Page 209 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

the left side of the thorax in a nearly horizontal
Electrocardiogram (ECG)
plane (! F). When used in combination with
The ECG records potential differences (few the aforementioned leads in the frontal plane,
m/V) caused by cardiac excitation. This pro- they provide a three-dimensional view of the
vides information on heart position, relative integral vector. To make recordings with the
chamber size, heart rhythm, impulse origin/ chest leads (different electrode), the three limb
propagation and rhythm/conduction distur- leads are connected to form an indifferent elec-
bances, extent and location of myocardial trode with high resistances (5 kΩ). The chest
ischemia, changes in electrolyte concentra- leads mainly detect potential vectors directed
tions, and drug effects on the heart. However, towards the back. These vectors are hardly de-
it does not provide data on cardiac contraction tectable in the frontal plane. Since the mean
or pumping function. QRS vector (see below) is usually directed
ECG potential differences arise at the inter- downwards and towards the left back region,
Cardiovascular System lated myocardial tissue does not generate any and V 6 are positive.
the QRS vectors recorded by leads V 1–V 3 are
face between stimulated and non-stimulated
myocardium. Totally stimulated or unstimu-
usually negative, while those detected by V 5
visible potentials The migration of the exci-
Intraesophageal leads and additional leads positioned
tatory front through the heart muscle gives
in the region of the right chest (V r3–V r6) and left back
rise to numerous potentials that vary in mag-
(V 7–V 9) are useful in certain cases (! F2).
nitude and direction.
These vectors can be depicted as arrows, where the
segments, and intervals (! B and p. 195 C). By
convention, upward deflection of the waves is
8 length of the arrow represents the magnitude of the An ECG depicts electrical activity as waves,
potential and the direction of the arrow indicates the
direction of the potential (arrowhead is +). As in a defined as positive (+), and downward deflec-
force parallelogram, the integral vector (summa- tion as negative (!). The electrical activity as-
tion vector) is the sum of the numerous individual sociated with atrial depolarization is defined
vectors at that moment (! A, red arrow). as the P wave (" 0.3 mV, " 0.1 s). Repolariza-
The magnitude and direction of the integral tion of the atria normally cannot be visualized
vector change during the cardiac cycle, pro- on the ECG since it tends to be masked by the
ducing the typical vector loop seen on a vector- QRS complex. The QRS complex (" 0.1 s) con-
cardiogram. (In A, the maximum or chief vec- sists of one, two or three components: Q wave
tor is depicted by the arrow, called the “electri- (mV " /4 of R, " 0.04 s), R wave and/or S wave
1
cal axis” of the heart, see below). (R+S # 0.6 mV). The potential of the mean QRS
Limb and chest leads of the ECG make it vector is the sum of the amplitudes of the Q, R
possible to visualize the course of the integral and S waves (taking their positive and negative
vector over time, projected at the plane deter- polarities into account). The voltage of the
mined by the leads (scalar ECG). Leads parallel mean QRS vector is higher (in most leads) than
to the integral vector show full deflection (R that of the P wave because the muscle mass of
wave ! 1–2 mV), while those perpendicular to the ventricles is much larger than that of the
it show no deflection. Einthoven leads I, II, and atria. The R wave is defined as the first positive
III are bipolar limb leads positioned in the fron- deflection of the QRS complex, which means
tal plane. Lead I records potentials between the that R waves from different leads may not be
left and right arm, lead II those between the synchronous. The QRS complex represents the
right arm and left leg, and lead III those be- depolarization of the ventricles, and the T wave
tween the left arm and left leg (! C1). represents their repolarization. Although op-
Goldberger leads are unipolar augmented limb posing processes, the T wave usually points in
leads in the frontal plane. One lead (right arm, the same direction as the R wave (+ in most
aVR, left arm aVL, or left leg, aVF; ! D2) acts as leads). This means that depolarization and re-
the different electrode, while the other two polarization do not travel in the same direction
limbs are connected and serve as the indiffer- (! p. 195 C, QRS and T: vector arrows point in
196 ent (reference) electrode (! D1). Wilson leads the same direction despite reversed polarity
(V 1–V 6) are unipolar chest leads positioned on during repolarization). The PQ (or PR) segment
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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