Electrocardiogram (ECG)
Electrocardiogram: Waves and Segments
Electrocardiogram shows the summed electrical activity generated by all cells of the heart.
It is an extracellular recording that represents the sum of multiple action potentials taking place in many
heart muscle cells.
The amplitudes of action potential and ECG recordings are very different. A ventricular action potential
has a voltage change of 110 mV, but the ECG signal has an amplitude of only 1 mV by the time it reaches
the surface of the body.
Einthoven Triangle: A hypothetical triangle created around the heart when electrodes are placed on both arms
and the left leg. The sides of the triangle are numbered to correspond with the three leads, or pairs of electrodes.
An ECG is recorded from one lead at a time. One electrode acts as the positive electrode of a lead, and a
second electrode acts as the negative electrode of the lead.
When an electrical wave moving through the heart is directed toward the positive electrode, the ECG
wave goes up from the baseline.
If net charge movement through the heart is toward the negative electrode, the wave points downward.
Waves of the ECG
There are two major components of the ECG: waves and segments.
Waves are the parts of the trace that go above or below the baseline.
Segments are sections of baseline between two waves.
Intervals are combinations of waves and segments.
Three major waves:
P Wave: Corresponds to the depolarization of the atria.
QRS Complex: Represents the progressive wave of ventricular depolarization. Q wave is sometimes
absent on normal ECGs. Atrial repolarization is incorporated into the QRS complex.
The first deflection is downward, and is called the Q-wave.
An upward deflection is called an R-wave whether it is preceded by a Q-wave or not.
Any deflection below the baseline following an R-wave is called an S-wave whether there has been
a Q-wave or not.
T Wave: Represents the repolarization of the ventricles.
During ventricular contraction, the last cells to depolarize are depolarized only for a short period
of time. Also, these are the cells that are first to repolarize. Therefore, T-wave occurs in the reverse
direction to what might be expected.
The P-wave is much smaller than the QRS complex because the atria have a much smaller mass of muscle
cells and generate less total electricity.
, There are three periods in the cardiac cycle when no current is flowing in the heart musculature. At these
periods, the heart is either fully depolarized or fully repolarized.
PR Segment: It corresponds to the delay in time between the end of the P-wave and the onset of the QRS
complex. During this period, current is flowing through the AV node, but it is too small to be detected.
ST Segment: It corresponds to the period when the entire ventricles are completely depolarized, i.e. the
plateau phase of the AP, and all the electrodes are detecting the same potential. As a result, no difference
is recorded. ST segment coincides with ventricular contraction following completion of the ventricular
activation represented by the QRS complex.
TP Segment: It corresponds to the period between the repolarization of the ventricles and next
depolarization of the atria. During the TP segment, the atria and ventricles are relaxed and filling.
Electrocardiogram: Recording and Electrode Placement
Nowadays, a 12-lead ECG is the standard for clinical use. It is recorded using various combinations of the
three limb electrodes plus another six electrodes placed on the chest and trunk.
Cardiac Cycle: Electrical Events
The cardiac cycle begins with both atria and ventricles at rest.
Atrial contraction starts during the latter part of the P-wave and continues during the P-R segment. During
the P-R segment, the electrical signal is slowing down as it passes through the AV node and AV bundle.
Ventricular contraction begins just after Q-wave and continues through the T-wave. The ventricles are
repolarizing during the T-wave, which is followed by ventricular relaxation.
During the T-P segment the hear is electrically quiet.
, Heart Structure and Cardiovascular Physiology
The cardiovascular system is composed of the heart, the blood, vessels, and the cells and plasma of the blood.
Structure of Heart
The heart is a muscular organ, about the size of a fist. It lies in the center of the thoracic cavity. The pointed
apex of the heart angles down to the left side of the body, while the broader base lies just behind the sternum.
The heart is encased in a tough membranous sac, the pericardium.
The pericardium is a fibroserous membrane composed of two layers: (a.) fibrous pericardium, and (ii.)
serous pericardium.
