The Electrocardiogram

The body is a good conductor of electricity because tissue fluids have a high concentration of ions that move (creating a current) in response to potential differences. Potential differences generated by the heart are thus conducted to the body surface, where they can be recorded by surface electrodes placed on the skin.

The recording thus obtained is called an electrocardiogram (ECG or EKG) (fig. 13.21); the recording device is called an electrocardiograph. Note that the ECG is not a recording of action potentials, but it does result from the production and conduction of action potentials in the heart. The correlation of an action potential produced in the ventricles to the waves of the ECG is shown in figure 13.21. This figure shows that the spread of depolarization through the ventricles corresponds to the plateau phase of the action potential, and thus to contraction of the ventricles.

A pair of surface electrodes placed directly on the heart will record a repeating pattern of potential changes. As action potentials spread from the atria to the ventricles, the voltage measured between these two electrodes will vary in a way that provides a "picture" of the electrical activity of the heart. By changing the position of the ECG recording electrodes on the body surface, a more complete picture of the electrical events can be obtained.

There are two types of ECG recording electrodes, or "leads." The bipolar limb leads record the voltage between electrodes placed on the wrists and legs. These bipolar leads include lead I


■ Figure 13.22 The electrocardiograph leads. The placement of the bipolar limb leads and the exploratory electrode for the unipolar chest leads in an electrocardiogram (ECG). The numbered chest positions correspond to V| through V6, as given in table 13.9. (RA = right arm; LA = left arm; LL = left leg.)

(right arm to left arm), lead II (right arm to left leg), and lead III (left arm to left leg). The right leg is used as a ground lead. In the unipolar leads, voltage is recorded between a single "exploratory electrode" placed on the body and an electrode that is built into the electrocardiograph and maintained at zero potential (ground).

The unipolar limb leads are placed on the right arm, left arm, and left leg, and are abbreviated AVR, AVL, and AVF, respectively. The unipolar chest leads are labeled 1 through 6, starting from the midline position (fig. 13.22). There are thus a total of twelve standard ECG leads that "view" the changing pattern of the heart's electrical activity from different perspectives (table 13.9). This is important because certain abnormalities are best seen with particular leads and may not be visible at all with other leads.

Each cardiac cycle produces three distinct ECG waves, designated P, QRS, and T. It should be noted that these waves represent changes in potential between two regions on the surface of the heart that are produced by the composite effects of action potentials in numerous myocardial cells. For example, the spread of depolarization through the atria causes a potential

Table 13.9

Name of Lead

Electrocardiograph (ECG) Leads

Placement of Electrodes

Bipolar limb leads


Right arm and left arm


Right arm and left leg


Left arm and left leg

Unipolar limb leads


Right arm


Left arm


Left leg

Unipolar chest leads


4"1 intercostal space to the right of the sternum


4"1 intercostal space to the left of the sternum


5"1 intercostal space to the left of the sternum


5"1 intercostal space in line with the middle of

the clavicle (collarbone)


5"1 intercostal space to the left of V4


5"1 intercostal space in line with the middle of

the axilla (underarm)

Heart and Circulation



(e) QRS complex: Ventricles depolarize and contract




(c) P wave: Atria depolarize and contract

(c) P wave: Atria depolarize and contract

(e) QRS complex: Ventricles depolarize and contract

(g) T wave: Ventricles repolarize and relax

Depolarization Repolarization

■ Figure 13.23 The relationship between impulse conduction in the heart and the ECG. The direction of the arrows in (e) indicates that depolarization of the ventricles occurs from the inside (endocardium) out (to the epicardium). The arrows in (g), by contrast, indicate that repolarization of the ventricles occurs in the opposite direction.

difference that is indicated by an upward deflection of the ECG line. When about half the mass of the atria is depolarized, this upward deflection reaches a maximum value because the potential difference between the depolarized and unstimulated portions of the atria is at a maximum. When the entire mass of the atria is depolarized, the ECG returns to baseline because all regions of the atria have the same polarity. The spread of atrial depolarization thus creates the P wave.

Conduction of the impulse into the ventricles similarly creates a potential difference that results in a sharp upward deflection of the ECG line, which then returns to the baseline as the entire mass of the ventricles becomes depolarized. The spread of the depolarization into the ventricles is thus represented by the QRS wave. The plateau phase of the cardiac action potential is related to the S-T segment of the ECG (see fig. 13.21). Finally, repolarization of the ventricles produces the T wave (fig. 13.23).

Correlation of the ECG with Heart Sounds

Depolarization of the ventricles, as indicated by the QRS wave, stimulates contraction by promoting the uptake of Ca2+ into the regions of the sarcomeres. The QRS wave is thus seen to occur at the beginning of systole. The rise in intraventricular pressure that results causes the AV valves to close, so that the first heart sound (Si, or lub) is produced immediately after the QRS wave (fig. 13.24).

Repolarization of the ventricles, as indicated by the T wave, occurs at the same time that the ventricles relax at the beginning of diastole. The resulting fall in intraventricular pressure causes the aortic and pulmonary semilunar valves to close, so that the second heart sound (S2, or dub) is produced shortly after the T wave begins in an electrocardiogram.

Test Yourself Before You Continue

1. Describe the electrical activity of the cells of the SA node and explain how the SA node functions as the normal pacemaker.

2. Using a line diagram, illustrate a myocardial action potential and the time course for myocardial contraction. Explain how the relationship between these two events prevents the heart from sustaining a contraction and how it normally prevents abnormal rhythms of electrical activity.

3. Draw an ECG and label the waves. Indicate the electrical events in the heart that produce these waves.

4. Draw a figure that shows the relationship between ECG waves and the heart sounds. Explain this relationship.

5. Describe the pathway of electrical conduction of the heart, starting with the SA node. How does damage to the AV node affect this conduction pathway and the ECG?

AV valves / S1 S2 Semilunar close valves close

AV valves / S1 S2 Semilunar close valves close

■ Figure 13.24 The relationship between changes in intraventricular pressure and the ECG. The QRS wave (representing depolarization of the ventricles) occurs at the beginning of systole, whereas the T wave (representing repolarization of the ventricles) occurs at the beginning of diastole.

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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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