Cardiac Electrophysiology & EKG

Pathway of blood

1. superior vena cava and inferior vena cava

2. Right atrium

3. Tricuspid valve (aka right AV)

4. Right ventricle

5. Pulmonary (semilunar) valve

6. Pulmonary trunk

7. R+L pulmonary artery

8. Lungs

9. R+L pulmonary vein

10. Left atrium

11. Bicuspid (mitral) valve

12. Left ventricle

13. Aorta

14. rest of the body

Which of the following statements is true?

a) All arteries transport oxygen rich blood.

b) The right side of the heart is the systemic circuit pump

c) Equal volumes of blood are pumped to the pulmonary and systemic circuits at any moment

d) The left side of the heart pumps blood to the lungs

c) Equal volumes of blood are pumped to the pulmonary and systemic circuits at any moment

Two kinds of myocytes

Contractile cells and pacemaker cells

Contractile cells

responsible for contraction

What kind of cells are non-contractile, spontaneously depolarize, initiate depolarization of entire heart, an do not need nervous system stimulation?

pacemaker cells

Approximately ____% of cardiac cells do not function in contraction but have specialized features that are essential for normal heart excitation.

1%

Do cardio myocytes contract as a unit or separately?

as a unit, all or none

The cardiac cells do not function in contraction constitute a network known as the __________ system of the heart and are in electrical contact with the cardiac muscle cells via ____________.

conducting; gap junctions

What does the conducting system do?

initiates heartbeat and helps spread an action potential rapidly throughout the heart

Three parts of action potential by pacemaker cells

pacemaker potential

depolarization

repolarization

Coordinated heartbeat is regulated by what?

-Presence of gap junctions

-Intrinsic cardiac conduction system

Intrinsic cardiac conduction system

-Network of non-contractile (autorhythmic) cells

-Initiate and distribute impulses to coordinate depolarization and contraction of heart

another name for non-contractile cells?

auto rhythmic cells

pacemaker potential

-K+ channels are closed, but slow Na+ channels are open

-causes interior to become more positive

depolarization

-AP begins when pacemaker potential reaches threshold

-Ca2+ channels open (around -40 mV), allowing huge influx of Ca2+

-leading to rising phase of action potential

Repolarization

-Ca2+ channels inactivate

-K+ channels open, allowing efflux of K+

-cell becomes more negative

Sequence of excitation: Cardiac pacemaker cells pass impulses in what order? What is this system called?

1. Sinoatrial node

2. Atrioventricular node

3. Atrioventricular bundle

4. Right and left bundle branches

5. Subendocardial conducting network

-Intrinsic conduction system

Cardiac pacemaker cells pass impulses across heart in ______ seconds.

0.22 seconds

What does the SA node do?

-pacemaker, generates impulses about 75/min

-Impulse spreads across atria and to AV node

Impulses pause at the ________ for ______ seconds.

AV node; 0.1

What connects the atria to the ventricles?

AV bundle

The bundle branches conduct the impulses through the _________.

inter-ventricular septum

What do the Purkinje fibers do?

depolarizes the contractile cells of both ventricles

Where is the SA node located?

in the right atrial wall near the opening of the superior vena cava

Where is the AV node located?

in inferior interatrial septum

What allows atrial contraction prior to ventricular contraction?

AV node

Only electrical connection between atria and ventricles

AV bundle

T/F: Atria and ventricles are connected via gap junctions.

false

Carry impulses toward apex of heart

Right & left bundle branches

Complete pathway through interventricular septum into apex and ventricular walls

Purkinje fibers

Purkinje fibers are more elaborate on what side of the heart?

left

an abnormal pacemaker that takes over pacing

ectopic focus

What causes ectopic focus?

defective SA node

ectopic focus of small region of heart that triggers impulse before SA node can, causing delay in next impulse

Extrasystole; premature contraction

What does premature contraction cause?

Heart has longer time to fill, so next contraction is felt as thud as larger volume of blood is being pushed out

What can cause a premature contraction?

excessive caffeine or nicotine

A defective AV node can cause what?

-heart block

-few impulses (partial block) or no impulses (total block) reach ventricles

-Ventricles beat at their own intrinsic rate which is too slow to maintain adequate circulation

Treatment for defective AV node?

artificial pacemaker

What makes up the bulk of the heart muscle and are responsible for the pumping action?

contractile muscle fibers

AP of contractile muscle fibers

-depolarization: fast voltage-gated Na+ channels open; Na+ enters cell (from -90 mV to +30 mV)

-Depolarization by Na+ also opens slow Ca2+ channels

-At +30 mV, Na+ channels close, but slow Ca2+ channels remain open, prolonging depolarization (seen as a plateau)

-After about 200 ms, slow Ca2+ channels are closed, and voltage-gated K+ channels are open

-Rapid efflux of K+ repolarizes cell to RMP

-Ca2+ is pumped both back into SR and out of cell into extracellular space

The plateau portion of the action potential in contractile cardiac muscle cells is due to:

a) an increased potassium permeability.

b) an influx of calcium ions.

c) an influx of sodium ions.

d) exit of calcium ions from the sarcoplasmic reticulum.

b) an influx of calcium ions

P wave

atrial depolarization initiated by SA node

QRS complex

ventricular depolarization and atrial repolarization

T wave

ventricular repolarization

beginning of atrial excitation to beginning of ventricular excitation

P-R interval

S-T segment

entire ventricular myocardium depolarized

beginning of ventricular depolarization through ventricular repolarization

Q-T interval

can detect electrical currents generated by heart

Electrocardiograph

graphic recording of electrical activity; Composite of all action potentials at given time; not a tracing of a single AP

Electrocardiogram (ECG or EKG)

The stimulus for the heart's rhythmic contractions comes from _________.

a) intercalated discs

b) acetylcholine

c) a neuromuscular junction

d) a pacemaker potential

d) a pacemaker potential

How many leads is typical for an ECG?

12

Electrodes for an ECG/EKG are placed at various points on body to measure _______________.

voltage differences

Problems that can be detected on ECG

-Enlarged R waves

-Elevated or depressed S-T segment

-Prolonged Q-T interval

what does enlarged R waves indicate?

enlarged ventricles

What does elevated or depressed S-T segment indicate?

cardiac ischemia

What does prolonged Q-T interval indicate? What does it increase the risk of?

a repolarization abnormality that increases risk of ventricular arrhythmias

In a normal heart, which of the following structures is responsible for setting the heart's pace?

a) Sinoatrial node

b) Atrioventricular node

c) Atrioventricular bundle

d) Purkinje fibers

a) Sinoatrial node

Damage to cells of the AV bundle would prevent the cardiac impulse from reaching this:

a. SA node

b. atria

c. AV node

d. ventricles

d. ventricles

Ventricular fibrillation

-Action potentials occur randomly throughout the ventricles

-chaotic, grossly abnormal ECG

-Seen in acute heart attack and after an electrical shock

Predict the nature of an ECG recording when the atrioventricular node becomes the pacemaker.

a) There would continue to be a normal sinus rhythm.

b) The P wave would be much larger than normal.

c) The rhythm would be slower.

d) The T wave would be much smaller than normal.

c) The rhythm would be slower.