The Q1 deflection of the electrocardiogram in bundle branch block and axis deviation

Abstract

An initial downward, or Q, deflection in Lead I is very uncommon in human left branch block. When this component occurs in an electrocardiogram otherwise characteristic of this conduction defect, a lesion of the ordinary muscle of the ventricular septum should be suspected, and a full set of precordial leads should be taken.A Q deflection in Lead I occurs in about one-half of all cases of left axis deviation, regardless of the criteria employed in selecting examples of this electrocardiographic abnormality. Left axis deviation accompanied by inversion of the T waves in Lead I may sometimes be due to incomplete left bundle branch block when Q1 is absent, but it is almost never due to this cause when this deflection is present.The incidence of Q1 in right branch block is similar to its incidence in left axis deviation. In right axis deviation, this deflection is extremely rare.