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The QT Interval: How Long Is Too Long?

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The QT Interval: How Long Is Too Long? heart matters The QT interval: How long is too long? By Natalie K. Cox, RN Staff Nurse • Coronary Care Unit • McLeod Regional Medical Center • Florence, S.C. What’s the QT interval and why’s it so the appearance that the QT interval is pro- important? In this article, I’ll answer these longed. However, a wide QRS complex questions plus show you how to measure represents depolarization, and LQTS is a dis- the QT interval and how to recognize the order of repolarization. Sometimes the end types of long QT syndrome (LQTS) and of a T wave isn’t clearly defi ned, which can their symptoms, causes, and treatments. make it diffi cult to get the QT measurement. Irregular rhythms also make it diffi cult to The long and short of it obtain a consistent QT interval measure- The QT interval on the ECG is measured ment. For example, atrial fi brillation makes from the beginning of the QRS complex to it extremely diffi cult to measure a QT inter- the end of the T wave (see ECG components). val because fi nding a T wave isn’t always It represents the time it takes for the ventricles of the heart to depolarize and repolarize, or to ECG components contract and relax. The QT interval is longer when First positive deflection after the the heart rate is slower and short- ECG components P or Q wave er when the heart rate is faster. So it’s necessary to calculate the corrected QT interval (QTc) using the Bazett formula: QT interval divided by the square root of the R-R interval. The R-R interval is measured from one R wave to the next R wave that comes before the QT interval being measured. First For example, if the QT interval negative measures 0.44 second and the R-R deflection after the interval measures 0.86 second, P wave then the QTc is 0.47. The normal QT interval varies depending on age and gender, but it’s usually 0.36 to 0.44 second First negative (see QT interval ranges). Anything deflection after the greater than or equal to 0.50 R wave second is considered dangerous for any age or gender; notify the healthcare provider immediately. There are several factors that make it diffi cult to measure the QT interval. A wide QRS 0.12 to 0.20 sec 0.06 to 0.10 sec 0.36 to 0.44 sec complex on an ECG may give www.NursingMadeIncrediblyEasy.com March/April 2011 Nursing made Incredibly Easy! 17 Copyright © 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. heart matters parents. In addition, there are 12 different QT interval ranges subtypes of LQTS, labeled LQT1 to LQT12. Age 1 to 15 Adult man Adult woman There are many drugs that can prolong the QT interval, such as some antibiotics, Normal Less than 0.44 Less than 0.43 Less than 0.45 second second second antidysrhythmics, antihistamines, antifun- gals, and antipsychotics. Other categories of Borderline 0.44 to 0.46 0.43 to 0.45 0.45 to 0.47 drugs that cause QT prolongation are some second second second heart medications, cholesterol-lowering Prolonged Greater than 0.46 Greater than 0.45 Greater than 0.47 drugs, and diabetes medications. This second second second doesn’t mean that all drugs in these catego- Source: Jacobson C. Long and short of it: What’s up with the QT interval? http://hosted.mediasite. ries can cause it, but many of them can. A com/mediasite/Viewer/?peid=9ed8856fcdab4bc0bb066c25a148435b1d. person is more likely to develop QT prolon- gation from these drugs if they have the possible with this rhythm. The best thing to following risk factors: renal dysfunction, do is measure fi ve or six QT intervals and hypokalemia, hypomagnesaemia, bradycar- average them together. dia, advanced age, female gender, underly- Each lead of an ECG will most likely give ing heart disease, and polypharmacy. Be you a slightly different QT measurement. aware of these risk factors in your patients When measuring QT intervals, it’s important and be aware of the drugs they’re on. to fi nd a lead on the ECG that has a T wave Whenever you start a patient on a new with a clearly defi ned end. Always use that drug that may prolong the QT interval, lead when measuring your QT interval, always document the baseline QT interval and remember to be consistent with your before administering the drug. Continue measurement technique (see Calculating QT monitoring and documenting the QT inter- interval duration). val at least once every 8 hours. Talk to the healthcare provider about correcting any How did it get so long? electrolyte imbalances, such as low potas- LQTS is a result of the heart’s electrical