Fibrous Pericardium: It is a tough external layer and serves to: (i.) protect and anchors the heart; and
(ii.) prevents overfilling.
Serous Pericardium: It contain two layers mainly composed of mesothelium. The parietal layer of
serous pericardium lines the internal surface of the fibrous pericardium, while the visceral layer of
serous pericardium is in contact with the external surface of the heart.
Pericardial Cavity: It is the space between the parietal and visceral layers of serous pericardium and
contains a thin layer of clear pericardial fluid (15-30 mL) which lubricates the external surface of the
heart, thus enabling it to beat in a frictionless environment.
The heart is divided by a central wall, or septum, into left and right halves. Each half functions as an
independent pump that consists of an atrium and a ventricle.
The septum prevents the mixing of blood from one side of the heart with the other.
The atrium receives blood returning to the heart from blood vessels, and the ventricle pumps blood out
into the blood vessels.
The right side of the heart receives blood from the tissues and sends it to the lungs for oxygenation.
The left side of the heart receives newly oxygenated blood from the lungs and pumps it throughout the body.
Blood Flow in the Cardiovascular System: Pulmonary Circulation
The right atrium receives de-oxygenated blood from the peripheral tissues through the inferior and
superior vena cava; and from the heart through coronary sinus. From the right atrium, blood flows
into the right ventricle of the heart.
The right ventricle pumps the blood through the pulmonary arteries to the lungs for oxygenation.
From the lungs, blood travels to the left side of the heart through the pulmonary veins.
The blood vessels that go from the right ventricle to the lungs and back to the left atrium are known
collectively as the pulmonary circulation.
Blood Flow in the Cardiovascular System: Systemic Circulation
The left atrium receives blood through the pulmonary veins after beings oxygenated in the lungs. The
received blood is then passed into the left ventricle.
, The left ventricle pumps the blood into aorta, the largest artery within the human body, which
branches into a series of smaller and smaller arteries that finally lead into networks of capillaries.
o Coronary Arteries: Supplies oxygenated blood to heart muscles.
o Ascending Aorta: Supplies oxygenated blood to the arms, head, and brain.
o Abdominal Aorta: Supplies oxygenated blood to the trunk, legs, and internal organs, such as liver
(hepatic artery), digestive tract, and kidneys (renal arteries).
After leaving the capillaries, blood flows into the venous side of the circulation, moving from small
veins into larger and larger veins.
The veins from the upper part of the body join to form the superior vena cava. Those from the lower
part of the body form the inferior vena cava. The two venae cavae empty into the right atrium.
The blood vessels that carry blood from the left side of the heart to the tissues and back to the right
side of the heart are known collectively as the systemic circulation.
Two sets of heart valves ensure one-way flow of blood: (a.) atrioventricular valves, between the atria and
ventricles; and (b.) semilunar valves, between the ventricles and the arteries.
Both sets of valves serves to prevent backward flow of blood.
Atrioventricular (AV) Valves
Structure
The opening between each atrium and its ventricle is guarded by an AV valve. It is formed from
thin flaps of tissue joined at the base to a connective tissue ring.
The flaps are connected to collagenous tendons, chordae tendineae, which, in turn, are connected
to papillary muscles on the ventricular side.
o The chordae tendineae prevent the valve from being pushed back into the atrium.
o Occasionally, the chordae fail, and the valve is pushed back into the atrium during ventricular
contraction, an abnormal condition known as prolapse.
o Papillary muscles provide stability for the chordae, but cannot actively open and close the AV
valves.
The valve that separates the right atrium and right ventricle has three flaps and is called the
tricuspid valve.
The valve between the left atrium and left ventricle has only two flaps and is called the bicuspid
valve. The bicuspid valve is also called the mitral valve.
Mechanism
As the ventricles relax, the AV valves between the atria and ventricles open. Blood flows by gravity
from the atria into the ventricles.
As the atria contact, the waves of depolarization move slowly through the conducting cells of the
AV node, and then down the Purkinje fibers to the apex of the heart.
