Cardiac Arrhythmias Flashcards
Define Bradycardia.
Slow heart rate <60/min potentially leading to a decrease in cardiac output, hypotension, heart failure and sx (dizziness, syncope, palpitations)
Define tachycardia.
Rapid heart rate >100/min potentially leading to:
• Palpitations
• Impairment of cardiac output, hypotension, heart failure, ischemia and sx
What are palpitations?
subjective sense of heart beating abnormally
What is the physiological term for normal heart rate?
Normal Sinus Rhythm
Define normal sinus rhythm.
Conventionally defined as 60-100/min, but people can be slightly below and slightly above
Define arrhythmia.
Literally “No Rhythm” but used to refer to any “Abnormal Rhythm.”
What is bradyarrhythmia?
- Abnormal Bradycardic Rhythm <60/min
* Excludes “normal bradycardia” such as physiologic sinus bradycardia at rest or during sleep
What is tachyarrhythmia?
- Abnormal Tachycardic Rhythm >100/min
* Excludes “normal tachycardia” such as phsyiologic sinus tachycardia during exercise or under stress.
Define supraventricular tachycardia (SVT).
Abnormal tachycardia which requires participation of either Atrial or AV Nodal Tissue
When supraventricular tachycardia (SVT) is chaotic what do you call it?
Atrial Fibrillation (AF)
Define ventricular tachycardia (VT).
- Abnormal tachycardia originating in the Ventricle or His-Purkinje System (= His Bundle + Bundle Branches + Purkinje Network)
- By definition, VT does not require involvement of either the Atrium or the AV Node.
When ventricular tachycardia (VT) is chaotic what do you call it?
Ventricular Fibrillation (VF)
All disorders of heart rhythm arise as a consequence of either _______ or _______.
•Alterations in Impulse Formation or Impulse Conduction (=Propagation)
Normal impulse formation comes from pacemaker cells which are what?
Specialized Cardiac Cells with intrinsic automaticity
Failure of faster structures may allows what?
subsidiary automatic tissue to exhibit automaticity at slower rates (Called “Escape” Rhythms or Pacemakers)
What provide redundancy in the conduction system and help protect from catastrophic bradycardia?
Subsidiary escape pacemakers
Which cells in the heart maintain a very
negative and static resting membrane potential (around -90 mV) and lack intrinsic automaticity?
most myocardial cells (working cells)
Sinus Node and AV Node have a higher diastolic membrane postentia, around what?
about -60 mV
What is so special about nodal cells?
exhibit spontaneous gradual diastolic depolarization
Spontaneous gradual diastolic depolarization is due to what channels?
(If): A Slow Inward Sodium Current
Relatively high resting membrane potential of SN and AVN cells keeps most Fast Na channels in the _________ State.
Inactivated
If fast Na channels are in an inactivated state, what channels do nodal cells depend on for depolarization?
dependence on the slower Ca++ Current
What is overdrive suppression?
Faster automatic sites normally preempt subsidiary slower potential pacemaker sites and transiently suppress the rate of subsidiary pacemakers
In spontaneously active pacemaker cells,
hyperpolarizing current of the electrogenic NaK-ATPase is offset by what?
the pacemaker current If
in passively activated subsidiary slower pacemaker cells, If is much smaller which leads to what?
hyperpolarization
So what regulates the sinus node?
regulated by autonomic tone to match cardiac output to metabolic demand
What 3 factors interplay to create normal automaticity in the node?
- Rate of Diastolic Depolarization (mostly If): Faster depolarization -> faster rate
- Maximum negative diastolic potential: More negative -> slower rate
- The Threshold Potential: More negative -> Faster rate & Less negtaive -> Slower rate
Near syncope preceded by nausea and cause by increased PNS tone is never lethal because?
PNS tone never stays high enough long enough the CNS usually fixes it
Two main effects of SNS on automaticity?
Increases rate of diastolic depolarization and makes the Threshold Potential More Negative both of which are excitatory
Main effects of the PNS on automaticity?