sys- sium, magnesium, calcium, and sodium, tem recharging abnormally. There are two before starting the drug. Keep the patient types: acquired and congenital. Acquired on a cardiac monitor and look for signs of LQTS is caused by an underlying medical impending torsades de pointes (TdP). condition, such as drugs that prolong the Congenital LQTS is a genetic mutation QT interval, electrolyte imbalances (such as of the ion channels in the cardiac cells. It caused by anorexia), and bradycardia. Con- affects the fl ow of potassium and sodium genital LQTS is inherited from one or both in and out of the cells. Genetic testing is Calculating QT interval duration Count the number of squares from the beginning of the QRS complex to the end of the T wave. Multiply this number by 0.04 second. The normal range is 0.36 to 0.44 second, or 9 to 11 small squares wide. 18 Nursing made Incredibly Easy! March/April 2011 www.NursingMadeIncrediblyEasy.com Copyright © 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. used to determine whether a person has the congenital form. Three of the congeni- TdP triggers in tal types are more common than the oth- congenital LQTS ers: LQT1, LQT2, and LQT3. People with • LQTS1: Emotional stress or exercise, espe- congenital LQTS are prone to go into TdP cially swimming or diving when there’s an external stimulus. The trig- • LQTS2: Extreme emotions or surprises, such gers for TdP are somewhat specifi c to the as harsh, sudden noises type of LQTS a person has (see TdP triggers • LQTS3: Slow heart rate while sleeping in congenital LQTS). Source: National Heart, Lung, and Blood Institute. What is long Beta-blockers are the main medication QT syndrome? http://www.nhlbi.nih.gov/health/dci/Diseases/ qt/qt_all.html. used for the management of congenital LQTS. If medication isn’t successful, then the patient may need dual chamber pacing at a to prevent adrenergic surges in heart rhythm rate to shorten the QT interval. For patients by avoiding loud noises, such as telephones, at increased risk or those who continue to doorbells, and alarm clocks. The patient may experience severe life threatening cardiac need to turn down the doorbell and phone events, an implantable cardioverter defi bril- volume or turn the phone ringer off at night. lator is strongly recommended. Also teach the patient to avoid overexertion. If you take care of a patient newly diag- Patients with confi rmed LQTS1 or LQRS2 nosed with congenital LQTS, patient educa- shouldn’t participate in competitive sports. tion needs to be a priority. Teach the patient Tell the patient to seek medical attention www.NursingMadeIncrediblyEasy.com March/April 2011 Nursing made Incredibly Easy! 19 Copyright © 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. heart matters if he or she has any illnesses that lower Review the patient’s drug list and stop any the potassium levels, such as vomiting or medications that prolong the QT interval. diarrhea, because this could trigger an epi- Obtain lab work to check electrolyte levels, Be aware sode of TdP. Ask the patient whether he or specifi cally potassium, sodium, magnesium, of signs of she has any family history of sudden cardiac and calcium. Notify the healthcare provider impending TdP. arrest. immediately. With any patient in your care, always There are various ways to manage TdP. consider LQTS if the patient has fainted and Give I.V. magnesium per the healthcare pro- it can’t be explained by any other medical vider’s orders. The healthcare provider may condition. start the patient on a beta-blocker to shorten the QT interval. Digoxin can also shorten Getting to the point of the QT interval. If the patient is still in TdP torsades de pointes after this, overdrive pacing will be needed. What can happen if the QT interval is too A transvenous pacemaker is inserted and long? If the QT interval lasts longer than set at a rate between 100 and 110 beats/ 0.50 second (500 milliseconds), then a pa- minute. This high heart rate prevents pauses tient’s heart rhythm is more likely to prog- and shortens the QT interval. As mentioned ress into TdP, an irregular chaotic heartbeat before, a patient in sustained TdP may prog- that’s a type of polymorphic ventricular ress to ventricular fi brillation and needs to tachycardia (VT). be defi brillated immediately. To determine whether a polymorphic VT is TdP, look at the beats around it. First the Going long QT interval will be prolonged, usually with a Remember, the key to good nursing care is pause in the heartbeat followed by a beat with to look at and listen to your patients. How a bizarre T wave.
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