Electrocardiogram: Waves and Segments
Electrocardiogram shows the summed electrical activity generated by all cells of the heart.
It is an extracellular recording that represents the sum of multiple action potentials taking place in many
heart muscle cells.
The amplitudes of action potential and ECG recordings are very different. A ventricular action potential
has a voltage change of 110 mV, but the ECG signal has an amplitude of only 1 mV by the time it reaches
the surface of the body.
Einthoven Triangle: A hypothetical triangle created around the heart when electrodes are placed on both arms
and the left leg. The sides of the triangle are numbered to correspond with the three leads, or pairs of electrodes.
An ECG is recorded from one lead at a time. One electrode acts as the positive electrode of a lead, and a
second electrode acts as the negative electrode of the lead.
When an electrical wave moving through the heart is directed toward the positive electrode, the ECG
wave goes up from the baseline.
If net charge movement through the heart is toward the negative electrode, the wave points downward.
Waves of the ECG
There are two major components of the ECG: waves and segments.
Waves are the parts of the trace that go above or below the baseline.
Segments are sections of baseline between two waves.
Intervals are combinations of waves and segments.
Three major waves:
P Wave: Corresponds to the depolarization of the atria.
QRS Complex: Represents the progressive wave of ventricular depolarization. Q wave is sometimes
absent on normal ECGs. Atrial repolarization is incorporated into the QRS complex.
The first deflection is downward, and is called the Q-wave.
An upward deflection is called an R-wave whether it is preceded by a Q-wave or not.
Any deflection below the baseline following an R-wave is called an S-wave whether there has been
a Q-wave or not.
T Wave: Represents the repolarization of the ventricles.
During ventricular contraction, the last cells to depolarize are depolarized only for a short period
of time. Also, these are the cells that are first to repolarize. Therefore, T-wave occurs in the reverse
direction to what might be expected.
The P-wave is much smaller than the QRS complex because the atria have a much smaller mass of muscle
cells and generate less total electricity.
, There are three periods in the cardiac cycle when no current is flowing in the heart musculature. At these
periods, the heart is either fully depolarized or fully repolarized.
PR Segment: It corresponds to the delay in time between the end of the P-wave and the onset of the QRS
complex. During this period, current is flowing through the AV node, but it is too small to be detected.
ST Segment: It corresponds to the period when the entire ventricles are completely depolarized, i.e. the
plateau phase of the AP, and all the electrodes are detecting the same potential. As a result, no difference
is recorded. ST segment coincides with ventricular contraction following completion of the ventricular
activation represented by the QRS complex.
TP Segment: It corresponds to the period between the repolarization of the ventricles and next
depolarization of the atria. During the TP segment, the atria and ventricles are relaxed and filling.
Electrocardiogram: Recording and Electrode Placement
Nowadays, a 12-lead ECG is the standard for clinical use. It is recorded using various combinations of the
three limb electrodes plus another six electrodes placed on the chest and trunk.
Cardiac Cycle: Electrical Events
The cardiac cycle begins with both atria and ventricles at rest.
Atrial contraction starts during the latter part of the P-wave and continues during the P-R segment. During
the P-R segment, the electrical signal is slowing down as it passes through the AV node and AV bundle.
Ventricular contraction begins just after Q-wave and continues through the T-wave. The ventricles are
repolarizing during the T-wave, which is followed by ventricular relaxation.
During the T-P segment the hear is electrically quiet.
, Heart Structure and Cardiovascular Physiology
The cardiovascular system is composed of the heart, the blood, vessels, and the cells and plasma of the blood.
Structure of Heart
The heart is a muscular organ, about the size of a fist. It lies in the center of the thoracic cavity. The pointed
apex of the heart angles down to the left side of the body, while the broader base lies just behind the sternum.
The heart is encased in a tough membranous sac, the pericardium.
The pericardium is a fibroserous membrane composed of two layers: (a.) fibrous pericardium, and (ii.)
serous pericardium.
