Heart (L1 and L2)

Question 1

What is 1st degree heart block?

Answer 1

abnormal prolongation in P-R interval (greater than normal 0.20 seconds) - AV nodal conduction time is too long

Question 2

What is 3rd degree heart block?

Answer 2

complete AV nodal block w/ no consistent P-R interval

Question 3

What is 2nd degree heart block?

Answer 3

some atrial impulses fail to reach ventricles; not all P waves followed by QRS complexes (=dropped beats)

Question 4

Describe the difference b/t Mobitz Type I and Type II of 2nd degree heart blocks.

Answer 4

Type I- defect of AV node; progressive increases in P-R interval until dropped beat, then cycle re-starts
Type II- defect of His-Purkinje system; fixed # of non-conducted P waves for every successfully conducted QRS (PR intervals prolonged by same amount, then dropped beat)

Question 5

What are the main autonomic neurotransmitters affecting heart rate?

Answer 5

acetylcholine (parasympathetic) and norepinephrine (sympathetic)

Question 6

Where do depolarizations in the heart start?

Answer 6

Depolarizations start at the SA node and spread through the atria. (the point: the heart is NOT stimulated by nerves, but the ANS can speed or slow heart rate)

Question 7

Explain the term “healing over.”

Answer 7

It is an increase in internal resistance resulting from a decrease in # of open gap junctions, which is caused by an increase in cytosolic calcium or hydrogen ions (decrease in pH). Neighboring cells essentially “pull in” gap junctions to prevent damage from spreading.

Question 8

What is the slow, positive increase in voltage across a SA nodal cell membranes known as?

Answer 8

pacemaker potential; this spontaneous depolarization triggers the AP (SA nodal cells do not have a true resting state)

Question 9

What is the main pacemaker in the heart?

Answer 9

SA node

Question 10

What are the two main factors that determine cardiac conduction rate?

Answer 10

1) Space (length) constant - lambda

2) Rate of rise AND amplitude of action potential

Question 11

What are the possible causes of a slurred QRS complex?

Answer 11

hyperkalemia, ischemia, ventricular tachycardia, PVC

Question 12

What are the possible causes of a notched QRS complex?

Answer 12

blockages in left and/or right bundle branches (ventricles not contracting in a coordinated manner)

Question 13

What takes up a large percentage of the volume of the cardiac myocyte?

Answer 13

mitochondria! (~40%)

Question 14

Why do Purkinje fibers need to be fast?

Answer 14

They need to active the cells of the ventricle all at once (~100 msec), which allows for coordinated ejection of blood vs. random sloshing of blood.

Question 15

Refers to failure of conduction somewhere in the cardiac conduction pathway.

Answer 15

heart block

Question 16

Does the sequence of electrical events in an EKG correspond to the action potential?

Answer 16

no!

Question 17

The sodium-potassium pump is specifically inhibited by _______.

Answer 17

digitalis

Question 18

Identify the key components of sodium-calcium exchange.

Answer 18

• Exchanges 3 sodium for 1 calcium ion
• Can move in two directions; the forward direction brings 3 sodium ions in and extrudes 1 calcium ion to maintain low intracellular [calcium]
• Driven by sodium gradient across membrane (thus, indirectly affected by Na-K pump alterations)

Question 19

Explain the process of “inward rectification” behavior associated with potassium.

Answer 19

• K+ in the heart affects its own permeability… K+ channels sense [K+], thereby increasing permeability with an increase in [K+] and decreasing permeability with a decrease in [K+]
• As potassium levels drop, K+ membrane permeability is decreased (less K+ leaking out corresponds to a less negative membrane potential)
• Essentially, this a way for the cell to conserve K+

Question 20

Why are K+ channels turned off during heart depolarization?

Answer 20

This is so that K+ ions stop fighting the upstroke of the AP, allowing the cell to remain in a depolarized state for a long enough duration. (it would be a waste of energy if K+ efflux were competing against Ca2+ influx during the plateau of the AP)