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Wolff-Parkinson-White (WPW) Syndrome-
Common Accessory Pathway
- -Alternate path around AV node(Bundle of Kent)
- -AP conducted directly: atria--> ventricle
- -Faster than normal AV nodal pathway
- -Ventricular depolarization is generally slower than normal
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Purkinje fibers
- -Arise from RBB and Ant + Post. LBB
- -Complext netowrk of conducting fibers spread out over subendocardial surfaces of R and L ventricles
- -Largest diameter cardiac cells
- -Fastest conduction velocity in the heart!
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In what order does repolarization occur.
- Opposite depolarization:
- Last Cells to depolarize are the first to repolarize.
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Fastest conduction
- Purkinje fibers
- (larger diameter equals less resistance
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Slowest Conduction
- AV node, SA node
- (small diameter=more resistance)
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Cardiac Muscle
- -Striated
- -mononucleated (sometimes binucleated)
- -intercalated disks(Gap Junctions)
- -Many Mitochondria
- -T-tubules and SR (CA2+ stores in ECF and SR)
- -Ca2+ regulation of contraction (binds troponin)
- -Relatively slow speed of contraction (~250 ms)
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Biomarkers of Myocardial Injury
- Troponin (cTnT, cTnI)
- -Increasingly utilized as biomarker for cardiac damage
- CK-MB
- -Creatine kinase isoform specific to cardiac muscle.
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Functional Syncytium
- -Electrical Syncytium
- -Intercalated disks
- -All or None Law for the Heart
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Electrical syncytium
All cardiac m. cells contract in a synchronous manner
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Intercalated disks
- Connect cardiac cells through mechanical junctions and electrical connections.
- -Desmosomes
- -Gap junctions
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All or None Law for the Heart
- Either all cardiac cells contract or none do
- -no variation in force production via motor unit recruitment
- Due to functional syncytium and conduction system
- Contractility (modified by altering sympathetic N.S. input) ((increase Ca2+ permeability)
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Extracellular Ca2+ in Cardiac Contraction
- -Influx of extracellular Ca2+ is required for additional Ca2+ release from the SR.
- -Ca2+ influx from ECF via voltage gated channels
- -Release of Ca2+ from the SR also required
- --Ca2+ induced ... not finished...
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Relaxation
- -Removal of Ca2+ to the ECF
- -Sequestering Ca2+ into the SR
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Removal of Ca2+ to the ECF
- -Sarcolemmal 3Na+ -1Ca2+ antiporter
- -Sarcolemmal Ca2+ pump
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Sequestering Ca2+ into the SR
- SR Ca2+ pump (SERCA)
- -Regulated by phospholamban
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Can we have tetanus in the heart?
No, it would be fatal
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Pacemaker cells
- -no resting potential
- -Spontaneous slow depolarization phase
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Non-pacemaker cells
True resting potential (~-80 to -90 mV)
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Fast Response AP
- Atrial, ventricular myocytes, and Purkinje fibers
- -Fast upstroke
- -Early, partial repolarization
- -Plateau
- -Final repolarization
- -Resting potential
- Threshold -70 mV
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Slow response AP
- SA and AV nodes
- -Gradual upstroke
- -Absent: early repolarization
- -Absent: Plateau is less prolonged and flat or absent
- -Transition from plateau to final repolarization is less distinct
- -no true resting potential
- Threshold -40mV
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4 Major Time-dependent and Voltage-gated Currents
- 1. Na+ current
- -rapid depolarization
- 2. Ca2+ current
- -rapid depolarization
- 3. K+ current
- Repolarization
- 4. Pacemaker ("funny") current
- -slow depolarization phase in SA and AV nodal cells
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Pase 0 Upstroke
Slow vs Fast
- Fast if upstroke is due to both Ina and Ica
- Slow is upstroke is only due to Ica
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Phase 1: Early, rapid (partial) repolarization
- Activation of minor K+ current
- Inactivation of Ina or Ica
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Phase 2: Plateau phase
Continued influx of Ca2+ countered by small K+ current
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Phase 3: Final Repolarization
Depends on Ik in all cells
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Phase 4: Electrical diastolic phase
- Changes in Ik, Ica, and If produce pacemaker activity in SA and AV nodal cells
- Atrial and ventricular muscle have no time-dependent currents during phase 4
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Conduction Velocity
- 1. Amplitude of action potential
- 2. Rate of change of potential during phase 0 (Slope of depolarization)
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Anatomic variables which can alter conduction velocity
- -Congenital accessory pathways
- -Degeneration of conduction system
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Ectopic Foci
- Generate action potentials that do not follow normal conduction pathways.
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Causes of Ectopic Foci
- -Local areas of ischemia
- -Mildly toxic conditions can irritate fibers of the A-V node, Purkinje system, or myocardium. (ie various drugs, nicotine, caffeine, alcohol.
- - Calcified plaques irritating adjacent cardiac fibers
- -Cardiac catheterization( mechanical initiation of premature contractions.)
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