QTc: how long is too long?

Abstract

Congenital long QT syndrome (LQTS) affects an estimated 1 in 2500 people and typically presents with syncope, seizures or sudden death. Whereas someone exhibiting marked prolongation of the QT interval with QTc exceeding 500 ms who was just externally defibrillated from torsades de pointes while swimming poses negligible diagnostic challenge as to the unequivocal probability of LQTS, the certainty is considerably less for the otherwise asymptomatic person who happens to host a QTc value coined "borderline" (QTc > or = 440 ms). Although a normal QT interval imparts a much lower risk of life-threatening events, it does not preclude a patient from nevertheless harbouring a potentially lethal LQTS-causing genetic mutation. Indeed, genetic testing exerts significant diagnostic, prognostic and therapeutic implications. However, the 12-lead ECG remains the universal initial diagnostic test in the evaluation of LQTS and is subject to miscalculation, misinterpretation and mishandling. This review discusses the components of accurate QTc measurement and diagnosis, re-examines what is known about factors affecting QT interval measurement, and clarifies current recommendations regarding diagnosis of so-called "borderline" QT interval prolongation. The current guideline recommendations for the athlete with LQTS are also summarised.

Figure 1

Distribution of QTc values for patients with and without long QT syndrome (LQTS). The “borderline” QTc level of 440 ms is shown with a solid line. Note the significant overlap between “normal” and QTc values of mutation-positive patients from Mayo’s LQTS Clinic. Also note that the average QTc value in normal postpubertal females is on average 10 ms longer than that of normal postpubertal males. Modified from Taggart et al with permission from the American Heart Association, copyright 2007.

Figure 2

A diagrammatic representation of two cardiac cycles as recorded on an ECG. The RR and QT intervals are identified, along with a typical U wave that should not be included in the QTc calculation.

Figure 3

A repeat diagrammatic representation of two cardiac cycles as recorded on an ECG, with demonstration of the “teach-the-tangent” method of QTc measurement as described by Postema et al.

Figure 4

An ECG rhythm strip demonstrating significant macroscopic T-wave alternans. Note the alternating T-wave morphology and vector.

Figure 5

Effect of clinical setting on the relative probability of having long QT syndrome (LQTS). Note the relatively low predictive value of a QTc >440 ms in any asymptomatic patient compared with the patient with personal symptoms or a family history suggestive of LQTS. TdP, torsades de pointes.