Electrocardiography is the most commonly used test for evaluating cardiac status, graphically records the electrical current (electrical potential) generated by the heart. This current radiates from the heart in all directions and, on reaching the skin, is measured by electrodes connected to an amplier and strip chart recorder. The standard resting ECG uses five electrodes to measure the electrical potential from 12 different leads; the standard limb leads (I,II,III), the augmented limb leads (aVf, aVL, and aVr), and the precordial, or chest, leads (V1 through V6).
ECG tracings normally consist of three identifiable waveforms: the P wave, the QRS complex, and the T wave. The P wave depicts atrial depolarization; the QRS complex, ventricular depolarization; and the T wave, ventricular repolarization.
Computerized ECG machines use small electrode tabs that peel off a sheet and adhere to the patient’s skin. The entire ECG tracing is displayed on a screen so abnormalities can be corrected before printing; then it’s printed on one sheet of paper. Electrode tabs can remain on the patient’s chest, arms, and legs to provide continuous lead placement for serial ECG studies.
Purpose of Electrocardiography (ECG)
- To help identify primary conduction abnormalities, cardiac arrhythmias, cardiac hypertrophy, pericarditis, electrolyte imbalances, myocardial ischemia, and the site and extent of myocardial infarction.
- To monitor recovery from an MI.
- To evaluate the effectiveness of cardiac medication.
- To assess pacemaker performance
- To determine effectiveness of thrombolytic therapy and the resolution of ST-segment depression or elevation and T-wave changes.
Electrocardiography (ECG) Procedure
Patient Preparation for Electrocardiography (ECG)
- Explain to the patient the need to lie still, relax, and breathe normally during the procedure.
- Note current cardiac drug therapy on the test request form as well as any other pertinent clinical information, such as chest pain or pacemaker.
- Explain that the test is painless and takes 5 to 10 minutes.
- Place the patient in a supine or semi-Fowler’s position.
- Expose the chest, ankles, and wrists.
- Place electrodes on the inner aspect of the wrists, on the medical aspect of the lower legs, and on the chest.
- After all electrodes are in place, connect the lead wires.
- Press the START button and input any required information.
- Make sure that all leads are represented in the tracing. If not, determine which electrode has come loose, reattach it, and restart the tracing.
- All recording and other nearby electrical equipment should be properly grounded.
- Make sure that the electrodes are firmly attached.
- Disconnect the equipment, remove the electrodes, and remove the gel with a moist cloth towel.
- If the patient is having recurrent chest pain or if serial ECG’s are ordered, leave the electrode patches in place.
- P wave that doesn’t exceed 2.5 mm (0.25 mV) in height or last longer than 0.12 second.
- PR interval (includes the P wave plus the PR segment) persisting for 0.12 to 0.2 second for heart rates above 60 beats/min.
- QT interval that varies with the heart rate and lasts 0.4 to 0.52 second for heart rates above 60 beats/min.
- Voltage of the R wave leads V1 through V6 that doesn’t exceed 27 mm.
- Total QRS complex lasting 0.06 to 0.1 second.
- Myocardial infarction (MI), right or left ventricular hypertrophy, arrhythmias, right or left bundle-branch block, ischemia, conduction defects or pericarditis, and electrolyte abnormalities.
- Abnormal wave forms during angina episodes or during exercise.
- The recording equipment and other nearby electrical equipment should be properly grounded to prevent electrical interference.
- Double-check color codes and lead markings to be sure connectors march.
- Make sure that the electrodes are firmly attached, and reattached them if loose skin contact is suspended. Don’t use cables that are broken, frayed, or bare.
- Improper lead placement.
- Skin sensitivity to the electrodes.