Stand and Deliver
Answer: D
This woman presents with a common complaint – dizziness – which includes both benign and dangerous etiologies. The most worrisome feature of her presentation is her prolonged QT interval. Based on population
studies conducted with tens of thousands of healthy volunteers in various settings (the U.S., Japan, Switzerland), the QT interval follows a Normal distribution. From these studies, a prolonged QTc was determined to be the value above the 95th percentile: for males, > 450 ms; for females, > 460 ms.
Below is a proposed scale for QT intervals:
This woman presents with a common complaint – dizziness – which includes both benign and dangerous etiologies. The most worrisome feature of her presentation is her prolonged QT interval. Based on population
studies conducted with tens of thousands of healthy volunteers in various settings (the U.S., Japan, Switzerland), the QT interval follows a Normal distribution. From these studies, a prolonged QTc was determined to be the value above the 95th percentile: for males, > 450 ms; for females, > 460 ms.
Below is a proposed scale for QT intervals:
When the diagnosis is in doubt, perform the standing QT test: in a patient with a borderline QT, repeat the ECG while the patient is supine, keep the ECG machine running and obtain another ECG immediately upon standing (within no more than 30 seconds).
The rationale behind this is the body’s natural response to an increase in heart rate (as seen simply in standing): in a healthy individual, an increase in heart rate will slightly shorten the QT interval. For that reason, various formulae exist to correct for heart rate, of which the most popular is Bazett’s formula: QTc = QT / √RR.
(Note: Bazett’s formula overinflates the QT at heart rates > 100 bpm and underestimates the QT at heart rates < 60 bpm. Outside the 60-100 bpm range, other formulae may be used, such as the Fredericia correction, which uses a cubed root of the RR interval, rather than squared as in Bazett’s).
In healthy individuals, the heart rate increase is relatively higher in magnitude than is the shortening of the QT interval, which yields a slightly longer QTc than the QT (see the above formula – both the numerator and
denominator decrease with higher heart rates, but the heart rate changes proportionally somewhat more than the QT). This explains why the QTc is only slightly longer than the QT in normal patients, but typically by no
more than 50 ms.
In patients with prolonged QT (either congenital or acquired), when the heart rate increases, there is either a significant lag in the shortening of the QT interval or even a slight increase (an innate problems in these individuals). That is, there is an abnormal interval response to an increase in heart rate – the heart rate increases, but the QT does not shorten as it should (see the above formula – as the RR interval decreases due to higher heart rate, the numerator does not change or even increases, causing a much larger QTc) . For that reason, the QTc on standing will be markedly longer than the normal increase in unaffected individuals, typically > 50 ms.
In fact, a common pitfall (due to anchoring bias) is always to ascribe positional dizziness to normal orthostatic changes in standing – this is actually a very typical trigger in individuals with prolonged QT: the QTc is markedly higher (due to their blunted reflex in the normal shortening of the QT with increased heart rate on standing).
Viskin et al found the standing QT test to have a 90% sensitivity and 86% specificity for long QT syndrome.
This woman’s ECG shows a baseline QT of 480 ms and QTc of 512 ms at a heart rate of 68 beats/minute:
The rationale behind this is the body’s natural response to an increase in heart rate (as seen simply in standing): in a healthy individual, an increase in heart rate will slightly shorten the QT interval. For that reason, various formulae exist to correct for heart rate, of which the most popular is Bazett’s formula: QTc = QT / √RR.
(Note: Bazett’s formula overinflates the QT at heart rates > 100 bpm and underestimates the QT at heart rates < 60 bpm. Outside the 60-100 bpm range, other formulae may be used, such as the Fredericia correction, which uses a cubed root of the RR interval, rather than squared as in Bazett’s).
In healthy individuals, the heart rate increase is relatively higher in magnitude than is the shortening of the QT interval, which yields a slightly longer QTc than the QT (see the above formula – both the numerator and
denominator decrease with higher heart rates, but the heart rate changes proportionally somewhat more than the QT). This explains why the QTc is only slightly longer than the QT in normal patients, but typically by no
more than 50 ms.
In patients with prolonged QT (either congenital or acquired), when the heart rate increases, there is either a significant lag in the shortening of the QT interval or even a slight increase (an innate problems in these individuals). That is, there is an abnormal interval response to an increase in heart rate – the heart rate increases, but the QT does not shorten as it should (see the above formula – as the RR interval decreases due to higher heart rate, the numerator does not change or even increases, causing a much larger QTc) . For that reason, the QTc on standing will be markedly longer than the normal increase in unaffected individuals, typically > 50 ms.
In fact, a common pitfall (due to anchoring bias) is always to ascribe positional dizziness to normal orthostatic changes in standing – this is actually a very typical trigger in individuals with prolonged QT: the QTc is markedly higher (due to their blunted reflex in the normal shortening of the QT with increased heart rate on standing).
Viskin et al found the standing QT test to have a 90% sensitivity and 86% specificity for long QT syndrome.
This woman’s ECG shows a baseline QT of 480 ms and QTc of 512 ms at a heart rate of 68 beats/minute:
The second panel (maximal tachycardia) shows artifact when she stands, then an unchanged QT but a markedly increased QTc: from 512 ms to 582 ms at a heart rate of 82 beats/minute.
With this new information (marked lengthening of the QTc above 50 ms with standing), our patient’s symptoms now seem more ominous. She should be admitted, monitored, and investigated.
Magnesium (B) and calcium (C) both affect the QT (hypomagnesemia and hypocalcemia both prolong the QT).
Hypokalemia (A) can be potentiated or exacerbated by hypomagnesemia; in addition, hypokalemia is associated with a U wave, which may be misinterpreted as a long QT.
Bottom line (and without the math!):
When prolonged QT is suspected, obtain a resting ECG while supine or seated, keep the leads and machine on, have the patient stand, and immediately obtain a repeat ECG (the standing ECG). A change in QTc of > 50 ms is suspicious for prolonged QT, and the patient’s presenting signs and symptoms should be interpreted through the lens of this new information.
References
Viskin S. The QT interval: too long, too short or just right. Heart Rhythm. 2009; 6(5):711-5. Epub 2009 Mar 3.
Viskin S et al. The Response of the QT Interval to the Brief Tachycardia Provoked by Standing: A Bedside Test for Diagnosing Long QT Syndrome. J Am Coll Cardiol. 2010 May 4; 55(18): 1955–1961.
With this new information (marked lengthening of the QTc above 50 ms with standing), our patient’s symptoms now seem more ominous. She should be admitted, monitored, and investigated.
Magnesium (B) and calcium (C) both affect the QT (hypomagnesemia and hypocalcemia both prolong the QT).
Hypokalemia (A) can be potentiated or exacerbated by hypomagnesemia; in addition, hypokalemia is associated with a U wave, which may be misinterpreted as a long QT.
Bottom line (and without the math!):
When prolonged QT is suspected, obtain a resting ECG while supine or seated, keep the leads and machine on, have the patient stand, and immediately obtain a repeat ECG (the standing ECG). A change in QTc of > 50 ms is suspicious for prolonged QT, and the patient’s presenting signs and symptoms should be interpreted through the lens of this new information.
References
Viskin S. The QT interval: too long, too short or just right. Heart Rhythm. 2009; 6(5):711-5. Epub 2009 Mar 3.
Viskin S et al. The Response of the QT Interval to the Brief Tachycardia Provoked by Standing: A Bedside Test for Diagnosing Long QT Syndrome. J Am Coll Cardiol. 2010 May 4; 55(18): 1955–1961.