Basic Principles
- Despite its limited sensitivity and specificity, the 12-lead ECG is still the standard for the evaluation of myocardial ischemia.
- 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
- The electrical energy sensed is then converted to a graphic display by the ECG machine. This display is referred to as the ECG.
- Each ECG lead consists of a positive and negative pole; each lead also has an axis that represents the direction in which current flows.
- Each lead takes a different view of the heart; therefore, the tracing will be different with each view obtained.
- The direction in which electrical current flows determines how the waveform will appear.
- There are three sets of leads:
- Standard limb or bipolar leads (I, II, III) utilize three electrodes; these leads form a triangle known as Einthoven's Triangle.
- Augmented unipolar leads (aVR, aVL, aVF).
- Precordial unipolar leads (V1, V2, V3, V4, V5, V6).
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- A heart contraction is represented on the ECG graph paper by the designated P wave, QRS complex, and T waves.
- The P wave is the first positive deflection and represents atrial depolarization or atrial contraction.
- The PR interval represents the time it takes for the electrical impulse to travel from the sinoatrial node to the AV node and down the bundle of His to the right and left bundle branches.
- 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. Atrial repolarization (relaxation) occurs during the QRS complex, but cannot be seen.
- The T wave follows the S wave and is joined to the QRS complex by the ST segment. The ST segment represents ventricular repolarization or relaxation. The point that represents the end of the QRS complex and the beginning of the ST segment is known as the J point.
- 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; it represents ventricular depolarization (contraction) and repolarization (relaxation).
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Indications
- The ECG is a useful tool in the diagnosis of conditions that may cause aberrations in the electrical activity of the heart. Examples of these conditions include:
- MI and other types of CAD such as angina.
- Cardiac dysrhythmias.
- Cardiac enlargement.
- Electrolyte disturbances (calcium, potassium, magnesium, and phosphorous).
- Inflammatory diseases of the heart.
- Effects on the heart by drugs, such as antiarrhythmics and tricyclic antidepressants.
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- Despite ECG's many advantages, it also has several shortcomings:
- Fifty percent of all patients with AMI have no ECG changes.
- A patient may have a normal ECG, present pain-free, and still have significant risk for myocardial ischemia.
- Several disease processes can mimic that of an AMI, including: left bundle-branch blocks, ventricular paced rhythms, and left ventricular hypertrophy.
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ECG Leads and Normal Wave Form Interpretation
- 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 second. P waves that exceed these measurements are considered to deviate 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 second. There is a built-in delay in time at the AV node to allow for adequate ventricular filling to maintain normal stroke volume.
- The QRS complex contains separate waves and segments, which should be evaluated separately. Normal QRS complex should be between 0.06 and 0.10 second.
- 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 a completed MI.
- 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.
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- The ST 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.
- 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.
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- The P, Q, R, S, and T waves all appear differently depending on which lead you are viewing.
Nursing and Patient Care Considerations
- Perform ECG or begin continuous ECG monitoring as indicated.
- Provide privacy, and ask 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 as movement may trigger alarms and false readings.
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- Interpretation of a rhythm strip. Develop a systematic approach to assist in accurate interpretation.
Determine the rhythm—is it regular, irregular, regularly irregular, or irregularly irregular? Use calipers, count blocks between QRS complexes, or measure the distance between R waves to determine regularity.
- Determine the rate, is it fast, slow, or normal?
- A gross determination of rate can be accomplished by counting the number of QRS complexes within a 6-second time interval (use the superior margin of ECG paper) and multiplying the complexes by a factor of 10.Note: 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 5-square blocks between each two QRS complexes into 300. Three hundred large blocks represent 1 minute on the ECG paper. Example: In Figure 12-3, the number of large square blocks between complexes #5 and #6 equals 5, and 300 ÷ 5 = 60, or a rate of 60.
- Evaluate the P wave are P waves present? Is there a P for every QRS complex? If there is not a P for every QRS, do the P waves have a normal configuration?
- Measure and evaluate the PR interval.
- Evaluate the QRS complex—Measure the QRS complex and examine its configuration.
- Evaluate the ST segment—An elevated ST segment heralds a pattern of injury and usually occurs as an initial change in acute MI. ST depression occurs in ischemic states. Calcium and potassium changes also affect the ST segment.
- Evaluate the T wave are T waves present? Do all T waves have a normal shape? Could a P wave be hidden in the T wave, indicating a junctional rhythm or third degree heart block? Is it positively or negatively deflected (inverted T waves indicate ischemia) or peaked (indicative of hyperkalemia)?
- Evaluate the QT interval should be less than one half the R-R interval. Prolonged QT interval may indicate digoxin toxicity, long-term quinidine (Quinaglute) or procainamide (Pronestyl) therapy, or hypomagnesemia.
