Physiology
Lec: 10 د. زيـد الاطرقجيCardiovascular system
Electro-cardiograph [ECG]
The electrical currents generated by cardiac muscle during depolarization and repolarization spreads into tissues around the heart and conducted through the body fluids. A small part of this electrical activity reaches the body surface where it can be detected using recording electrodes the record produced is called electrocardiograph or ECG.The atria pump blood over only very short distances (across the AV valves into the ventricles); therefore, they are very thin-walled structures with very little muscle mass. Conversely, the right ventricle must pump blood all the way through the lungs, and therefore has a thicker wall and more muscle mass. Finally, the left ventricle pumps blood out to the entire body, and thus has the thickest wall of all the chambers, three to four times thicker than the wall of the right ventricle.
One determinant of the size of an electrical complex on the ECG is how much voltage is generated by depolarization of a given portion of the heart. Thus, the QRS complex is normally larger than the P wave because depolarization of the greater muscle mass of the ventricles generates more voltage than does depolarization of the thinner walls of the atria.
Two important points when considering what an ECG represents:
1. An ECG is a recording of that part of the electrical activity induced in body fluids by the cardiac impulses that reaches the body surface not a direct recording of the actual electrical activity.
2. The ECG is a complex recording representing the overall spread of activity throughout the heart during depolarization and repolarization.
It is not a recording of a single action potential in a single cell at a single point in time. The record at any given time represents the sum of electrical activity in all the cardiac muscle cells some of which may be undergoing action potential while others may not be activated. For example immediately after the S-A node fires, the atrial cells are undergoing action potential while the ventricular cells are still at resting potential. At a later point the electrical activity will have spread to the ventricular cells while the atrial cells will be repolarizing. Therefore the overall pattern of cardiac electrical activity varies with time as the impulse passes throughout the heart.
Parts of ECG:
1. P-wave represents atrial depolarization. Because there is relatively little atrial muscle mass, only low voltages are normally produced.
P Wave Characteristics:
Duration: 0.08-0.11 sec.
Amplitude: Limb leads < 2.5 mm
Morphology: Upright in I, II, aVF , Upright or biphasic in III, aVL, V1,V2
2. QRS complex represents ventricular depolarization. The QRS is naturally the largest complex on the ECG because it corresponds to depolarization of the ventricles, with their larger muscle mass. Therefore, QRS amplitude may normally reach as high as 25mm or more (five big boxes) in large individuals, or in those with thin chest walls that actually allow the precordial electrodes to be closer to the heart.
Normal QRS duration: <0.10 sec
QRS Nomenclature
1. The first deflection of the complex is called a Q wave if it is negative.
2. The first positive deflection of the complex is called an R wave.
3. A negative deflection coming after an R wave is called an S wave.
3. T- wave represents ventricular repolarization. It is normally inscribed in the same direction as the predominant deflection of the QRS, and has less amplitude than the QRS. Abnormalities of the T wave predominantly take the form of inversion.
Direction, normally it is:
-Upright in I,II,V3-V6
-Inverted in aVR, V1
-Upright, flat or biphasic in III, aVL, aVF, V1, V2
The PR interval corresponds to the time it takes an impulse to travel from the SA node all the way down through the conduction system to the first muscle fibers stimulated in the ventricles, from the onset of atrial depolarization to the onset of ventricular depolarization. Therefore, it is measured from the beginning of the P wave to the beginning of the QRS. Normal duration : 0.12-0.20 sec
during the A-V nodal delay this delay is represented by the interval of time between the end of P-wave and the onset of QRS. This segment of the ECG is called PR segment .current is flowing through the A-V node but the magnitude is too small for the ECG electrode to detect.
The ST segment represents the time between the completion of depolarization of the ventricles and the onset of repolarization of the ventricles. It is normally isoelectric, meaning neither positive nor negative. The ST segment plays a very important role in the diagnosis of ischemic heart Disease.
when the ventricles are completely depolarized and the cardiac contractile cells are undergoing the plateau phase of their action potential before they repolarize represented by the S-T segment.
The QT interval is measured from the beginning of the QRS to the end of The T wave, and normal intervals vary with heart rate and the persons sex. The primary potential abnormality of the QT interval is prolongation, reflecting delays in ventricular repolarization.
TP interval is when the heart muscle is completely repolarized and at rest and ventricular filling is taking place after the T-wave and before the next P-wave.
