T3 C

The Electrocardiogram (ECG)

The ECG is a recording of potential differences (in mV) that are generated by the excitation within the heart. It can provide information about the position of the heart and its rate and rhythm as well as the origin and spread of the action potential, but not about the contraction and pumping action of the heart.

The ECG potentials originate at the border between excited and nonexcited parts of the myocardium. Nonexcited or completely excited (i.e., depolarized) myocardium does not produce any potentials which are visible in the ECG. During the propagation of the excitation front through the myocardium, manifold potentials occur, differing in size and direction. These vectors can be represented by arrows, their length representing the magnitude of the potential, their direction indicating the direction of the potential (arrow head: +). The many individual vectors, added together, become a summated or integral vector (^ A, red arrow). This changes in size and direction during excitation of the heart, i.e., the arrow head of the summated vector describes a loop-shaped path (^ A) that can be recorded oscillographi-cally in the vectorcardiogram.

The limb and precordial leads of the ECG record the temporal course of the summated vectors, projected onto the respective plane (in relation to the body) of the given lead. A lead parallel to the summated vector shows the full deflection, while one at a right angle to it shows none. The Einthoven (or standard limb) leads I, II, and III are bipolar (^C1,2) and lie in the frontal plane. For the unipolar Goldberger (limb) leads, aVL, aVR, and aVF (a = augmented) (^C3), one limb electrode (e.g., the left arm in aVL) is connected to the junction of the two other limb electrodes. These leads, too, lie in the frontal plane. The unipolar precordial leads V—V6 (Wilson leads; ^ C4) lie approximately in the horizontal plane (of the upright body). They mainly record those vectors that are directed posteriorly. As the mean QRS vector (see below) mainly points downward to the left and posteriorly, the thoracic cage is divided into a positive and a negative half by a plane which is vertical to this vector. As a result, the QRS vector is usually negative in V,-V3, positive in V5-V6.

An ECG tracing (^ B and p. 183 C) has waves, intervals, and segments (deflection upward +, downward -). The P wave (normally < 0.25 mV, < 0.1 s) records depolarization of the two atria. Their repolarization is not visible, because it is submerged in the following deflections. The Q wave (mV<J'4ofR), the R and S waves (R + S >0.6 mV) are together called the QRS complex (< 0.1 s), even when one of the components is missing. It records the depolarization of the ventricles; the Twave records their repolarization. Although the two processes are opposites, the T wave is normally in the same direction as that of the QRS complex (usually + in most leads), i.e., the sequence of the spread of excitation and of repolarization differs: theAPs in the initially excited fibers (near the endocardium) last longer than those excited last (near the epicardium). The PQ segment (fully depolarized atria) and the ST segment (fully depolarized ventricles) are approximately at the zero mV level (isoelectric line). The PQ interval (<0.2s; ^ B) is also called (atrioventricular) transmission time. The QT interval (^ B) depends on heart rate. It is normally 0.35-0.40 seconds at 75 beats per minute (time taken for ventricular depolarization and repolarization).

The six frontal limb leads (standard and augmented) are included in the Cabrera circle (^ C3). The simultaneous summated vector in the frontal plane, for example, the mean QRS vector, can be determined by using the Einthoven triangle or the Cabrera circle (^C2, red arrow). When the spread of excitation is normal, its position corresponds approximately to the anatomic longitudinal axis of the heart (electrical axis of the heart). The potential of the mean QRS vector is calculated (taking the positivity and negativity of the deflections into account) from the height of the Q, R, and S deflections. The normal positional type of the electrical axis extends from ca. + 90° to ca. - 30° (for arrangement of degrees ^ C3). Abnormal positional types are marked right axis deviation (> + 120°), for example, in right ventricular hypertrophy, and marked left axis deviation (more negative than - 30°), for example, in left ventricular hypertrophy. Extensive myocardial infarcts can also change the electrical axis.

Horizontal

(after Antoni)

Horizontal

(after Antoni)

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