This was a relatively weak warning, because in the New England Journal azithromycin study, the patients who comprised the highest-risk cohort (who had about a 1 in 4,000 observed chance of sudden cardiac death after a 5-day course of azithromycin) included patients not only with QT prolongation or known arrhythmias, but those with a wide range of moderate to severe cardiac disease (coronary artery disease, congestive heart failure, etc), without any history of arrhythmias.
The zpack warning is overblown to begin with.