Right Bundle Branch Block

With any of the bundle branch blocks, activation is delayed in that bundle, causing the EKG to display more of its depolarization as it is isolated (the QRS is usually a combination of multiple bundles/fascicles). With all of the fascicular/bundle branch blocks, the beginning of the QRS is normal. This delayed end of the QRS with a RBBB produces an R' in the right-sided chest leads (V1-V3) and a wide S wave in the left-sided chest leads (V4-V6). It is classically taught that a RBBB will produce an RSR' in the right-sided leads, though there may instead be a qR pattern. These do not always fall in V1-V3 exactly (sometimes the RSR' extends further). As with both of the bundle branch blocks, the QRS will be wide as well (>120ms). An incomplete RBBB meets the above criteria, except the QRS is between 100-120ms. A RBBB does not necessarily mean the patient will have right axis deviation.

These depolarization abnormalities will result in repolarization abnormalities, producing ST depression/T wave inversion in the right-sided precordial leads. The usual changes are discordant (opposite direction to the major QRS axis). Patients do not commonly have these repolarization abnormalities in the left-sided precordial leads.  If concordant changes are found on the EKG, that is highly concerning for ischemia. Sometimes, the ST segment being isoelectric in the right-sided precordial leads is concerning for early ischemia if the patient's prior EKG's demonstrate ST depression in these areas. Leads with prominent S waves should not have ST depression or T wave inversions. An OMI with a new RBBB is highly concerning for either proximal LAD occlusion or RCA occlusion and carries a worse prognosis. 

Examples:

Rate-Related Bundle Branch Block

Patients can develop a rate-related bundle branch block when the heart rate exceeds the refractory period. This is more commonly seen in older patients. Normally, the refractory period is dependent on the heart rate: the faster the heart rate, the shorter the refractory period will be. The EKG with a normal heart rate will not have a bundle branch block, but when the patient develops a tachycardia, the patient will develop a bundle branch block. The heart rate that this occurs at is patient dependent and usually signifies some underlying disease to the affected bundle; the lower the heart rate that this appears, the more pathology is typically present at that bundle. In other words, a rate-related bundle branch block occurs when an acceleration of the heart rate produces conduction failure due to early refractoriness. Sometimes, patients have an underlying incomplete bundle branch block that becomes complete when the heart rate exceeds a certain threshold. Rarely, patients may develop a rate-related bundle branch block that occurs with bradycardia. 

This most often causes confusion for a ventricular origin of conduction or for ischemia. However, patients will still have a supraventricular origin (ie P waves if it is a sinus rhythm). Sometimes, this rate-related bundle branch block can be captured on an EKG also with their native underlying conduction in a patient with varying heart rates. This is commonly seen in patients with premature atrial contractions ("PAC with aberrant conduction"). In patients without underlying disease, the right bundle has a slightly longer refractory period and is more commonly seen in patients with significantly high heart rates with supraventricular conduction through the AV node (ie extreme sinus tachycardia or SVT). One can even sometimes see a tachycardia-dependent alternans. This is when a patient usually has an alternating heart rate (intermittently tachycardic) with a rate-related bundle branch block, so the QRS complexes alternate between a normal conduction and a rate-related bundle branch block. 

Examples:

Further Reading:

https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/

http://hqmeded-ecg.blogspot.com/search/label/right%20bundle

https://www.onlinejacc.org/content/accj/16/1/240.full.pdf

www.ahajournals.org/doi/pdf/10.1161/01.CIR.55.2.242