II. Definitions
- QT Interval
- EKG interval between the begining of the QRS interval and the end of the T Wave
- Represents ventricular electrical activity (from ventricular depolarization to repolarization)
III. Interpretation: Normal QT Duration
- Always confirm Prolonged QT with a manual measurement (QTc >500 ms)
- Computer read EKG can over-measure QTc (e.g. in presence of U Waves)
- QT modifying factors
- Normal QT decreases with increasing Heart Rate
- QT is longer in leads V2 and v3
- Calculation of QTc or corrected QT (Bazett's Formula)
- QTc = QT/(sqrt RR Interval)
- QTc is normally <0.45 in men (<0.46 in women and children)
- Prolonged QTc (esp. >0.5) is at risk of Torsades de Pointes (see below)
- Approximation of normal QT
- QT Interval shortens with decreasing RR Interval
- QT = 0.5 x preceding RR Interval (if normal rate, 60 to 100 bpm)
- QT should be less than half the duration of the R-R interval (rate 60 to 100 bpm)
- Approximate normal QT Interval
- QT <= 0.38 if Heart Rate 80 bpm or greater
- Add 0.02 sec for every 10 bpm below 80
- Example: Normal QT <= 0.46 if Heart Rate 40-50
-
Heart Rate determined QT
- 115 - 84 bpm: QT 0.30 to 0.37 seconds
- 83 - 72 bpm: QT 0.32 to 0.40 seconds
- 71 - 63 bpm: QT 0.34 to 0.42 seconds
- 62 - 56 bpm: QT 0.36 to 0.43 seconds
- 55 - 45 bpm: QT 0.39 to 0.46 seconds
IV. Causes: Prolonged QT (QTc > 500 ms)
- See Prolonged QT Interval due to Medication
- Familial Long QT Syndrome (congenital Prolonged QT, cardiac ion channel abnormalities)
- Congestive Heart Failure
- Cardiomyopathy
- Myocardial Infarction
- Hypocalcemia
- Hypokalemia
- Hypomagnesemia
- Type I Antiarrhythmic drugs (Sodium channel blocking drugs)
- Rheumatic Fever
- Myocarditis
- Congenital Heart Disease
- Hypothermia
- Increased Intracranial Pressure
-
Subarachnoid Hemorrhage (SAH)
- Mechanism thought related to SAH induced Catecholamine surge
V. Complications: QT Prolongation resulting in Torsades de Pointes
- See Torsades de Pointes for management
- QT Prolongation risks R-On-T Phenomenon (PVC on T Wave) provoking polymorphic VT (Torsades)
- Consider baseline EKG prior to administering medications that may increase QTc
- Hospital protocols may require baseline EKG before certain medications (e.g. Droperidol)
- Caution in patients with pre-existing QT Prolongation (QTc >450 ms) when prescribing new medications
- See Prolonged QT Interval due to Medication
- Select medications that do not prolong the QT Interval further
- Stop medication causes of Prolonged QT when QTc rises above 500 ms
- Avoid Beta Blockers, due to Syncope risk, precipitating Torsades (3.6 fold increased risk)
- Not all Prolonged QT intervals significantly predispose to Torsades
- Degree of QT Prolongation does not correlate with risk of Torsades
- Medication specific effects (e.g. some medications are more likely to result in Torsades)
- See Prolonged QT Interval due to Medication for likelihood of Torsades
- Avoid combining multiple medications each of which increase QT Interval
- Coexisting conditions which make Drug-Induced Torsades de Pointes more likely
- Baseline QT Prolongation
- Bradycardia
- Acute Myocardial Infarction
- Low Ejection Fraction
- Hypokalemia
- Hypomagnesemia
- Hypocalcemia
- Volume depletion (Vomiting, Diuretics)
- Female gender (long QT Interval at baseline)
- Evaluate context of QT Prolongation
- Asymptomatic with an extremely Long QTc (600 to 700 ms)
