Monroe Community College - State University of New York

PSTC Logo

Public Safety Training Center

Monroe Community College
Rochester, New York

Understanding the QT/QTc Measurement

by Peter Bonadonna

What is a QT?

The QT interval is a measurement that represents the total time from ventricular depolarization to complete repolarization. This process begins at the start of the q wave and extends to the end of the T wave. If there is no q wave, then the starting point would be the very beginning of the R wave. The QT interval is affected by many things. Heart rate, autonomic nervous tone, sympathomimetics, electrolytes esp. calcium, some drugs, age, sex of the patient and even sleep affect this duration. Because of the complexities of assessment and interpretation, this measurement is often overlooked by Paramedics. This is unfortunate because the QT measurement can help you to recognize life threatening problems before they happen. Charts have been devised to make QT assesment easier. Most charts look at heart rate, and often, sex and age of the patient. These charts usually give the normal and upper acceptable measurements.

How do you measure it?

Find a lead with the tallest T wave and count the little boxes from the start of the QRS complex to the point where the T wave comes back down to the isoelectric line. Multiply the number of little boxes by 0.04 seconds. Example if you counted 8 boxes then 8 * 0.04 = 0.32 seconds. At this point you can say that your patient has a QT interval of 0.32 seconds (320 milliseconds) but you can't determine whether this is normal or not until you consider other parameters. This is where the chart comes in.

QT Chart

Heart Low Normal High Rate Normal Normal 40 0.42989 0.47765 0.52542 41 0.42461 0.47179 0.51897 42 0.41953 0.46614 0.51275 43 0.41462 0.46069 0.50676 44 0.40988 0.45542 0.50096 45 0.40530 0.45033 0.49537 46 0.40087 0.44541 0.48995 47 0.39658 0.44065 0.48471 48 0.39243 0.43603 0.47964 49 0.38840 0.43156 0.47472 50 0.38450 0.42722 0.46995 51 0.38071 0.42301 0.46532 52 0.37703 0.41893 0.46082 53 0.37346 0.41496 0.45645 54 0.36999 0.41110 0.45221 55 0.36661 0.40734 0.44808 56 0.36332 0.40369 0.44406 57 0.36012 0.40013 0.44014 58 0.35700 0.39667 0.43633 59 0.35396 0.39329 0.43262 60 0.35100 0.39000 0.42900 61 0.34811 0.38679 0.42547 62 0.34529 0.38366 0.42202 63 0.34254 0.38060 0.41866 64 0.33985 0.37762 0.41538 65 0.33723 0.37470 0.41217 66 0.33467 0.37185 0.40904 67 0.33216 0.36906 0.40597 68 0.32971 0.36634 0.40298 69 0.32731 0.36368 0.40004 70 0.32496 0.36107 0.39718 71 0.32267 0.35852 0.39437 72 0.32042 0.35602 0.39162 73 0.31822 0.35357 0.38893 74 0.31606 0.35118 0.38629 75 0.31394 0.34883 0.38371 76 0.31187 0.34652 0.38118 77 0.30984 0.34427 0.37869 78 0.30785 0.34205 0.37626 79 0.30589 0.33988 0.37387 80 0.30397 0.33775 0.37152 81 0.30209 0.33566 0.36922 82 0.30025 0.33361 0.36697 83 0.29843 0.33159 0.36475 84 0.29665 0.32961 0.36257 85 0.29490 0.32767 0.36043 86 0.29318 0.32575 0.35833 87 0.29149 0.32388 0.35627 88 0.28983 0.32203 0.35424 89 0.28820 0.32022 0.35224 90 0.28659 0.31843 0.35028 91 0.28501 0.31668 0.34835 92 0.28346 0.31495 0.34645 93 0.28193 0.31326 0.34458 94 0.28043 0.31158 0.34274 95 0.27895 0.30994 0.34093 96 0.27749 0.30832 0.33915 97 0.27606 0.30673 0.33740 98 0.27464 0.30516 0.33568 99 0.27325 0.30361 0.33398 100 0.27188 0.30209 0.33230 101 0.27053 0.30059 0.33065 102 0.26920 0.29912 0.32903 103 0.26789 0.29766 0.32743 104 0.26660 0.29623 0.32585 105 0.26533 0.29481 0.32429 106 0.26408 0.29342 0.32276 107 0.26284 0.29204 0.32125 108 0.26162 0.29069 0.31976 109 0.26042 0.28935 0.31829 110 0.25923 0.28803 0.31684 111 0.25806 0.28673 0.31541 112 0.25691 0.28545 0.31400 113 0.25577 0.28418 0.31260 114 0.25464 0.28294 0.31123 115 0.25353 0.28170 0.30987 116 0.25244 0.28049 0.30853 117 0.25136 0.27928 0.30721 118 0.25029 0.27810 0.30591 119 0.24924 0.27693 0.30462 120 0.24819 0.27577 0.30335

QT Graph

Often the QT is measured by the 12 lead machine and printed on the EKG record. You will need to consult a QT chart or graph to draw conclusions from this measurement.

A useful rule of thumb

The QT interval should be less than half the preceding R-R interval. This only works for regular rates between 65 and 90.

What is a QTc?

Because we have the need to create a number that is normal, a mathamatical formula, known as Bazet's formula, was devised to correct for the heart rate of the patient. The "c" in QTc stands for corrected. If you plug in a few variables into a QTc formula, you will get a number result. You then compare this number with the two normal values (male 0.39, female 0.41). This method allows computers in 12 lead equipment to quickly calculate the QTc and print it on the EKG record. It also allows Paramedics to quickly assess the QT interval. Click here for our QTc Calculator

What does it mean?

QT or QTc intervals that are long suggest abnormal effects on the myocardium. QT prolongation is usually divided into two categories, congenital and aquired. Congenital QT prolongation is an inherited disorder which places these people at increased risk of sudden death usually as children or young adults. Aquired QT prolongation may be due to ischemia, electrolyte imbalance esp hypokalemia or hypomagnesemia, CVA, medications like Quinidine or Amiodarone or Seldane + an oral antifungal, or toxic ingestions such as tricyclic antidepressant (TCA) overdose. Most aquired forms result from medications. Extremely prolonged QT/QTcs can increase the patients chances of sudden cardiac death (SCD). The usual etiology of this death is Torsade de Pointes (TDP). When a patient has QT prolongation and bouts of syncope, TDP, or cardiac arrest, it is called "Long QT Syndrome".

How can I use this information as a Paramedic?

As a Paramedic, your goals are to quickly arrive, assess, treat, and stabilize your patient. Once life threatening conditions have been cared for, a more complete assessment of the EKG can be made. Lives have been saved when astute Paramedics have noticed abnormally long QTs, and notified the Emergency Physician. Any patient with syncope who is taking Quinidine, Amiodarone, or Seldane requires cardiac monitoring and Emergency Department Evaluation regardless of the QT measurement.