Note
In normal ECG no separate wave for atrial repolarization is visible .the electrical activity associated with atrial repolarization normally occurs simultaneously with ventricular depolarization and is masked by the QRS complex.
The Limb Leads
The limb leads view the heart in a vertical plane called the frontal plane. The frontal plane can be envisioned as a giant circle superimposed on the patient's body. The limb leads view electrical forces (waves of depolarization and repolarization) moving up , down , left and right through this circle. Lead I, II, III
The three standard limb leads are defined as follows:
Lead I is created by making the left arm positive and the right arm negative.Its angle of orientation is 0. ( RA LA )
Lead II is created by making the left leg positive and the right arm negative. Its angle of orientation is 60.( RA LL)
Lead III is created by making the left leg positive and the left arm negative. Its angle of orientation is 120. ( LA LL )
The augmented limb leads
They are called augmented leads because the ECG machinery must amplify the tracings to get an adequate recording. Lead aVL, aVR, aVF
Lead aVL is created by making the left arm positive and the other limbs negative. Its angle of orientation is -30.
Lead aVR is created by making the right arm positive and the other limbs negative. Its angle of orientation is -150.
Lead aVF is created by making the left leg positive and the other limbs negative. Its angle of orientation is +90.
The Six Precordial ( Chest ) Leads
The six precordial leads, or chest leads. They are arranged across the chest in a horizontal plane. Whereas the leads of the frontal plane ( limb leads ) view electrical forces moving up and down and left and right. The precordial leads record forces moving anteriorly and posteriorly, ( Transverse or horizontal plane ).
The chest leads are located at the:
V1right sternal border, 4th interspace
V2left sternal border, 4th interspace
V3midway between V2 and V4
V4midclavicular line, 5th interspace
V5anterior axillary line, 5th interspace
V6midaxillary line, 5th interspace
Note that the right ventricle lies anteriorly and medially within the thoracic cavity, and the left ventricle lies posteriorly and laterally. Lead V1 lies directly over the right ventricle, V2 and V3 over the interventricular septum, V4 over the apex of the left ventricle, and V5 and V6 over the lateral side of the left ventricle.
Leads Look View V1,V2, V3, V4 Anterior I, aVL, V5, V6 Left lateral II, III, aVF Inferior aVR, V1 Right atrium, Right Vent.
ECG Interpretation:
1- Heart Rate 2- Rhythm 3- P-wave 4- PR interval
5- QRS interval 6- QT interval
7- ST segment 8- T wave
Heart Rate:
Regular Rhythm: Each large square= 0.20 sec. Each small square= 0.04 sec. so 1 minute= 300 large square = 1500 small square
Atrial rate=300/ number of large boxes between 2 P waves Ventricular rate= 300/ number of large boxes between 2 R waves
OR: Atrial rate=1500/ number of small boxes between 2 P waves
Ventricular rate= 1500/ number of small boxes between 2 R waves
2- Irregular or slow rhythm: Identify 6 second tracing strip ( 30 large boxes ), count the number of QSR complexes and multiply by 10 =vent rate
For Atrial rate: count the number of P waves in this 6 sec strip and multiply by 10 = atrial rate
QRS Axis (Electrical Axis of The Heart)
Is the mean direction of the action potential travelling through the ventricles during ventricular activation (depolarization).
Clinically the electrical axis of the heart is usually determined from the slandered limp leads of electrocardiogram. After recording the standard leads, one determines the maximum potential and polarity of the recording in two of the lead ( I ) , in lead one the recording is positive as shown in the figure below , and in lead III the recording is mainly positive but negative in part of the cycle. If any part of the recording is negative, this negative potential is subtracted from the positive potential to determine the net potential for that lead. After subtraction of the negative portion of the QRS wave in lead III from the positive portion , each net potential is plotted on the axes of the respective leads , with the base of the potential at the point of intersection of the axes. If the net potential is positive, it is plotted in a positive direction along the line depicting lead I. on the other hand, if this potential is negative, it is plotted in the negative direction. Also the same is done for lead III.
To determine the actual vector of the ventricle mean electrical potential, one draws perpendicular lines from the apices of the two net potential of leads I and III, respectively. The point of intersection of these two perpendicular lines represents the actual mean QRS in the ventricles.
Normal range is from 20 to 100 degree,(59 degree).
Normal ECG electrocardiograms recorded from the six standard chest leads
II
Normal ECG electrocardiograms recorded from the
three augmented unipolar limb leads
III
Normal ECG electrocardiograms recorded from the three standard electrocardiographic leads