Qrs
However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of 'Wilson's central terminal', which is derived by adding leads I, II, and III together and plugging them into the negative terminal of the ECG machine. Wilson's central terminal paved the way for the development of the augmented limb leads aVR, aVL, aVF and the precordial leads V1, V2, V3, V4, V5, and V6.
Lead aVR or "augmented vector right" has the positive electrode (white) on the right arm.
The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm.
Lead aVL or "augmented vector left" has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm.
Lead aVF or "augmented vector foot" has the positive (red) electrode on the left leg.
The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg.
The augmented limb leads aVR, aVL, and aVF are amplified in this way because the signal is too small to be useful when the negative electrode is Wilson's central terminal. Because of their close proximity to the heart, they do not require augmentation.
Wilson's central terminal is used for the negative electrode, and these leads are considered to be unipolar (recall that Wilson's central terminal is the average of the three limb leads. This will approximate ground).The precordial leads view the heart's electrical activity in the so-called horizontal plane.
The baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave.
This turns into the P wave on the ECG, which is upright in II, III, and aVF (since the general electrical activity is going toward the positive electrode in those leads), and inverted in aVR (since it is going away from the positive electrode for that lead). In addition, because the His/Purkinje system coordinates the depolarization of the ventricles, the QRS complex tends to look "spiked" rather than rounded due to the increase in conduction velocity.
However, correct interpretation of difficult ECGs requires exact labeling of the various waves. Some authors use lowercase and capital letters, depending on the relative size of each wave.
For example, an Rs complex would be positively deflected, while a rS complex would be negatively deflected. If both complexes were labeled RS, it would be impossible to appreciate this distinction without viewing the actual ECG.
The duration, amplitude, and morphology of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
Q waves can be normal (physiological) or pathological.
It is far smaller in magnitude than the QRS and is therefore obscured by it.
Animation of a normal ECG wave.
PR/PQ interval
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the T wave.
It should be essentially level with the PR and TP segment.
The normal ST segment has a slight upward concavity.
Flat, downsloping, or depressed ST segments may indicate coronary ischemia.
ST segment elevation may indicate myocardial infarction. An elevation of >1mm and longer than 80 milliseconds following the J-point.
This measure has a false positive rate of 15-20% (which is slightly higher in women than men) and a false negative rate of 20-30%.
T wave
The T wave represents the repolarization (or recovery) of the ventricles. The last half of the T wave is referred to as the relative refractory period (or vulnerable period).
In most leads, the T wave is positive.
In addition, it is not uncommon to have an isolated negative T wave in lead III, aVL, or aVF.
Inverted (or negative) T waves can be a sign of coronary ischemia, Wellens' syndrome, left ventricular hypertrophy, or CNS disorder.
Tall or "tented" symmetrical T waves may indicate hyperkalemia. Flat T waves may indicate coronary ischemia or hypokalemia.
The earliest electrocardiographic finding of acute myocardial infarction is sometimes the hyperacute T wave, which can be distinguished from hyperkalemia by the broad base and slight asymmetry.
When a conduction abnormality (e.g., bundle branch block, paced rhythm) is present, the T wave should be deflected opposite the terminal deflection of the QRS complex.
Normal values for the QT interval are between 0.30 and 0.44 seconds. The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome. Long QT intervals may also be induced by antiarrythmic agents that block potassium channels in the cardiac myocyte.
The QT interval represents on an ECG the total time needed for the ventricles to depolarize and repolarize.
The most commonly used method for correcting the QT interval for rate is the one formulated by Bazett and published in 1920. Bazett's formula is , where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, this formula tends to be inaccurate, and over-corrects at high heart rates and under-corrects at low heart rates.
QTc may also be found via the following formula: QTc = QT + 1.75(Ventricular Rate - 60).
U wave
An electrocardiogram of an 18-year-old man showing U waves, most evident in lead V3.
The U wave is not always seen.
Two leads that look at the same anatomical area of the heart are said to be contiguous (see color coded chart).
The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior (or diaphragmatic) surface.
The lateral leads (I, aVL, V5 and V6) look at the electrical activity from the vantage point of the lateral wall of left ventricle. Because the positive electrodes for leads V5 and V6 are on the patient's chest, they are sometimes referred to as the low lateral leads.
The septal leads, V1 and V2 look at electrical activity from the vantage point of the septal wall of the ventricles.
The anterior leads, V3 and V4 look at electrical activity from the vantage point of the anterior surface of the heart.
In addition, any two precordial leads that are next to one another are considered to be contiguous.
For example, even though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another.
Lead aVR offers no specific view of the left ventricle. Rather, it views the inside of the endocardial wall to the surface of the right atrium, from its perspective on the right shoulder.
Axis
Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.
The heart's electrical axis refers to the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane.
Recent research suggests that ECG heterogeneity often precedes dangerous cardiac arrhythmias.
Background
There are over 350,000 cases of sudden cardiac death (SCD) in the United States each year, and over twenty percent of these cases involve people with no outward signs of serious heart disease.