Advanced EKGs - The QT Interval and Long QT Syndrome

Summary

This video explains the significance of the QT interval on an EKG and the implications of Long QT Syndrome (LQTS). The QT interval represents the time between ventricular depolarization and repolarization, with its accurate measurement being crucial for diagnosing arrhythmias. The video discusses methods of measuring the QT interval, adjusting for heart rate, and identifying prolonged QT, which can lead to fatal arrhythmias like torsades de pointes. It covers both congenital and acquired causes of prolonged QT, including medication, electrolyte disturbances, and genetic conditions, offering valuable insights for healthcare professionals and students.

Takeaways

• 😀 The QT interval represents the time delay between ventricular depolarization and repolarization, reflecting the balance between potassium and calcium currents in myocardial cells.
• 😀 Accurate measurement of the QT interval is crucial, as misinterpretation can occur due to low-amplitude T waves, atrial fibrillation/flutter, or signal artifact.
• 😀 The QT interval should be corrected for heart rate, with the most commonly used formula being Bazett's formula, which adjusts the QT interval for a heart rate of 60 bpm.
• 😀 A QTc of under 440 ms is generally considered normal, with values above 500 ms being clinically relevant and associated with a higher risk of fatal arrhythmias.
• 😀 QT prolongation can be caused by both congenital and acquired conditions, with acquired causes often linked to medications, electrolyte imbalances, or metabolic issues.
• 😀 The most common fatal arrhythmia associated with prolonged QT is Torsades de Pointes, which is a polymorphic ventricular tachycardia triggered by a long-short pattern in the RR interval.
• 😀 Long QT syndrome (LQTS) can be congenital (hereditary) or acquired, with the former often related to genetic mutations in potassium or sodium ion channels.
• 😀 In congenital LQTS, LQT1, LQT2, and LQT3 are the most prevalent types, each with unique triggers like exercise, emotional stress, or sleep disturbances.
• 😀 Medications such as antiarrhythmics, antipsychotics, and some antibiotics are significant contributors to acquired long QT syndrome.
• 😀 Electrolyte abnormalities like hypokalemia, hypomagnesemia, and hypocalcemia can also cause QT prolongation, with hypocalcemia uniquely affecting the ST segment rather than the T wave.
• 😀 Short QT syndrome, characterized by an unusually short QT interval, is a rare condition associated with a high risk of sudden cardiac death due to gain-of-function mutations in potassium channels or loss-of-function mutations in calcium channels.

Q & A

What is the QT interval and what does it represent physiologically?

The QT interval represents the time delay between depolarization and repolarization of the ventricular myocardium. It spans from the beginning of the QRS complex to the end of the T wave on an electrocardiogram (EKG). This interval reflects the time it takes for the heart's electrical system to trigger and reset the cells of the heart's ventricles.

Why is measuring the QT interval considered an advanced topic?

Measuring the QT interval is considered advanced because there are many nuances involved in its accurate measurement and interpretation. It requires an understanding of various electrophysiological concepts, the effects of heart rate, irregular rhythms, and certain conditions like a wide QRS complex or abnormal T waves.

How is the QT interval measured on an EKG?

The QT interval is typically measured from the beginning of the QRS complex to the end of the T wave. However, measurement can be challenging in cases where T waves are low amplitude, or when there is signal interference, such as in atrial fibrillation or flutter.

What is the problem with using the rule of thumb that the QT interval should be half the RR interval?

The rule that the QT interval should be half the RR interval is only accurate at normal heart rates. It becomes problematic at higher or lower heart rates, as the QT interval must be adjusted to account for heart rate differences. A faster heart rate can cause the QT interval to appear shortened, while a slower heart rate can make it appear prolonged.

How is the QT interval corrected for heart rate?

The QT interval is corrected for heart rate using formulas like Bazett’s formula. This formula calculates the QT corrected (QTc) by dividing the QT interval by the square root of the RR interval (in seconds). This gives an estimate of what the QT interval would be at a heart rate of 60 beats per minute.

What is the normal range for the QTc interval in adults?

For adults, a QTc under 440 milliseconds is considered normal. A QTc between 440 and 450 milliseconds is borderline in men, and between 440 and 461 milliseconds in women. A QTc above 500 milliseconds is generally considered clinically relevant and may indicate a higher risk for life-threatening arrhythmias.

What are the potential risks associated with a prolonged QT interval?

A prolonged QT interval can lead to a potentially fatal arrhythmia known as Torsades de Pointes (Tdp), which is a polymorphic ventricular tachycardia. It is commonly triggered by a long-short pattern of heartbeats and can lead to syncope or sudden cardiac death.

What factors or conditions contribute to long QT syndrome?

Long QT syndrome can be congenital (genetic) or acquired (due to medications or metabolic disorders). Acquired factors include medications that prolong the QT interval, such as certain antiarrhythmics, antipsychotics, and antibiotics. Electrolyte disturbances, like hypokalemia and hypomagnesemia, can also contribute to QT prolongation.

What is Torsades de Pointes and how is it related to the QT interval?

Torsades de Pointes is a type of polymorphic ventricular tachycardia that is associated with a prolonged QT interval. It is often triggered by a preceding long-short interval pattern, where a long RR interval is followed by a short one, leading to early after depolarizations that can initiate the arrhythmia.

How do medications affect the QT interval and the risk of Torsades de Pointes?

Certain medications, especially antiarrhythmics, antipsychotics, and some antibiotics, can prolong the QT interval. However, not all QT-prolonging medications carry the same risk for Torsades de Pointes. Some, like amiodarone, can prolong the QT interval without significantly increasing the risk for Torsades, while others have a much higher association with this arrhythmia.