Fibrous Pericardium: It is a tough external layer and serves to: (i.) protect and anchors the heart; and
(ii.) prevents overfilling.
Serous Pericardium: It contain two layers mainly composed of mesothelium. The parietal layer of
serous pericardium lines the internal surface of the fibrous pericardium, while the visceral layer of
serous pericardium is in contact with the external surface of the heart.
Pericardial Cavity: It is the space between the parietal and visceral layers of serous pericardium and
contains a thin layer of clear pericardial fluid (15-30 mL) which lubricates the external surface of the
heart, thus enabling it to beat in a frictionless environment.
The heart is divided by a central wall, or septum, into left and right halves. Each half functions as an
independent pump that consists of an atrium and a ventricle.
The septum prevents the mixing of blood from one side of the heart with the other.
The atrium receives blood returning to the heart from blood vessels, and the ventricle pumps blood out
into the blood vessels.
The right side of the heart receives blood from the tissues and sends it to the lungs for oxygenation.
The left side of the heart receives newly oxygenated blood from the lungs and pumps it throughout the body.
Blood Flow in the Cardiovascular System: Pulmonary Circulation
The right atrium receives de-oxygenated blood from the peripheral tissues through the inferior and
superior vena cava; and from the heart through coronary sinus. From the right atrium, blood flows
into the right ventricle of the heart.
The right ventricle pumps the blood through the pulmonary arteries to the lungs for oxygenation.
From the lungs, blood travels to the left side of the heart through the pulmonary veins.
The blood vessels that go from the right ventricle to the lungs and back to the left atrium are known
collectively as the pulmonary circulation.
Blood Flow in the Cardiovascular System: Systemic Circulation
The left atrium receives blood through the pulmonary veins after beings oxygenated in the lungs. The
received blood is then passed into the left ventricle.
, The left ventricle pumps the blood into aorta, the largest artery within the human body, which
branches into a series of smaller and smaller arteries that finally lead into networks of capillaries.
o Coronary Arteries: Supplies oxygenated blood to heart muscles.
o Ascending Aorta: Supplies oxygenated blood to the arms, head, and brain.
o Abdominal Aorta: Supplies oxygenated blood to the trunk, legs, and internal organs, such as liver
(hepatic artery), digestive tract, and kidneys (renal arteries).
After leaving the capillaries, blood flows into the venous side of the circulation, moving from small
veins into larger and larger veins.
The veins from the upper part of the body join to form the superior vena cava. Those from the lower
part of the body form the inferior vena cava. The two venae cavae empty into the right atrium.
The blood vessels that carry blood from the left side of the heart to the tissues and back to the right
side of the heart are known collectively as the systemic circulation.
Two sets of heart valves ensure one-way flow of blood: (a.) atrioventricular valves, between the atria and
ventricles; and (b.) semilunar valves, between the ventricles and the arteries.
Both sets of valves serves to prevent backward flow of blood.
Atrioventricular (AV) Valves
Structure
The opening between each atrium and its ventricle is guarded by an AV valve. It is formed from
thin flaps of tissue joined at the base to a connective tissue ring.
The flaps are connected to collagenous tendons, chordae tendineae, which, in turn, are connected
to papillary muscles on the ventricular side.
o The chordae tendineae prevent the valve from being pushed back into the atrium.
o Occasionally, the chordae fail, and the valve is pushed back into the atrium during ventricular
contraction, an abnormal condition known as prolapse.
o Papillary muscles provide stability for the chordae, but cannot actively open and close the AV
valves.
The valve that separates the right atrium and right ventricle has three flaps and is called the
tricuspid valve.
The valve between the left atrium and left ventricle has only two flaps and is called the bicuspid
valve. The bicuspid valve is also called the mitral valve.
Mechanism
As the ventricles relax, the AV valves between the atria and ventricles open. Blood flows by gravity
from the atria into the ventricles.
As the atria contact, the waves of depolarization move slowly through the conducting cells of the
AV node, and then down the Purkinje fibers to the apex of the heart.
