Change in the frontal QRS axis with changes in the anatomic positions of the heart.

Summary Changes in the frontal QRS axis with change in heart position were observed in individuals in two positions: standing at deep inspiration and supine after quiet expiration. Heart position was described by the predicted QRS axis, a value derived from the frontal anatomic (long) axis of the heart corrected for an estimate of horizontal anatomic deviation away from the frontal plane. Change in heart position was associated with a magnified shift of the frontal QRS axis. For every degree of change in the anatomic axis, the frontal QRS axis varied by about 3°. This relation held at frontal QRS axes from −30° to +105°. When only the frontal anatomic axis was measured, the association between the long axis and frontal QRS axis held until the frontal QRS axis was decreased to about 20°. From that point on, further decrease in the frontal QRS axis could occur without apparent change in the long axis of the heart. On this basis a hypothetical model was developed to show that longitudinal rotation could cause the frontal QRS axis to deviate from +30° to −30° with counterclockwise longitudinal heart rotation. However, the opposite longitudinal rotation was observed at surgery and in cadaver hearts with elevation of the diaphragm. From these findings it was concluded that: o 1. Change in heart position will cause deviation of the frontal QRS axis throughout its normal range. 2. About 3° of change in the frontal QRS axis is caused by each degree of positional change of the heart. 3. With shift of the frontal QRS axis to the left, the QRS loop undergoes counter-clockwise longitudinal rotation. This rotation of the loop is in the opposite direction to the longitudinal rotation which occurs as the heart is rotated toward the horizontal.