- Obtain labs for underlying cause (e.g. Serum Calcium, Magnesium, Potassium)
- Review causes of Prolonged QT Interval due to Medication
- Hold suspected causative medications
- Coordinate with providers who prescribed causative medications
- May discharge with very close interval follow-up (days)
- QT Prolongation and reassuring findings that require no change in regimen
- Chronic, stable, asymptomatic use of a medication associated with QT Prolongation
- Avoid prescribing any new medications that affect the QTc
- QT Prolongation and red flag findings that suggest further evaluation or regimen change
- Family History of Sudden Cardiac Death
- Symptoms (high risk group that requires thorough evaluation including monitoring)
- Syncope (admit to telemetry)
- Palpitations
- Spells or Seizures (presenting event in 10% of pediatric cases)
- Asymptomatic with an extremely Long QTc (600 to 700 ms)
VI. Causes: Shortened QT (<330 ms)
- Digoxin (Digitalis)
- Hypercalcemia
- Hyperkalemia
- Phenothiazines
- Sympathomimetics
- Familial (cardiac ion channel abnormalities)
VII. Complications: Shortened QT (<330 ms)
- Syncope
- Paroxysmal Atrial Fibrillation
- Ventricular Fibrillation
VIII. References
- Joshi and Dermark (2016) Crit Dec Emerg Med 30(8):3-12
- Mirvis in Braunwald (2001) Cardiovascular, p. 92-3
- Rollings (1984) Facts and Formulas, p. 64
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Related Studies
| Definition (NCI_CDISC) | An electrocardiographic finding in which the QT interval not corrected for heart rate is prolonged. Thresholds for different age, gender, and patient populations exist. |
| Definition (NCI) | An electrocardiographic finding in which the QT interval not corrected for heart rate is prolonged. Thresholds for different age, gender, and patient populations exist. (CDISC) |
| Concepts | Finding (T033) |
| SnomedCT | 123332001, 142130001, 111975006 |
| English | QT PROLONGED, Electrocardiogram QT prolonged, QT increased, QT interval prolonged, Long QT, QT prolonged, interval prolong qt, interval prolonged qt, intervals prolong qt, long qt, increased qt, prolong qt, qt prolonged, intervals prolonged qt, prolonged qt interval, prolonged qt, qt interval prolonged, Increased Q-T interval, Increased Q-T interval (finding), PROLONGED QT, Prolonged QT, Prolonged QT Interval by EKG Finding, Prolonged QT Interval, Prolonged QT Interval by ECG Finding, Prolonged QT interval, Increased QT interval, Prolonged QT interval (finding), Increased Q-T interval -RETIRED- |
| Italian | Intervallo QT dell'elettrocardiogramma, prolungato, QT prolungato, Intervallo QT prolungato, QT lungo, QT aumentato |
| Dutch | QT verhoogd, lang QT, QT-interval verlengd, QT verlengd, elektrocardiogram QT verlengd |
| French | QT long, Intervalle QT prolongé, QT augmenté, QT prolongé, ALLONGEMENT DU QT, Intervalle QT prolongé à l'électrocardiogramme |
| German | QT verlaengert, QT-Intervall verlaengert, langes QT, QT erhoeht, QT-VERLAENGERUNG, Elektrokardiogramm QT verlaengert |
| Portuguese | QT longo, Intervalo QT prolongado, QT sumentado, QT prolongado, QT PROLONGADO, Intervalo QT prolongado (ECG) |
| Spanish | QT largo, Intervalo QT alargado, QT aumentado, QT prolongado, Intervalo QT del electrocardiograma prolongado, QT ALARGADO, aumento del intervalo QT, intervalo Q - T prolongado - RETIRADO -, intervalo Q - T aumentado, intervalo Q - T prolongado - RETIRADO - (concepto no activo), intervalo QT prolongado (hallazgo), intervalo QT prolongado |
