ECG

The electrocardiogram (ECG) provides a graphic representation, or picture, of cardiac electrical activity. The cardiac electrical currents are transmitted to the body surface. Electrodes, consisting of a conductive gel on an adhesive pad, are placed on specific sites on the body and attached to cables connected to an ECG machine or a monitor. The cardiac electrical current is transmitted via the electrodes and through the lead wires to the machine or monitor, which displays the cardiac electrical activity. A lead provides one view of the heart’s electrical activity. Multiple leads, or views, can be obtained. Electrode placement is the same for male and female patients.

Lead systems are made up of a positive pole and a negative pole. An imaginary line joining these two poles is called the lead axis. The direction of electrical current flow in the heart is the cardiac axis. The relationship between the cardiac axis and the lead axis is responsible for the deflections seen on the ECG pattern:

• The baseline is the isoelectric line. It occurs when there is no current flow in the heart after complete depolarization and also after complete repolarization. Positive deflections occur above this line, and negative deflections occur below it. Deflections represent depolarization and repolarization of cells.

• If the direction of electrical current flow in the heart (cardiac axis) is toward the positive pole, a positive deflection (above the baseline) is viewed (Fig. 28.2A).

• If the direction of electrical current flow in the heart (cardiac axis) is moving away from the positive pole toward the negative pole, a negative deflection (below the baseline) is viewed (Fig. 28.2B).

Lead Systems

The standard 12-lead ECG consists of 12 leads (or views) of the heart’s electrical activity. Six of the leads are called limb leads because the electrodes are placed on the four extremities in the frontal plane. The remaining six leads are called chest (precordial) leads because the electrodes are placed on the chest in the horizontal plane.

Standard bipolar limb leads consist of three leads (I, II, and III) that each measure the electrical activity between two points and a fourth lead (right leg) that acts as a ground electrode. Of the three measuring leads, the right arm is always negative, the left leg is always positive, and the left arm can be either positive or negative.

Other lead systems include the 18-lead ECG, which adds six leads placed on the horizontal plane on the right side of the chest to view the right side of the heart. This is sometimes referred to as a right-sided ECG. The extra leads are sometimes placed on the back. Unipolar limb leads consist of a positive electrode only. The unipolar limb leads are aVR, aVL, and aVF, with a meaning augmented; V is a designation for a unipolar lead. The third letter denotes the positive electrode placement: R for right arm, L for left arm, and F for foot (left leg). The positive electrode is at one end of the lead axis. The other end is the center of the electrical field, at about the center of the heart.

There are six unipolar (or V) chest leads, determined by the placement of the chest electrode. The four limb electrodes are placed on the extremities, as designated on each electrode (right arm, left arm, right leg, and left leg). The fifth (chest) electrode on a monitor system is the positive, or exploring, electrode and is placed in one of six designated positions to obtain the desired chest lead. With a 12-lead ECG, four leads are placed on the limbs and six are placed on the chest, eliminating the need to move any electrodes about the chest (Fig. 28.3).

Positioning of the electrodes is crucial in obtaining an accurate ECG. Comparisons of ECGs taken at different times will be valid only when electrode placement is accurate and identical at each test. Positioning is particularly important when working with patients with chest deformities or large breasts. Patients may be asked to move the breasts to ensure proper electrode placement. For serial ECGs, a surgical marker may be used to mark the electrode placement site to allow for accurate placement. It is important to remove the electrodes following the ECG because skin breakdown can occur.

While obtaining a 12-lead ECG, remind the patient to be as still as possible in a semi-reclined position, breathing normally. Any repetitive movement will cause an artifact and could lead to an inaccurate interpretation of the ECG.

Nurses are sometimes responsible for obtaining 12-lead ECGs, but more commonly, technicians are trained to perform this task. Remind the technician to notify the nurse or primary health care provider of any suspected abnormality. A nurse may direct a technician to take a 12-lead ECG on a patient experiencing chest pain to observe for diagnostic changes, but it is the primary health care provider’s responsibility to definitively interpret the ECG.

