Electrocardiography

Question 1

Pacemaker potential

Answer 1

1) Phase 4 depolarization: membrane potential slowly rises to potential
2) Depolarization: action potential begins when threshold is reached
3) Repolarization to resting potential

Question 2

phase 4 depolarization – ionic basis

Answer 2

[membrane potential slowly rises to potential]

1) decrease in outward flux of K+, I(k) [[[slowly close K+ channels]]]
2) I(f), funny current: inward slow flux of Na+, thru nonspecific channels
3) gradual influx of Ca++ as threshold is approached I(Ca)

Question 3

Depolarization (pacemaker potential) – ionic basis

Answer 3

Opening of voltage-gated L-type Ca++ channels

[[[influx of Ca++]]]

Question 4

Repolarization (pacemaker potential) – ionic basis

Answer 4

Opening of voltage-gated K+ channels

[[[influx of K+]]]

Question 5

hierarchy of pacemakers

Answer 5

1) SA node (60-100 bpm)
2) AV node (40-50 bpm)
3) His-Purkinje Fibers (20 bpm)

Question 6

parasympathetic control of heart rate

Answer 6

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.

Question 7

sympathetic control of heart rate

Answer 7

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).

Question 8

non-pacemaker cell action potentials

Answer 8

resting potential: -90 mV

0) rapid depolarization
1) slight repolarization
2) plateau phase
3) repolarization
4) maintenance of resting potential

Question 9

phase 0 of non-pacemaker cell AP

Answer 9

Rapid influx of Na+ through voltage gated channels.

Rapid depolarization.

Question 10

phase 1 of non-pacemaker cell AP

Answer 10

Inactivation of Na+ channels.
Opening of outward K+ channel.

Starts depolarization.

Question 11

phase 2 of non-pacemaker cell AP

Answer 11

Plateau phase.
Inward flux of Ca++ (L-type voltage gated channels).
Slow outward flux of K+.

Balance each other, stay close to 0 mV.

Question 12

phase 3 of non-pacemaker cell AP

Answer 12

Repolarization.
Ca++ channels close.
Potassium efflux.
Membrane returns to resting potential.

Question 13

phase 4 of non-pacemaker cell AP

Answer 13

Small fluxes of Na+ and K+ maintain resting potential.

Question 14

Effective Refractory Period (ERP)

Answer 14

Phase 0-1 of non-pacemaker cell AP.
New AP cannot be elicited.
Allows cell to pump out blood and refill before the next beat.

Question 15

Sequence of heart activation

Answer 15

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

Question 16

P wave

Answer 16

ECG
Atrial depolarization.
Triggers contraction of atria.

Question 17

QRS complex

Answer 17

ECG
Ventricular depolarization.
Triggers contraction of ventricles.

Question 18

T wave

Answer 18

Repolarization of ventricles.

Question 19

Wiggers Diagram

Answer 19

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

Question 20

Wiggers Diagram – atrial systole

Answer 20

Triggered by P wave.
Small pressure rise in atrium squeezes blood into ventricle.
Tops off ventricle.

Question 21

Wiggers Diagram – isovolumetric contractions

Answer 21

Triggered by QRS complex.
Mitral valve closes.
Aortic valve closed.
Pressure rises rapidly.

Question 22

Wiggers Diagram – opening of aortic valve

Answer 22

Ventricular pressure exceeds aortic pressure.
Begins ejection phase.
Aortic pressure rises.

Question 23

Wiggers Diagram – reduction in ejection rate

Answer 23

Ventricle approaches end of contraction (T wave).
Outward flow declines.
Ventricular/aortic pressures begin to fall.

Question 24

Wiggers Diagram – isovolumetric relaxation

Answer 24

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.

Question 25

Wiggers Diagram – opening of mitral valve

Answer 25

Rapid filling of ventricle from blood accumulated in atrium.

Rapid filling then reduced filling from pulmonary veins.

Question 26

end diastolic volume (EDV)

Answer 26

Most full the ventricle will be for that beat.

Filled by atrial systole.

Question 27

End systolic volume (ESV)

Answer 27

Least full the ventricle will be during that beat.

Volume remaining in ventricle at end of ejection.

Question 28

stroke volume

Answer 28

SV = EDV - ESV

Question 29

S1 heart sound

Answer 29

Closure of mitral/tricuspid valves.
Sudden rise in ventricular pressure.
Turbulence of blood in ventricles.

Question 30

S2 heart sound.

Answer 30

Closure of aortic/pulmonary valves.

Vibrations of blood in the high pressure vessels.

Question 31

S3 heart sound

Answer 31

Rapid filling in some people.

Common in children

Question 32

S4 heart sound

Answer 32

Vibration of ventricular walls during atrial systole in some people.

Question 33

ejection fraction

Answer 33

ejection fraction = SV/EDV