The Electrocardiogram

The Electrocardiogram (ECG)

Basic Principles
• Electrical activity is generated by the cells of the heart as ions are exchanged across cell membranes.
• Electrodes that are capable of conducting electrical activity from the heart to the ECG machine are placed at strategic positions on the extremities and chest precordium (Fig. 1).
• The electrical energy sensed is then converted to a graphic display by the ECG machine. This display is referred to as the electrocardiogram.
• A heart contraction is represented by wave forms on the ECG graph paper that are designated P, Q, R, S, and T waves.
• Wave forms are referred to as deflections relative to an isoelectric line (a line that expresses no energy). The isoelectric line can be determined by looking at the T-to-P interval.
 The P wave is the first positive deflection and represents atrial depolarization.
 The Q wave is the first negative deflection after the P wave; the R wave is the first positive deflection after the P wave.
 The S wave is the negative deflection after the R wave.
 The QRS wave form is generally regarded as a unit and represents ventricular depolarization.
 The T wave follows the S wave and is joined to the QRS complex by the ST segment. The T wave represents the return of ions to the appropriate side of the cell membrane. This signifies relaxation of the muscle fibers and is referred to as repolarization of the ventricles.
 The QT interval is the time between the Q wave and the T wave.

Indications The ECG
is a useful tool in the diagnosis of those conditions that may cause aberrations in the electrical activity of the heart. Examples of these conditions are as follows:
• MI and other types of coronary artery diseases, such as angina
• Cardiac dysrhythmias
• Cardiac enlargement
• Electrolyte disturbances, especially of calcium and potassium levels
• Inflammatory diseases of the heart
• Effects on the heart by drugs such as digitalis (Lanoxin) and tricyclic antidepressants

ECG Leads and Normal Wave Form Interpretation (Fig. 2)
• The standard ECG consists of 12 leads (I, II, III, AVR, AVL, AVF, V1, V2, V3, V4, V5, V6).
 Each lead records the heart’s electrical activity from a different anatomic position.
 Identification of specific myocardial changes on certain leads assists in defining pathologic conditions.
• The normal amplitude of the P wave is 3 mm or less; the normal duration of the P wave is 0.04 to 0.11 seconds. P waves that exceed these measurements are considered to be a deviation from normal.
• The PR interval is measured from the upstroke of the P wave to the QR junction and is normally between 0.12 and 0.20 seconds.
 The PR interval represents the time of impulse transmission from the SA node to the AV node.
 There is a built-in delay in time at the AV node to allow for adequate ventricular filling to maintain normal stroke volume (the amount of blood ejected with each contraction).
• The QRS complex contains separate waves and segments, which should be evaluated separately. Normal QRS complex should be between 0.06 and 0.10 seconds.
o The Q wave, or first downward stroke after the P wave, is usually less than 3 mm in depth. A Q wave of significant deflection is not normally present in the healthy heart. A pathologic Q wave usually indicates an old MI.
o The R wave is the first positive deflection after the P wave, normally 5 to 10 mm in height. Increases and decreases in amplitude become significant in certain disease states. Ventricular hypertrophy produces very high R waves because the hypertrophied muscle requires a stronger electrical current to depolarize.
• The S–T segment begins at the end of the S wave, the first negative deflection after the R wave, and terminates at the upstroke of the T wave.
• The T wave represents the repolarization of myocardial fibers or provides the resting state of myocardial work; the T wave should always be present.
o Normally, the T wave should not exceed a 5-mm amplitude in all leads except the precordial (V1 to V6) leads, where it may be as high as 10 mm.

Nursing/Patient Care Considerations
• Perform ECG or begin continuous ECG monitoring as indicated.
 Provide privacy and ask the patient to undress, exposing chest, wrists, and ankles. Assist with draping as appropriate.
 Place leads on chest and extremities as labeled, using self-adhesive electrodes or water-soluble gel or other conductive material.
 Instruct patient to lie still, avoiding movement, coughing, or talking while ECG is recording to avoid artifact.
 Make sure ECG machine is plugged in and grounded, and operate according to manufacturer’s directions.
 If continuous cardiac monitoring is being done, advise patient on the parameters of mobility and not to panic if an alarm sounds.
• Interpret ECG (Fig. 3). Develop a systematic approach to assist in accurate interpretation for dysrhythmias, myocardial damage, or other changes.
o Determine the rate. Is it fast, slow, or normal?
 A gross determination of rate can be determined by counting the number of QRS complexes within a 6-second time interval (use the superior margin of ECG paper) and multiply the complexes by a factor of 10.
Note: One must be cautioned that this method is accurate only for rhythms that are occurring at normal intervals and should not be used for determining rate in irregular rhythms. Irregular rhythms are always counted for 1 full minute for accuracy.
 Another means of obtaining rate is to divide the number of large five-square blocks between each two QRS complexes into 300. Three hundred large blocks represent 1 minute on the ECG paper.
Example: In Figure 3, the number of large square blocks between complexes #5 and #6 equals 5, or a rate of 60.
o Next, determine the rhythm. Is it regular, irregular, regularly irregular, or irregularly irregular?
 Use calipers or count blocks between QRS complexes to determine regularity.
 Finally, examine each wave and segment for abnormality.
 Find the P waves. Is one present for each QRS complex? Are they absent as in junctional rhythm? Are they replaced by other wave forms? What is the configuration like? Are they identical, well-formed, or do they change shape as in atrial fibrillation or paroxysmal atrial tachycardia?
 Measure the PR interval. Prolonged PR interval may be a precursor to a variety of heart blocks due to drug therapy or myocardial disease.
 Look for pathologic Q waves, or one that is greater than 0.04 seconds in time and greater than 3 mm in depth or greater than one third the height of the R wave.
 Measure the QRS complex. Are they identical in configuration? Do they fall early? Does the configuration vary? Are any wide and bizarre, representing a premature ventricular contraction?
 Examine the S–T segments. Elevation of the S–T segment heralds a pattern of injury and usually occurs as an initial change in acute MI. S–T depression occurs in ischemic states. Calcium and potassium changes also affect the S–T segment.
 Look at the T wave. Is it positively or negatively deflected? Is it peaked? Inverted T waves may indicate ischemia.
 Measure the QT interval. The normal QT interval should be less than one half the RR interval. Prolonged QT interval may indicate digitalis toxicity, long-term quinidine (Quinaglute) or procainamide (Pronestyl) therapy, or hypomagnesemia.