Heart Physiology
Conduction System of the Heart and Electrocardiography
The Intrinsic Conduction System: contraction of the heart is the result of electrical
changes in the heart
Cardiac cells are connected by gap junctions
B/c of this the entire myocardium acts as a single unit
Ability to beat is intrinsic, does not depend on impulses
from the nervous system
However, the ANS does play a role in acceleration and
deceleration of rate
Intrinsic Conduction System or Nodal System: made up of noncontractile
tissue
The make up ensures that the heart depolarizes in a
sequential manner
Sinoatrial (SA) Node: found in the right atrium, inferior to the vena
cava entrance
Discharges at a high rate and provides main stimulus for
contraction
Sets the rate for the heart; called the pacemaker
Impulse spreads through the atrium and to the AV node
Impulse is immediately followed by atrial contraction
Atrioventricular (AV) Node: found in the lower part of the atrual
septum at the junction of the atria and ventricles
Is a delay in the impulse beginning (.1 sec); allows the atria
to finish
Damage here can totally stop the ventricles from having SA
node influence
AV Bundle (Bundle of His): found in the interventricular septum
Bundle Branches: 2 main branches off the Bundle of His
Purkinje Fibers: long strands of cells
Spread out in the walls of the ventricles
After the impulse leaves the AV node, it passes through the above
structures which causes ventricular contractions
Electrocardiography: process by which impulses from the heart can be measured
on the body’s surface
Electrocardiogram (ECG): recording of the impulses in graph form
Uses 12 standard leads
Made up of 3 waves:
P Wave: indicates depolarization of the atria right before atrial
contraction
QRS Complex: indicates ventricular depolarization
Shape is due to difference in size of the ventriculars and the
time differences present for contractions
T Wave: ventricular repolarization
What an ECG does and does not show:
1. Is a record of voltage and time, nothing else.
We infer muscle contraction is occurring because of
the impulses, but sometimes it does not and the
graph does not show that.
2. Records electrical events in large amounts of muscle
tissue, not the activity of the nodes themselves.
3. Is able to show abnormalities based on shape and
changes in time.
Stuff about intervals/Other Info.
PQ Interval: represents the time between the beginning of atrial
depolarization and ventricular depolarization
A long interval can indicate a partial or total AV heart
block
In total heart block, no impulses are transmitted by the AV
node and the atria and ventricles beat independently
of each other- the atria at the SA node rate (usually
fast) and ventricles at intrinsic rate (usually slow)
QRS Interval: around .08 seconds in length
If longer, indicates a right or left bundle branch block
where one ventricle is contracting later than the
other
QT Interval: beginning of ventricular depolarization through
repolarization, including ventricular contraction
An increase in rate decreases the interval
Tachycardia: a heart rate over 100bpm
Bradycardia: a heart rate below 60bpm
Usually a positive finding in athletes because it indicates an
increase in the efficiency of the heart
In athletes, stroke volume (the amount of blood ejected by
the heart with each contraction) increases with conditioning
which means the heart can beat slower and still meet
adequate circulation
Fibrillation: can be caused by long term tachycardia
Condition where the heart does not contract in the normal
synchronized manner
Renders the heart useless as a pump