| Japanese | ＱＴ間隔延長, 心電図ＱＴ延長, ＱＴ延長, QTｴﾝﾁｮｳ, QTｶﾝｶｸｴﾝﾁｮｳ, ｼﾝﾃﾞﾝｽﾞQTｴﾝﾁｮｳ |
| Czech | QT interval prodloužený, QT prodloužení, Prodloužený QT interval na elektrokardiogramu, Dlouhý QT interval, Prodloužení QT |
| Hungarian | Hosszú QT, QT megnyúlt, Elektrokardiogram QT megnyúlt, QT idő megnyúlt, QT emelkedett |
Ontology: QT interval feature (observable entity) (C0429028)
| Definition (NCI) | The time interval between the start of the Q wave and the end of the T wave in the cardiac cycle. |
| Concepts | Clinical Attribute (T201) |
| SnomedCT | 123331008, 81435004, 142127008 |
| LNC | LP31070-3, MTHU008256 |
| Italian | Intervallo QT dell'elettrocardiogramma, Intervallo QT |
| Dutch | QT-interval, elektrocardiogram QT-interval |
| French | Intervalle QT, Intervalle QT à l'électrocardiogramme |
| German | QT-Intervall, Elektrokardiogramm QT-Intervall |
| Portuguese | Intervalo QT, Intervalo QT (ECG) |
| Spanish | Intervalo QT, intervalo Q - T - RETIRADO - (concepto no activo), intervalo Q - T - RETIRADO -, intervalo QT (entidad observable), intervalo QT, Intervalo QT del electrocardiograma |
| Japanese | 心電図ＱＴ間隔, ＱＴ間隔, QTｶﾝｶｸ, ｼﾝﾃﾞﾝｽﾞQTｶﾝｶｸ |
| English | Q-T interval, NOS, Electrocardiogram QT interval, QT interval feature (observable entity), QT Interval, intervals qt, q-t interval, interval q-t, interval qt, q t interval, qt interval, QT interval (observable entity), Q-T interval (observable entity), Q-T interval, QT interval, QT interval feature, QT interval, NOS, Q-T interval -RETIRED- |
| Czech | QT interval na elektrokardiogramu, QT interval |
| Hungarian | QT idő, Elektrokardiogram QT távolság |
Ontology: QT interval duration (C0577807)
| Concepts | Finding (T033) |
| SnomedCT | 301117000 |
| English | QT interval duration, QT interval duration (observable entity) |
| Spanish | duración del intervalo QT (entidad observable), duración del intervalo QT |
Ontology: Electrocardiogram QT corrected interval (C0855331)
| Concepts | Finding (T033) |
| English | Electrocardiogram QT corrected interval |
| Spanish | Intervalo QT corregido del electrocardiograma |
| French | Intervalle QT corrigé à l'électrocardiogramme |
| Dutch | elektrocardiogram gecorrigeerd QT-interval |
| Portuguese | Intervalo QT corrigido (ECG) |
| German | Elektrokardiogramm QT-korrigiertes Intervall |
| Italian | Intervallo QT corretto dell'elettrocardiogramma |
| Japanese | ｼﾝﾃﾞﾝｽﾞQTﾎｾｲｶﾝｶｸ, 心電図ＱＴ補正間隔 |
| Czech | Korigovaný QT interval na elektrokardiogramu |
| Hungarian | Elektrokardiogram korrigált QT távolság |
Ontology: QTc (C0860814)
| Definition (NCI) | The time interval between the start of the Q wave and the end of the T wave in the cardiac cycle as corrected with a non-specified correction formula. |
| Definition (NCI_CDISC) | The time interval between the start of the Q wave and the end of the T wave in the cardiac cycle as corrected with a non-specified correction formula. |
| Concepts | Quantitative Concept (T081) |
| English | QTc, Corrected QT Interval, QTC, QT Interval, Corrected |
| Spanish | QTc |
| French | QTc |
| Dutch | QTc |
| Portuguese | QTc |
| German | QTc |
| Italian | QT corretto |
| Japanese | QTﾎｾｲｶﾝｶｸ, ＱＴ補正間隔 |
| Czech | QTc |
| Hungarian | QTc |
Ontology: Prolonged QTc interval (C1560305)
| Concepts | Pathologic Function (T046) |
| English | Prolonged QTc interval |
