INTERPRETATION OF THE ELECTROCARDIOGRAM
The electrical impulse that travels through the heart can be viewed by means of electrocardiography, the end product of which is an electrocardiogram (ECG). Each phase of the cardiac cycle is reflected by specific waveforms on the screen of a cardiac monitor or on a strip of ECG graph paper.
An ECG is obtained by slightly abrading the skin with a clean dry gauze pad and placing electrodes on the body at specific areas. Electrodes come in various shapes and sizes, but all have two com-ponents: (1) an adhesive substance that attaches to the skin to secure the electrode in place and (2) a substance that reduces the skin’s elec-trical impedance and promotes detection of the electrical current.
The number and placement of the electrodes depend on the type of ECG needed. Most continuous monitors use two to five electrodes, usually placed on the limbs and the chest. These elec-trodes create an imaginary line, called a lead, that serves as a ref-erence point from which the electrical activity is viewed. A lead is like an eye of a camera; it has a narrow peripheral field of vision, looking only at the electrical activity directly in front of it. There-fore, the ECG waveforms that appear on the paper or cardiac monitor represent the electrical current in relation to the lead (see Fig. 27-1). A change in the waveform can be caused by a change in the electrical current (where it originates or how it is con-ducted) or by a change in the lead.
Electrodes are attached to cable wires, which are connected to one of the following:
· An ECG machine placed at the patient’s side for an imme-diate recording (standard 12-lead ECG)
· A cardiac monitor at the patient’s bedside for continuous reading; this kind of monitoring, usually called hardwire monitoring, is associated with intensive care units
· A small box that the patient carries and that continuously transmits the ECG information by radio waves to a central monitor located elsewhere (called telemetry)
· A small, lightweight tape recorder-like machine (called a Holter monitor) that the patient wears and that continu-ously records the ECG on a tape, which is later viewed and analyzed with a scanner
The placement of electrodes for continuous monitoring, te-lemetry, or Holter monitoring varies with the type of technol-ogy that is appropriate and available, the purpose of monitoring, and the standards of the institution. For a standard 12-lead ECG, 10 electrodes (six on the chest and four on the limbs) are placed on the body (Fig. 27-2). To prevent interference from the electrical activity of skeletal muscle, the limb electrodes are usually placed on areas that are not bony and that do not have significant movement. These limb electrodes provide the first six leads: leads I, II, III, aVR, aVL, and aVF. The six chest electrodes are attached to the chest at very specific areas. The chest electrodes provide the V or precordial leads, V1 through V6. To locate the fourth intercostal space and the placement of V1, locate the sternal angle and then the sternal notch, which is about 1 or 2 inches below the sternal angle. When the fin-gers are moved to the patient’s immediate right, the second rib can be palpated. The second intercostal space is the indentation felt just below the second rib.
Locating the specific intercostal space is critical for correct chest electrode placement. Errors in diagnosis can occur if electrodes are incorrectly placed. Sometimes, when the patient is in the hospital and needs to be monitored closely for ECG changes, the chest electrodes are left in place to ensure the same placement for follow-up ECGs.
A standard 12-lead ECG reflects the electrical activity pri-marily in the left ventricle. Placement of additional electrodes for other leads may be needed to obtain more complete information. For example, in patients with suspected right-sided heart dam-age, right-sided precordial leads are required to evaluate the right ventricle (see Fig. 27-2).
The ECG waveform represents the function of the heart’s con-duction system, which normally initiates and conducts the elec-trical activity, in relation to the lead. When analyzed accurately, the ECG offers important information about the electrical activity of the heart. ECG waveforms are printed on graph paper that is divided by light and dark vertical and horizontal lines at standard intervals (Fig. 27-3). Time and rate are measured on the hori-zontal axis of the graph, and amplitude or voltage is measured on the vertical axis. When an ECG waveform moves toward the top of the paper, it is called a positive deflection. When it moves to-ward the bottom of the paper, it is called a negative deflection. When reviewing an ECG, each waveform should be examined and compared with the others.
The ECG is composed of waveforms (including the P wave, the QRS complex, the T wave, and possibly a U wave) and of seg-ments or intervals (including the PR interval, the ST segment, and the QT interval) (see Fig. 27-3).
