What’s your QT Interval?
What is the QT interval? From the beginning of the Q to the end of the T is the QT interval.
What does the QT represent? The QT interval measures the complete ventricular contraction and relaxation cycle.
Why
do we have to look for the QT Interval?
The longer the QT interval the higher risk of lethal cardiac arrhythmias, specifically Polymorphic Ventricular Tachycardia known at “Torsades de Points”.
Torsades
de Pointes
Thousands of apparently health children and adults die annually due to this under recognized condition.
What
are the symptoms? Sudden loss of consciousness (the medical term
is 'syncope') and sudden death are the common symptoms and usually occur during physical exertion or emotional excitement like
anger, fear or startle, but may occur during sleep, or arousal from sleep.
Common startle events include sirens, the telephone and the alarm clock. It
is less common for the syncope or sudden death to occur when the person is
awake and at rest. The particular trigger for the symptoms depends to some
degree on the specific gene abnormality (see the section on genetics). Exercise
induced syncope usually occurs right during the exercise, but occasionally
occurs within a few seconds or a minute or two after the exertion. In patients
who experience syncope the torsade de pointes rhythm reverts spontaneously
to normal, usually within about 1 minute or less. When this occurs, the patient
quickly regains consciousness, usually without disorientation or residual
symptoms, although fatigue may be present. When the torsade rhythm persists
for a longer time, however, it degenerates into ventricular fibrillation and
the outcome is death unless electrical defibrillation is provided.
Not
all patients who have this condition have symptoms; at least one-third, and
probably more, never develops any symptoms. In the others, some have just
one or two syncopal spells as children, and none thereafter, whereas, some
have many episodes over a number of years. The symptoms may begin as early
as the first days or weeks of life, or as late as middle age. Most commonly,
however, the symptoms first occur during pre-teen and teenage years. The symptoms
start earlier in males than females, beginning on average at approximately
8 years in males and 14 years in females. Because many affected persons never
have symptoms, the absence of a history of syncope or sudden death in a family
does not at all guarantee the absence of LQTS in the family.
How
do you get Long QT?
Ionic Channelopathies. “A dysfunction
of special heart cells called ion channels” These channels control the flow
of ions like potassium, sodium and calcium molecules in and out of the heart
cells. This flow produces the electrical activity of the heart.
You can generally get this disorder in two ways.
1. Inherited
from genetics, eg. passed on from mom or dad to child.
2. Acquired
through medication use.
What
Drugs Prolong the QT
interval?
These drugs have a risk of Torsades de Pointes *,**
*As
of 3/02/2006 ** From www.qtdrugs.org
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Drugs that
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Is the QT interval hard to Identify? No, it only takes about 5 seconds to measure the QT interval. A simple QT measurement can help you recognize the precursor to LQTS and prevent life threatening problems.
How
do you measure the QT interval?
The QT interval should be less than half of the R-R interval. This is generally accurate for regular rates between 65 and 90. If you have a rate less than 65 and have a prolonged QT interval consider a 12 lead ecg. This doesn’t work on irregular rhythms.
1) Measure the QT interval.
2) Compare the QT interval to the R to R interval
3) If the QT interval is greater than 50% of the R to R, it suggests LQTS.
4) Notify the MD in charge.
5) Request a 12 lead EKG to monitor for QTc > 440ms in men and >450/ms in women and suggest cardiology screen for LQTS.
Frequently asked Questions.
Q.
What does the QTc mean?
A. The QT interval corrected for the rate.
Q.
Why does the QT interval need to be
corrected for the rate?
A. The faster the rate the more narrow the QT interval.
The slower the rate the wider the QT interval.
The QT interval is not a static number that we can tell you to look for.
Q. Where do you find the QTc?
A. The QTc is calculated by all 12 lead EKG machines and is in the same area that contains all other measurements on a 12 lead EKG. (eg. Rate, axis, PR interval, QRS interval etc… QTc is included in the same area; just look for it and you will find it).
Q. What is normal QTc?
A. Normal QTc; less than 440ms in men
less than 450ms in women.
If your pt has a prolonged QTc generally it’s a cardiology evaluation to rule out Long QT syndrome. We don’t panic for QTc’s that are a little long. The longer the QTc gets the more concern we have.
Example; A QTc of 580ms is definitely a cause for concern and needs a cardiology evaluation along with a review of current medications. A QTc of 450ms is less of a concern but still could be enough to consider a cardiology evaluation.
Q. Do you need to document Qt interval and QTc?
A. Yes and also document that you informed the MD in charge of the pt’s care and pass it on to your coworkers in shift change report.
Q.
What formula is used to calculate QTc?
A. Bazett's formula:
QT interval corrections in the literature use Bazett's
formula, defined as the observed QT interval divided by the square root of
the R-R interval in seconds. A corrected QT interval of > 440 msec is defined
as abnormal. Bazett's formula corrects or normalizes the measured QT interval
for a heart rate of 60 BPM. Thus, the QT is measured at the given heart rate,
and the QTc estimates what the QT interval would be if the heart rate were
60. Bazett's formula works reasonably
well at "normal" heart rates, but is less accurate when the heart
rate is slow or fast.
Rautaharju’s
formula:
A
more accurate method to correct the QT interval for the rate was developed
by Rautaharju et al., who developed the formula. This method is not widely used by clinicians.
Some great websites: www.qtsyndrome.ch