Page 194 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

The Electrocardiogram (ECG)
The ECG is a recording of potential differences An ECG tracing (→ B and p.183 C) has waves,
(in mV) that are generated by the excitation intervals, and segments (deflection upward +,
within the heart. It can provide information downward –). The P wave (normally < 0.25 mV,
about the position of the heart and its rate < 0.1 s) records depolarization of the two atria.
and rhythm as well as the origin and spread of Their repolarization is not visible, because it is
the action potential, but not about the contrac- submerged in the following deflections. The Q
tion and pumping action of the heart. wave (mV < ⁄4 of R), the R and S waves (R + S
1
The ECG potentials originate at the border > 0.6 mV) are together called the QRS complex
between excited and nonexcited parts of the (< 0.1 s), even when one of the components is
myocardium. Nonexcitedorcompletelyexcited missing. It records the depolarization of the
(i.e., depolarized) myocardium does not pro- ventricles; the T wave records their repolariza-
tion. Although the two processes are opposites,
duce any potentials which are visible in the
Heart and Circulation front through the myocardium, manifold po- that of the QRS complex (usually + in most
the T wave is normally in the same direction as
ECG. During the propagation of the excitation
leads), i.e., the sequence of the spread of excita-
tentials occur, differing in size and direction.
tion and of repolarization differs: the APs in the
These vectors can be represented by arrows,
initially excited fibers (near the endocardium)
their length representing the magnitude of the
last longer than those excited last (near the
potential, their direction indicating the direc-
individual vectors, added together, become a
ized atria) and the ST segment (fully depolar-
ized ventricles) are approximately at the zero
summated or integral vector (→ A, red arrow).
7 tion of the potential (arrow head: +). The many epicardium). The PQ segment (fully depolar-
This changes in size and direction during exci- mV level (isoelectric line). The PQ interval
tation of the heart, i.e., the arrow head of the (< 0.2 s; → B) is also called (atrioventricular)
summated vector describes a loop-shaped transmission time. The QT interval (→ B) de-
path (→ A) that can be recorded oscillographi- pends on heart rate. It is normally 0.35–0.40
cally in the vectorcardiogram. seconds at 75 beats per minute (time taken for
The limb and precordial leads of the ECG re- ventricular depolarization and repolarization).
cord the temporal course of the summated The six frontal limb leads (standard and
vectors, projected onto the respective plane augmented) are included in the Cabrera circle
(in relation to the body) of the given lead. A (→ C3). The simultaneous summated vector
lead parallel to the summated vector shows in the frontal plane, for example, the mean
the full deflection, while one at a right angle QRS vector, can be determined by using the
to it shows none. The Einthoven (or standard Einthoven triangle or the Cabrera circle
limb) leads I, II, and III are bipolar (→ C1,2) (→ C2, red arrow). When the spread of excita-
and lie in the frontal plane. For the unipolar tion is normal, its position corresponds ap-
Goldberger (limb) leads, aVL, aVR, and aVF proximately to the anatomic longitudinal axis
(a = augmented) (→ C3), one limb electrode of the heart (electrical axis of the heart). The
(e.g., the left arm in aVL) is connected to the potential of the mean QRS vector is calculated
junction of the two other limb electrodes. (taking the positivity and negativity of the de-
These leads, too, lie in the frontal plane. The flections into account) from the height of the
unipolar precordial leads V 1 –V 6 (Wilson leads; Q, R, and S deflections. The normal positional
→ C4) lie approximately in the horizontal type of the electrical axis extends from ca.
plane (of the upright body). They mainly re- + 908 to ca. – 308 (for arrangement of degrees
cord those vectors that are directed posterior- → C3). Abnormal positional types are marked
ly. As the mean QRS vector (see below) mainly right axis deviation (> + 1208) , for example, in
points downward to the left and posteriorly, right ventricular hypertrophy, and marked left
the thoracic cage is divided into a positive and axis deviation (more negative than – 308), for
a negative half by a plane which is vertical to example, in left ventricular hypertrophy. Ex-
184 this vector. As a result, the QRS vector is usual- tensive myocardial infarcts can also change
ly negative in V 1 –V 3 , positive in V 5 –V 6 . the electrical axis.
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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