Page 148 - 84 human physiolofy part-1

Chapter 8

discovered in 1839 by Jan Evangelista Purkinje, who gave them his name.

Pacemaker
The contractions of the heart are controlled by electrical impulses, these fire at a rate which controls the
beat of the heart. The cells that create these rhythmical impulses are called pacemaker cells, and they
directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the
body's intrinsic conduction system to produce these impulses synthetically.

Fibrillation

Fibrillation is when the heart flutters abnormally. This can be detected by an electrocardiogram
which measures the waves of excitation passing through the heart and plotting a graph of potential
difference (voltage) against time. If the heart and cardiac cycle is functioning properly the
electrocardiogram shows a regular, repeating pattern. However if there is fibrillation there will be no
apparent pattern. In a hospital the monitor would make a sound and alert the doctors to treat the
fibrillation by passing a huge current through the chest wall and shocking the heart out of its
fibrillation. This causes the cardiac muscle to stop completely for 5 seconds and when it begins to beat
again the cardiac cycle would have resumed to normal and the heart will be beating in a controlled
manner again. Fibrillation is an example of "circus movement" of impulses through the heart muscle.

Circus movement occurs when an impulse begins in one part of the heart muscle and spreads in a
circuitous pathway through the heart then returns to the originally excited muscle and "re-enters" it to
stimulate it once more. The signal never stops. A cause of circus movement is long length pathway in
which the muscle is no longer in a refractatory state when the stimulus returns to it. A "flutter" is a
circus movement in coordinated, low frequency waves that cause rapid heart rate. If the Bundle of HIS
is blocked, it will result in dissociation between the activity of the atria and that of the ventricles,
otherwise called a third degree heart block. The other cause of a third degree block would be a block of
the right, left anterior, and left posterior bundle branches. A third degree block is very serious medical
condition that will most likely require an artificial pacemaker.

The EKG

Also know as the Electrocardiogram. Cardiac electrophysiology is the science of the mechanisms,
functions, and performance of the electrical activities of specific regions of the heart. The EKG is the
recording of the heart's electrical activity as a graph. The graph can show the heart's rate and rhythm, it
can detect enlargement of the heart, decreased blood flow, or the presence of current or past heart
attacks. EKG's are inexpensive, Non-invasive, quick, and painless. Depending of the results, the
patient’s medical history, and a physical exam; further tests or a combination of medications and
lifestyle changes may be ordered.

How To Read An EKG

EKG Waveform

148 | Human Physiology