Question 30.1

Created on Thu, 09/14/2017 - 06:35
Last updated on Tue, 12/19/2017 - 16:42
Pass rate: 49%
Highest mark: 7.0

Other SAQs in this paper

Other SAQs on this topic

a) List the ECG criteria that are helpful in distinguishing ventricular tachycardia (VT) from supraventricular tachycardia (SVT) with aberrant conduction. For each listed criterion, indicate which diagnosis it makes more likely. (30% marks)

b) List the specific management strategies that may be used to treat torsades de pointes. (30% marks)
 

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College answer

a)                                                         
•    Capture beats: VT 
•    Fusion beats: VT 
•    Concordance in chest leads (or absence of RS complex): VT 
•    Typical RBBB or LBBB morphology: SVT 
•    R to S interval >100ms: VT 
 
(Note: there are some more specific criteria from diagnostic algorithms – if correct these should receive credit.) 
b)                                                          
•    Correction of electrolyte abnormalities or hypothermia 
•    Magnesium 
•    Isoprenaline 
•    Phenytoin 
•    Sodium Bicarbonate 
•    Lignocaine 
•    Electrical cardioversion 
•    Atrial overdrive pacing 
•    Cessation of provoking drugs 
 

Discussion

Some of the criteria are somewhat duplicated because the features are listed according to the society guideline being quoted. It makes sense that most of the guideline-makers would agree on such obvious things as "wide QRS" and "regular", etc.

How to Tell VT from SVT with Aberrancy
Criterion Findings associated with SVT Findings associated with VT
ACC/AHA Guidelines (2003)
QRS duration <120 msec > 120 msec
Rhythm Irregular Regular
A-V relationship Atrial rate faster than ventricular rate Ventricular rate faster than atrial rate
Axis Normal, right or left axis Bizarre axis (+90 to -90)
QRS morphology in the precordial leads Typical RBBB or LBBB Concordance; no R/S pattern; onset of R to nadir is longer than 100 msec.

In RBBB pattern: 
- qR, Rs or Rr patter in V1

In LBBB pattern:
- R in V1 longer than 30msec
- R to nadir of S in V1 longer than 60 msec
- qR or qS in V6
     
Brugada algorithm (1991)
RS complex in precordial leads Present Absent
R-S interval in one precordial lead <100 msec >100 msec
A-V relationship Associated Dissociated
QRS morphology criteria for VT Not met Met
Brugada QRS morphology criteria for LBBB pattern
Initial R period <100 msec >100 msec
S-wave  in
V1 or V2
Normal downwards leg Slurred or notched downwards leg
Q to nadir QS in V1 or V2 <100 msec >100 msec
Q or QS in V6 Absent Present
Brugada QRS morphology criteria for RBBB pattern
R or qR in V1 Normal Monophasic
R to R' size  R shorter than R' R taller than R'
R in V6 No Rs Rs present in V6
Vereckei algorithm (2007)
A-V relationship Associated Dissociated
R in aVR Absent Present
QRS morphology Like a RBBB or LBBB Unlike RBBB or LBBB
Vi/Vt V(initial QRS upstroke y-axis distance during the first 40 msec) is greater than V(terminal QRS downstroke y-axis distance during the last 40 msec of the QRS) Vi is smaller than Vt

Management of torsades is somewhat less complex. Thomas and Behr (2015) have published a good article which describes the management strategies for torsades:

  • Preventative strategies
    • Stop the QT-prolonging drugs
    • Keep the serum K+ around 4.7 - 5.2 mmol/L
  • Immediate treatment
    • IV magnesium sulfate
    • Isoprenaline (to increase heart rate to 100-110)
    • Overdrive pacing
    • Lignocaine
  • Experimental treatments and last resort measures
    • Clonidine
    • Ranolazine