Electrocardiography
I. Historical
II. General Applications
III. The Normal Electrocardiogram
IV. Diagnosis of Specific Conditions
V. Recent Discoveries
GLOSSARY
arrhythmia general term for an irregularity or rapidity of the heartbeat, an abnormal heart rhythm.
hypertrophy increase in thickness of muscle.
ischemia temporary lack of blood and oxygen to an area of cells, for example, the heart muscle, usually due to severe obstruction of the artery supplying blood to this area of cells.
left ventricular dysfunction poor contractility of the ventricle, this leads to heart failure.
myocardial infarction death of an area of heart muscle due to blockage of a coronary artery by blood clot and atheroma; medical term for a heart attack or coronary thrombosis.
myocardium the heart muscle.
pericarditis inflammation of the pericardium or sac surround¬ing the heart; this is not a heart attack.
- I. HISTORICAL
The development of the modern electrocardiographic instrument illustrates how medicine, a branch of the bio¬logic sciences, took advantage of the concepts of physical science and its instrumentation. The invention of the electrocardiographic instrument would not have been possible without the notable orchestrated work of engi¬neers, physicists, physiologists, and researchers. Of parti¬cular importance was the development of the string galvanometer by Ader, a French electrical engineer. - II. GENERAL APPLICATIONS
Despite the advent of expensive and sophisticated cardio-logic tests, the electrocardiogram remains the most reliable tool for the confirmation of acute myocardial infarction. The electrocardiogram — not the cardiac enzymes, tropo-nins, CK–MB, echocardiogram, or Sector PET scan — dictates the rapid administration of lifesaving thrombolytic therapy. There is no test to rival the electrocardiogram in the diagnosis of arrhythmia, which is a common problem seen in clinical practice. Also, the diagnosis of acute pericarditis can only be confirmed by electrocardiographic findings. The very common condition of myocardial ischemia which causes chest pain in patients with angina can be confirmed in some by electrocardiographic findings at rest and particularly during exercise. - III. THE NORMAL ELECTROCARDIOGRAM
The electrocardiogram picks up the heart’s electrical impulses transmitted through the skin of the chest. Figure 2 gives a simplified concept of ion exchange — the polarized, depolarized, and repolarized state of the myocardial cell; and the action potential. An electrical current arriving at the cell causes positively charged irons to cross the cell membrane (depolarization), followed by repolarization which generates an action potential: phase 0, 1, 2, 3, and 4. This electrical event traverses the heart and initiates mechanical systole or a heartbeat. - IV. DIAGNOSIS OF SPECIFIC CONDITIONS A. Acute Myocardial Infarction
The ECG diagnosis of acute myocardial infarction is revealed by an elevation of the ST segment in the patient who has acute chest pain. Figure 6 shows the ECG of a patient with infarction of the anterior wall of the heart. The typical diagnostic ECG finding is ST-segment elevation in the leads taken over the chest, precordial leads V1 through V6 (ST-segment elevation myocardial infarction). Figure 7 shows involvement of the inferior myocardium during inferior myocardial infarction. - V. RECENT DISCOVERIES
A. Microvolt T-wave Alternans - BIBLIOGRAPHY
Bloomfield, D. M., Steinman, R. C., Namerow, P. B. et al. Microvolt T-wave alternans distiguishes between patients likely and patients not likely to benefit from implanted cardiac defibrillator therapy. A solution to the multicenter automatic defibrillator implan¬tation trial (MADIT)II conundrum. Circulation, 110:1885–1889, 2004.