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Introduction ⬇

An electrocardiogram (ECG) records the electrical impulses that stimulate the heart to contract. It also records dysfunctions that influence the conduction ability of the myocardium. The ECG is helpful in diagnosing and monitoring the origins of pathologic rhythms; myocardial ischemia; myocardial infarction (MI); atrial and ventricular hypertrophy; atrial, atrioventricular (AV), and ventricular conduction delays and abnormalities; and pericarditis. It can be helpful in diagnosing systemic diseases that affect the heart, determining cardiac drug effects (especially digitalis and antiarrhythmic agents), evaluating disturbances in electrolyte balance (especially potassium and calcium), and analyzing cardiac pacemaker or implanted defibrillator functions.

An ECG provides a continuous picture of electrical activity during a complete cardiac cycle. Heart cells are charged or polarized in the resting state, but they depolarize and contract when electrically stimulated. The intracellular body fluids are excellent conductors of electrical current and are an important component of this process. When the depolarization (stimulation) process sweeps in a wave across the cells of the myocardium, the electrical current generated is conducted to the body's surface, where it is detected by special electrodes placed on the patient's limbs and chest. An ECG tracing shows the voltage of the waves and the time duration of waves and intervals. By studying the amplitude of the waves and measuring the duration of the waves and intervals, disorders of impulse formation and conduction can be diagnosed.

A vectorcardiogram (VCG) records the magnitude and direction of the action currents of the heart as a series of vectors that form a curved line around a center. A VCG may help to diagnose and monitor left-sided hypertrophy and hyperfunction, bundle branch blocks, and MI.

A VCG records a three-dimensional display of the heart's electrical activity, whereas the ECG is a single-plane representation (see Table 16.1) The following are the three planes of the VCG:

  1. Frontal plane (combines the Y and X axes)

  2. Sagittal plane (combines the Y and Z axes)

  3. Horizontal plane (combines the X and Z axes)

  1. The P wave is normally upright; it represents atrial depolarization and indicates electrical activity associated with the original impulse that travels from the sinus node through the atrial sinus. If P waves are present; are of normal size, shape, and deflection; have normal conduction intervals to the ventricles; and demonstrate rhythmic timing variances between cardiac cycles, it can be assumed that they began in the sinoatrial node.

  2. The Ta or Tp designation is used to differentiate atrial repolarization, which ordinarily is obscured by the QRS complex, from the more conventional T wave, which signifies ventricular repolarization (see number 8 below).

  3. The Q(q) wave is the first downward/negative deflection in the QRS complex; it results from ventricular depolarization. The Q(q) wave may not always be apparent.

  4. The R(r') wave is the first upright/positive deflection after the P wave (or in the QRS complex); it results from ventricular depolarization.

  5. The S(s') wave is the downward/negative deflection that follows the R wave.

  6. The Q and S waves are negative deflections that do not normally rise above the baseline.

  7. The T wave is a deflection produced by ventricular repolarization. There is a pause after the QRS complex, and then a T wave appears. The T wave is a period of no cardiac activity before the ventricles are again stimulated. It represents the recovery phase after the ventricular contraction.

  8. The U wave is a deflection (usually positive) following the T wave. It represents late ventricular repolarization of Purkinje fibers or the intraventricular papillary muscles. This wave may or may not be present on an ECG. If it appears, it may be abnormal, depending on its configuration.

  1. The RR interval (normally, 0.83 second at a heart rate of 72 beats/minute) is the distance between successive R waves. In normal rhythms, the interval, in seconds or fractions of seconds, between two successive R waves divided into 60 seconds provides the heart rate per minute.

  2. The PP interval (normally, 0.83 second at a heart rate of 72 beats/minute) will be the same as the RR interval in normal sinus rhythm. The responsiveness of the sinus node to physiologic activity (e.g., exercise, rest, respiratory cycling) produces a rhythmic variance in PP intervals.

  3. The PR interval (~0.16 second) measures conduction tone and includes the time it takes for atrial depolarization and normal conduction delay in the AV node to occur. It terminates with the onset of ventricular depolarization. It is the period from the start of the P wave to the beginning of the QRS complex. This interval represents the time it takes for the impulse to traverse the atria, proceed through the AV node, and reach the ventricles and initiate ventricular depolarization.

