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ECG Basics

The QT Interval



  • The QT interval indicates of the time from ventricular depolarisation (phase 0) to ventricular repolarisation (phases 1-3). It is the duration of activation and recovery of the ventricular myocardium.
      • Normal Range

      • Men 390 - 450ms
      • Women - 390 - 460ms

Measuring the QT Interval

  • The QT interval is best measured on lead II, V5 or V6. Measure the distance between the start of the Q wave and the end of the T wave.
  • Compare QT intervals between sequential ECGs in order to determine change in QT.
    • Measuring the QT Interval
    • Tangent Method

    • In order to accurately measure the QT interval, draw a tangent to the steepest part of the downsloping portion of the T wave and find the intersection between this tangent and the baseline - this is the true end of the T wave.
    • Measuring the QT Interval

Correcting the QT Interval (QTc)

  • The QT interval shortens with rapid heart rates and lengthens at slower heart rates, as a compensatory mechanism. In order to truly estimate risk it is important to correct for the heart rate, i.e. estimatinng what the QT interval would be at 60 beats per minute.
  • Adjust the QT interval according the to heart rate using the Bazett, Fridericia or Framingham method. The Bazett formula is the most commonly used, and is validated for heart rates between 60-100bpm. In the presence of tachycardia or bradycardia, one of the other two formulae should be used.
    • Bazett

    • QTc = QT √RR Interval
    • Fridericia

    • QTc =  QT RR 1/3
    • Framingham

    • QTc = QT + 0.154(1000 - RR Interval)

QT Prolongation

    • QT Prolongation
  • A prolonged QT interval represents delayed ventricular repolarisation, and increases the risk of a re-entry circuit from forming (i.e. Torsade de Pointes).
  • The QT interval is prolonged if it is >450ms in men, >460ms in women, or if there is an increase of >30ms in sequential ECG recordings.
    • Causes of Prolonged QT Interval

    • Congenital

    • Congenital Long QT Syndrome (LQTS)
    • Jervel and Lange-Nielsen syndrome
    • Drugs

    • Antiarrhythmics - amiodarone, sotalol, procainamide, quinidine
    • Antidepressants - amitryptiline, dothiapine, citalopram, escitalopram
    • Antipsychotics - risperidone, haloperidol, clozapine, droperidol, chlorpromazine
    • Antiemetics - ondansetron, domperidone
    • Macrolides - azithromycin, clarithromycin, erythromycin
    • Quinolones - ciprofloxacin, moxifloxacin
    • Antifungals - fluconazole, ketoconazole
    • Antimalarials - chloroquine, mefloquine
    • Anaesthetic gases – halothane, sevoflurane
    • Methadone
    • Other

