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How to Measure a QT Interval

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How to Measure a QT Interval Medical education How to measure a QT interval A standard approach in QT measurement improves communication between clinicians n abnormally prolonged QT interval is associated For uncertain cases, obtain a second or with an increased risk of sudden cardiac death.1 third electrocardiogram ASome professional bodies recommend national The QT interval varies constantly and under different population-based screening programs to detect QT influences.9,12,18 Therefore, an individual may have a 2 prolongation. Familial long QT syndrome (LQTS) normal or borderline QTc one moment or day, and a may remain undetected because of misdiagnosis (eg, prolonged one the next. A repeat ECG on the same day 3 as a seizure disorder) or through failure to measure may be helpful, but if there is no urgency, two to three 4 the QT interval correctly. Psychiatrists fear the QT repeat ECGs over a few days or weeks may help in prolongation caused by many psychotropic arriving at the correct conclusion in borderline cases. medications,5 and it may also be seen during periods of hypothermia; electrolyte imbalance (such as Manually measure the QT hypokalaemia, hypomagnesaemia and hypocalcaemia); in the setting of raised intracranial pressure or post- While the automatic algorithms used to measure the cardiac arrest; with other medications, such as type 1A, QT interval on commercial ECG machines are mostly accurate, significant measurement errors are not 1C and III antiarrhythmic agents; and with 15 antihistamines and macrolide antibiotics. uncommon. Likewise, some automatic reporting systems suppress the QT prolongation statement if the Yet, despite its importance, research shows that two heart rate is greater than 100 beats per minute, which is physicians cannot measure the same QT interval and get misleading for people who rely on the automatic reports the same result; an observation that also includes cardiac for guidance. Therefore, we recommend manual electrophysiologists.4,6 There is no universally accepted measurement to the nearest 5 milliseconds (one quarter agreement about how to measure the QT interval, of a small square)19 and referencing to normal values.20 which 12-lead electrocardiogram (ECG) leads to To increase accuracy, some specialists advocate taking the include,7,8 how many beats to measure,9 or which heart average QT of three to five consecutive QRS-T complexes rate correction formula to use.9 Two large international from a single lead at a paper speed of 50 mm per familial LQTS registries measure the end of the T wave in second.9,21 This process is time consuming and has not different ways, such that one has an average QT interval always been done routinely; taking instead the longer of (454 milliseconds)10 40 milliseconds below the other two measurements, one in lead II and one in V5 and using (494 milliseconds).11 Moreover, the QT interval has been a normal paper speed (ie, 25 mm per second). shown to vary minute to minute, beat to beat, and day by day.9,12 Because of these problems, guidance as to Become accustomed to normal T wave how to obtain accurate and reproducible QT appearance and be suspicious of unusual shapes measurements is a matter of opinion and experience. One way to diagnose familial LQTS is with quantitative QTc assessment, when no other QT prolonging influences or medications are present (QTc > 500 milliseconds on Principles for measuring and assessing two occasions, or > 480 milliseconds with symptoms),1,22 the QT interval but qualitative T wave assessment is also useful.23 An unusually shaped T wave should raise the suspicion of familial LQTS when other causes are absent. Make measurements on a good quality Characteristic T wave findings in the three most common electrocardiogram equipment fi 4 types of LQTS include broad T waves in LQT1, bi dT Corrected QT (QTc) calculation is prone to error, so in waves in LQT2 and late onset T waves in LQT323 (Box 1,A, fl Kathryn ensuring a quality ECG with at baseline, a minimal B and C respectively). Waddell-Smith1 artefact with high resolution (not faxed) helps to obtain an MJA 207 (3) Robert M Gow2 accurate measurement. When possible, QTc measurement is best done during stable sinus rhythm. Focus on leads II and V5 Jonathan R This is because the heart rate changes due to emotional or Measurements are made in leads II and V5 because QTc Skinner1 postural changes; moreover, sinus arrhythmia, in these leads correlates best with genotype status in 24 j LQTS, either alone or in combination, and results are 7 August 2017 1 Starship ventricular ectopic beats and arrhythmias (such as atrial 13-15 19 Children’s Health, fibrillation) affect the accuracy of the QTc, as does