Electrical activity of the heartElectrical activity of the heart

Interatrial pathwayRight atrium Left atrium

SA node

Internodalpathway

AV node

Purkinjefibers

Right ventricle Left ventricle

Bundleof His

Sinoatrial(SA) node

Rightatrium

Internodalpathway

Rightbranchof bundleof His Right

ventricle Purkinjefibers

Leftbranchof bundleof His

Leftatrium

Atrioventricular(AV) node

Interatrialpathway

Artificial Pacemaker • External Defibrillator

Conduction Of Cardiac Excitation Wave:-Conduction Of Cardiac Excitation Wave:-

The Sinoatrial node (SA node) is the normal Pace maker of the heart

(the heart rate is set by SA node=70 beats/minute)• In cases of damage of the SA node the next slower

pace maker will take the lead i.e. the AV node and the heart rate will be slower (about 50/minute)

• If the AV node is damaged ,and SA node is functional ,the atria will contract according to SA node rate (70/minute) and the ventricles will contract according to the purkinje rate (30/minute) this condition occur in complete heart block (complete atrio-ventricular dissociation) and it is live threatening condition unless treated by implantation of artificial pace maker set at normal SA node rate to drive the whole heart

Ectopic focus: Sometimes other area than SA node depolarize at faster

rate may arise from hyperexcited purkinje fiber or any other cardiac tissue and it causes an abnormal impulse (premature beat or extrasystole)

Cardiac Muscle can not be tetanized:Cardiac Muscle can not be tetanized:

Tetanus: this is continuous maximal contraction without relaxation due to stimulation by rapid repetitive stimuli

• Tetanus can not occur in cardiac muscle but can occur in skeletal muscle

• Tetanus is fatal if occur in cardiac muscle because the heart muscle relax to fill with blood before it contracts to eject this blood

Refractory period: Period of time during which excitable tissues can not be re -excited whatever the strength of the 2nd stimulus is

The heart has a long refractory period (250 msec) almost all the period of contraction (300msec) ,unlike the skeletal muscle which has a shorter refractory period and a 2nd stronger stimulus can produce another contraction

NB) Cause of long refractory period of cardiac muscle in inactivation of Na+ channels during the long plateau phase of action potential and remain inactive till the membrane potential repolarizes back to the resting level

ARRYTHMIA : Irregular heart beat

TACHYCARDIA : Heart beat over 100 beats per minute

BRADYCARDIA : Heart beat less than 60 beats per minute

SINOATRIAL NODE FAILURE : P waves are missing , erratic QRS complex

T wave inversion can be seen when one suffers from CVD

ST segment lies below the normal during angina

P wave elevation Pulmonary hypertension , blood pressure in the lungs is much higher than normal

Ventricullar fibrillation

Is a condition in which the hearts electrical activity becomes disordered

Collapse and sudden death will follow in minutes unless medical help is provided