Physiology Physiology of the of the

Cardiovascular System Cardiovascular System

The Conduction System of the The Conduction System of the HeartHeart

Modified cardiac muscle that specializes in Modified cardiac muscle that specializes in contraction contraction

There are four main structures that compose the There are four main structures that compose the conduction system of the heart conduction system of the heart Sinoatrial (SA) node Sinoatrial (SA) node Atrioventricular (AV) node Atrioventricular (AV) node AV bundle AV bundle Purkinje system Purkinje system

Sinoatrial (SA) Node Sinoatrial (SA) Node

Initiates the mechanical contraction of the heart Initiates the mechanical contraction of the heart (known as the pacemaker)(known as the pacemaker)

Located in the right atrium just below the Located in the right atrium just below the junction of the superior vena cava junction of the superior vena cava

Possesses an intrinsic rhythm. This means that Possesses an intrinsic rhythm. This means that without any stimulation by nerve impulses from without any stimulation by nerve impulses from the brain and cord, they themselves initiate the brain and cord, they themselves initiate impulses at regular intervals.impulses at regular intervals.

Atrioventricular (AV) Node Atrioventricular (AV) Node

The impulse generated in the SA node travels The impulse generated in the SA node travels swiftly throughout the muscle fibers of each swiftly throughout the muscle fibers of each atria. atria.

The stimulated atria begin to contract– they will The stimulated atria begin to contract– they will completely contract before the impulse reaches completely contract before the impulse reaches the ventricles the ventricles

The action potential enters the AV node by way The action potential enters the AV node by way of three internodal bundles of conducting fibers of three internodal bundles of conducting fibers

AV Bundle and Purkinje Fibers AV Bundle and Purkinje Fibers

The impulse slowly passes through the AV The impulse slowly passes through the AV node then speeds up as the impulse is node then speeds up as the impulse is relayed through the AV bundle (bundle of relayed through the AV bundle (bundle of His) into the ventricles His) into the ventricles

It is the right and left bundle branches and It is the right and left bundle branches and the Purkinje fibers that conduct the impule the Purkinje fibers that conduct the impule through the ventricles, causing them to through the ventricles, causing them to contract simultaneously. contract simultaneously.

Conduction System of the HeartConduction System of the Heart

Artificial Pacemakers Artificial Pacemakers

Devices that electrically Devices that electrically stimulate the heart stimulate the heart

Electrodes sewn directly Electrodes sewn directly into the epicardium or into the epicardium or directly inserted into the directly inserted into the heart chamber heart chamber

Inferior to the heart’s Inferior to the heart’s natural pacemakernatural pacemaker

Electrocardiogram (ECG)Electrocardiogram (ECG)

Impulse conduction generates tiny electrical Impulse conduction generates tiny electrical currents in the heart that spread through currents in the heart that spread through surrounding tissues to the surface of the body. surrounding tissues to the surface of the body.

An electrocardiogram (ECG OR EKG) is a An electrocardiogram (ECG OR EKG) is a graphic record of the hearts electrical activity—graphic record of the hearts electrical activity—the impules that preced the actual contractionthe impules that preced the actual contraction

Electrodes are attached to a voltmeter and to the Electrodes are attached to a voltmeter and to the limbs and chest of the subject. limbs and chest of the subject.

Theory of Theory of

ElectrocardiographyElectrocardiography A.A. At rest (baseline)At rest (baseline)B.B. Action potential reaches Action potential reaches

the first electrode the first electrode (relatively negative)(relatively negative)

C.C. Action potential reaches Action potential reaches the second electrode (pen the second electrode (pen returns to baseline)returns to baseline)

D.D. End of action potential End of action potential passes the first electrode passes the first electrode (relatively positive)(relatively positive)

E.E. Action potential passes Action potential passes second electrode (returns second electrode (returns to baseline)to baseline)

ECG WavesECG Waves

Represent the Represent the dynamic events that dynamic events that happen during the happen during the contraction of the contraction of the heart heart

Letters are arbitrary Letters are arbitrary and do not stand for and do not stand for any wordsany words

P Wave P Wave

A.A. Heart wall is Heart wall is completely relaxedcompletely relaxed—no change in —no change in electrical activity electrical activity

B.B. P wave represents P wave represents the depolarization of the depolarization of the atriathe atria

QRS ComplexQRS Complex

C.C. Atrial walls are Atrial walls are completely depolarizedcompletely depolarized—no change recorded —no change recorded

D.D. QRS Complex occurs QRS Complex occurs as atrial walls repolarize as atrial walls repolarize and ventricular walls and ventricular walls depolarize (massive depolarize (massive depolarization of depolarization of ventricles overshadows ventricles overshadows atrial repolarization)atrial repolarization)

T Wave T Wave

E. Atrial walls are E. Atrial walls are completely completely repolarized and repolarized and ventricular walls are ventricular walls are completely completely depolarizeddepolarized

F.F. T wave appears as T wave appears as the ventricular walls the ventricular walls replarizereplarize

Back to BaselineBack to Baseline

G.G. Once the ventricles Once the ventricles are completely are completely repolarized it is back repolarized it is back to the baseline ECGto the baseline ECG

Bradycardia Bradycardia

Slow heart rhythm—Slow heart rhythm—below 60 beats per below 60 beats per minute minute

Slight bradycardia is Slight bradycardia is normal during sleep and normal during sleep and in conditioned athletes in conditioned athletes when awakewhen awake

Can be caused by Can be caused by damage to the SA nodedamage to the SA node

Tachycardia Tachycardia

Very rapid heart rhythm—Very rapid heart rhythm—more than 100 beats per more than 100 beats per minute minute

Normal during and after Normal during and after exercise and during the exercise and during the stress response stress response

Can result from improper Can result from improper autonomic control of the autonomic control of the heart, blood loss, shock heart, blood loss, shock and a host of other and a host of other factorsfactors

Ventricular Fibrillation Ventricular Fibrillation

Complete disruption Complete disruption of the normal heart of the normal heart rhythmrhythm

Death may occur Death may occur within minutes if the within minutes if the heart beat is not heart beat is not corrected by corrected by defibrillation or other defibrillation or other meansmeans