The P wave represents the electrical impulse starting in the sinus node and spreading through the atria. Therefore, the P wave represents atrial muscle depolarization. It is normally 2.5 mm or less in height and 0.11 second or less in duration.
The QRS complex represents ventricular muscle depolariza-tion. Not all QRS complexes have all three waveforms. The first negative deflection after the P wave is the Q wave, which is nor-mally less than 0.04 second in duration and less than 25% of the R wave amplitude; the first positive deflection after the P wave is the R wave; and the S wave is the first negative deflection after the R wave. When a wave is less than 5 mm in height, small letters (q, r, s) are used; when a wave is taller than 5 mm, capital letters (Q, R, S) are used. The QRS complex is normally less than 0.12 sec-onds in duration.
The T wave represents ventricular muscle repolarization (when the cells regain a negative charge; also called the resting state). It follows the QRS complex and is usually the same direction as the QRS complex.
The U wave is thought to represent repolarization of the Purkinje fibers, but it sometimes is seen in patients with hypo-kalemia (low potassium levels), hypertension, or heart disease. If present, the U wave follows the T wave and is usually smaller than the P wave. If tall, it may be mistaken for an extra P wave.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and represents the time needed for sinus node stimulation, atrial depolarization, and con-duction through the AV node before ventricular depolarization. In adults, the PR interval normally ranges from 0.12 to 0.20 sec-onds in duration.
The ST segment, which represents early ventricular repolar-ization, lasts from the end of the QRS complex to the beginning of the T wave. The beginning of the ST segment is usually iden-tified by a change in the thickness or angle of the terminal por-tion of the QRS complex. The end of the ST segment may be more difficult to identify because it merges into the T wave. The ST segment is normally isoelectric (see discussion of TP interval). It is analyzed to identify whether it is above or below the iso-electric line, which may be, among other signs and symptoms, a sign of cardiac ischemia.
The QT interval, which represents the total time for ventric-ular depolarization and repolarization, is measured from the be-ginning of the QRS complex to the end of the T wave. The QT interval varies with heart rate, gender, and age, and the measured interval needs to be corrected for these variables through a spe-cific calculation. Several ECG interpretation books contain charts of these calculations. The QT interval is usually 0.32 to 0.40 sec-onds in duration if the heart rate is 65 to 95 beats per minute. If the QT interval becomes prolonged, the patient may be at risk for a lethal ventricular dysrhythmia called torsades de pointes.
The TP interval is measured from the end of the T wave to the beginning of the next P wave, an isoelectric period (see Fig 27-3).
When no electrical activity is detected, the line on the graph re-mains flat; this is called the isoelectric line. The ST segment is compared with the TP interval to detect changes from the line on the graph during the isoelectric period.
The PP interval is measured from the beginning of one P wave to the beginning of the next. The PP interval is used to determine atrial rhythm and atrial rate. The RR interval is measured from one QRS complex to the next QRS complex. The RR interval is used to determine ventricular rate and rhythm (Fig. 27-4).
Heart rate can be obtained from the ECG strip by several meth-ods. A 1-minute strip contains 300 large boxes and 1500 small boxes. Therefore, an easy and accurate method of determining heart rate with a regular rhythm is to count the number of small boxes within an RR interval and divide 1500 by that number. If, for example, there are 10 small boxes between two R waves, the heart rate is 1500 ÷ 10, or 150; if there are 25 small boxes, the heart rate is 1500 ÷ 25, or 60 (see Fig. 27-4A).
An alternative but less accurate method for estimating heart rate, which is usually used when the rhythm is irregular, is to count the number of RR intervals in 6 seconds and multiply that number by 10. The top of the ECG paper is usually marked at 3-second intervals, which is 15 large boxes horizontally (see Fig. 27-4B). The RR intervals are counted, rather than QRS complexes, because a computed heart rate based on the latter might be inaccurately high.
The same methods may be used for determining atrial rate, using the PP interval instead of the RR interval.
The rhythm is often identified at the same time the rate is deter-mined. The RR interval is used to determine ventricular rhythm and the PP interval to determine atrial rhythm. If the intervals are the same or nearly the same throughout the strip, the rhythm is called regular. If the intervals are different, the rhythm is called irregular.