181
Electrocardiogram
This indicates a direct dependence between the three recordings, stating that these
three measurements are only in a single two-dimensional (2-D) plane and therefore
fail to reveal the whole depolarization story. As can be seen, in order to address
this issue, one can include more electrode recordings in forming the heart vector.
For instance, including both the Winston and Goldman electrode placements will
provide a 3-D heart vector.
9.3.4 PERIODICITY OF ECG: HEART RATE
The leading ECG feature deciphering the hemodynamic phenomena is the frequency
of relaxation and contraction of the heart muscle, i.e., the pulse or heart rate. As mentioned before, a complete heart cycle is started by the atrial contraction, associated
with the pulse represented by the P wave. After the atrial contraction, the ventricular
contraction occurs, which is preceded by the QRS complex marking the systole. The
cycle ends with rest in which both atria and ventricles are in relaxed state or diastole.
This rest state then leads to atrial contraction and repeat of the cycle. The repetition
of this entire cycle makes ECG and all other heart signals periodic. As mentioned in
Part I of the book, the most informative measure in periodic signal is the frequency
of the variations. This is why the heart rate is the most indicative measure of the
cardiovascular system.
In an average person, the heart rate is approximately 75 beats per minute, which
yields a period of 0.8 s on the ECG. The various stages of the contraction are spread
out over this period in the following sequence. The atrial contraction in this case
lasts approximately 0.1 s, followed by a ventricular contraction that lasts 0.3 s, with
an end pause of 0.4 s. This means the atrial diastole lasts 0.7 s in this example, and
the ventricular diastole lasts 0.5 s.
The heart rate is sensitive to internal and external stimuli that can increase or
decrease the heart rate. Two different types of mechanisms that can affect the heart
rate can be distinguished: intrinsic and extrinsic. The intrinsic mechanisms are
due to the changes (e.g., stretching) in the S-A node, which directly alters the heart
rate. Another intrinsic effect is temperature, which can affect the heart rate both in
an upward and downward direction depending on raised or lowered temperature,
respectively. The extrinsic regulatory mechanism includes both parasympathetic
and sympathetic nervous systems. These autonomic nerve systems affect the release
of acetylcholine or noradrenaline–adrenaline that changes the heart rate. The parasympathetic and orthosympathetic nervous system affects the heart rate through the
nervi vagi. The final and most well-known mechanism in heart rate control is the
hormone adrenaline released by the adrenal glands, which increases the heart rate
for the fight-or-flight reaction.
Various deviations in the typical heart rhythm are often caused by either impulse
generation malfunctioning or conduction distortion. The activation in the atria may
be fully irregular and chaotic, producing irregular fluctuations in the baseline. As
a consequence, the ventricular rate becomes rapid and irregular, even though the
QRS contour may still look normal. This electric phenomenon is referred to as atrial
fibrillation (AF). When the electric disturbance is confined to the ventricles, the
resulting disease is referred to as ventricular arrhythmias.
Electrocardiogram
This indicates a direct dependence between the three recordings, stating that these
three measurements are only in a single two-dimensional (2-D) plane and therefore
fail to reveal the whole depolarization story. As can be seen, in order to address
this issue, one can include more electrode recordings in forming the heart vector.
For instance, including both the Winston and Goldman electrode placements will
provide a 3-D heart vector.
9.3.4 PERIODICITY OF ECG: HEART RATE
The leading ECG feature deciphering the hemodynamic phenomena is the frequency
of relaxation and contraction of the heart muscle, i.e., the pulse or heart rate. As mentioned before, a complete heart cycle is started by the atrial contraction, associated
with the pulse represented by the P wave. After the atrial contraction, the ventricular
contraction occurs, which is preceded by the QRS complex marking the systole. The
cycle ends with rest in which both atria and ventricles are in relaxed state or diastole.
This rest state then leads to atrial contraction and repeat of the cycle. The repetition
of this entire cycle makes ECG and all other heart signals periodic. As mentioned in
Part I of the book, the most informative measure in periodic signal is the frequency
of the variations. This is why the heart rate is the most indicative measure of the
cardiovascular system.
In an average person, the heart rate is approximately 75 beats per minute, which
yields a period of 0.8 s on the ECG. The various stages of the contraction are spread
out over this period in the following sequence. The atrial contraction in this case
lasts approximately 0.1 s, followed by a ventricular contraction that lasts 0.3 s, with
an end pause of 0.4 s. This means the atrial diastole lasts 0.7 s in this example, and
the ventricular diastole lasts 0.5 s.
The heart rate is sensitive to internal and external stimuli that can increase or
decrease the heart rate. Two different types of mechanisms that can affect the heart
rate can be distinguished: intrinsic and extrinsic. The intrinsic mechanisms are
due to the changes (e.g., stretching) in the S-A node, which directly alters the heart
rate. Another intrinsic effect is temperature, which can affect the heart rate both in
an upward and downward direction depending on raised or lowered temperature,
respectively. The extrinsic regulatory mechanism includes both parasympathetic
and sympathetic nervous systems. These autonomic nerve systems affect the release
of acetylcholine or noradrenaline–adrenaline that changes the heart rate. The parasympathetic and orthosympathetic nervous system affects the heart rate through the
nervi vagi. The final and most well-known mechanism in heart rate control is the
hormone adrenaline released by the adrenal glands, which increases the heart rate
for the fight-or-flight reaction.
Various deviations in the typical heart rhythm are often caused by either impulse
generation malfunctioning or conduction distortion. The activation in the atria may
be fully irregular and chaotic, producing irregular fluctuations in the baseline. As
a consequence, the ventricular rate becomes rapid and irregular, even though the
QRS contour may still look normal. This electric phenomenon is referred to as atrial
fibrillation (AF). When the electric disturbance is confined to the ventricles, the
resulting disease is referred to as ventricular arrhythmias.