Electrocardiography Flashcards
Pacemaker potential
1) Phase 4 depolarization: membrane potential slowly rises to potential
2) Depolarization: action potential begins when threshold is reached
3) Repolarization to resting potential
phase 4 depolarization – ionic basis
[membrane potential slowly rises to potential]
1) decrease in outward flux of K+, I(k)
2) I(f), funny current: inward slow flux of Na+, thru nonspecific channels
3) gradual influx of Ca++ as threshold is approached I(Ca)
Depolarization (pacemaker potential) – ionic basis
Opening of voltage-gated L-type Ca++ channels
Repolarization (pacemaker potential) – ionic basis
Opening of voltage-gated K+ channels
hierarchy of pacemakers
1) SA node (60-100 bpm)
2) AV node (40-50 bpm)
3) His-Purkinje Fibers (20 bpm)
parasympathetic control of heart rate
Vagus nerve releases ACh to SA node.
Voltage gated K+ channels close more slowly. Hyperpolarize resting potential.
Decreased funny current.
Reduce slope of phase 4 depolarization to slow heart rate.
sympathetic control of heart rate
Cardiac plexus releases norepi to SA/AV nodes and myocardium.
Increases I(f) and I(Ca) in all myocardial cells.
Alters thrshold toward more negative voltage.
Increases heart rate and increases force of contraction (positive ionotropic effect).
non-pacemaker cell action potentials
resting potential: -90 mV
0) rapid depolarization
1) slight repolarization
2) plateau phase
3) repolarization
4) maintenance of resting potential
phase 0 of non-pacemaker cell AP
Rapid influx of Na+ through voltage gated channels.
Rapid depolarization.
phase 1 of non-pacemaker cell AP
Inactivation of Na+ channels.
Opening of outward K+ channel.
Starts depolarization.
phase 2 of non-pacemaker cell AP
Plateau phase.
Inward flux of Ca++ (L-type voltage gated channels).
Slow outward flux of K+.
Balance each other, stay close to 0 mV.
phase 3 of non-pacemaker cell AP
Repolarization.
Ca++ channels close.
Potassium efflux.
Membrane returns to resting potential.
phase 4 of non-pacemaker cell AP
Small fluxes of Na+ and K+ maintain resting potential.
Effective Refractory Period (ERP)
Phase 0-3 of non-pacemaker cell AP.
New AP cannot be elicited.
Allows cell to pump out blood and refill before the next beat.
Sequence of heart activation
1) SA node fires AP
2) Signal travels thru atrial muscle via gap junctions
3) Signal reaches AV node (tiny fibers –> slow velocity –> delays signal)
4) Signal enters bundle of His, then R/L bundle branches, then Purkinje fibers
5) Signal spreads cell to cell via gap junctions
septum –> apex –> base
endocardium –> epicardium
P wave
ECG
Atrial depolarization.
Triggers contraction of atria.
QRS complex
ECG
Ventricular depolarization.
Triggers contraction of ventricles.
T wave
Repolarization of ventricles.
Wiggers Diagram
Pressure tracings.
1) Atrial systole
2) Isovolumetric contraction
3) Opening of Aortic valve
4) Reduction in ejection rate
5) Isovolumetric relaxation
6) Opening of mitral valve
Wiggers Diagram – atrial systole
Triggered by P wave.
Small pressure rise in atrium squeezes blood into ventricle.
Tops off ventricle.
Wiggers Diagram – isovolumetric contractions
Triggered by QRS complex.
Mitral valve closes.
Aortic valve closed.
Pressure rises rapidly.
Wiggers Diagram – opening of aortic valve
Ventricular pressure exceeds aortic pressure.
Begins ejection phase.
Aortic pressure rises.
Wiggers Diagram – reduction in ejection rate
Ventricle approaches end of contraction (T wave).
Outward flow declines.
Ventricular/aortic pressures begin to fall.
Wiggers Diagram – isovolumetric relaxation
Movement of blood reverses, closing valves.
Ventricular pressure falls rapidly.
Atrial pressure rises as venous inflow fills atrium.
Aortic pressure falls as ejected blood drains away from heart.
Wiggers Diagram – opening of mitral valve
Rapid filling of ventricle from blood accumulated in atrium.
Rapid filling then reduced filling from pulmonary veins.
end diastolic volume (EDV)
Most full the ventricle will be for that beat.
Filled by atrial systole.
End systolic volume (ESV)
Least full the ventricle will be during that beat.
Volume remaining in ventricle at end of ejection.
stroke volume
SV = EDV - ESV
S1 heart sound
Closure of mitral/tricuspid valves.
Sudden rise in ventricular pressure.
Turbulence of blood in ventricles.
S2 heart sound.
Closure of aortic/pulmonary valves.
Vibrations of blood in the high pressure vessels.
S3 heart sound
Rapid filling in some people.
Common in children
S4 heart sound
Vibration of ventricular walls during atrial systole in some people.
ejection fraction
ejection fraction = SV/EDV
a wave
atrial systole
atrial pressure wave
c wave
closure of mitral valve
atrial pressure wave
v wave
atrial filling anf emptying
atrial pressure wave
