ECG Basic:Ventricular Conduction Disturbances-Credit Cardio Rhythm Online Blogspot

When someone talks about "heart blocks" in most cases what they are referring to are blocks or conduction delays at the AV node i.e. first, second, and third degree heart block. However, blocks can also occur in the ventricular conduction system; perhaps the most well-known of these - right and left bundle branch block. But what of these other blocks? When I was a student I used to hear terms like "hemiblock" and “bifascicular block" and quite frankly it scared the life out of me. I reckoned because it sounded complicated it must be far too complex for my tiny brain to understand. When I first started working in the pacing lab as a technician I was more than familiar with most of the reasons for inserting a pacemaker, but I remember looking at this one ECG and rather embarrassingly asking the cardiologist why the patient needed pacing, because all I could see was a right bundle branch block! I now know it was a trifascicular block. 

To become proficient at reading these types of ECGs you really need to know how to calculate the cardiac axis. Once you can do this everything becomes so much easier. Below is a summary of what works best in my classes, and a good place to start if you are a beginner…

NORMAL VENTRICULAR CONDUCTION 

Basically we have three main fascicles: The right bundle branch, the left anterior fascicle and the left posterior fascicle (the latter two fascicles make up the left bundle branch). If all of these fascicles are working correctly both ventricles will contract at the same time; this is a very rapid process and equates to a narrow QRS complex. 

RIGHT BUNDLE BRANCH BLOCK

As the terminology implies this is a block in the right bundle branch. Does this cause the heart to slow down like we see in some AV blocks? No, because we still have the left bundle working the electrical impulse simply travels down the left side and then spreads across to the right ventricle. Ok, it's not as efficient as both bundles working at the same time, but it's still enough to make both ventricles contract albeit in a different direction from the norm and with a slight delay. How does this manifest on the ECG? Well, perhaps the most obvious sign is a change in the QRS morphology in the right precordial leads - namely the typical RSR pattern. Why the RSR pattern? Well, it's all about vectors. The second R wave is produced by the wave of depolarisation spreading from the left ventricle to the right ventricle i.e. toward the right precordial leads. Anything that moves toward a lead will produce a positive complex. Don't forget that in a normal ECG V1 should be predominantly negative. There aren't that many causes of a positive complex in V1, so right bundle branch block is at the top of your list. 

Please bear in mind that there are many presentations of right bundle branch block that won't necessarily fit this text book example, but if you are a beginner this type of pattern recognition is a good place to start. There are also other causes of RSR patterns.

LEFT BUNDLE BRANCH BLOCK

If the left bundle is blocked then the impulse simply travels down the right bundle and then spreads across to the left side. With the left side being so much larger there tends to be a more obvious delay in conduction through the left ventricle i.e. the delay manifests as a broad QRS complex. V1 will be negative (as in a normal ECG), but the QRS complex will appear wide throughout. You may also see notching in the lateral leads.

Of course bundle branch blocks make life rather difficult because if you are a beginner you have probably already learnt that a wide QRS indicates a rhythm initiated in the ventricles! To further complicate matters bundle branch blocks may also show associated T wave changes; these are not necessarily indicative of ischaemic heart disease, but it doesn't mean the patient hasn't got underlying coronary disease either, it is just very difficult, if not impossible, to determine this from an ECG when a bundle branch block is present. As a beginner it's easy to become fixated on ST/T wave changes and bundle branch block ECGs often get mistaken for MIs. Remember if your QRS complex is wide consider that it might be a bundle branch block - this is especially so if your heart rate is normal and/or you can see one P wave before each QRS. If you are ever asked to determine between right or left bundle branch block simply look at V1 - if it is positive it is RBBB, if it is negative it is LBBB.

HEMIBLOCKS
The left bundle branch splits into the anterior and posterior fascicles. A hemiblock is a block in either one of these fascicles. Is this enough to cause a wide QRS like we see in left bundle branch block? No, because one of the other fascicles is still in contact with the left ventricle there is not enough of a conduction delay for the QRS to appear as wide as it does in left bundle branch block. What it does do though is cause a shift in axis.  

LEFT ANTERIOR HEMIBLOCK

The impulse travels down the posterior fascicle and then spreads in an upward leftward direction to depolarise the anterior region. This manifests as a left axis deviation. 

LEFT POSTERIOR HEMIBLOCK 

The impulse travels along the anterior fascicle and spreads in a downward rightward direction to depolarise the posterior region. This manifests as a right axis deviation.

Hemiblocks by themselves can be quite tricky to diagnose because you need to exclude other causes of left and right axis deviation, such as an inferior MI (left axis) or right heart enlargement (right axis) before arriving at a lone hemiblock conclusion. Where I think it becomes interesting though is when a hemiblock is combined with another block – namely right bundle branch block…

BI & TRI FASCICULAR BLOCKS

The terminology implies a block in either two or three fascicles. A bifascicular block is usually classed as a right bundle branch block with either a left anterior or left posterior hemiblock. The ECG will show a right bundle branch block pattern with either a left or right axis deviation respectively. This means that there is only one remaining fascicle to bring about ventricular contraction. You will note that the ECG shown above for right bundle branch block demonstrates a normal axis – I can’t remember where I appropriated this term, but I often refer to these as “uncomplicated” i.e. a right bundle branch block with a normal axis. The ECG below, however, demonstrates a right bundle branch block with a left axis deviation (suggesting a left anterior hemiblock also). 

If you add into the above mix a first degree heart block this is often referred to as trifascicular block. In other words: right bundle branch block + hemiblock + prolonged PR interval = trifascicular block  

However, a complete trifascicular block (where all three fascicles are blocked) would actually manifest as a complete heart block, so some authors now refer to the pattern I've just described as an “incomplete” or “impending” trifascicular block. Furthermore, the AV node is not a fascicle so the terminology can be a bit of a misnomer; hence there are some recommendations against using it altogether. Either way, a small percentage of these patients, especially those who experience syncope, may require pacing.

There are other scenarios I need to briefly mention. Some authors also refer to left bundle branch block as bifascicular block due to the fact that it involves both the anterior and posterior fascicles. In the past I have also come across left bundle branch block + first degree heart block, and alternating left and right bundle branch block also being referred to as a trifascicular block