Arrhythmias and EKGs

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Arrhythmias and EKGs

• Part 3

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Outline

• Triggered Activity and Torsade de Pointes
• Sinus Node Block
• Differentiating VT from SVT with aberrancy

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Mechanisms of Arrhythmogenesis
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Triggered Activity

• A third general mechanism for generating
  tachyarrhythmias (in addition to
  enhanced/abnormal automaticity and reentry).
• When triggered activity occurs, a normal action
  potential is followed by oscillations of the
  membrane potential, known as afterdepolarizations.
  This triggers additional depolarization which
  have the potential to lead to extra atrial or
  ventricular contractions, and/or
  tachyarrhythmias.

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Triggered Activity(early afterdepolarizations)
Early afterdepolar-izations occur during either
phase 2 or phase 3 of the action potential, and
are seen most commonly in QT prolongation.
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Triggered Activity(late afterdepolarizations)
Late afterdepolar-izations occur shortly after
completion of repolarization, and are seen most
commonly in digitalis intoxication and high
catecholamine states.
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Torsade de Pointes
EKG Characteristics Irregular wide-complex
tachycardia The morphology,
amplitude, and axis of the QRS complexes
cycle through a sinusoidal pattern No
discernable P waves
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Torsade de Pointes

• Torsade de pointes means twisting of the points
  
• It is most commonly seen in the setting of a
  prolonged QT interval (either congenital or
  acquired), and is caused by early
  afterdepolarizations.
• This rhythm is usually short lived, and resolves
  spontaneously within seconds, but can progress to
  ventricular fibrillation if prolonged.

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Sinoatrial (SA) Nodal Block

• Less common than AV nodal block, and more
  difficult to detect
• Is classified similarly to AV block into 1st,
  2nd, and 3rd degree, although only 2nd degree SA
  block is detectable on EKG.

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1st Degree SA Block

• Since the discharge of the sinus node cannot be
  measured externally, this will appear to be
  normal sinus rhythm on EKG.
• Can only be diagnosed via EP study.

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Type 1 2nd Degree SA Block
Braunwald's Heart Disease A Textbook of
Cardiovascular Medicine, 7th ed., 2005.
EKG Characteristics Progressive shortening of
the PP interval until a pause in the
sinus rhythm appears. The pause
will be less than the two preceding PP
interval. The PP interval following the
pause is greater than the PP interval
just before the pause.
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Type 2 2nd Degree SA Block
Braunwald's Heart Disease A Textbook of
Cardiovascular Medicine, 7th ed., 2005.
EKG Characteristics Abnormal pauses between 2
sinus P waves Length of pause is
a multiple of the shortest PP interval
(usually 2x) PP interval is otherwise
constant.
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3rd Degree SA Block

• Indistinguishable from complete sinus arrest on
  EKG.
• Can only be diagnosed via EP study.

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Distinguishing VT from SVT with aberrancy

• SVT can occasionally present as an unknown
  wide-complex tachycardia if if occurs in the
  presence of
• Preexisting bundle branch block
• Rate related bundle branch block
• An accessory pathway
• Treatment with class IA or IC antiarrhythmics

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Distinguishing VT from SVT with aberrancy

• VT accounts for 80 of all cases of regular
  wide-complex tachycardias, and 95 of all cases
  of regular wide-complex tachycardias which occur
  in patients with a history of MI.
• One of the most common lethal errors made in
  arrhythmia diagnosis is to mistake VT for SVT and
  treat with verapamil, diltiazem, and adenosine,
  all of which can precipitate ventricular
  fibrillation in patients in VT, even if initially
  stable.

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Distinguishing VT from SVT with aberrancy

• Therefore, all wide-complex tachycardias should
  be assumed to be VT until proven otherwise.

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EKG features moderately suggestive of VT

• QRS duration gt 160ms
• An extreme QRS axis (-90 to -180 degrees)
• Precordial QRS concordance
• Variations in the QRS and ST-T morphologies
• Slight irregularity at the onset of the
  arrhythmia

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EKG features highly suggestive of VT

• Fusion beats
• Capture beats (aka Dressler beats)
• Dissociated P waves

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Physical findings highly suggestive of VT

• Signs of AV dissociation, including
• Canon A waves in the jugular venous pulsations
• Varying BP measurement from beat to beat
• Varying intensity of S1