Decreases the “Open Probability” of the Pacemaker Current channel (If) and makes Threshold Potential Less Negative
during diastole, if adjacent cells have differing diastolic potentials what can happen because they are connected by gap junctions?
current may bleed
from one cell to the other following the potential gradient
Automaticity will be _______ in Pacemaker
cells tightly coupled to adjacent working
myocardium due to current bleed
decreased
in the setting of pathologic fibrosis leading to cell decoupling, automaticity may be ________ in pacemaker cells normally inhibited by adjacent working myocardium
enhanced
A failure of sinus node automaticity is often moderated by emergence of normally suppressed secondary pacemakers in the AV node or, if this has failed too, what takes over?
infranodal conduction system
An escape rhythm arising in the ______ (termed a Junctional Escape)
will have a narrow QRS without a preceding P-wave and typically a rate
of 50-60/min
AV node
An escape rhythm arising in the ________ (Ventricular Escape) will have a Wide QRS.
Infranodal tissue
Increase in rate of tissue normally capable of pacemaker activity is what type of tachycardia mechanism?
Enhanced Automaticity
Abnormal impulse formation or automaticity in tissue not normally capable of pacemaker activity is what type of tachycardia mechanism?
Abnormal Automaticity
Abnormal Automaticity is normally caused by what?
Often a consequence of cellular injury. May be seen in setting of coronary ischemia, electrolyte disturbances, myocardial disease
What is the triggered activity mechanism of tachycardia?
Single or repetitive cellular activity following a prior action potential. Due to oscillations in membrane potential
There are two types of triggered activity, what are they?
Early Afterdepolarizations (EAD’s) and Delayed Afterdepolarizations (DAD’s)
What are EAD’s?
Membrane Oscillations which occur within the action potential, either in the Plateau (Phase 2) or During Repolarization (Phase 3) related to Inward Ca++ Current (in Phase 2) or reactivation of Fast Na Current (in Phase 3)
What are DAD’s?
Membrane Oscillations occurring after completion of Full Repolarization (Phase 4) promoted by conditions which lead to high intracellular calcium
What do you treat DAD’s with?
inhibited by Ca++ channel blockers
What is a conduction block?
Occurs when a propagating wavefront encounters tissue which is unexcitable
What are the three types of conduction block?
permanent or fixed
intermittent
functional
What does permanent or fixed block mean for conduction?
All impulses fail to propagate
What does intermittent block mean for conduction?
Ability to conduct is variable and changes over time
What does functional block mean for conduction?
Block may be present only at critical rates which are faster than tissue refractory period will accommodat
Block in the _____ or _____ can result in interruption of atrioventricular conduction
AV Node or His Bundle
What are the classifications of the 3 degrees of AV block?
1st Degree: Delay without failure of conduction
2nd Degree: Some but not all beats fail to conduct
3rd Degree: No propagation from Atrium to Ventricle (Also called Complete Heart Block)
What does block in the right or left bundle branches do?
does not interrupt AV conduction but results in abnormal sequence of ventricular activation reflected in “Bundle Branch Block” patterns on ECG
What is electrical reentry?
represents abnormal endless loop myocardial propagation and is the the primary mechanism of many important pathologic tachycardias
What kind of tissue does reentry occur in?
myocardial tissue composed of many myocytes working in sequence
What prevents the wavefront from looping back on itself?
refractory tissue
What are the requirements for reentry to occur?
Two Distinct Paths for Propagation, Slowed Conduction in at Least One Path or Unidirectional Block: Tissue capable of conduction in one but not the opposite direction
All arrhythmias arise as a consequence of altered impulse what?
formation or conduction
The normal heart rhythm is initiated by the fastest normal pacemaker tissue in the heart because of what?
“Gradient of Automaticity”
Abnormal slowing of the sinus node may allow secondary pacemakers to emerge where?
AV Node or Infranodal His-Purkinje System
Working myocytes are not capable of automaticity but may exhibit _______ or _______ under pathologic conditions which results in ectopic (abnormally located) beats and tachycardia.
Abnormal Automaticity or Triggered Activity
Failure of conduction may lead to bradycardia by causing _______ or
tachycardia by the mechanism of ________
heart block
unidirectional block and Reentry
The requirements for reentry are what?
- Two distinct pathways for conduction
- An area of slowed conduction
- Functional or Permanent Unidirectional Block