  4. The QRS interval (normally, 0.12 second) represents ventricular depolarization time and tracks the electrical impulse as it travels from the AV node through the bundle branches to Purkinje fibers and into the myocardial cells. Normal waves consist of an initial downward deflection (Q wave), a large upward deflection (R wave), and a second downward deflection (S wave). It is measured from the onset of the Q wave (or R if no Q is visible) to the termination of the S wave.

  5. QT interval measures the duration of ventricular activation and recovery. It is measured from the beginning of the QRS complex to the end of the T wave. The QT interval varies with the heart rate, gender, and time of day. Normal QT interval is 350-430 msec.

  1. The PR segment is normally isoelectric and is the portion of the ECG tracing from the end of the P wave to the onset of the QRS complex.

  2. The J junction (or J point) is the point at which the QRS complex ends and the ST segment begins.

  3. The ST segment is that part of the ECG from the J point to the onset of the T wave. Elevation or depression is determined by comparing its location with the portion of the baseline between the end of the T wave and the beginning of the P wave or relating it to the PR segment. This segment represents the period between the completion of depolarization and onset of repolarization (i.e., recovery) of the ventricular muscles.

  4. The TP segment (~0.25 second) is the portion of the ECG record between the end of the T wave and the beginning of the next P wave. It is usually isoelectric.

  1. Voltage from the top of the R wave to the bottom of the S wave is 1 mV. Voltage of the P wave is ~0.1-0.3 mV. Voltage of the T wave is ~0.2-0.3 mV. Upright deflection voltage is measured from the upper part of the baseline to the peak of the wave.

  2. Negative deflection voltage is measured from the lower portion of the baseline to the nadir of the wave.

  1. Because cardiac electrical forces extend in several directions at the same time, a comprehensive view of heart activity is possible only if the flow of current in several different planes is recorded.

  2. For a 12-lead ECG, 12 leads are simultaneously used to present this comprehensive picture. Limb leads (I, II, III, AVL, AVF, AVR) record events in the frontal plane of the heart. Chest leads (V1, V2, V3, V4, V5, and V6) record a horizontal view of the heart's electrical activity.

  3. Occasionally, an esophageal lead, which is swallowed or placed in the esophagus, can supply additional information. This type of lead is frequently used during surgical procedures.

  4. His bundle electrography is a very specialized procedure that requires placement of an IV catheter, which is then advanced into the heart. An ECG is simultaneously being recorded while the electrical activity of the bundle of His is measured by a sensor at the end of the catheter. This test measures the electrical activity between contractions (Figure 16.3).

Procedure ⬆ ⬇

The following steps apply to both the ECG and the VCG:

  1. Have the patient assume a supine position; however, recordings can be taken during exercise.

  2. Prepare the skin sites and, if necessary, shave, and place electrodes on the four extremities and on specific chest sites. Ensure that the right leg is the ground.

  3. A typical rhythm strip is a 2-minute recording from a single lead, usually lead II. It is frequently used to evaluate dysrhythmias.

  4. Follow guidelines in Chapter 1 for safe, effective, informed intratest care.

Procedural Alert

  1. Chest pain, if present, should be noted on the ECG strip.

  2. The presence of a pacemaker and the use of a magnet in testing should be documented.

  3. Marking the position on the chest wall in ink ensures a reproducible precordial lead placement.

Clinical Implications ⬆ ⬇

  1. ECG:

    1. The ECG does not depict the actual mechanical state of the heart or functional status of the valves.

    2. An ECG may be normal in the presence of heart disease unless the pathologic process disturbs the electrical forces. It cannot predict future cardiac events.

    3. An ECG should be interpreted and treatment ordered within the context of a comprehensive clinical picture.

    4. ECG abnormalities are categorized according to five general areas:

      1. Heart rate

      2. Heart rhythm

      3. Axis or position of the heart

      4. Hypertrophy

      5. Infarction/ischemia

    5. Typical abnormalities include the following:

      1. Pathologic rhythms

      2. Conduction system disturbances

      3. Myocardial ischemia

      4. MI

      5. Hypertrophy of the heart

      6. Pulmonary infarction

      7. Altered potassium, calcium, and magnesium levels

      8. Pericarditis

      9. Effects of drugs

      10. Ventricular hypertrophy

  2. VCG:

    1. The VCG is more sensitive than the ECG for diagnosing MI; it is probably not any more specific.

    2. VCG is more specific than the ECG in determining hypertrophy or ventricular dilation.

    3. Differentiation of intraventricular conduction abnormalities is possible.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose, procedure, and interfering factors. Emphasize that the test is painless and does not deliver electrical current to the body.

