A portion of the electrical current generated by the heart beat flows away from the heart through the surrounding tissues and reaches the body surface. Using electrodes placed on the skin, this current can be measured and used to produce a recording referred to as an electrocardiogram (ECG). An important point to remember regarding the ECG is that it represents the sum of all electrical activity throughout the heart at any given moment, not individual action potentials. Therefore, an upward deflection of the recording does not necessarily represent depolarization, nor does a downward deflection represent repolarization. Furthermore, a recording is made only when current is flowing through the heart during the actual process of depolarization or repolarization. No recording is made when the heart is completely depolarized (during the plateau phase of the ventricular action potential) or completely repolarized (between heart beats). The ECG provides information concerning:

• Relative size of heart chambers

• Various disturbances of rhythm and electrical conduction

• Extent and location of ischemic damage to the myocardium

• Effects of altered electrolyte concentrations

• Influence of certain drugs (e.g., digitalis and antiarrhythmic drugs)

The ECG provides no information concerning contractility or, in other words, the mechanical performance of the heart as a pump.

The normal ECG is composed of (Figure 13.4):

Figure 13.4 Electrocardiogram. The electrocardiogram (ECG) is a measure of the overall electrical activity of the heart. The P wave is caused by atrial depolarization, the QRS complex is caused by ventricular depolarization, and the T wave is caused by ventricular repolarization.

• P wave, caused by atrial depolarization

• QRS complex, caused by ventricular depolarization

• T wave, caused by ventricular repolarization

The ECG has several noteworthy characteristics. First, the firing of the SA node, which initiates the heart beat, precedes atrial depolarization and therefore should be apparent immediately prior to the P wave. However, due to its small size, it does not generate enough electrical activity to spread to the surface of the body and be detected by the electrodes. Therefore, there is no recording of the depolarization of the SA node.

Second, the area under the curve of the P wave is small compared to that of the QRS complex. This is related to the muscle mass of the chambers. The ventricles have significantly more muscle than the atria and therefore generate more electrical activity. Furthermore, although it may not appear to be the case given the spike-like nature of the QRS complex, areas under the QRS complex and the T wave are approximately the same. This is because these recordings represent electrical activity of the ventricles even though one is caused by depolarization and the other by repolarization. Either way, the muscle mass involved is the same.

Third, no recording takes place during the PR segment when the electrical impulse is being conducted through the AV node. As with the SA node, not enough tissue is involved to generate sufficient electrical activity to be detected by the electrodes. The length of the PR segment is determined by the duration of the AV nodal delay.

Finally, no recording occurs during the ST segment — the period between ventricular depolarization and ventricular repolarization. In other words, the ventricles are completely depolarized and the muscle cells are in the plateau phase of the action potential. As mentioned earlier, unless current is actually flowing through the myocardium, there is no recording.

Using the ECG, the heart rate may be determined by calculating the time from the beginning of one P wave to the beginning of the next P wave, or from peak to peak of the QRS complexes. A normal resting heart rate in adults is approximately 70 beats/min. A heart rate of less than 60 beats/min is referred to as bradycardia and a heart rate of more than 100 beats/min is referred to as tachycardia.

Pharmacy application: antiarrhythmic drugs

Normal cardiac contraction depends on the conduction of electrical impulses through the myocardium in a highly coordinated fashion. Any abnormality of the initiation or propagation of the impulse is referred to as an arrhythmia. These disorders are the most common clinical problem encountered by a cardiologist. There is a wide range of types of arrhythmias with multiple etiologies and a variety of symptoms. In this section, two types of cardiac tachyarrhythmias are discussed. The most common treatment for these conditions is drug therapy.

Verapamil (Class IV antiarrhythmic drug) is an effective agent for atrial or supraventricular tachycardia. A Ca++ channel blocker, it is most potent in tissues where the action potentials depend on calcium currents, including slow-response tissues such as the SA node and the AV node. The effects of verapamil include a decrease in heart rate and in conduction velocity of the electrical impulse through the AV node. The resulting increase in duration of the AV nodal delay, which is illustrated by a lengthening of the PR segment in the ECG, reduces the number of impulses permitted to penetrate to the ventricles to cause contraction.

Procainamide (Class IA antiarrhythmic drug) is an effective agent for ventricular tachycardia. Its mechanism of action involves blockade of the fast Na+ channels responsible for phase 0 in the fast response tissue of the ventricles. Therefore, its effect is most pronounced in the Purkinje fibers. The effects of this drug's activity include a decrease in excitability of myocardial cells and in conduction velocity. Therefore, a decrease in the rate of the phase 0 upstroke and a prolonged repolarization are observed. As a result, duration of the action potential and the associated refractory period is prolonged and the heart rate is reduced. These effects are illustrated by an increase in the duration of the QRS complex.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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