Cardiovascular Physiology

Dr. Hussein Farouk Sakr

Excitability • The ability of the cardiac muscle to respond to a stimulus, this

response is in the form of action potential which is propagated to the different muscle fibers to produce contraction.• The RMP of the cardiac muscle is -90 m.v. due to:• Selective permeability of the membrane.• Na+ -K+ pump (an electrogenic pump).

Action potential of the ventricular muscle fiber

Phase 0: - Fast channels open, Na influx

depolarization.- The channels open and close

quickly.

Action potential of the ventricular muscle fiber Phase 1: • Partial repolarization is due to closure of

Na channels and transient K efflux through special type of K channel (transient outward) and the inactivation of the Na channel.

Action potential of the ventricular muscle fiber Phase 2: - Plateau phase, at which the membrane potential

is kept constant by:a) Ca influx through the L-type Ca channels.b) K efflux through the ungated K channels

Action potential of the ventricular muscle fiber Phase 3: - Repolarization appear due to closure

of the voltage activated Ca channels and opening of the voltage gated K channels rapid K efflux (delayed rectifier).

Action potential of the ventricular muscle fiber Phase 4: - Return of the membrane potential to the resting

state.

Electrical, excitability and mechanical changes in the ventricular muscle fibers

Excitability changes

• Absolute refractory period • During this period the excitability is

completely lost.• No stimulus whatever its strength can

stimulate the cardiac muscle during this period.• Coincides with rapid depolarization,

repolarization and plateau.• With the mechanical changes coinciding

with systole.• No cardiac tetanization.

• Relative refractory period • During this period the excitability

increases gradually from 0-100%.• It coincides with the rapid

repolarization and first half of diastole.• The cardiac muscle can be stimulated

by superminimal stimulus.

Abnormal Action Potentials

Early afterdepolarizationsoccur during phase 3 and are more likely to occur when action potential durations are prolonged. Because these afterdepolarizations occur at a time when fast Na channels are still inactivated, slow inward Ca carries the depolarizing current. Another type of afterdepolarization

Abnormal Action Potentials

delayed afterdepolarization,occurs at the end of phase 3 or early in phase 4. It, too, can lead to self sustaining action potentials and tachycardia.This form of triggered activity appears to beassociated with elevations in intracellular calcium,as occurs during ischemia, digitalis toxicity,and excessive catecholamine stimulation

Conductivity • The ability of the cardiac muscle to conduct action potential from one fiber to the

other.

Conducting system of the heart

Atrial muscle fiber 0.3 m/sec.

Internodal fibers 1-4 m/sec.

AVN 0.03 -0.05 m/sec.

4-5 m/sec.

Ventricular muscle fiber 0.3 m/sec.

Conduction of impulse from SAN to AVN

• The impulses arising from the SAN spread in different directions:• Through the atrial muscle fibers at rate 0.3

m/sec.• Through the anterior interatrial band at rate

1 m/sec.• Through the anterior, middle and

posterior internodal fibers at rate 1-4 m/sec.

Conduction of the impulse through the AVN(AVN delay)

• When the impulse reaches the AVN, there is delay in impulse conduction.• The duration of delay is 0.16 sec.• The velocity of conduction though he AVN is

0.03-0.05 m/sec.Causes of AVN delay:

• 1- Decreased number of gap junction between the muscle fibers.• 2-Small size of the cardiac muscle fibers in contrast

with the atrial muscle fibers.• 3-The less negative resting membrane potential

slow depolarization.

• Significance of the AVN delay:• 1-Allows sufficient time for atrial contraction

before ventricular contraction sufficient ventricular filling.• 2-It protects the ventricle from high atrial

rhythm by producing partial block as in atrial flutter and atrial fibrillation.

Transmission in the Purkinje system

• The ventricular muscle fibers is depolarized from the end of Purkinje fibers and the impulse is conducted from the inner surface to the outer surface.• The conduction velocity is 4-5 met/sec.• The wave of excitation passes spirally along

the direction of the ventricular muscle fibers.