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 assessment 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 400.429890.477650.52542 410.424610.471790.51897 420.419530.466140.51275 430.414620.460690.50676 440.409880.455420.50096 450.405300.450330.49537 460.400870.445410.48995 470.396580.440650.48471 480.392430.436030.47964 490.388400.431560.47472 500.384500.427220.46995 510.380710.423010.46532 520.377030.418930.46082 530.373460.414960.45645 540.369990.411100.45221 550.366610.407340.44808 560.363320.403690.44406 570.360120.400130.44014 580.357000.396670.43633 590.353960.393290.43262 600.351000.390000.42900 610.348110.386790.42547 620.345290.383660.42202 630.342540.380600.41866 640.339850.377620.41538 650.337230.374700.41217 660.334670.371850.40904 670.332160.369060.40597 680.329710.366340.40298 690.327310.363680.40004 700.324960.361070.39718 710.322670.358520.39437 720.320420.356020.39162 730.318220.353570.38893 740.316060.351180.38629 750.313940.348830.38371 760.311870.346520.38118 770.309840.344270.37869 780.307850.342050.37626 790.305890.339880.37387 800.303970.337750.37152 810.302090.335660.36922 820.300250.333610.36697 830.298430.331590.36475 840.296650.329610.36257 850.294900.327670.36043 860.293180.325750.35833 870.291490.323880.35627 880.289830.322030.35424 890.288200.320220.35224 900.286590.318430.35028 910.285010.316680.34835 920.283460.314950.34645 930.281930.313260.34458 940.280430.311580.34274 950.278950.309940.34093 960.277490.308320.33915 970.276060.306730.33740 980.274640.305160.33568 990.273250.303610.33398 1000.271880.302090.33230 1010.270530.300590.33065 1020.269200.299120.32903 1030.267890.297660.32743 1040.266600.296230.32585 1050.265330.294810.32429 1060.264080.293420.32276 1070.262840.292040.32125 1080.261620.290690.31976 1090.260420.289350.31829 1100.259230.288030.31684 1110.258060.286730.31541 1120.256910.285450.31400 1130.255770.284180.31260 1140.254640.282940.31123 1150.253530.281700.30987 1160.252440.280490.30853 1170.251360.279280.30721 1180.250290.278100.30591 1190.249240.276930.30462 1200.248190.275770.30335
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 mathematical 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.
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 acquired. Congenital QT prolongation is an inherited disorder which places these people at increased risk of sudden death usually as children or young adults. Acquired 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 acquired 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.
