Electrocardiogram PDF

Electrocardiogram ----------------- As action potentials propagate through the heart, they generate electrical currents that can be detected at the body\'s surface. An electrocardiogram (e-lek′-trō-KAR-dē-ō-gram), abbreviated either ECG or EKG (from the German word elektrokardiogram), is a recordin...

Electrocardiogram ----------------- As action potentials propagate through the heart, they generate electrical currents that can be detected at the body\'s surface. An electrocardiogram (e-lek′-trō-KAR-dē-ō-gram), abbreviated either ECG or EKG (from the German word elektrokardiogram), is a recording of these electrical signals. The ECG is a composite record of action potentials all heart muscle fibers produce during each heartbeat. In a typical ECG, three recognizable waves appear with each heartbeat (Figure 14.17). The first, the P wave, is a slight upward deflection on the ECG. The P wave represents atrial depolarization, which spreads from the SA node through contractile fibers in both atria. The second wave, called the QRS complex, begins as a downward deflection, continues as a large, upright, triangular wave, and ends as a downward wave. The QRS complex represents ventricular depolarization, as the action potential spreads through ventricular contractile fibers. The third wave is a dome-shaped upward deflection called the T wave. It indicates ventricular repolarization and occurs just as the ventricles are starting to relax. The T wave is smaller and broader than the QRS complex because repolarization occurs more slowly than depolarization. During the plateau phase of steady depolarization, the ECG tracing is flat. In reading an ECG, the size of the waves can provide clues to abnormalities. Larger P waves indicate enlargement of an atrium; an enlarged Q wave may indicate a myocardial infarction; and an enlarged R wave generally indicates enlarged ventricles. The T wave is flatter than normal when the heart muscle is receiving insufficient oxygen---for example, in coronary artery disease. The T wave may be elevated in hyperkalemia (high blood K+ level). Analysis of an ECG also involves measuring the time spans between waves, called intervals or segments. For example, the P--Q interval, also known as the P--R interval, is the time from the beginning of the P wave to the beginning of the QRS complex. It represents the conduction time from the beginning of atrial excitation to the beginning of ventricular excitation. Put another way, the P--Q interval is the time required for the action potential to travel through the atria, AV node, and the remaining fibers of the conduction system. The P- Q interval lengthens when the action potential is forced to detour around scar tissue caused by disorders such as coronary artery disease and rheumatic fever. The S--T segment, which begins at the end of the S wave and ends at the beginning of the T wave, represents the time when the ventricular contractile fibers are depolarized during the plateau phase of the action potential. The S--T segment is elevated (above the baseline) in acute myocardial infarction and depressed (below the baseline) when the heart muscle receives insufficient oxygen. The Q--T interval extends from the start of the QRS complex to the end of the T wave. It is the time from the beginning of ventricular depolarization to the end of ventricular repolarization. The Q--T interval may be lengthened by myocardial damage, myocardial ischemia (decreased blood flow), or conduction abnormalities. Correlation of ECG Waves with the Timing of Atrial and Ventricular Diastole and Systole --------------------------------------------------------------------------------------- As you have learned, the atria and ventricles depolarize and contract simultaneously because the conduction system routes cardiac action potentials along a specific pathway. The term systole (SIS-tō-lē = contraction) refers to the phase of contraction; the phase of relaxation is diastole (dī-AS-tō-lē = dilation or expansion). The ECG waves predict the timing of atrial and ventricular systole and diastole. At a heart rate of 75 beats per minute, the timing is as follows (Figure 14.18): 1. A cardiac action potential arises in the SA node. It propagates throughout the atrial muscle and down to the AV node in about 0.03 sec. The P wave appears on the ECG as the atrial contractile fibers depolarize. 2. After the P wave begins, the atria contract (atrial systole). Conduction of the action potential slows at the AV node because the fibers there have much smaller diameters and fewer gap junctions. (Traffic slows in a similar way where a four-lane highway narrows to one lane in a construction zone!) The resulting 0.1-sec delay gives the atria time to contract, thus adding to the volume of blood in the ventricles, before ventricular systole begins. 3. The action potential propagates rapidly again after entering the AV bundle. About 0.2 sec after onset of the P wave, it has propagated through the bundle branches, Purkinje fibers, and the entire ventricular myocardium. Depolarization progresses down the septum, upward from the apex, and outward from the endocardial surface, producing the QRS complex. At the same time, atrial repolarization is occurring, but it is not usually evident in an ECG because the larger QRS complex masks it. 4. Contraction of ventricular contractile fibers (ventricular systole) begins shortly after the QRS complex appears and continues during the S--T segment. As contraction proceeds from the apex toward the base of the heart, blood is squeezed upward toward the semilunar valves. 5. Repolarization of ventricular contractile fibers begins at the apex and spreads throughout the ventricular myocardium. This produces the T wave in the ECG about 0.4 sec after the onset of the P wave. 6. Shortly after the T wave begins, the ventricles start to relax (ventricular diastole). By 0.6 sec, ventricular repolarization is complete and ventricular contractile fibers are relaxed. During the next 0.2 sec, contractile fibers in both the atria and ventricles are relaxed. At 0.8 sec, the P wave appears again on the ECG, the atria begin to contract, and the cycle repeats.