Electrical Activity of the Heart

Question 1

Which are the fibers specialized for slow conduction in the heart? And what is its function?

Answer 1

• AV node

- slow the conduction ▶️ delay

Question 2

What mechanisms of the non-nodal cells contribute to the resting membrane potential of the heart?

Answer 2

• ungated potassium channels ▶️ always open

- inward K rectifying channels (IK1) ▶️ open at rest, depolarization closes, try open when begins repolarization

Question 3

Effect of stimulation of B1 receptor in phase 0 of action potential at non-nodal cells?

Answer 3

• increases the slope ▶️ ⬆️ conduction velocity

* epinephrine and norepinephrine

Question 4

What type of arrhythmia is associated with a prolonged QT?

Answer 4

Torsades de pointes (ventricular tachycardia)

*ex: congenital long QT syndrome: genetic alteration ▶️ significant Na current in phase 2 (delays repolarization) ▶️ prolonged QT
Or Na channels slow to inactivate

Question 5

What type of channel is absent in nodal cells? How do they make the phase 4 (initial but not the upstroke) of action potential?

Answer 5

• fast Na channels

- Ca current (no phases 1 nor 2) ▶️ T-type Ca channel

Question 6

What is the difference between L-type and T-type Ca channels of the non nodal and nodal cells respectively?

Answer 6

• L-type: open at -40mV

- T-type: open at -70mV (almost at the resting membrane potential ▶️ explain part of the automaticity

Question 7

What mediate the upstroke of the action potential (phase 0) in the nodal cells?

Answer 7

L-type Ca channels

Question 8

What is the effect of sympathetic stimulation on the QT interval?

Answer 8

Decrease QT interval

*Duration of QT interval has inverse relation with heart rate, sympathetic stimulation ⬆️ HR

Question 9

Causes left axis deviation in ekg

Answer 9

• left heart enlargement (left ventricular hypertrophy or dilation)
• conduction defects in left ventricle (except posterior bundle branch)
• acute MI on right side shift axis to left (unless right ventricle dilates)

Question 10

Causes of right axis deviation in ekg

Answer 10

• right ventricle enlargement (hypertrophy or dilation)
• conduction defects of right ventricle or the posterior left bundle branch
• acute MI on left side shift axis right (unless left ventricle dilates)

Question 11

What alteration in electrolytes can lead in a prolonged QT?

Answer 11

• hypocalcemia

- hypokalemia

Question 12

Effect of class I antiarrhythmic drugs

Answer 12

Blocks fast Na channels - change in phase 0

*reduces conduction velocity (ex, lidocaine)

Question 13

Effect of class III antiarrhythmic drugs

Answer 13

Block K channels

*delays repolarization (long QT interval)

Question 14

Effect of class II and IV antiarrhythmic drugs

Answer 14

• class II: Beta-blockers
• class IV: Ca channel blockers
• reduce automaticity and conduction through AV node ▶️ useful in tachyarrhythmias