In an adult with a healthy heart, the heart rate is usually about 72 beats per
minute.
The excitatory and electrical conduction system of the heart is responsible for
the contraction and relaxation of the heart muscle. The sinoatrial node (SA
node) is the pacemaker where the electrical impulse is generated. This node is
located along the posterior wall of the right atrium right beneath the opening of
the superior vena cava. It is crescent shaped and about 3 mm wide and 1 cm
long.
The impulse travels from the SA node through the internodal pathways to the
atrioventricular node (AV node). The AV node is responsible for conduction of
the impulse from the atria to the ventricles. The impulse is delayed slightly at
this point to allow complete emptying of the atria before the ventricles
contract.The impulse continues through the AV bundle and down the left and
right bundle branches of the Purkinje fibers. The Purkinje fibers conduct the
impulse to all parts of the ventricles, causing contraction (Guyton, 1982).
Abnormal heart rhythms occur for several reasons.
, 1. The vagal stimulation of the parasympathetic nervous system can cause
a decrease in the rate at the SA node and can also decrease the
excitability of the AV junction fibers. This causes a slowing of the heart
rate, and in severe cases a complete blockage of the impulse through the
AV junction.
2. Sympathetic stimulation also effects cardiac rhythm and conduction. It
increases the rate at the SA node and increases the rate of conduction
and excitability throughout the heart. It also increases the force of
myocardial contraction. Subsequently, the overall workload on the heart
is increased.
3. A small area of the heart can become more excitable than normal, which
causes abnormal heart beats called ectopy. Ectopic foci are usually
caused by an irritable area in the heart. This irritability can be caused by
ischemia, stimulants such as nicotine and caffeine, lack of sleep or
anxiety (Guyton, 1982).
Instant Feedback:
Parasympathetic stimulation of the heart can:
A. Increase the heart rate
B. Increase contractility of the heart
C. Decrease the heart rate
Please visit this "Cardiac Care" website for educational information
about the heart geared towards patients/clients.
How to Read an EKG Strip
EKG paper is a grid where time is measured along the horizontal axis.
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