Biomedical Engineering Reference
In-Depth Information
it is pointing top to bottom. As everything repolarizes, the dipole direction heads
back upwards and ends up going bottom to top. The ECG is recorded by measur-
ing the potential difference between any two points on the body. Generally, ECG
is recorded at a paper speed of 25 mm/s. The stretch between two points is called
a LEAD and it is modeled as a direction. The potential difference measured by the
lead depends on the size of the electric dipole (wave of electrical depolarization),
the direction of the electrodes and the distance of the electrodes from the dipole.
To reduce variation in distance during clinical evaluations, standard positions
for electrode placement have been generated. Dutch physiologist Willem Einthoven
in 1903 developed the first practical device for recording cardiac potentials, called
the string galvanometer, which became the electrocardiograph. He developed the
classical limb lead system, whereby an electrode is placed at each corner of an im-
aginary equilateral triangle, known as Einthoven's triangle, superimposed on the
front of a person with the heart at its center. The three corners represented right
arm (RA), left arm (LA), and left leg (LL). Einthoven's lead system is conventionally
found based on the assumption that the heart is located in an infinite, homogene-
ous volume conductor. The stretch between two limb (arm or leg) electrodes consti-
tutes a lead. Each side represents the three standard limb leads of the ECG [Figure
3.9(a)]: Lead I records the cardiac potentials between RA and LA, lead II records
between RA and LL and lead III records between LA and LL.
Einthoven studied the relationship between these electrodes, forming a triangle
where the heart electrically constitutes the null point. Einthoven's triangle is used
when determining the electrical axis of the heart. According to Kirchhoff's law,
these lead voltages have the following relationship: lead I + lead III = lead II. Hence,
only two are independent at any time. Using this basis,
scalar ECG
is measured us-
ing only two electrodes plus a ground (right arm, left arm, and right leg). However,
more electrodes are necessary to display the cardiac vector (referred to as
vector
ECG
)
as a function of time at particular leads (directions). In 1934, American
physiologist Frank N. Wilson developed an improved system, which amplified the
action potentials, known as the augmented (as the signal is increased) lead system
(leads 4, 5, and 6). These are referred to as aVR (right arm), aVL (left arm), and
aVF (foot) where V stands for voltage (the voltage seen at the site of the electrode).
These leads record a change in electric potential in the frontal plane. Augmented
leads [Figure 3.9(b)] are unipolar in that they measure the electric potential at one
point with respect to a null point (NP, one which does not register any significant
variation in electric potential during the contraction of the heart). This null point
is obtained for each lead by adding the potential from the other two leads. For
example, in lead aVR, the electric potential of the right arm is compared to a null
point, which is obtained by adding together the potential of lead aVL and lead
aVF. A wave traveling
towards
the positive (+) lead inscribes an upward deflection
of the EKG; conversely, a wave traveling
away
from the positive lead inscribes a
downward deflection. For example, a wave traveling from the head to the feet is
shown as an upwards deflection in aVF, since it is going towards the aVF+ lead.
Waves that are traveling at a 90° angle to a particular lead will create no deflection
and are called
isoelectric
leads.
Six electrodes record the electric potential changes in the heart in a cross-sec-
tional plane [Figure 3.9(c)]. These are called precordial leads (V1 through V6),
