Mechanism of arrythmias

1. Mechanism of ArrhythmiasMechanism of Arrhythmias
Dr Fuad FarooqDr Fuad Farooq

2. Implies normal sequence of conduction, originating in the sinus node and
proceeding to the ventricles via the AV node and His-Purkinje system
EKG Characteristics: Regular narrow-complex rhythm
Rate 60-100 bpm
Each QRS complex is proceeded by a P wave
P wave is upright in lead II & downgoing in lead aVR
Normal Sinus RhythmNormal Sinus Rhythm

3. The mechanisms responsible for cardiac The mechanisms responsible for cardiac
arrhythmias are generally divided intoarrhythmias are generally divided into
–Disorders of impulse formationDisorders of impulse formation
–Disorders of impulse conductionDisorders of impulse conduction
–Combinations of both Combinations of both

4. Can beCan be
–Disorder in Automaticity Disorder in Automaticity
–Trigger activityTrigger activity
Disorders of Impulse FormationDisorders of Impulse Formation

5. Automaticity is the property of a fiber to initiate an Automaticity is the property of a fiber to initiate an
impulse spontaneously, without need for prior stimulationimpulse spontaneously, without need for prior stimulation
Characterized by Characterized by
–Inappropriate discharge rate of the normal pacemaker, the Inappropriate discharge rate of the normal pacemaker, the
SA node (e.g., sinus rates too fast or too slow for the SA node (e.g., sinus rates too fast or too slow for the
physiologic needs of the patient)physiologic needs of the patient)
–Discharge of an ectopic pacemaker that controls atrial or Discharge of an ectopic pacemaker that controls atrial or
ventricular rhythmventricular rhythm
Ectopic pacemakers are often called latent or subsidiary Ectopic pacemakers are often called latent or subsidiary
pacemakerspacemakers
AutomaticityAutomaticity

6. Ectopic pacemakers are usually suppressed by Ectopic pacemakers are usually suppressed by
overdrive suppression by the more rapidly firing SA node overdrive suppression by the more rapidly firing SA node
or by concealed conduction from neighboring fibersor by concealed conduction from neighboring fibers
–Manifested when the SA discharge rate slows or block Manifested when the SA discharge rate slows or block
occurs at some level between the SA node and the ectopic occurs at some level between the SA node and the ectopic
pacemaker sitepacemaker site
–The discharge rate of the latent pacemaker can speed The discharge rate of the latent pacemaker can speed
inappropriately and usurp control of cardiac rhythm from inappropriately and usurp control of cardiac rhythm from
the SA nodethe SA node
AutomaticityAutomaticity

7. Recognizing Altered Automaticity on EKGRecognizing Altered Automaticity on EKG
Gradual onset (warming) and termination of the Gradual onset (warming) and termination of the
arrhythmiaarrhythmia
The P wave of the first beat of the arrhythmia is The P wave of the first beat of the arrhythmia is
typically the same as the remaining beats of the typically the same as the remaining beats of the
arrhythmia (if a P wave is present at all)arrhythmia (if a P wave is present at all)

8. Sinus Bradycardia
Decreased AutomaticityDecreased Automaticity

9. Sinus tachycardia
Junctional tachycardia
Ectopic atrial tachycardia
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Increased/Abnormal AutomaticityIncreased/Abnormal Automaticity

10. •Triggered activity is initiated by after-depolarizations, which Triggered activity is initiated by after-depolarizations, which
are depolarizing oscillations in membrane voltage induced by are depolarizing oscillations in membrane voltage induced by
one or more preceding action potentialsone or more preceding action potentials
•Thus, triggered activity is pacemaker activity that results Thus, triggered activity is pacemaker activity that results
consequent to a preceding impulse or series of impulses, consequent to a preceding impulse or series of impulses,
without which electrical quiescence will occurswithout which electrical quiescence will occurs
TriggeredTriggered ActivityActivity

11. Phase 4: Resting
phase
•Associated with
diastole portion of
heart cycle
•Potassium outward
current through
open, inwardly
rectifying K
+
channels
( I
kl
)
Phase 0 : Rapid
depolarization

•Opening of fast Na
channels
Phase 1: Initial
rapid
repolarization
•Closure of the
fast Na
+
channels
•Phase 0 and 1
together
correspond to
the R and S
waves of the
ECG
Phase 2 Plateau
phase
•Balance between
the inward
movement of Ca
+
and
outward movement
of K
+

•Corresponds to ST
segment of the ECG
Phase 3 Repolarization
•K
+
channels remain open
•Allow K
+
to build up outside the cell,
causing the cell to repolarize
•Corresponds to T wave on the ECG

12. Can be: Can be:
•Early after-depolarizations (EADs), arise from a reduced level of Early after-depolarizations (EADs), arise from a reduced level of
membrane potential during phases 2 (type 1 – augmented membrane potential during phases 2 (type 1 – augmented
opening of Ca channels) and 3 (type 2 – augmented opening of opening of Ca channels) and 3 (type 2 – augmented opening of
Na channels) of the cardiac action potentialNa channels) of the cardiac action potential
•Late or delayed after-depolarizations (DADs), occur after Late or delayed after-depolarizations (DADs), occur after
completion of repolarization (phase 4 – secondary to increase completion of repolarization (phase 4 – secondary to increase
cytosolic Cacytosolic Ca
++++
levels), generally at a more negative membrane levels), generally at a more negative membrane
potential than that from which EADs arisepotential than that from which EADs arise

14. •Causes of EAD include:Causes of EAD include:
1.1.Low potassium blood levelLow potassium blood level
2.2.Slow heart rateSlow heart rate
3.3.Drug toxicity (i.e. quinidine causing torsades de Drug toxicity (i.e. quinidine causing torsades de
pointes)pointes)
•DAD may cause triggered activity due to:DAD may cause triggered activity due to:
1.1.Premature beatsPremature beats
2.2.Increased serum CaIncreased serum Ca
++++
level level
3.3.Increased adrenaline levels Increased adrenaline levels
4.4.Digitalis toxicityDigitalis toxicity

15. Can be:Can be:
–Conduction blockConduction block
–Re-entryRe-entry
Disorder of Impulse ConductionDisorder of Impulse Conduction

16. Mechanism of ReentryMechanism of Reentry

17. Mechanism of ReentryMechanism of Reentry

18. AV nodal reentrant tachycardia (AVNRT)AV nodal reentrant tachycardia (AVNRT)
AV reentrant tachycardia (AVRT)AV reentrant tachycardia (AVRT)
–OrthodromicOrthodromic
–AntidromicAntidromic
Atrial flutterAtrial flutter
Atrial fibrillationAtrial fibrillation
Ventricular tachycardiaVentricular tachycardia
Reentrent RhythmsReentrent Rhythms

19. Abrupt onset and termination of the arrhythmiaAbrupt onset and termination of the arrhythmia
The P wave of the first beat of the arrhythmia is The P wave of the first beat of the arrhythmia is
different as the remaining beats of the different as the remaining beats of the
arrhythmia (if a P wave is present at all)arrhythmia (if a P wave is present at all)
Recognizing Reentry on EKG

20. Example of AVNRTExample of AVNRT

21. Most cases of atrial flutter are caused by a large reentrant circuit in the wall of the
right atrium
Atrial FlutterAtrial Flutter

22. Atrial fibrillation is caused by numerous wavelets of depolarization spreading
throughout the atria simultaneously, leading to an absence of coordinated
atrial contraction
Atrial Fibrillation

23. Thank YouThank You