ABSTRACT
In practice, axis deviation which might indicate pathology is determined by applying ‘rules of thumb’ to the heights of the R and S waves in leads I, II, and III.
Left axis deviation is significant when there is a deep S wave in III and the S wave is of greater amplitude than the R wave in II (Figure 47.5). It is usually caused by a conduction defect (especially left anterior hemi-block, Figure 47.5) rather than by left ventricular hypertrophy. Minor degrees of left axis deviation are found in any condition in which the heart is more horizontal than normal (e.g. short fat subjects, pregnancy, all the causes of a raised diaphragm), and rarely are associated with tricuspid atresia and ostium primum atrial septal defects.