    • Hypokalaemia
    • Hypomagnesaemia
    • Hypocalcaemia
    • Severe hypothermia
    • Severe bradycardia - sick sinus syndrome, complete heart block
    • Cardiovascular disease - tako-tsubo cardiomyopathy, MI, CCF
    • Cerebrovascular disease - intracranial / subarachnoid haemorrhage, stroke
    • Hypothyroidism
  • The congenital long QT syndromes are a diverse group of diseases that predispose to ventricular tachyarrhythmias. The three most common are LQTS1, in which events are triggered by exercise; LQTS2, in which events are triggered by stress and loud noises; and LQTS3, in which events often occur during sleep. Other congenital causes are much rarer.
  • Many drugs can cause a prolonged QT interval; particularly certain antiarrhythmics, many antimicrobials, and many psychoactive drugs.
  • Electrolyte disturbances can cause prolonged QT intervals, as can severe hypothermia, sevre bradycardia, cardiovascular stress and cerebrovascular pathology.
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 Balci B. Tombstoning ST-elevation myocardial infarction. Current cardiology reviews. 2009 Nov 1;5(4):273-8. Baranchuk A, Enriquez A, Garcia-Niebla J, Bayes-Genis A, Villuendas R, de Luna AB. Differential diagnosis of rSr’pattern in leads V1-V2. Comprehensive review and proposed algorithm. Ann Noninvasive Electrocardiol. 2015 Jan 1;20(1):7-17.
Benjamin EJ, Levy D, Vaziri SM, D'Agostino RB, Belanger AJ, Wolf PA. Independent risk factors for atrial fibrillation in a population-based cohort: the Framingham Heart Study. Jama. 1994 Mar 16;271(11):840-4.
Brady WJ, Skiles J. Wide QRS complex tachycardia: ECG differential diagnosis. The American journal of emergency medicine. 1999 Jul 31;17(4):376-81.
 Chung EK. Appraisal of multifocal atrial tachycardia. British heart journal. 1971 Jul;33(4):500. Coppola G, Carita P, Corrado E, Borrelli A, Rotolo A, Guglielmo M, Nugara C, Ajello L, Santomauro M, Novo S. ST segment elevations: Always a marker of acute myocardial infarction?. indian heart journal. 2013 Aug 31;65(4):412-23. Edhouse J, Thakur RK, Khalil JM. ABC Of Clinical Electrocardiography: Conditions Affecting The Left Side Of The Heart.BMJ: British Medical Journal. 2002; 324: 1264-1267. Fuchs RM, Achuff SC, Grunwald L, Yin FC, Griffith LS. Electrocardiographic localization of coronary artery narrowings: studies during myocardial ischemia and infarction in patients with one-vessel disease. Circulation. 1982;66:1168-1176. H. Abdollah, J.A. Milliken. Recognition of electrocardiographic left arm/left leg lead reversal. Am J Cardiol, 80 (1997), p. 1247. Hanna EB, Glancy DL. ST-segment depression and T-wave inversion: classification, differential diagnosis, and caveats. Cleve Clin J Med. 2011;78:404-414. Harrigan RA, Jones K. ABC of clinical electrocardiography. Conditions affecting the right side of the heart. BMJ. 2002; 324(7347): 1201-4. Hayden GE, Brady WJ, Perron AD, Somers MP, Mattu A. Electrocardiographic T-wave inversion: differential diagnosis in the chest pain patient. The American journal of emergency medicine. 2002 May 31;20(3):252-62. Herring N, Paterson DJ. ECG diagnosis of acute ischaemia and infarction: past, present and future. QJM : monthly journal of the Association of Physicians. 2006; 99: 219-230. Ishikawa K, Pipberger HV. Critical evaluation of the twin peaked P wave (“Pseudo-P mitrale”) in orthogonal electrocardiograms. Journal of Electrocardiology. 1980;13:181-184.
Kandolin R, Lehtonen J, Kupari M. Cardiac sarcoidosis and giant cell myocarditis as causes of atrioventricular block in young and middle-aged adults. Circulation: Arrhythmia and Electrophysiology. 2011 Jun 1;4(3):303-9.
 Kastor JA. Multifocal atrial tachycardia. New England Journal of Medicine. 1990 Jun 14;322(24):1713-7. Katritsis DG, Camm AJ. Atrioventricular nodal reentrant tachycardia. Circulation. 2010 Aug 24;122(8):831-40. Khan, IA Clinical and therapeutic aspects of congenital and acquired long QT syndrome.Am J Med2002;112,58-66. Lewin RF, Sclarovsky S, Strasberg B, Arditti A, Erdberg A, Agmon J. Right axis deviation in acute myocardial infarction. Clinical significance, hospital evolution, and long-term follow-up. Chest. 1984 Apr;85(4):489-93.
Lynch R. ECG lead misplacement: A brief review of limb lead misplacement. African Journal of Emergency Medicine. 2014 Sep 30;4(3):130-9.
 McCord J, Borzak S. Multifocal atrial tachycardia. CHEST Journal. 1998 Jan 1;113(1):203-9. Mieghem CV, Sabbe M, Knockaert D. The Clinical Value of the ECG in Noncardiac Conditions. Chest. 2004; 125: 1561-1576. Nora Goldschlager, and Galen Wagner. Electrocardiogram interpretation. Journal of Electrocardiology, 2007-07-01, Volume 40, Issue 4, Pages 326-326.
Okada M, Yotsukura M, Shimada T, Ishikawa K. Clinical implications of isolated T wave inversion in adults: Electrocardiographic differentiation of the underlying cause of this phenomenon. Journal of the American College of Cardiology. 1994 Sep 1;24(3):739-45.
 Porela P, Kyto V, Nikus K, Eskola M, Airaksinen KE. PR depression is useful in the differential diagnosis of myopericarditis and ST elevation myocardial infarction. Annals of Noninvasive Electrocardiology. 2012 Apr 1;17(2):141-5. Rashford S. Acute Pleuritic Chest Pain. Australian Family Physician. 2001; 30(9). Rautaharju, P.M., Surawicz, B. and Gettes, L.S., 2009. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society Endorsed by the International Society for Computerized Electrocardiology. Journal of the American College of Cardiology, 53(11), pp.982-991. Riddle W. Misleading. The clinical implications of misplaced ECG leads. JEMS: a journal of emergency medical services. 2008 Sep;33(9):56-62. Salman S, Bajwa A, Gajic O, Afessa B. Paroxysmal atrial fibrillation in critically ill patients with sepsis. Journal of intensive care medicine. 2008 Apr 28. Seidenberg PH, Haynes J. Pericarditis: diagnosis, management, and return to play. Current sports medicine reports. 2006 Mar 1;5(2):74-9. Somers MP, Brady WJ, Perron AD, Mattu A. The prominant T wave: electrocardiographic differential diagnosis. The American journal of emergency medicine. 2002 May 31;20(3):243-51. Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society endorsed by the International Society for Computerized Electrocardiology. Journal of the American College of Cardiology. 2009 Mar 17;53(11):976-81. Vincent, GM Long QT syndrome.Cardiol Clin2000;18,309-325. W.M. Jackman, K.J. Friday, J.L. Anderson, E.M. Aliot, M. Clark, R. Lazzara. The long QT syndrome: a critical review, new clinical observations and a unifying hypothesis. Prog Cardiovasc Dis, 31 (1988), p. 115. Wellens HJ. Ventricular tachycardia: diagnosis of broad QRS complex tachycardia. Heart. 2001 Nov 1;86(5):579-85. Zema MJ, Kligfield P. ECG poor R-wave progression: review and synthesis. Archives of internal medicine. 1982 Jun 1;142(6):1145-8.