# Understanding the QT/QTc Measurement

```
```
```
```
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.

Back

URL: /depts/pstc/backup/parasqt.htm

Updated: Sept. 15, 1999