LBBB is seen with all except: (A) Acute MI, (B) Ashmann syndrome, (C) Hypokalemia, (D) Hyperkalemia
Answer is C. Ans. is 'c' i.e., Hypokalemia Causes of LBBB are :- Aoic stenosis Ischaemic hea disease Hypeension Dilated cardiomyopathy Anterior MI Primary degenerative disease (fibrosis) of the conducting system (Lenergre disease) Hyperkalaemia Digoxin toxicity Ashmann phenomenon (has both LBBB and RBBB) Atrial fibrillation has a narrow complex qRS but Ashmann phenomenon seen in atrialfibrillation is characterized by broad complex qRS with usually a RBBB morphology. Thus if an impulse lands on the bundle of HIS and finds the right bundle refractory then RBBB will occur. Also remember that the refractory period of right fascicle ix more than that of the left fascicle resulting in RBBB mostly in these patients. ECG findings of LBBB Normally the septum is activated from left to right, producing small Q waves in the lateral leads. In LBBB, the normal direction of septal depolarisation is reversed (becomes right to left), as the impulse spreads first to the RV the right bundle branch and them to the LV the septum. This sequence of activation extends the qRS duration to > 120 ms and eliminates the normal septal Q waves in the lateral leads. The overall direction of depolarisation (from right to left) produces tall R waves in the lateral leads (IaVLV5V6) deep S waves in the right precordial leads (V4 R) and usually leads to left axis detion. As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (`W-shaped) R wave in the lateral leads. Non-concordance in ST segment and T wave changes. The point is that the two fasciles of bundle of his have different refractory periods with the right fascicle having higher refractory period than the left. This means that if an impulse lands on the bundle of HIS and finds the right bundle refractory then RBBB will occur. In atrial fibrillation because of faster conduction, normally we see narrow complex qRS but sometimes we may see broad complex qRS also which is technically called Ashmann phenomenon. Mostly in Ashmann phenomenon RBBB is seen based on the physiological principle of refractory period of right fascicle more than the left one. However, rarely LBBB can also be seen if the impulse lands to find the left fascicle refractory. Irrespective of RBBB or LBBB, broad complex qRS will occur in case of atrial fibrillation and Ashmann phenomenon is occurrence of broad complex qRS in atrial fibrillation and not the occurrence of RBBB as is the popular perception. Hyperkalemia can cause defective repolarization and hence cause Bundle branch block that culminates in sine wave pattern. In acute MI, ischemia can damage the left bundle leading to LBBB.