The QT interval is that sneaky part of the ECG measured from the start of the QRS complex to the end of the T wave.
I call it sneaky as as clinicians we rarely pay it much attention, then
it just happens to get a bit on the long side and opppss… we have a
ventricular arrhythmia on our hands. It represents the time taken for complete contraction and relaxation of the ventricles.
The first thing to learn about the QT interval is that it decreases as heart rate increases. It is therefore necessary to correct it, which is achieved by using either Bazetts or Fridericias formula.
Bazetts = corrected QT = QT/square root of RR interval
Fridericias = corrected QT = QT/cube root of RR interval
Upper limit of normal is generally taken to be 450ms in a man and 470ms in a woman
(for some bizarre reason I remember 450 by singing the song 'brimful of
asha on the 45' then just remember that females are 20 more)
Classical pattern of deterioration in long QT is long QT leading to torsade de pointes then either VT or VF.
There are multiple causes of long QT:
1. Familial
2. Metabolic
- low Mg/Ca/K
- low temperature
- low thyroid (also known as hypothyroidism…)
3. Drugs
- class I and III antiarrhythmics
- macrolides (erthyromycin, clarithromycin)
- quinolones (ciprofloxacin)
- TCA/haloperidol/risperidone/SSRIs
- antiemetics (ondansetron, prochlorperazine, domperidone)
- cocaine, methadone
- organophosphates
- antihistamines
4. Other
- MI
- myocarditis
- SAH
In
terms of remembering the drugs which can cause long QT I think of it as
lots of the ‘antis’ – antibiotics, antiarrhythmics, antiemetics,
antihistamines, antidepressants and antipyscotics.
Treatment is by treating the underlying cause/stopping underlying medication or in familial cases with beta blockers and possibly an ICD.
Note that if VT occurs due to long QT amiodarone should not be given as a treatment.
Small print gem:
The mechanism for most potential QT-prolonging medications is
inhibition of the KCNH2-encoded HERG potassium channel. HERG stands for
human ether-à-go-go related gene – and no, I didn’t make that name up.
References
