Biomedical Engineering Reference
In-Depth Information
heart as possible. The drawback is that the exact electrode
positions become critical, and it is difficult to obtain the
same positions next time by another person or clinic. Six
unipolar pick-up electrodes are positioned on the skin
around the heart in a transverse plane. The lead vector of
each of the six electrodes has the direction of the line from
the heart center to the unipolar electrode in the trans-
versal plane. The ''reference'' electrode system may be
simply the RAelectrode alone (CR lead). Amore common
''reference'' is obtained (V lead) by summing the voltage
from all three extremities, this is the Wilson reference.
The cardiac cycle in the normal human heart is initi-
ated by the sinoatrial (SA) node. The excitation wave
spreads through the atria at a velocity of about 1 m/s. The
electroanatomy of the heart separates the atria and
ventricles (two chamber heart) so that the excitatory
wave can be delayed before it reaches the ventricles. This
delay of about 0.15 s is performed in the zone of the
atrioventricular (AV) node, and permits a filling of the
ventricles before their contraction. Beyond the AV node
the excitation spreads rapidly (2-4 m/s) in the networks
of His and Purkinje. These networks consist of special-
ized muscular (not nerve) tissue. From these networks
the excitation wave spreads in the myocard at a much
lower speed (0.3 m/s). The myocard cells communicate
by channels that connect the intracellular electrolytes
directly. Each muscle cell is therefore triggered by its
neighbor myocard cell but guided by the Purkinje net-
work. Myocard lacks direct nerve control, although the
SA node is under nerve control. The intracellular volume
is like one volume of cytoplasm with many nuclei (
syn-
cytium
), and electrical models are based on the concept
of the
bidomain,
the extracellular and intracellular do-
mains. The tissue mass of the network is much smaller
than the mass of the myocard and the electrical signal
from myocard dominates the surface ECG.
Important clinical use of ECG data is in the study of
beat-to-beat differences, both waveform and repetition
rate (variation in the beat-to-beat Q-T interval is called
dispersion
), arrhythmias, blocks, detection of ischemia
and infarcted muscle volumes and their positions.
Other unipolar leads
Invasive electrodes
During open heart surgery a net of 11 sterile electrodes are
used for epicardialmapping. They have direct contactwith
the surface of the heart, and the spread of signal on the
heart surface is examined by sampling at millisecond in-
tervals in the systole. The signal amplitude is large, and the
discriminative power also. The position of infarcted re-
gions is revealed by abnormal epicardial potential spread.
Another invasive ECG recording is with intracardial
catheter electrodes (e.g. for
His
3
-
bundle
transmission).
Both unipolar and bipolar leads are used, and the catheter
is always advanced into the right atrium via the venous
vessels. From there it is further advanced through the
tricuspidal valve into the ventricle. Being so near to
the source, only small position changes have a large in-
fluence on the recorded waveform. This is acceptable
because usually the time intervals are of greatest interest.
Since the electrodes are very near or on the myocard, the
recorded signals have several millivolt of amplitude and
with a frequency content up to above 500 Hz.
4.1.1.5 Vector cardiography
Figure 4.1-5
actually shows a 2D vector cardiogram in the
frontal
plane. 3D vector cardiography according to Frank
is based on the heart vector in a 3D Cartesian diagram.
Five strategically positioned skin surface electrodes
define the heart vector in the
transversal
plan, and two
additional electrodes on the head and LL take care of the
vertical vector component. The heart vector is calculated
from the recorded lead voltages and projected into the
three body planes. Three loci curves of the vector tip in
the three planes are the basis for the doctor's description.
In principle the 12 channel registration is reduced to
three, even so this 3D data set contains more information
than the 2D data set from lead I, II and III. However, the
problem for vector cardiography is that it is very difficult
to throw overboard the long tradition of interpreting
curves obtained with the old standardized electrode po-
sitions. From long experience a clinical information bank
has been assembled giving the relation between wave-
form and diagnosis. The reason for these relationships
4.1.1.3 Standard 12 lead ECG
The 12 lead ECG clinical diagnostic test is composed of
the six limb leads (see
Fig. 4.1-3
) and the six unipolar
precordial leads.
4.1.1.4 Cardiac electrophysiology
The heart is a large muscle group (myocard) driven by
a single firing unit followed by a special network for
obtaining the optimal muscle squeeze and blood accel-
eration. It is a very important organ and a very well-
defined signal source; therefore all sorts of recording
electrodes have been taken into use: from invasive intrac-
ardiac catheter electrodes for local His-bundle recording,
to multiple sterile electrodes placed directly on the heart
surface during open heart surgery (epicardial mapping).
3
Wilhelm His Jr. (1863-1934), Swiss physician.
