Transcript The Electrocardiogram

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
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.
2.
3.
4.
Blood Volume
Strength of heart contractions
Heart rate
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
Electrophysiology cont.....
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.....
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