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Editor,—Dr Davies’s comments on the relation between QT prolongation and sudden infant death1 are based on accepting the report by Schwartz et al at face value.2 In that paper 9725 of 34 442 ECGs recorded over 19 years were analysed. Twenty four infants had unexplained sudden death and their ECGs were analysed retrospectively to show a mean difference in QT interval of 35 ms (< 1 mm). The paper gives no indication that the observers were blinded to the occurrence of sudden death. QT measurement depends on identification of the end of the T wave and retrospective measurement in the knowledge of sudden infant death raises doubts about the findings.
The report by Schwartz et al also goes against what is known about familial long QT syndrome. In long QT syndrome there is a strong association between the degree of prolongation of the QT interval and risk,3 yet Schwartzet al reported the QTc only slightly prolonged in all but two of their cases. International studies have failed to demonstrate any excess of sudden infant death in families with known long QT syndrome.4 Studies of first degree relatives of victims of sudden infant death5 and survivors of “near miss” sudden infant death have also shown no evidence of QT prolongation.
The changes that occur in QT interval with age during infancy are normal and should not be taken as evidence of “tendency towards reduced cardiac stability”. There is also no evidence of any differential risk related to different QT intervals within the normal range. Unless evidence can be provided from a blinded measurement of QT intervals in infancy it will be difficult to accept that there is any association between QT interval and sudden infant death syndrome.