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Monroe Community College
Rochester, New York


Understanding the QT/QTc Measurement

by Peter Bonadonna


QT

What is a QT?

image

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?image

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

QT Chart

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.


image 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?

image 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.


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Updated: Sept. 15, 1999
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