Lecture 7: Heart Rhythmicity & ECG

Question 0

What are characteristics of AV node?

Answer 0

Receives signal from SA node

Slow rate of depolarization/amplitude action potential b/c of small size

Goes through penetrating bundles

Question 1

What are the characteristics of the SA node?

Answer 1

Composed of special cardiac muscle fibers

Fibers connect directly to atrial fibers

Question 2

What are penetrating bundles?

Answer 2

Fibers going through small canal in connective tissue between atrium and ventricles

Question 3

What is the time frame for signal to pass from SA node to AV node to ventricles?

Answer 3

AV node receives signal from SA node 0.03 seconds after origin

Signal delayed 0.09 seconds in AV node then passed to penetrating fibers

Signal delayed 0.04 seconds in penetrating fibers

Total=0.16 seconds

Question 4

Why does the conductance slow through the process?

Answer 4

Due to a diminished number of gap junctions along pathway resulting in increased conductive resistance

Question 5

What is the flow of action potential through the heart?

Answer 5

Base—>Apex (via midline)—>Base (via sides of ventricles)

Question 6

What is the resting membrane potential of SA node?

Answer 6

-55mV to -60mV

Question 7

Explain the process of SA nodal depolarization

Answer 7

Slow Na+/Ca++ channels open allowing leakage into cell At -40 mV

Na+/Ca++ channels are activated for 100-150 msec

When Na+/Ca++ channels close, K+ channels open

K+ remains open for a few tenths of a second

Question 8

What is the ventricular fiber action potential?

Answer 8

-85 mV to -90 mV

Question 9

What is the term for an action potential originating from the SA node?

Answer 9

Sinus rhythm

Question 10

What is the name for an action potential originating anywhere else?

Answer 10

Ectopic

Question 11

What is the parasympathetic modulation of heart rate/action potential?

Answer 11

Vagus nerve uses Ach to slow SA rhythm and decrease AV excitability

Increases permeability to K+ ions

Question 12

What is the sympathetic modulation of heart rate/action potential?

Answer 12

Norepinephrine stimulates beta-1 and increases depolarization rate by increasing permeability to Na+/Ca++

Question 13

What does the P wave represent?

Answer 13

Atrial depolarization

Question 14

What does the QRS waver represent?

Answer 14

Ventricular depolarization

Question 15

What does the T wave represent?

Answer 15

Ventricular repolarization

Question 16

What is an ECG representing?

Answer 16

Extracellular potential

Question 17

T/F: No potential is recorded when the ventricle is completely polarized or depolarized

Answer 17

True

Question 18

How much time lapses from P wave–>QRS wave and Q wave–>T wave?

Answer 18

0.35 seconds for both

Question 19

What does limb lead I attach to and which way does it read?

Answer 19

Both arms

Reads left to right

Question 20

What does limb lead II attache to and which way does it read?

Answer 20

Negative terminal on right arm, positive terminal on left leg

Reads upper right to lower left

Question 21

What does limb lead III attach to and which way is it read?

Answer 21

Negative on left arm, positive on left leg

Reads upper left to lower left

Question 22

What is Einthoven’s triangle?

Answer 22

The triangle formed by the 3 leads

Question 23

What is Einthoven’s law?

Answer 23

If any two electrical potentials of the three leads are known, the third can be determined by summing the first two

Question 24

Which way does current flow in Einthoven’s triangle?

Answer 24

From negative to positive (typically base to apex)

Question 25

What is the vector directional angle of Leads I, II, & III?

Answer 25

Lead I- 0˚

Lead II- 60˚

Lead III- 120˚

Question 26

Which portion becomes repolarized first in the atria?

Answer 26

SA node

Question 27

What is the electrical axis?

Answer 27

The direction of electrical potential (- to +) from the base of the ventricles to the apex

Question 28

What conditions cause abnormal ventricular axis deviations?

Answer 28

Change in position of heart in chest

Hypertrophy of one ventricle

Bundle branch block

Fluid in pericardium

Pulmonary emphysema

Question 29

How long is a normal QRS wave?

Answer 29

0.06-0.08 seconds

Question 30

What is the current of injury?

Answer 30

The heart remaining partially or totally depolarized all the time

Question 31

What is the current of injury caused by?

Answer 31

Mechanical trauma Infectious processes Ischemia (most common)

Question 32

What is the “J” point?

Answer 32

The point where all parts of ventricles, including injured areas, are depolarized

Reference point for analyzing current of injury

At very end of QRS wave