3.1.4. Cardiac Electrophysiology

Question 1

What are the three types of cardiac action potentials?

Answer 1

Atrial, nodal and ventricular.

Question 2

What are the phases of the 3 cardiac action potentials?

Answer 2

Ventricular and atrial have phases 0-4.

Nodal has phases 0, 3 and 4.

Question 3

Explain the phases of the atrial action potential.

Answer 3

Phase 0: rapid depolarization @ the start of the action potential; responsible ion: Na+

Phase 1: brief, partial repolarization of action potential.

Phase 2: Abbreviated in atrial action potential.

Phase 3: Repolarization that returns membrane potential to its resting level.

Phase 4: Resting potential

Question 4

Explain the phases of the ventricular action potential.

Answer 4

Phase 0: Rapid depolarization @ the start of the action potential; responsible ion: Na+

Phase 1: Brief, partial repolarization of action potential.

Phase 2: Plateau of ventricular action potential.

Phase 3: Repolarization that returns membrane potential to its resting level

Phase 4: resting potential
-non-gated, inwardly rectifying K+ channels (behaves like
voltage-gated)

Question 5

Explain the phases of the nodal action potential.

Answer 5

Phase 0: Rapid depolarization @ the start of the AP; responsible ion: Ca++

Phase 3: Repolarization that returns membrane potential to its pacemaker level.

Phase 4: Pacemaker potential.

Question 6

What are refractory periods?

Answer 6

Periods of time where initiation of AP is difficult or impossible

Question 7

What is an absolute refractory period and when does it occur?

Answer 7

It is when the initiation of new action potentials is impossible.

Last from beginning phase 0 → end phase 2.

Question 8

What is a relative refractory period?

Answer 8

Initiation of action potentials is possible with large stimuli
produce new reduced amplitude action potentials

Occur from the end of phase 2 → ~ end of phase 3.

Question 9

What is a supranormal period?

Answer 9

When the initiation of new action potentials requires less depolarization than normal.
- reduced amplitude stimuli can produce new AP during this period

Last from end of phase 3 till the end of phase 4.

Question 10

What are the parasympathetic effects on cardiac muscle?

Answer 10

Vagus nerve stimulation releases ACh which:

1. Slows the heart rate
2. Promotes hyperpolarization.
3. Decreases Na+ permeability (during pacemaker potential)
4. Decreases Ca++ permeability (during pacemaker potential)
5. Increases K+ permeability (during pacemaker potential)

Question 11

What are the sympathetic effect of cardiac muscle?

Answer 11

Release of norepinephrine changes ion permeabilities:

1. Increases Na+
2. Increases Ca++
3. Decreases K+
4. To promote depolarization

Question 12

What is an ECG lead axis?

Answer 12

The point of attachment of the negative electrode to the point of attachment of the positive electrode (negative electrode → positive electrode)

Question 13

What are leads I, II, and III that make up Einthoven’s

triangle?

Answer 13

Lead I: Right arm (-) –> Left arm (+)
Lead II: Right arm (-) –> Left leg (+)
Lead III: Left arm (-) –> Left leg (+)

Question 14

How is a cardiac strip depolarization vector is resolved onto a lead axis?

Answer 14

Resolve or project the depolarization or repolarization vector onto the lead axis

• Drop lines from the tail and tip of the depolarization or repolarization vector perpendicular to the lead axis
• Length of the projection on the lead axis is the voltage in the lead

Question 15

If the arrows of the lead axis and the depolarization vector are pointed in the same direction, the voltage is:

Answer 15

Positive

Question 16

If the arrows of the lead axis and the depolarization vector are pointed in opposite directions, the voltage is:

Answer 16

Negative

Question 17

How does the mean cardiac vector depend on the orientation and degree of polarization of individual cardiac myocytes?

Answer 17

a. Depolarization follows a pattern that starts with the atria and proceeds to the ventricles.
b. all repolarized (at rest): mean cardiac of zero.
c. all depolarized (contracting): mean cardiac vector of zero.
d. some repolarized/some depolarized: non-zero mean cardiac vector.