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Lesson # 6. The Heart-2. Chapter 20. Objectives:. 1- Explain the events of an action potential in cardiac muscle. 2 - Identify the electrical events associated with the electrocardiogram (ECG). 3 - Explain the events of the cardiac cycle. 4- Defining cardiac output and how it is regulated.

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slide1

Lesson # 6

The Heart-2

Chapter 20

Objectives:

1- Explain the events of an action potential in cardiac muscle.

2- Identify the electrical events associated with the electrocardiogram (ECG).

3- Explain the events of the cardiac cycle.

4- Defining cardiac output and how it is regulated.

slide2

Atrioventricular bundle or bundle of His

Right and left bundle branches

Sinoatrial node (SA node)

Atrioventricular node (AV node)

Purkinje fibers

The Conducting System

It connects electrically the atria to the ventricles.

They conduct the impulse to the Purkinje fibers.

It establishes the heart rate (pacemaker).

It delays the impulses to allow the atria to finish contracting before the ventricles start to contract.

They conduct the impulse to the lateral walls of the ventricles allowing the contraction to spread from the apex to the base.

slide3

Na+

The Sinoatrial (SA) Node

K+

It is a reduction in the membrane potential because the interior of the cell becomes less negative or more positive.

DEPOLARIZATION

0 mV

Depolarization

Depolarization is produced when gated sodium channels are open and sodium ions enter the cell.

Repolarization

It is an increase in the membrane potential because the interior of the cell becomes more negative.

HYPERPOLARIZATION

-90 mV

Hyperpolarization is produced when gated potassium channels are open and potassium ions exit the cell.

Hyperpolarization

slide4

+50

+50

+50

Action Potential

Pacemaker Potential

Resting Potential

0

0

Fast K+ outflow

0

Fast Ca+ and Na+ inflow

-60 mV

-40

-40

-40 mV

-60

-60

-60

-70

-70

-70

Slow Na+ inflow

SA node does not have a stable resting membrane potential. It starts at -60 mV.

  • It drifts upward because of a slow inflow of Na+ .
  • When it reaches a threshold of -40 mV, voltage-gated Ca2+ and Na+ channels open and a faster depolarization occurs peaking at 0 mV.
  • The K+ channels then open and K+ leaves the cell causing repolarization.

Action Potentials: They are changes in the transmembrane potential that, once initiated, affect an entire excitable membrane.

Each depolarization of the SA node sets off one heartbeat. At rest, fires every 0.8 seconds or 75 bpm.

slide5

Threshold

Prepotential

(spontaneous depolarization)

Time (sec)

Changes in the membrane potential of a pacemaker

cell in the SA node that is establishing a heart rate of

72 beats per minute. Note the presence of a

prepotential, a gradual spontaneous depolarization.

slide6

The Electrocardiogram or ECG (EKG)

  • An ECG is a composite of all action potentials of nodal and myocardial cells, detected, amplified and recorded by electrodes on arms, legs and chest
slide7

The Electrocardiogram

R

Waves and Segments

QRS complex

Depolarization of ventricles.

+1

Depolarization of atria.

T

P

Repolarization of ventricles

0

Q

S

PQ segment

ST segment

The ST segment represents the time during which the ventricles contract and eject blood.

Millivolts

100 msec

Atrial systole

Ventricular systole

slide8

The Cardiac Rhythm

Sinus rhythm:

It is the normal heartbeat triggered by the SA node

At rest, the sinus rhythm is about 70 to 80 times per minute (rates from 60 to 100 bpm).

Tachycardia:

It is a persistent, resting adult heart rate above 100 bpm.

Bradycardia:

It is a persistent, resting adult heart rate below 60 bpm.

Extra-systoles:

Extra heart beats produced in any region of spontaneous firing other than the SA node.

If the SA node is damaged, other part of the myocardium may take over the governance of the heart rhythm.

Any region of spontaneous firing other than the SA node. The most common ectopic focus is the AV node, which produces a nodal rhythm.

Ectopic focus:

Nodal rhythm:

It is the cardiac rhythm produced by the AV node. It is a slower heartbeat of 40 to 50 bpm.

If neither the SA nor AV nodes is functioning, an artificial pacemaker is required

Arrhythmia:

It is any abnormal cardiac rhythm.

