[2] Lecture 8:Heart rhythmicity/Norm ECG Flashcards
Conduction system of the heart
Sinus(SA) node-intermodal pathways-Atrioventricular node- AV bundle- bundle branches [l/r]
Describe the delay in heart’s electrical system:
AV node receives impulse from SA after .03 s.
Signal is delayed in AV for .09 s.
Signal is delayed in penetrating bundles for .04 s.
=.16 delay from initial SA signal to ventricular contraction
Why the delay from SA to AV?
Takes .03 s to get signal through atrial fibers that are directly connected to SA.
D/t small size of cells, low amplitude of AP, and slow rate of depolarization during excitation the signal is delayed .09 s.
What’s main cause of slow conduction along the electrical pathway of heart?
Slow conduction is d/t diminished # gap junction in pathway resulting in the resistance to conduction
Resting membrane potential and threshold of SA node
Resting: -55 to -60 mV
Threshold: -40mV
What is open @ -55 to -60 mV in SA node
Slow sodium-calcium channels-making membrane more positive.
Atrial nodal AP is slower
What happens @ -40 mV?
Sodium-calcium channels become activated
Inactivated again in 100-150 msec
What happens to SA node @ approx. 0 mV?
Repolarization by means of large potassium channels open [after Na-Ca channels close]
Differ ventricular and SA node resting potential
SA=-55 to -60 mV
Ventricular=-85 to -90 mV
Difference btw sinus rhythm and ectopic focus
AP originating in SA node=sinus
Generated anywhere else is ectopic
Vagus nerve and parasympathetic effects on HR:
Acetylcholine involves muscarinic receptors…decreases rate of rhythm of SA d.t decreased excitable AV junctional fibers.
This is done by increasing permeability of potassium ions in fiber membranes. Hyper polarization.
Vagus nerve and sympathetic effects on HR:
Noepinephrine stimulates beta-1 adrenergic receptors, increases depolarization rate.
May increase permeability of NA+/Ca++in fiber membranes.
Positive chronotropic effect
Increases depolarization rate
Hyperpolarization: - chronogenic effect
What do the P, QRS, and T waves represent on typical ECG?
P- atrial depolarization [contraction]
QRS-ventricular depolarization [contraction]
T- ventricular repolarization
- can’t see atrial repolarization; it’s behind QRS complex.
Difference between ECG recording and transmembrane potential recording
ECG measures extracellular potential; not the same as transmembrane potential: only concerned w/ what is happening across membrane interface.
When there is current flow between regions of the heart what doe the ECG look like
Deflection from 0. Current flows between different membrane potentials. Monophasic potential of ventricular muscle
What potential is recorded when ventricle is completely polarized or depolarized?
No potential.
It is only when the muscle is partially polarized/ depolarized does current flow from one part of the ventricle to another.
PR interval
0.16 s
QT interval
0.35 s
Beginning of atria contraction
Phase 0 of AP through atrial muscle
P wave
Beginning of the ventricle contraction
Phase 0 of the AP’s through the atrial muscle
QRS complex
Repolarization of ventricles occurs after end of
Represents phase 3 of repolarization of ventricle
T wave
- terminal= R arm
+ terminal= L arm
Looks at heart from R to L
Limb lead I
- terminal=R arm
+ terminal=L leg
Looks at heart from R to L
Limb lead II