the highly repeatable. Moreover, a consistent approach is Auckland, New Zealand. presence of bundle branch block and required because T wave projection and noise vary 16 8 2 Children’s Hospital WolffeParkinsoneWhite syndrome — in these two between the leads. We use the longer of these two of Eastern Ontario, cases, specialist measurement of the QT is advised. measurements.2 Ottawa, Canada. jskinner@ In athletes, when the heart rate is less than 50 beats per adhb.govt.nz minute, some light aerobic activity increasing the heart Use the tangent technique rate to 60e90 beats per minute improves the accuracy of The end of the T wave (which can be difficult to define)7 107 17 doi: 10.5694/mja16.00442 the Bazett correction. can be most reproducibly ascertained using the tangent Medical education 1 LQT1, broad T waves (A); LQT2, bifid T waves (B); and LQT3, late onset T waves (C) 7 August 2017 j technique, where a line or tangent is drawn down the Be aware of bifid T waves and U waves MJA 207 (3) steepest slope of the terminal limb of the T wave. The end WhereaTwaveisbifid, the second peak should fi of the T wave is de ned by the intersection of this line with be included if it is at least half the amplitude of 25 the baseline, which is measured as the TP or PR the first using the tangent technique (Box 3,A).27 26 segments on a standard ECG, or on an exercise test as the When a U wave is present, if large (ie, > 50% amplitude 108 19 QQ line (Box 2, A and B respectively). of the T wave) and merging into the T wave, it Medical education Consider techniques to expose latent QT 2 QT correction method on 12-lead ECG (A): fi measure the QT interval using the tangent prolongation in individuals who have rst-degree technique and the preceding RR interval. QT relatives with familial long QT syndrome and a correction method exercise stress test (B): define normal or borderline QTc the end of the T wave as the intersection of a Up to 30% of individuals with familial LQTS may have tangent from the steepest slope of the last limb QTc intervals within the normal range, and unmasking of the T wave and the baseline.* QTc prolongation may occur with provocative tests. Standing test. This is a relatively new technique, requiring validation and more normative data (especially in children), but may add diagnostic information if a high pre-test probability of LQTS exists. Record a 12-lead ECG when supine, ask the patient to briskly stand, and record another ECG about 10 seconds later. At this time, a tachycardia has resulted, and in patients with LQTS, the QT does not adapt and shorten to the reduced RR interval as quickly as in people without LQTS. Therefore, the QTc interval increases much more in patients with LQTS than in patients without it. Characteristic LQTS T wave morphology (such as bifid T waves) may also be revealed.28 * The baseline is defined as the QQ line to provide a consistent reference Exercise test. On formal exercise testing, the QTc for each QT interval. The QT measurement derived from the tangent line is divided by the square root of the previous RR interval (shown intervals of patients with LQTS recorded during the by the colour pairs). u recovery period are usually longer than for non-carrier patients. A post-exercise recovery QTc taken at 4 minutes that exceeds 480 milliseconds29 is supportive evidence of should probably be included, but excluded if it is LQTS in adults.30 In children, a highly reproducible smallerorseparate9 (Box 3,B). technique averages four consecutive QT intervals in lead II (where the end of the T wave is defined as the intersection of the tangent and a baseline of the QQ line; Consider referencing neighbouring leads Box 2, B). A QTc equal to or exceeding 500 milliseconds The end of the T wave may still be difficult to define after suggests LQTS. Because children’s heart rates recover using the tangent technique, such as when the T wave is more slowly than adults’,19,31 consider monitoring very flat. When this occurs, several approaches are another 7 minutes of recovery: a carefully measured possible. Drawing a vertical line down through the two average QTc exceeding 470 milliseconds at this point adjacent leads may help to determine the T wave end; or (after a bicycle ergometry test) suggests LQTS.31 measuring the QT in a non-traditional lead (such as leads Holter monitor. Holter recordings are not yet standard in I, III or V6) and taking the longest of these may provide an 26 the diagnosis of LQTS, but are promising. Holter QTc is alternative (Box 4). Leads V1eV3 should be avoided, significantly longer in patients who carry LQTS compared particularly in children and young adults (especially 32,33 26 with their gene-negative relatives, and assessment of females) where T wave morphology is greatly variable.
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