Since the bundle branchesThe bundle of His terminates in the right and left bundle branches, insulated rapidly conducting electrical pathways that connect with the Purkinje network and thus begin depolarizing waves across the ventricles; the left bundle branch splits into three smaller branches... are insulated – they are encapsulated with a fibrous sheath – an obstacle to conduction in any bundle (i.e. ischemiaInsufficient supply of oxygen to meet the oxygen demands of tissue. Anaerobic metabolism becomes increasingly important during periods of ischemia. Ischemia results from an inadequate blood flow that fails to meet the oxygen demands (energy demands) of tissues. If tissues... or infarct) results in the impulse not carried through to the ventricle; as a result, the depolarizing wave from the other bundle branch must travel further to depolarize the remaining ventricle; due to the extra distance for the wave to travel, more time is taken to depolarize and the QRS is wider than normal.
A bundle branch block reduces the speed by which the ventriclesThe larger chambers of the heart (3 times the volume and muscle thickness than the atria), responsible for the pumping of blood to the lungs and the rest of the body. depolarize, resulting in a wide QRS complexThe electrical representation of ventricular depolarization; the atrial repolarization is also a part of the QRS. ECG interpretation relies heavily on the QRS complex. The QRS complex represents the depolarization of the ventricles. The repolarization of the atria is also... (>.12 seconds or 3 mm). SupraventricularLocated above the ventricle – includes the bundle of His, AV node, atria and the SA node. 1. Six Second ECG Guidebook (2012), T Barill, p. 206 rhythms with a bundle branch block (with its wide QRS complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194) can appear to be ventricular rhythms, especially for rapid rhythms where P wavesA wave generated from the depolarization of the atria; the P wave is upright when originating from the SA node. The P wave represents the depolarization of the right and left atria. The P wave begins with the first deviation... are difficult to identify. Fortunately, left bundle branch blockA bundle branch block must satisfy two criteria - a wide QRS complex and a notch in the QRS complex. To distinguish between a RBBB and a LBBB, first make certain that the rhythm is indeed supraventricular (P waves before... (LBBB) and right bundle branch blockA bundle branch block must satisfy two criteria - a wide QRS complex and a notch in the QRS complex. To distinguish between a RBBB and a LBBB, first make certain that the rhythm is indeed supraventricular (P waves before... (RBBB) are easily determined with a 12 lead ECGElectrocardiogram; also called an EKG; a representation of electrical voltage measured across the chest over a period of time. 1. Six Second ECG Guidebook (2012), T Barill, p. 196.
An incomplete block of the anterior or posterior fascicleNormal ventricular depolarization begins with the septal fascicle of the left bundle branch (causing a Q wave) followed by a simultaneous depolarization of the remaining ventricular walls via the right and left bundle branches. The left bundle branch splits into... of the left bundle branch is called a hemiblock. A hemiblock has a normal QRS duration of less than 0.12 seconds (unless a RBBB coexists). Left anterior hemiblock (LAHB) is diagnosed if the net QRS deflectionWhile often a quick look is sufficient to determine whether the QRS complex is upright or inverted, occasionally a QRS complex requires a simple calculation to arrive at a net deflection. Figure 6.10 outlines the steps required to arrive at... in lead II is negative (deeper S waveAn S wave is the first wave after the R wave that dips below the baseline (isoelectric line). The end of the S wave occurs where the S wave begins to flatten out. This is called the J point. Figure... than height of R waveFollowing the depolarization of the interventricular septum, ventricular depolarization then progresses from the endocardium through to the epicardium across both ventricles producing an R wave and an S wave. An R wave is the first positive deflection of the QRS...). About 98% of all hemiblocks are anterior hemiblocks.
Both bundle branch blocks cause the ventricles to depolarize out of sync. The ventricle with the intact bundle branch depolarizes before its counterpart. As a result, two R waves form, an R wave and R prime resulting in a notch in the QRS complex.
A bundle branch block must satisfy two criteria – a wide QRS complex and a notch in the QRS complex. To distinguish between a RBBB and a LBBB, first make certain that the rhythmOften refers to a discernible pattern in time or distance between QRS complexes and/or P waves. is indeed supraventricular (P waves before each QRS) and that the QRS complex is wide (at least 0.12 seconds in duration).
A left bundle branch block (LBBB) is best distinguished using leads V5 or V6 due to their close proximity to the left ventricleThe left ventricle ejects blood into the aortic arch to the body. Within the arch, the coronary arteries branch off first followed by three main arteries that branch to the brain (carotids) and the upper thorax (subclavian artery). The chambers.... If a supraventricular rhythmIf the QRS is narrow, the rhythm originates from a supraventricular site. Otherwise, if the QRS is wide, chances are, the rhythm is a ventricular rhythm. Remember that the ECG is a two dimensional plotting of voltage (height or amplitude)... has wide and notched QRS complexes in leads V5 and/or V6, then a LBBB is evident. Similarly, a supraventricular rhythm with a right bundle branch block (RBBB) is diagnosed with a wide and notched QRS complex in leads V1 and V2 (closest to the right ventricleThe right ventricle ejects blood through the main branches of the left and right pulmonary arteries to the lungs. The chambers of the heart are the main drivers within an intricate pathway, delivering blood to the lungs for gas exchange...).
Occasionally, the QRS complex meets only one of the criteria (i.e. wide but not notched or notched but not wide). If not a hemiblock (see inset note above), this is commonly called an intraventricular conduction delayOccasionally, the QRS complex meets only one of the criteria (i.e. wide but not notched or notched but not wide). If not a hemiblock, this is commonly called an intraventricular conduction delay. sinus rhythm, HR 88, wide QRS, non-conducted PAC,....
Both left and right bundle branch blocks are commonly associated with ST depressionThe downward placement of the ST segment greater than 1 mm from the isoelectric line; suggestive of cardiac ischemia, but also may point to ventricular hypertrophy, digoxin use, and pericarditis among other possibilities. Between the QRS complex and the T... and T wave inversionAn inverted T wave can point to cardiac ischemia among other causes. Ischemia to the epicardium prolongs ventricular repolarization to this area. This extended repolarization of the epicardium removes the delay between the repolarization of the endocardium and the repolarization... (with or without ischemia). This makes the identification of cardiac ischemiaThe ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac deaths are associated with an ischemic episode. Patient deaths due to an acute myocardial infarction (MI) typically... difficult. In addition, a LBBB often presents with ST elevationThe upward placement of the ST segment greater than 1 mm from the isoelectric line; suggestive of cardiac infarction or ischemia. Between the QRS complex and the T wave, lies the ST segment. The ST segment usually follows the isoelectric... in leads V1-V3 (and other leads), making the identification of an acute MI almost impossible in the presence of a LBBB. With clinical symptoms congruent with an MI, the appearance of a new onset LBBB is considered equivalent to a STEMI.
A quick reference for the difference between RBBB and LBBB.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 151, 184, 192