS morphology of PACs, and ST-T wave morphology of the beat that follows the PAC.
  - See also ECG Blog #3 (on BBB) - and Blog #15 (on aberrant conduction) - 
 - Please check out our ECG Video on the Basics of BBB (


  1. thats a god post i like much ............This interesting 12-lead ECG obtained on an apparently asymptomatic 72-year-old woman shows underling sinus arrhythmia with PACs (some of which conduct with aberration)....i think its a big achivement

  2. Thank you Faseeh for your kind words. Glad this post was useful.

    Please note that I have MUCH MORE on BBB - free downloads - GO TO:

    All the best - Ken Grauer, MD (

  3. thanks a lot, thats quite demonstrative example

  4. Should not QRS be narrow at hemiblocks?

  5. @ GB - GOOD question you ask! Usually - the presence of an isolated hemiblock (either LAHB or LPHB) will produce only minimal QRS widening (by 0.01-to-0.02 second). However in this case - there is ALSO a complete RBBB (See the rsR' complex in lead V1) - and this is the reason the QRS complex is wide.