Heart rate, gender differences, and presence versus absence of diagnostic ST elevation as determinants of spatial QRS|T angle widening in acute coronary syndrome.

We explored the directions of the repolarization sequence (RS) and excitation sequence (ES) as determinants of QRS|T angle widening in 126 patients with acute coronary syndrome (ACS) with and 658 patients without diagnostic ST-segment elevation myocardial infarction and in a reference group of 5,376 normal subjects. The initial and terminal RS and ES directions were derived from the initial and terminal T and QRS vectors from quintiles 1 to 3 and 5 of time-normalized QRS and T. In addition to the QRS|T angle, we evaluated the RS and ES deviation angles from the normal RS and ES directions. The normal zones for the ES and RS directions in the reference group were used to classify the ES and RS in the group with and without ST-segment myocardial infarction as normal or deviant. The initial and terminal QRS|T angles were rate invariant in the normal group but correlated with the heart rate (r = 0.33) in the ACS group. Adjusted for a heart rate of 70 beats/min, the initial QRS|T angle increased progressively in the ACS group from 41° among those with normal RS and ES directions to 121° when both were deviant. This apparently ischemia-induced initial QRS|T angle increase was associated with an initial RS direction from the anterior-right to the posterior-left producing a progressively greater peak TV1 amplitude. The terminal ES orientation in the ACS group was toward the right ventricular outflow tract with a QRS|T angle of 124° in 48% and leftward and posterior, with the QRS|T angle decreasing to 59°, in the remaining 52%. In conclusion, the initial and terminal RS and ES orientations differ drastically and their deviant orientation accounts for the widened QRS|T angle in patients with ACS, differences not revealed by evaluation of the mean QRS|T angle.