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Cardiac Rhythm Assessment and Management 287

Research vignette
Pickham D, Helfenbein E, Shinn J, Chan G, Funk M, Drew B. How significant corrected QT interval of 500 msecs or greater, with high
many patients need QT interval monitoring in critical care units? risk drug administration the most prevalent indicator and the pro-
Preliminary report of the QT in practice study. Journal of Electro- longation incidence increasing with the number of separate crite-
cardiography, 2010; 43: 572–6. ria present. Within the entire sample population, 8.7% of patients
who did not possess any of the monitoring indicators had devel-
Abstract oped a prolonged QT. This later finding differed significantly from
Recent Scientific Statement from the American Heart Association the AHA research outcomes, which reported a QT prolongation
(AHA) recommends that hospital patients should receive QT inter- incidence of 2.7% in patients not meeting any AHA monitoring
val monitoring if certain conditions are present: QT-prolonging indicators. The authors note that they were broader in their appli-
drug administration or admission for drug overdose, electrolyte cation of the AHA criteria than the original recommendations and
disturbances (K, Mg), and bradycardia. No studies have quantified for the purpose of measurement accuracy they excluded patients
the proportion of critical care patients that meet the AHA’s indica- with atrial fibrillation, significant artefact and a widened QRS dura-
tions for QT interval monitoring. This is a prospective study of 1039 tion. The number of patients excluded on this basis was not speci-
critical care patients to determine the proportion of patients that fied, but it can be assumed that this represented a significant
meet the AHA’s indications for QT interval monitoring. Secondary proportion of the reference population, given the high incidence
aim is to evaluate the predictive value of the AHA’s indications of such electrocardiographic abnormalities in critically ill patients.
in identifying patients who actually develop QT interval Improved methods for accurately determining QT measurement in
prolongation. these patients would lead to a more precise understanding of the
Methods prevalence of repolarisation delay in the wider critical care patient
Continuous QT interval monitoring software was installed in all population.
monitored beds (n = 154) across five critical care units. This system Polymorphic ventricular tachyarrhythmias, particularly torsades de
uses outlier rejection and median filtering in all available leads to pointes, are associated with the prior presence of pathological or
construct a root-mean-squared wave from which the QT measure- acquired ventricular repolarisation delay, as measured by QT inter-
ment is made. Fridericia formula was used for heart rate correction. val prolongation on the surface electrocardiograph. Whilst the
A QT interval greater than 500 milliseconds for 15 minutes or development of such arrhythmias is a relatively uncommon phe-
longer was considered prolonged for analyses. To minimise false nomenon, their occurrence can be potentially catastrophic, par-
positives all episodes of QT prolongation were manually over read. ticularly in patients with significant underlying cardiovascular
Clinical data was abstracted from the medical record. dysfunction.
Results Continuous bedside QT interval monitoring is not universally avail-
Overall 69% of patients had 1 or more AHA indications for QT able in all critical care settings, however the above findings, and its
interval monitoring. More women (74%) had indications than men associated AHA recommendations highlight the need for closer
(64%, P = 0.001). One quarter (24%) had QT interval prolongation bedside vigilance of the electrocardiographic repolarisation status
(>500 ms for ≥15 minutes). The odds for QT interval prolongation of critically ill patients, including its specific evaluation from routine
increased with the number of AHA indications present; 1 indica- 12 lead ECG recordings. Early detection of established or evolving
tion, odds ratio (OR) = 3.2 (2.1–5); 2 indications, OR = 7.3 (4.6–11.7); QT interval prolongation can prompt pre-emptive measures to
and 3 or more indications OR = 9.2 (4.8–17.4). Positive predictive reduce its associated risk, such as the reappraisal and possible
value of the AHA indications for QT interval prolongation was modification of causative drug therapies such as the Class IA and
31.2%; negative predictive value was 91.3%. III antiarrhythmic drugs and some antipsychotic agents, amongst
Conclusion others. Similarly, patients found to have QT prolongation should be
Most critically ill patients (69%) have AHA indications for QT inter- subjected to close serum electrolyte monitoring and control and
val monitoring. One quarter of critically ill patients (24%) devel- enhanced clinical vigilance if bradycardiac or experiencing chronic
oped QT interval prolongation. The AHA indications for QT interval or increasing ventricular ectopic activity. Whilst it was beyond the
monitoring successfully captured the majority of critically ill stated aims of this study to measure the actual incidence of tors-
patients developing QT interval prolongation. ades de pointes or other polymorphic ventricular tachyarrhythmias
in those patients with QT prolongation, this broader risk-benefit
Critique factor remains the key question when assessing the ultimate clini-
The aims of this study were to identify the number of critical care cal worth of implementing continuous repolarisation interval
patients who met the American Heart Association (AHA) clinical monitoring in critical care patients. This is a particularly important
indicators for continuous QT monitoring and to assess the predic- consideration given the additional cost, training and focus required
tive value of these indicators to the development of a prolonged in undertaking such an initiative. Subfactor analysis of the QT pro-
QT interval. It was found that a significant proportion (69%) of the longation risk indicators, e.g. anti-arrhythmic vs other QT prolong-
sampled population had one or more of the AHA continuous QT ing drugs, will further add to the understanding of this evolving
monitoring criteria and that of this subgroup, 31.2% had a clinically area of arrhythmogenic risk monitoring.