Tachyarrytmias Flashcards
VT vs SVT
1- AV dissociation
2- fusion and capture beats
Fusion Beats
• Definition: A fusion beat occurs when a ventricular beat (from the VT focus) and a supraventricular impulse (e.g., from the sinus node) depolarize the ventricles simultaneously.
• Mechanism:
• The impulse from the VT focus starts to depolarize the ventricles.
• At the same time, a supraventricular impulse manages to partially penetrate the ventricles before the VT impulse fully takes over.
• This results in a QRS complex that is partially normal (narrower) and partially wide, appearing as a hybrid between a normal beat and a ventricular beat.
• Appearance: A QRS complex that looks intermediate in morphology between the wide QRS complexes of VT and the patient’s normal QRS complexes.
- Capture Beats
• Definition: A capture beat occurs when a supraventricular impulse (e.g., from the sinus node) “captures” the ventricles, fully depolarizing them despite the ongoing VT.
• Mechanism:
• During VT, the ventricles are typically refractory to other impulses because they are continuously depolarized by the VT focus.
• Occasionally, a supraventricular impulse arrives at the ventricles during a moment when they are no longer refractory.
• This supraventricular impulse completely depolarizes the ventricles, resulting in a normal (narrow) QRS complex amid the wide QRS complexes of VT.
• Appearance: A single normal QRS complex in the midst of wide QRS complexes.
Why Do These Features Suggest VT?
Fusion and capture beats indicate AV dissociation, which is a hallmark of VT. In VT:
• The ventricles are being driven by an independent focus, separate from the atria.
• Occasionally, an atrial impulse may compete with or interrupt the ventricular focus, producing these unique beats.
In SVT with aberrant conduction, the impulses originate from the atria and propagate through the ventricles via a consistent pathway, so you wouldn’t see fusion or capture beats.
VT vs SVT
Positive QRS Concordance in Chest Leads
Positive QRS concordance is a strong indicator of ventricular tachycardia (VT). Let’s break it down:
What Is QRS Concordance?
• Definition: Concordance refers to whether all the QRS complexes in the precordial (chest) leads (V1–V6) point in the same direction—either all positive or all negative.
• Positive concordance: All QRS complexes in leads V1–V6 are entirely upright (positive).
• Negative concordance: All QRS complexes in leads V1–V6 are entirely downward (negative).
Why Positive Concordance Suggests VT
In normal cardiac conduction (e.g., during sinus rhythm or SVT with aberrancy), the activation of the ventricles follows the normal His-Purkinje system, creating a gradual transition of QRS morphologies in the precordial leads:
• Leads V1–V3 typically show negative QRS complexes (over the right ventricle).
• Leads V4–V6 typically show positive QRS complexes (over the left ventricle).
This results in a normal progression (from negative to positive) across the precordial leads.
In VT, the ventricles are activated abnormally from a focus outside the normal conduction system, often leading to a wide QRS and abnormal patterns:
• A ventricular focus that propagates toward all chest leads creates positive concordance, as the depolarization moves uniformly in that direction.
• This is a highly specific feature of VT, especially in wide-complex tachycardias.
Mechanism of Positive Concordance in VT
• The depolarization of the ventricles starts from an ectopic ventricular focus (not the atria or AV node).
• If the focus is located low in the ventricles, the electrical activity spreads upward and outward, creating QRS complexes that are consistently upright across V1–V6.
Differentiating VT from SVT with Aberrant Conduction
• Positive concordance is rarely (if ever) seen in SVT with aberrant conduction because conduction through the normal pathways maintains a more typical QRS transition.
• In VT, this pattern results from disorganized ventricular activation.
Summary
• Positive QRS Concordance: All QRS complexes in V1–V6 are positive and upright, which is highly suggestive of VT.
• This feature reflects abnormal ventricular depolarization originating from a ventricular ectopic focus, rather than supraventricular origins.
VT vs SVT
Positive QRS Concordance in Chest Leads
Positive QRS concordance is a strong indicator of ventricular tachycardia (VT). Let’s break it down:
What Is QRS Concordance?
• Definition: Concordance refers to whether all the QRS complexes in the precordial (chest) leads (V1–V6) point in the same direction—either all positive or all negative.
• Positive concordance: All QRS complexes in leads V1–V6 are entirely upright (positive).
• Negative concordance: All QRS complexes in leads V1–V6 are entirely downward (negative).
Why Positive Concordance Suggests VT
In normal cardiac conduction (e.g., during sinus rhythm or SVT with aberrancy), the activation of the ventricles follows the normal His-Purkinje system, creating a gradual transition of QRS morphologies in the precordial leads:
• Leads V1–V3 typically show negative QRS complexes (over the right ventricle).
• Leads V4–V6 typically show positive QRS complexes (over the left ventricle).
This results in a normal progression (from negative to positive) across the precordial leads.
In VT, the ventricles are activated abnormally from a focus outside the normal conduction system, often leading to a wide QRS and abnormal patterns:
• A ventricular focus that propagates toward all chest leads creates positive concordance, as the depolarization moves uniformly in that direction.
• This is a highly specific feature of VT, especially in wide-complex tachycardias.
Mechanism of Positive Concordance in VT
• The depolarization of the ventricles starts from an ectopic ventricular focus (not the atria or AV node).
• If the focus is located low in the ventricles, the electrical activity spreads upward and outward, creating QRS complexes that are consistently upright across V1–V6.
Differentiating VT from SVT with Aberrant Conduction
• Positive concordance is rarely (if ever) seen in SVT with aberrant conduction because conduction through the normal pathways maintains a more typical QRS transition.
• In VT, this pattern results from disorganized ventricular activation.
Summary
• Positive QRS Concordance: All QRS complexes in V1–V6 are positive and upright, which is highly suggestive of VT.
• This feature reflects abnormal ventricular depolarization originating from a ventricular ectopic focus, rather than supraventricular origins.
VT vs SVT
Features suggesting VT rather than SVT with aberrant conduction
AV dissociation
fusion or capture beats
positive QRS concordance in chest leads
marked left axis deviation
history of IHD
lack of response to adenosine or carotid sinus massage
QRS > 160 ms
VT vs SVT
Features suggesting VT rather than SVT with aberrant conduction
AV dissociation
fusion or capture beats
positive QRS concordance in chest leads
marked left axis deviation
history of IHD
lack of response to adenosine or carotid sinus massage
QRS > 160 ms