Continuous Electrocardiographic Monitoring

For continuous ECG monitoring, the electrodes are not placed on the limbs because movement of the extremities causes “noise,” or motion artifact, on the ECG signal. Place the electrodes on the trunk, a more stable area, to minimize such artifacts and to obtain a clearer signal. If the monitoring system provides five electrode cables, place the electrodes as follows:

• Right arm electrode just below the right clavicle

• Left arm electrode just below the left clavicle

• Right leg electrode on the lowest palpable rib, on the right midclavicular line

• Left leg electrode on the lowest palpable rib, on the left midclavicular line

• Fifth electrode placed to obtain one of the six chest leads

With this placement, the monitor lead-select control may be changed to provide lead I, II, III, aVR, aVL, aVF, or one chest lead. The monitor automatically alters the polarity of the electrodes to provide the lead selected.

The clarity of continuous ECG monitor recordings is affected by skin preparation and electrode quality. To ensure the best signal transmission and decrease skin impedance, clean the skin and clip hairs if needed. Make sure that the area for electrode placement is dry. The gel on each electrode must be moist and fresh. Attach the electrode to the lead cable and then to the contact site. The contact site should be free of lotion or other substance that increases skin impedance. Electrodes cannot be placed on irritated skin or over scar tissue. Electrodes may be applied by assistive personnel (AP), but the nurse determines which lead to select and checks for correct electrode placement. Assess the quality of the ECG rhythm transmission to the monitoring system.

The ECG cables can be attached directly to a wall-mounted monitor (a hardwired system) if the patient’s activity is restricted to bed rest and sitting in a chair, as in a critical care unit. For an ambulatory patient, the ECG cable is attached to a battery-operated transmitter (a telemetry system) held in a pouch. The ECG is transmitted to a remote monitor via antennae located in strategic places, usually in the ceiling. Telemetry allows freedom of movement within a certain area without losing transmission of the ECG.

Most acute care facilities have monitor technicians who are educated in ECG rhythm interpretation and are responsible for:

• Watching a bank of monitors on a unit

• Printing ECG rhythm strips routinely and as needed

• Interpreting rhythms

• Reporting the patient’s rhythm and significant changes to the nurse

The technical support is particularly helpful on a telemetry unit that does not have monitors at the bedside. The nurse is responsible for accurate patient assessment and management. The use of telemetry monitors on medical-surgical units is increasing, and nursing assessment is critical to initiate appropriate interventions.

Some units have full-disclosure monitors, which continuously store ECG rhythms in memory up to a certain amount of time. This system allows nurses and health care providers to access and print rhythm strips for more thorough patient assessment. Routine strips and any changes in rhythm are printed and documented in the patient’s record.

Health care providers are responsible for determining when monitoring can be suspended, such as during showering. They will also determine whether monitoring is needed during off-unit testing procedures and for transportation to other facilities. Clinical alarms, such as those associated with continuous ECG monitoring, have been identified as one of the top 10 technology hazards.

Prehospital personnel, such as paramedics and emergency medical technicians (EMTs) with advanced training, frequently monitor ECG rhythms at the scene and on the way to a health care facility. They function under medical direction and protocols but may also be communicating with a nurse in the emergency department.

Recording the ECG

The ECG strip is printed on graph paper (Fig. 28.4), with each small block measuring 1 mm in height and width. ECG recorders and monitors are standardized at a speed of 25 mm/sec. Time is measured on the horizontal axis. At this speed, each small block represents 0.04 second. Five small blocks make up one large block, defined by darker bold lines and representing 0.20 second. Five large blocks represent 1 second, and 30 large blocks represent 6 seconds. Vertical lines in the top margin of the graph paper are usually 15 large blocks apart, representing 3-second segments (Fig. 28.5).

Electrocardiographic Complexes, Segments, and Intervals

Complexes that make up a normal ECG consist of a P wave, a QRS complex, a T wave, and possibly a U wave. Segments include the PR segment, ST segment, and TP segment. Intervals include the PR interval, QRS duration, and QT interval (Fig. 28.6).