Electrocardiography - Diagnostic Evaluation of Cardiovascular Function

ELECTROCARDIOGRAPHY

The ECG is a diagnostic tool used in assessing the cardiovascular system. It is a graphic recording of the electrical activity of the heart; an ECG can be recorded with 12, 15, or 18 leads, showing the activity from those different reference points. The ECG is obtained by placing disposable electrodes in standard positions on the skin of the chest wall and extremities. The heart’s electrical impulses are recorded as a tracing on special graph paper.

The standard 12-lead ECG is the most commonly used tool to diagnose dysrhythmias, conduction abnormalities, enlarged heart chambers, myocardial ischemia or infarction, high or low calcium and potassium levels, and effects of some medications. A 15-lead ECG adds 3 additional chest leads across the right precordium and is a valuable tool for the early diagnosis of right ventricular and pos-terior left ventricular infarction. The 18-lead ECG adds 3 poste-rior leads to the 15-lead ECG and is very useful for early detection of myocardial ischemia and injury (Wung & Drew, 1999). To en-hance interpretation of the ECG, the patient’s age, gender, BP, height, weight, symptoms, and medications (especially digitalis and antiarrhythmic agents) should be noted on the ECG requisi-tion.

Continuous Electrocardiographic Monitoring

Continuous ECG monitoring is standard for patients who are at high risk for dysrhythmias. Two continuous ECG monitoring techniques are hardwire monitoring, found in critical care units and specialty step-down units, and telemetry, found in specialty step-down units and general nursing care units. Patients who are receiving continuous ECG monitoring need to be informed of its purpose and cautioned that this monitoring method will not de-tect symptoms such as dyspnea or chest pain. Therefore, patients need to be advised to report symptoms to the nurse whenever they occur.

HARDWIRE CARDIAC MONITORING

The patient’s ECG can be continuously observed for dysrhyth-mias and conduction disorders on an oscilloscope at the bedside or at a central monitoring station by a hardwire monitoring sys-tem. This system is composed of three to five electrodes posi-tioned on the patient’s chest, a lead cable, and a bedside monitor. Hardwire monitoring systems vary in sophistication but in gen-eral can do the following:

·      Monitor more than one lead simultaneously

·        Monitor ST segments (ST-segment depression is a marker of myocardial ischemia; ST-segment elevation provides evidence of an evolving MI)

·      Provide graded visual and audible alarms (based on prior-ity, asystole would be highest)

·      Computerize rhythm monitoring (dysrhythmias are inter-preted and stored in memory)

·      Print a rhythm strip

·       Record a 12-lead ECG

Two leads commonly used for continuous monitoring are leads II and V1 or a modification of V1 (MCL1) (Fig. 26-10). Lead II provides the best visualization of atrial depolarization (represented by the P wave). Leads V1 and MCL1 best visualize the ventricle responsible for ectopic or abnormal ventricular beats.

TELEMETRY

In addition to hardwire monitoring systems, the ECG can be continuously observed by telemetry, the transmission of radio-waves from a battery-operated transmitter worn by the patient to a central bank of monitors. Although telemetry systems have the same capabilities as hardwire systems, they are wireless, al-lowing the patient to ambulate while being monitored. Following the guidelines for electrode placement will ensure good conduc-tion and a clear picture of the patient’s rhythm on the monitor:

·       Clean the skin surface with soap and water and dry well (or as recommended by the manufacturer) before applying the electrodes. If the patient has much hair where the electrodes need to be placed, shave or clip the hair.

·      Apply a small amount of benzoin to the skin if the patient is diaphoretic (sweaty) and the electrodes do not adhere well.

·      Change the electrodes every 24 to 48 hours and examine the skin for irritation. Apply the electrodes to different locations each time they are changed.

·      If the patient is sensitive to the electrodes, use hypoaller-genic electrodes.

SIGNAL-AVERAGED ELECTROCARDIOGRAM

For some patients who are considered to be at high risk for sudden cardiac death, a signal-averaged ECG is performed. This high-resolution ECG assists in identifying the risk for life-threatening dysrhythmias and helps to determine the need for invasive diag-nostic procedures. Signal averaging works by averaging about 150 to 300 QRS waveforms (QRS waveforms represent depolarization of the ventricle). The resulting averaged QRS complex is analyzed for certain characteristics that are likely to lead to lethal ventric-ular dysrhythmias. The recording is performed at the bedside and requires about 15 minutes.

CONTINUOUS AMBULATORY MONITORING

In ambulatory ECG monitoring, which may occur in the hos-pital but is more commonly prescribed for outpatients, one lead of the patient’s ECG can be monitored by a Holter monitor. This monitor is a small tape recorder that continuously (for 10 to 24 hours) documents the heart’s electrical activity on a mag-netic tape. The tape recorder weighs approximately 2 pounds and can be carried over the shoulder or worn around the waist day and night to detect dysrhythmias or evidence of myocardial ischemia during activities of daily living. The patient keeps a diary of activity, noting the time of any symptoms, experiences, or unusual activities performed. The tape recording is then exam-ined with a special scanner, analyzed, and interpreted. Evidence obtained in this way helps the physician diagnose dysrhythmias and myocardial ischemia and evaluate therapy, such as anti-arrhythmic and antianginal medications or pacemaker function.

TRANSTELEPHONIC MONITORING

Another method of evaluating the ECG of a patient at home is by transtelephonic monitoring. The patient attaches a specific lead system for transmitting the signals and places a telephone mouthpiece over the transmitter box; the ECG is recorded and evaluated at another location. This method is often used for di-agnosing dysrhythmias and in follow-up evaluation of permanent cardiac pacemakers.