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Understanding the QT/QTc Measurement

What is a QT

Normal QT=0.39 times the square root of RR (sec) +/-10%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

Image of QT measurementFind 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

 Image of 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

imageThe 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?

Bazet's formula 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.