Download Laboratory Exercise 13: Cardiac Physiology

Laboratory Exercise 13: Cardiac Physiology
The work of the heart is performed by cardiac muscle. It is unique in its ability to
depolarize itself (start an electric current) and initiate its own contraction. To coordinate
the electrical impulses, the heart evolved a conduction system. The conduction system’s
purpose is to ensure that the contractile cardiac muscle contracts in an orderly and
sequential manner and that the impulse reaches all parts of the cardiac muscle
simultaneously to give a forceful contraction.
The rate and force of the cardiac muscle contraction can be adjusted by influences
coming to it by the nervous and endocrine systems.
A. The Electrocardiogram
The heart generates small electrical currents at regular intervals of time. These cardiac
electrical impulses travel from the heart to the body surface and can be recorded by
electrodes placed on the skin. The amplified electrical impulses are recorded on moving
chart paper is an electrocardiogram (ECG). The patterns of the cardiac impulses through
the atria and ventricles’ walls recorded on the ECG reveal significant clinical variables in
magnitude, rhythm and direction of these electrical impulses.
Cardiac Conduction System
The sequence of electrical changes in a cardiac cycle begins when an impulse (action
potential) arises from a small collection of specialized conductile muscle cells high in the
right atrium, the sinoatrial (SA) node. Since the SA node normally initiates cardiac
rhythm, it is considered the “pacemaker” of the heart. The impulse from the SA node
first spreads through the atria to depolarize (reverse electrical charge on the cell
membrane’s surface) the atrial myocardium. This causes atrial contraction of systole.
Atrial depolarization is indicated on the ECG by the P wave.
The impulse then travels to the atrioventricular node at the top of the interventricular
septum. Here the impulse experiences a brief delay, to permit atrial depolarization and
systole to occur in advance of ventricular depolarization. Beyond the AV node, the
impulse enters a conduction system, the bundle of His. The bundle of His then descends
the interventricular septum via the left and right bundle branches. The bundle branches
end in the Purkinje fibers penetrating among the contractile cardiac muscle cells. The
Purkinje fibers carry impulses into the ventricle myocardium causing it to contract
simultaneously to give a forceful contraction.
The wave produced by ventricular depolarization is the QRS complex or interval.
Repolarization of the ventricles is recorded as the T wave. Atrial repolarization is obscured by
the QRS complex. Repolarization is a period of electrical recovery (refractory period) a time
for re-establishing the responsiveness of the cardiac muscle. As the cardiac muscle awaits the
next impulse, the myocardium briefly relaxes in diastole.
1
The heart rate, the number cardiac cycles or beats/minute, is determined by how often the SA
node produces depolarizing impulses. Under the influence of hormones and nerves the SA
node rhythm adapts to the needs of the body. Orderly timed relationships of electrical changes
are important to maintain the orderly sequence of the mechanical contraction of the chambers
to give a forceful contraction.
Variable
Duration (sec)
Amplitude (mv)
Atrial depolarization, P wave
0.08-0.10
0.2
________________________________________________________________________
Conduction time for
impulse to travel
over atria to AV node,
conduction system of
ventricles and
atrial systole, P-R interval
0.10-0.20
at base line, because
of depolarization of
the atria
P-R interval include P wave + P-R segment
_______________________________________________________________________
Depolarization of
ventricular myocardium,
atrial repolarization obscured
QRS complex or interval
End of spread of
depolarization,
ventricular systole,
beginning of ventricular
repolarization, S-T segment
0.08-0.12
1
0.08-.0.12
at base line, because
of depolarization of
the ventricles
< 0.2 mv deflection
Ventricular
repolarization, T wave
0.16-0.27
0.2-0.3
Q-T interval includes QRS complex + S-T segment
Heart rate
60-100/minute
Time for one beat 0.8 second, average heart rate, 72-75 beats/minute
2
B. Effect of Posture on Heart Rate
The heart rate of a resting individual varies with body position, some postures place more
stress on the cardiovascular system than others.
A move from a horizontal (supine) to an upright position (sitting or standing) causes an
immediate drop in the blood volume within vessels of the upper body due to gravity.
This reduction in volume causes a decrease in blood pressure that is monitored by
baroceptors within blood vessels of the neck and chest. The nervous system responds by
almost instantaneous acceleration of heart rate to restore normal blood pressure. During
this time of changing from horizontal to upright position the baroceptors are stimulated
less, send more excitatory afferent impulses to the cardiovascular center, to stimulate the
cardioacceleratory center to send sympathetic impulses to the heart, releasing epinephrine
and norepinephrine. The secretion of these hormones increases heart rate, force of
contraction and vasoconstriction to restore normal blood pressure.
C. Effect of Exercise on Heart Rate
A rise in cardiac output is required to meet the increased metabolic demands made by
muscle tissue during exercise. The increase in cardiac output is explained in part by
acceleration of the heartbeat.
D. Effect of Cigarette Smoke on Heart Rate
Nicotine decreases vagus nerve activity, increases heart rate.
E. Heart Sounds
The vibrations caused by closure of the valves of the heart are the cause of the heart
sounds.
“Lubb” sound, a low pitch sound, is the systolic sound due to the AV valves closing
during ventricular systole. It occurs at the R wave and S-T segment.
“Dupp” sound, a high pitch sound, due to semi-lunar valves closing during ventricular
diastole. It occurs at the T wave.
3