It has been considered that the most common cause of marked left axis deviation (LAD) is abnormal conduction through the anterior branches of the left bundle. This concept has been so much emphasized that there is a tendency to point out too easily an existence of left anterior hemiblock (LAH) when LAD is found in ECG. Thereupon to investigate this correlation, vectorcardiographic studies were conducted with
Frank lead system in 101 subjects, who had electrical QRS axis with LAD (-30°～-90°) and QRS duration less than 0.12 sec. in ECG. Half the cases had ischemic heart disease and/or hypertension, and one third of them had clinically no abnormal cardiopulmonary findings. In 40 subjects, abnormalities in ECG were not recognized except for LAD. The patterns of QRS loop were classified into 7 types in frontal plane (F-1～F-7) and 6 types in horizontal plane (H-1～H-6), from the point of the direction of main and terminal QRS vectors. F-1, 2 types represent elongations of QRS loop from left-inferior to right- superior quadrant, F-3 type; extention to leftward and F-4, 5, 6 types ; locating chiefly in left superior quadrant. Each group consisted of one third of the all subjects. Pattern with characteristics of LAH was found in F-5, 6. F-3, 4 showed less superior displacement of QRS loop. Adding to these findings, F-5 showed a close relation to left ventricular hypertrophy in ECG. On the other hand, F-1, 2 showed LAD considered to be results of a delayed activation of the posterobasal resion of right and/or left ventricles. Twenty five cases showed S(Ⅰ) S(Ⅱ) S(Ⅲ) types ECG in which presence of S(Ⅰ) resulted in LAD. Their horizontal QRS loops showed H-1, 2, 6 types and 76 % of them except four cases with pseudo-LAD had narrow frontal QRS loops elongated from left-inferior to right-superior quadrant. Therefore, their spatial QRS loops were elongated from left antero-inferior to right postero-superior quardrant. Thirty five of the examined subjects satisfied Kulbertus' electrocardiographic criteria for LAH in which the limit of electrical QRS axis was broad, and 24 subjects did Rosenbaum's one which was more restrictive. Cases which satisfied any one of those criteria had various patterns in frontal QRS loop. Moreover, one third of them in each criteria showed F-1, 2 types. All of 9 cases showing F-5 satisfied the criteria of Kulbertus, but only 2 cases of them did that of Rosenbaum. Therefore, it was concluded that studies on morphorogic pattern of QRS loop are more useful than electrical QRS axis for diagnosis of LAH.