ECG Screening of QTC
Briefly, the QT interval represents the period of cardiac repolarization of the ventricular action potential. If the observed QT interval is too long, it suggests that cardiac repolarization is too slow and therefore unstable (Haddad and Anderson, 2002; Khan, 2002).
The range of normal QTC values in children and adolescents has roughly a mean of 400 msec with a standard deviation of 25 msec to 30 msec. Therefore, a QTC value that exceeds two standard deviations ( >450 msec to 460 msec) is too long. A QTC >450 msec is associated with increased mortality in the elderly (Labellarte et al., 2003a; Robbins et al., 2003), and QTC >500 msec is associated with increased mortality at all ages (Labellarte et al., 2003a; Moss, 1993).
Similarly, any prolongation from baseline that exceeds two standard deviations is too much; an increase in QTC >60 msec from baseline is also associated with increased mortality (Haddad and Anderson, 2002; Labellarte et al., 2003a).
However, measuring QTC duration is a limited clinical screening tool in pediatric psychopharmacology because of variability in rate-correction formulas (Tisdale et al., 2001); variability in clinician accuracy (Labellarte et al., 2003b); variability between expert clinician readings and ECG computerized printouts (Miller et al., 2001); and normal QTC variability related to fear, postural changes or time of day. Additionally, ECG measurement may not be sensitive enough to detect QTC disturbance. Most important, documentation of baseline QTC duration within the normal range is not a guarantee that environmental factors (including drug effects) will not cause QTC prolongation.
Psychiatrists should be able to screen ECGs for changes associated with QTC prolongation before and during treatment, but are not required to be experts on ECG interpretation. In addition, the computer readout of an ECG is not reliable when confounding factors are present (i.e., tachycardia, bradycardia, unusual T-wave morphology, any arrythmia including bundle branch block or significant U-waves).
Clinical guidelines suggest consulting a cardiologist if the QTC stays prolonged after discontinuing medication, if cardiovascular symptoms are present, or if pre-existing cardiac disease or strong family history is a factor (Wilens et al., 1996). Several other QTC scenarios occurring during treatment also merit consultation: QTC duration >480 msec; QTC prolongation >60 msec over pretreatment duration, or QTC duration >450 msec after 10% to 15% increase (Labellarte et al., 2003a).
A child and adolescent psychiatrist should consult a cardiologist familiar with QTC measurement if they are uncertain about the clinical significance of QTC status before treatment or during careful monitoring of patients on medications that can prolong QTC. If the utility of medications that can prolong QTC is in question, perhaps safer alternatives should be prescribed.
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