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Electrocardiogram - EKG (ECG)
An Electrocardiogram (abbreviated as either
ECG or EKG (from German,
Elektrokardiogramm)) is a graphic produced by an
electrocardiograph, which records the
electrical current in the
heart in the form of a continuous strip graph. The ECG results
provide the following:
- Determine whether the heart is performing normally or suffering
from abnormalities (eg. extra or skipped heartbeats -
- May indicate coronary artery blockages (during or after a
- Can be used for detecting calcium, magnesium and other
- Allows the detection of conduction abnormalities (heart blocks).
- Indicates the physical shape of a patient during stress tests.
- Can provide information on the physical condition of the heart (ie:
left ventricular hypertrophy).
The EKG complex. P=P wave, PR=PR segment, QRS=QRS
complex, QT=QT interval, ST=ST segment, T=T wave.
A typical ECG tracing of a normal heartbeat consists of a P wave, a
QRS complex and a T wave. The P wave is the electrical signature of
the current that causes
atrial contraction. Both the left and right
atria contract simultaneously. The QRS complex corresponds to the
current that causes contraction of the
right ventricles, which is much more forceful than that of the
atria and involves more muscle area, thus resulting in a much greater
ECG deflection. The QRS complex contains the atrial repolarization
current. The T wave represents the repolarization of the ventricles.
Electrically, the cardiac muscle cells are like loaded springs. A
small impulse sets them off, they depolarize and contract. Setting the
spring up again is repolarization (more at
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The QT interval is measured from the beginning of the QRS complex
to the end of the T wave. The QT interval as well as the corrected QT
interval are important in the diagnosis of
long QT syndrome and
short QT syndrome. The QT interval varies based on the heart rate,
and various correction factors have been developed to correct the QT
interval for the heart rate.
The most commonly used method for correcting the QT interval for
rate is the one formulated by Bazett. 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 not
be accurate, and over-corrects at high heart rates and under-corrects
at low heart rates.
A more accurate method to correct the QT interval for the rate was
developed by Rautaharju et al., who developed the formula
This method is not widely used by clinicians.
EKG lead placement
A typical ECG report shows the cardiac cycle from 12 different
vantage points (I, II, III, aVR, aVL, aVF, V1-V6),
like viewing the event electrically from 12 different locations.
Understanding the usual and abnormal directions, or vectors, of
depolarization and repolarization yields important diagnostic
information. The directions, or vectors, are known as leads. aVR is
placed on the right arm (or shoulder), aVL is placed on the left arm
(or shoulder), and aVF is placed on the left leg (or hip). Lead I
represents activity that is going from the right arm to the left arm.
Lead II represents activity that is going from the right arm to the
left leg. Lead III represents activity that is going from the left arm
to the left leg.
Electrical vectors and how they apply to the heart
The inferior leads (leads II, III and aVF) look at electrical
activity from the vantage point of the inferior region (wall) of the
heart. The lateral leads (I, aVL, V5 and V6)
look at the electrical activity from the vantage point of the lateral
wall of the heart. The anterior leads, V1 through V6,
and represent the anterior wall of the heart. aVR is rarely used for
diagnostic information, but indicates if the ECG leads were placed
correctly on the patient.
The inferior leads record events from the apex of the left
ventricle. The lateral and anterior leads record events from the left
wall and front walls of the left ventricle, respectively. The right
ventricle has very little muscle mass. It leaves only a small imprint
on the ECG, making it more difficult to diagnose than changes in the
The leads measure the average electrical activity generated by the
summation of the action potentials of the heart at a particular moment
in time. For instance, during normal atrial
systole, the summation of the electrical activity produces an
electrical vector that is directed from the SA node towards the AV
node, and spreads from the right atrium to the left atrium (since the
SA node resides in the right atrium). This turns into the P wave on
the EKG, which is upright in II, III, and aVF (since the general
electrical activity is going towards those leads), and inverted in aVR
(since it is going away from that lead).
The EKG has become so familiar to the general population with its
depiction in various
television medical dramas that the reading of no cardiac
electrical activity (nicknamed flatline) is often used as a symbol of death or at
least extreme medical peril. This is technically known as