Cardiac Cycle and Conduction System of the Heart

The Cardiac Cycle

The mechanical events that take place in one heartbeat.

The Cardiac Cycle !  Contraction of a chamber - systole !  Relaxation of a chamber - diastole !  1 full contraction and relaxation - cardiac

cycle

Physiology of Contraction (Atrial diastole)

!  AV valves are open, Semilunar closed !  Blood draining into atria !  Blood draining into ventricles (gravity/

pressure gradient) !  SA node fires ending atrial diastole !  AV valves open further. Atria push

remaining blood to ventricles then go into diastole

Cont...... !  Increased pressure in ventricles

!  Pressure depends on volume and chamber size

!  Isometric contraction of ventricles (why?) !  Semi lunar valves open AV valves close

due to back pressure !  Ventricles contract and force blood out

Cont...... !  Ventricles relax - decreased pressure causes

semilunar valves to close !  cycle repeats !  intra-atrial pressures !  intra-ventricular pressure

Systole and Diastole !  Peak pressure of a chamber or vessel !  Stroke volume

!  amount of blood ejected by the heart (LV) in one contraction

•  aprox. 70 cc (60-100 cc)

!  Relaxation of a chamber !  Pressure at its lowest

Cardiac Output !  Cardiac Output

!  amount of blood ejected every minute •  aprox. 5-6L •  SV X HR = CO •  The strength of contraction is directly related to

venous return

Starling’s Law !   The more a muscle is stretched the harder it

will contract (up to a certain point) !  Similar to a spring !  Muscle fibers will stretch and contract

harder when there is a increased blood return

Blood Pressure !  The pressure exerted by blood against the

walls of a given artery.

Factors Affecting Blood Pressure 1.  Blood Volume 2.  Strength of heart contractions 3.  Heart rate 4.  Blood viscosity – thickness of blood

Electrophysiology !  Before the heart can contract it must be

stimulated. !  This is accomplished through a specialized

network of cells called the conduction system.

!  There are numerous “pacemaker” cells to stimulate the heart to contract.

Sinoatrial Node (SA) !  Located in high RA !  Main pacemaker of heart !  Sends out regular impulses at 60- 100 times

per minute !  Regulated by the Autonomic Nervous

System (vagus nerve)

Atrioventricular Node (AV) !  Located on floor of TA next to septum and

behind CS os !  Has 2 main jobs

!  Delay the impulse from the SA node long enough so the ventricles can fill with blood

!  Protect the ventricles from rapid atrial rates !  Inherent rate of 40-70 bpm

Bundle of His (HB) !  A fibrous ring around the 4 valves !  Only muscular connection between the atria

and ventricles !  AV node and HB collectively are called the

AV junction !  The AV junction can pace the heart at rate

of 40-60 bpm

Bundle Branches !  HB branches into rt.... And lt.. Branches !  Lt.. Bundle branch (LBB) then divides into

anterior and posterior sections

Purkinje Fibers !  Terminal portion of the conduction system !  Located in ventricular muscle !  Can pace the heart at 20-40 bpm

Electrical Properties of Cardiac Muscle

!  Automaticity !  Property by which a cell can reach threshold potential

(depolarize) and send an impulse without help from an external source

!  Excitability !  Ability of cell to reach threshold potential in response

to an outside stimulus !  Conductivity

!  The propagation of an impulse form cell to cell through intercalated disks

!  Contractility !  Ability of a muscle to shorten (contract) when

stimulated

Electrophysiology cont.....

Electrophysiology Cont..... !  The end result of contraction of the heart is

electrochemical in nature. !  The impulse is sent along the conduction

system because of changes in cellular polarity.

Cardiac Emergencies !  A thrombosis or

embolism may plug the arteries causing a myocardial infarction or “heart attack”

!  Angina pectoris is used to describe the severe chest pain

infarct