QT interval is the time between the onset of depolarization to repolarization.
It is affected by diet, gender, alcohol, time of the day, menstrual cycle and heart rate.
QTc = QT msec/square root of RR
Calculation of QTd
QTd was calculated as the difference between the longest QTc and the shortest QTc in the 12 leads.
An easier way is to calculate the RR interval (number of large squares) and if QT interval is longer than 50% of the RR interval
(again check the number of large squares between beginning of Q and end of T) it is an indication that it is prolonged.
Potential consequences of QT prolongation include torsade de pointes (syncope),
ventricular fibrillation and sudden death. If QTc prolongation is associated with T wave changes refer to the cardiologist.
BACKGROUND
Long QT syndrome (LQTS) is a congenital disorder characterized by a prolongation of the QT interval on ECG
and a propensity to ventricular tachyarrhythmias, which may lead to syncope, cardiac arrest, or sudden death.
The QT interval on the ECG, measured from the beginning of the QRS complex to the end of the T wave,
represents the duration of activation and recovery of the ventricular myocardium.
QT intervals corrected for heart rate (QTc) longer than 0.44 seconds are generally considered abnormal,
though a normal QTc can be more prolonged in females (up to 0.46 sec).
The Bazett formula is used to calculate the QTc, as follows: QTc = QT/square root of the R-R interval.
The QT interval should be measured in lead II, because the influence of a possible U-wave are smallest in this lead.
If lead II is cannot be used for measurements, than leads V4 or V5 can be used alternatively.
CAUSES of LQTS
Inherited Long QT Syndrome
Faulty genes that control the production of certain types of ion channels in your heart cells cause inherited long QT syndrome (LQTS).
These LQTS genes may cause the body to produce too few ion channels or ion channels that don't work as they should, or both.
If you have inherited LQTS, you received the faulty gene(s) from one or both of your parents. Inherited LQTS is a lifelong condition.
There are seven known types of inherited LQTS (types 1 though 7). The most common types of inherited LQTS are types 1, 2, and 3.
Some LQTS types stem from faulty or lacking potassium ion channels, while others are due to faulty or lacking sodium channels.
If you have LQTS 1 and LQTS 2, the flow of potassium ions through the ion channels in your heart cells isn't normal,
which may cause problems when you exercise or when you're surprised or experience emotional stress.
During these situations, you may develop a rapid and uncontrollable heart rhythm that may prevent your heart from pumping blood.
This will result in death if it's not quickly brought under control.
If you have LQTS 3, the flow of sodium ions through ion channels in your heart cells isn't normal,
which also may trigger a rapid and uncontrollable heart rhythm that can result in death.
In LQTS 3, problems usually develop when your heart beats slower than normal, such as during sleep.
Noninherited Long QT Syndrome
Acquired, or noninherited, LQTS may be brought on by certain medicines and other medical conditions.
Side effects from certain medicines are the most common cause of noninherited LQTS.
Medication-Induced Long QT Syndrome
Presently, more than 50 medicines have been recognized to cause LQTS. Some of the commonly used medicines
that may cause LQTS include:
1. Antihistamines and decongestants
2. Diuretics (pills that remove excess water from your body)
3. Antibiotics
4. Antidepressants
5. Cholesterol-lowering medicines and some diabetes medicines
Some people with medication-induced LQTS may also have an inherited form of the syndrome that doesn't cause symptoms
unless they take certain medicines that lengthen the QT interval or lower potassium levels in the blood.
This is sometimes called silent LQTS.
Other Causes of Acquired Long QT Syndrome
Excessive diarrhea or vomiting that causes a major loss of potassium or sodium ions from the bloodstream may cause LQTS.
The syndrome lasts until the levels of these ions in the blood return to normal.
The eating disorder anorexia nervosa and certain thyroid disorders may cause a drop in potassium ion levels in the blood, causing LQTS.
