Transcript File

Conduction in the Human Heart
How a Heart Beats
• Heart has specific cells that do not
contract that specialize in forming and
distributing electrical impulses
throughout the heart
– Called Intrinsic Cardiac Conduction System
(individual cells called autorhythmic cells)
– Causes regulated depolarization and
contraction
Sequence of Firing
• There are 4 places autorhythmic cells reside in
the heart
– Sinoatrial (SA) Node – place of original electrical
stimulus. Called the pacemaker, generates impulses
75 times every minute. Found in the right atrium
– Atrioventricular (AV) Node – After current passes
through the SA node, it moves here. Delays impulse
to allow atria to complete contraction before
ventricles contract. Located in the septum above the
tricuspid valve.
Firing (cont.)
– Right and Left Bundle Branches –
Progresses here after AV node. Moves
pulse through the septum to the apex of
the heart
– Purkinje Fibers – Moves here after bundle
branches. Causes the rest of ventricular
muscles to contract.
Heart Readings
• We can read the electrical impulses of the
heart via an electrocardiogram or EKG. EKG’s
consist of three waves, two small waves and
a large spike…any more or less than 3 with
more or less of an amplitude and your heart
has a problem.
Waves within an EKG
• First small wave is called a P-wave…lasts .08
seconds. Results from depolarization of SA
node in atria.
– .1 seconds after P wave, atria contracts.
• Next comes the QRS complex. This is the
large spike we see. The Q is the little dip, R is
the large spike, S is the dip at the end.
– The QRS complex is the rapid depolarization of
the right and left ventricles. Large muscle mass
of the ventricles causes the QRS complex to have
a much larger amplitude than the P-wave.
Waves (cont.)
• Lastly we have the T-wave. The T-wave is
caused by ventricle repolarization.
Repolarization is slow, so it has a much lower
amplitude than the QRS.
– Apex of T-wave is heart’s absolute refractory
period
– Last half of the T-wave is heart’s relative
refractory period
– Heart cannot begin a new impulse until past the
refractory periods
Intervals
• From the beginning of the P-wave to the
beginning of the Q in the QRS complex,
we have the P-Q interval. This is the
time from beginning of atrial
contraction to beginning of ventricular
contraction (.16 seconds)
• S-T interval – The interval from the
beginning of S to the beginning of T.
Whole myocardium is depolarized
Intervals (cont.)
• Q-T interval – From the beginning of
Q to end of T (Q-T interval includes
the S-T interval). Marks time from
ventricular depolarization to
repolarization. (Something to know
for your quiz: this is the period
where the ventricles contract.)
Lasts .38 seconds
Video explaining an EKG