Friday, 10 September 2010

MRCP revision battle 7.3: long QT

The QT interval is that sneaky part of the ECG measured from the start of the QRS complex to the end of the T way.  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 oh whoopsie we have a ventricular arrhythmia on our hands.

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 Fridericas formula.

Bazetts = corrected QT = QT/square root of heart rate
Fridericas = corrected QT = QT/cube root of heart rate

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)

There are multiple causes of long QT:

  1. familial
    • Romano-Ward Syndrome (to be covered tomorrow)
    • Jervell-Lange-Nielson Syndrome (also to be covered tomorrow)
  2. metabolic
    • low Mg/Ca/K
    • low temp
    • low thyroid (also known as hypothyroidism.. :) )
  3. drugs
    • class I and III antiarrhythmics (which after the last revision battle you should be able to name)
    • erythromycin
    • TCA/haloperidol/risperidone/SSRIs
    • cocaine
    • organophosphates
    • antihistamines
  4. other
    • MI
    • myocarditis
    • SAH

Treatment is possibly beta blockers, possibly ICD.

Now on to revision battle 7.4, to meet a drug that fell into class V...