Biomedical Engineering 403

More Electrical Conduction in the Heart

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5
More Electrical Conduction;
The Electrocardiogram

Spreading Excitation

Atrial
From the SA node, excitation spreads across the two atria, passing from
cell to cell at a rate of approx 1 m/sec and initiating atrial systole.  

After passing down the atrial septum, the electrical impulse reaches the
AV node.  The impulse is delayed in the node approx .1 sec (at resting
heart rates).  The resulting delay is important because it allows the
atria sufficient time to contract before the ventricals are activated.
AN, N, NH Regions.

AV nodal cells have relative refractory periods that extend long beyond
complete repolarization time.  If atria are depolarized at very high
frequencies, not all impulses will get through because of this.  Another
protection against tetanus.  

Vagal activity hyperpolarizes cells and fibers in AV node, which  may
prolong AV conduction time or even block some impulses.

Sympathetic nerves decrease conduction time in the AV node.  NE increases
amp and slope of AV nodal APs.


Ventricular
The bundle of His is a bundle of fast-conducting muscle fibers transmits
excitation from the A/V node to the ventricular septum and on to the
Purkinge fibers (large diameter fibers that transmit very quickly because
of their large caliber, and thereby spread excitation throughout the
ventricles very quickly).

Bundle of His splits into right and left bundle branches, which proceed
down the septum. Left much thicker and breaks into anterior and posterior
divisions.  Block of either bundle branch is called bundle branch block,
or block of one division of left is called hemiblock (i.e. left anterior
hemiblock).

Purkinge Fibers--The broadest cells in the heart, large diameter
accounting for greater conduction velocity.  Conduction in Purkinge cells
can be as fast as 4 m/sec, so entire ventricular myocardium activated very
quickly.  AP of Purkinge cell looks like that of any other ventricular
myocardial cell.  

Long refractory periods
Premature Atrial Contraction--premature activation of the atria causes
premature atrial contraction,
                impulse passes through AV node
                blocked by Purkinge fibers, so not followed by ventricular
contraction

The septum and Papillary muscles are activated first, but spread to other
parts very rapid.

Reentry
Re-excitation of an area.  Caused by unidirectional block (caused by
ischemia or other injury).


Electrocardiogram

Reflects only electrical activity in the heart.  Heart may or may not be
contracting properly.

Recording Issues
                Electrode Placement--Einthoven Triangle
                Standard 12-lead ECG
                quick and dirty ECG

P-Wave          Excitation of Atria cell mass (depolarization of the SA
node involves too few cells to cause a visible          wave);
Corresponds with the upstroke of the atrial action potentials, so precedes
slightly the atrial          contraction.  Atrial contraction ocurs during
the PR interval.
PR interval     Reflects the time taken for excitation to spread over the
atria and through the conduction system to            reach the
ventricular septum.  Atrial repolarization is too slow and diffuse to
register on the ECG.
QRS Complex     Excitation of Ventricles.  A large mass of muscle is
involved, resulting in the huge spike.  Q and S            waves may or
may not be present based on the placement of electrodes.
ST segment      Plateau of the ventricular action potential.  Rapid
ejection occurs during this period
T-Wave          Ventricular Repolarization--usually positive indicating
that repolarization occurs in the opposite              direction of
depolarization.

                Sometimes a U--any wave between a T and the next P.

Arrhythmias
All cardiac tissue has the potential of becoming (temporarily or
permanently) a pacemaker for the heart and usurp the   role of the normal
pacemaker.  Such ectopic foci are responsible for most cardiac
arrhythmias.
Premature Atrial Contraction--Results from a non-nodal atrial pacemaker; P
wave may look different depending on         the location of the pacemaker
responsible.   QRS complex usually normal.
Premature Ventricular Contraction--Caused by an ectopic focus in the
ventricle.  QRS complex and T are completely       different because the
activation origninates at an unusual place on the myocardium.
Compensatory pause         because atrium and SA node are still
functioning normally.  Ventricular impulse does not conduct in
retrograde direction, and/or atrial didn't get there quickly.   Missed SA
nodal impulse did not affect ventricle        because it was still
refractory.
Heart Block--not all p-waves may be followed by QRS complexes, because a
block in the myocardial tissue keeps   some atrial impulses from passing
on the ventricles.
        1st degree heart block--prolonged P-R interval (over .2 seconds in
adult)
        2nd degree heart block--not all P waves are followed by QRS
complexes
        3rd degree heart block--complete heart block--impulses unable to
pass from atria to ventricles.  Atrial and             ventricular rhythms
are completely independent.  Because of slow ventricular rhythms,
circulation is            usually inadequate.  Artificial pacemakers often
inserted for this.

Paroxysmal tachycardia
Both may be caused by metabolic or electrolyte disturbances or ischemic
injury or cardiomyopathy.
Supraventricular tachycardia--originates in atrium or AV node.  Ectopic
focus maintains a very high rate.  QRS  complexes usually normal.  May be
caused by a circular wave of excitation traveling around a damaged area.
Vagus stimulation (i.e., valsalva maneuver, diving reflex) often will
abort such episodes.
Ventricular tachycardia--ectopic focus in the ventricles.  Repeated
bizarre QRS complexes

Fibrillation--
        Atrial Fibrillation--fairly normal but irregular QRS complexes.
No P's.  May be reverted by drugs that                 prolong the
refractory period.

        Ventricular Fibrillation--leads to loss of consciousness within a
few seconds.  Uncoordinated twitchings of             ventricular muscle
fibers pumps no blood.  Rarely reverts to normal spontaneously.  Caused by
multiple reentrant waves, which continue to propagate over the ventricles.
Each time a cell            completes its refractory period, it is
activated again.  Self sustaining.  A brief, strong electric
current leaves all cells refractory at the same time, breaking the cycle.
Then, hopefully, the SA              node will take back over as
pacemaker.

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