Heart block:

It is the failure of any part of the of the cardiac conducting system to transmit signals.

slide9

Heart Sounds

During ventricular systole (contraction) the two AV close at the same time and produce the first sound referred aslubb.

Lubb

Dupp

Lubb

Dupp

When the ventricles relax (diastole) the two semilunar valves close at the same time and produce the second sound referred asdubb.

slide10

The Cardiac Cycle

At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes).

In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed.

Pressure

Pressure

slide11

The Cardiac Cycle

At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes).

Once pressure in the ventricles exceeds that in the arterial trunks (pulmonary and aortic), the semilunar valves open and blood flows into the pulmonary and aortic trunks. This point marks the beginning of the period of ventricular ejection.

Pressure

Pressure

slide12

At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes).

It is the period of isovolumetric relaxation, the ventricles relax and the pressure drops, but blood does not flow because all the valves are closed.

Pressure

Pressure

slide13

At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes).

Once pressure in the atria the AV valves open and blood flows into the ventricles. This point marks the beginning of the ventricular filling.

Pressure

Pressure

slide14

The Cardiac Cycle

The cardiac cycle consist of the events during a complete heart beat.

Start

Atrial systole begins:

Atrial contraction forces a small

amount of additional blood into

relaxed ventricles.

Atrial systole ends,

atrial diastole

begins

Ventricular

diastole—late:

All chambers are

relaxed.

Ventricles fill

passively.

Ventricular systole—

first phase: Ventricular

contraction pushes AV

valves closed but does

not create enough

pressure to open

semilunar valves.

Cardiac

cycle

Ventricular systole—

second phase: As

ventricular pressure rises

and exceeds pressure

in the arteries, the

semilunar valves

open and blood

is ejected.

Ventricular diastole—early:

As ventricles relax, pressure

in ventricles drops; blood

flows back against cusps of

semilunar valves and forces

them closed. Blood

flows into the relaxed atria.

slide15

At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow.

(b)

At the end of the atrial systole, each ventricle contains a maximum amount of130 mL of blood:

End-diastolic volume

(a)

1- Ventricular Filling

- Passive (70%)

(c)

- Active (30%)

In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed.

2- Isovolumetric Contraction

(d)

(e)

This point marks the beginning of the period of ventricular ejection.

4- Isovolumetric Relaxation

3- Ventricular Ejection

slide16

At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow.

(f)

(a)

Atrial Systole

A small amount of blood (30 %) is forced to the ventricles

Fist Phase:

Ventricular contraction closes the AV valves (first sound). Isometric contraction.

Second Phase:

Ventricular Systole

Pressure increases and semilunar valves open. Ventricular ejection.

Early:

Pressure decreases in the ventricles and semilunar valves close (second sound).

Ventricular Diastole and Atrial Diastole

Late:

Atria are also in diastole. Passive blood flow fills the ventricles (70%).

slide18

QRS complex

Depolarization of ventricles

Depolarization of atria

Repolarization of ventricles

T

P

PQ segment

ST segment

ATRIAL

ATRIAL

ATRIAL

DIASTOLE

VENTRICULAR

VENTRICULAR

VENTRICULAR

SISTOLE

DIASTOLE

DIASTOLE

SISTOLE

DIASTOLE

1- Ventricular Filling

2- Isovolumetric Contraction

3- Ventricular Ejection

4- Isovolumetric Relaxation

1- Ventricular Filling

Passive (70%)

Active (30%)

slide19

Second sound

Ventricular ejection

Isovolumetric relaxation

Isovolumetric contraction.

First sound

Ventricular filling

Ventricular filling

slide20

End-Diastolic Volume

Stroke Volume

End-Sistolic Volume

End-Diastolic Volume (EDV)

It is the volume of blood that each ventricle contains at the end of ventricular filling (about 130 mL).

Stroke Volume (SV)

It is the volume of blood that each ventricle ejects during ventricular ejection (about 70 - 80 mL).

End-Systolic Volume (ESV)

It is the volume of blood left behind in the ventricles after ventricular ejection.

EDV – SV =

(ESV)

Ejection fraction

It is the percentage of the end-diastolic volume (EDV) that is ejected (about 54%).

Cardiac Output (CO)

The amount of blood pumped by the left ventricle in one minute

Cardiac Output (CO) =

Stroke Volume (SV) x Heart Rate (HR)

75 bpm x 80 mL/beat = 6000 mL/min (6L/min)