  2. Have the patient completely relax to ensure a satisfactory tracing.

  3. Be aware that, ideally, the person should rest for 15 minutes before ECG recording. Have the patient avoid heavy meals and smoking for at least 30 minutes before the ECG and longer if possible.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results and counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Modify the nursing care plan as needed.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Clinical Alert

  1. When an ECG shows changes that indicate myocardial ischemia, injury, or infarction, these changes must be reported and acted on immediately. The goal of diagnosis and treatment is to increase myocardial blood supply and reduce oxygen demand.

    1. When ECG changes represent stages of myocardial ischemia, injury, or necrosis and symptoms of possible acute MI appear, the primary concern is balancing myocardial oxygen supply and demand as follows:

      1. Nitroglycerin dilates blood vessels.

      2. Narcotics relieve pain and anxiety.

      3. Calcium channel blockers relieve coronary spasm.

      4. Oxygen increases O2 supply available to the myocardium.

      5. Beta-blocking drugs slow rapid heart rates.

      6. Antiarrhythmic agents correct abnormal rhythms.

      7. Frequent reassurances alleviate patient anxiety.

    2. Monitoring for cardiac rhythm disturbances is an essential component of care. Potentially lethal dysrhythmias, especially ventricular tachyarrhythmias, require immediate intervention and may signal the need for possible cardiopulmonary resuscitation.

  2. Serious diagnostic errors can be made if the ECG is not interpreted in the broader context of the patient's history, signs, and symptoms.

  3. The electrical axis is not synonymous with the anatomic position of the heart.

Interfering Factors ⬆ ⬇

  1. Ra ST elevation with T-wave inversion is more common in African Americans but disappears with maximal exercise effort.

  2. Food intake: High carbohydrate content is especially associated with an intracellular shift of potassium in association with intracellular glucose metabolism. Nondiagnostic ST depression and T-wave inversion are evident with hypokalemia.

  3. Anxiety: Episodic anxiety and hyperventilation are associated with prolonged PR interval, sinus tachycardia, and ST depression with or without T-wave inversion. This may be due to autonomic nervous system imbalances.

  4. Deep respiration: The position of the heart in the chest shifts more vertically with deep inspiration and more horizontally with prolonged expiration.

  5. Exercise/movement: Strenuous exercise before the test can produce misleading results. Muscle twitching can also alter the tracing.

  6. Position of heart within the thoracic cage: There may be an anatomic cardiac rotation in both horizontal and frontal planes.

  7. Position of precordial leads: Inaccurate placement of the bipolar chest leads and the transposition of right and left arm and left leg electrodes will affect test results. In normal persons, lead reversal produces the typical ECG findings of dextrocardia (congenital anomaly resulting in the heart being on the right side of the chest) in frontal plane leads and can mimic a MI pattern.

  8. A leftward shift in the QRS axis occurs with excess body weight, ascites, and pregnancy.

  9. Age: At birth and during infancy, the right ventricle is hypertrophied because the fetal right ventricle performs more work than the left ventricle. T-wave inversion in leads V1 to V3 persists into the second decade of life and into the third decade in African Americans.

  10. Gender: Women exhibit slight ST-segment depression.

  11. Chest configuration and dextrocardia: In this congenital anomaly in which the heart is transposed to the right side of the chest, the precordial leads must also be placed over the right side of the chest.

  12. Severe drug overdose, especially with barbiturates, and many other medications can influence ECG configuration. Antiarrhythmic agents, antihistamines, and antibiotic agents can widen QT intervals.

  13. The serious effects of electrolyte imbalances show up on the ECG as follows:

    1. Increased Ca2+: Shortened QT; less frequently, prolonged PR interval and QRS complex.

    2. Decreased Ca2+: Prolonged QT.

    3. Alterations in K+ may produce cardiac arrhythmias.

Reference Values ⬆

Normal

  • Normal positive and negative deflections in an ECG recording

  • Normal cardiac cycle components (one normal cardiac cycle is represented by the P wave, QRS complex, and T wave; additionally, a U wave may be observed). This cycle is repeated continuously and rhythmically (Figure 16.2).

  • The P wave indicates atrial depolarization; QRS complex indicates ventricular depolarization; T wave indicates ventricular repolarization/resting stage between beats; and U wave indicates nonspecific recovery after potentials.