EARLIER TODAY, Mr. S, 67, was admitted to receive I.V. antibiotics to treat a significant case of lower extremity cellulitis secondary to a cut he sustained fishing. Responding to the call bell, his nurse finds him sitting up in bed and complaining of chest discomfort. The nurse takes his vital signs and performs a pain assessment, which includes documenting the onset, location, quality, intensity, duration, and any radiation of the discomfort. The nurse asks about associated signs and symptoms and factors that aggravate or relieve the pain. Following facility protocol, the nurse administers supplemental oxygen at 4 L/minute via nasal cannula and pages the physician on call, who orders stat serum cardiac biomarkers, a 12-lead ECG, and sublingual nitroglycerin.
A nurse who can independently interpret a 12-lead ECG can anticipate and prepare for any emergency care the patient may need. This article explains the basics of 12-lead ECG interpretation, focusing on a normal ECG. Next month, the second part of this article will discuss ECG abnormalities.
What's happening in the heart
The heart's internal conduction system initiates each heartbeat and coordinates all parts of the heart to contract at the proper time. A normal heartbeat is initiated in the sinoatrial (SA) node, a specialized group of cells in the right atrium. (See Taking anatomy to heart.) The SA node depolarizes at a rate of 60 to 100 times/minute, causing the atria to contract and propel blood into the ventricles.
Atrial depolarization produces the first element on the ECG waveform: the P wave. The first part of the cardiac cycle, the P wave appears as a small, semicircular bump (see Tracing a normal ECG waveform).
The wave of depolarization continues through the atria until it encounters the next important structure, the atrioventricular (AV) node, which receives the atrial impulse. After a brief pause to let the ventricles fill, the AV node transmits the impulse to the ventricles via the bundle of His. A collection of cardiac conduction fibers, the bundle of His splits into the right and left bundle branches.
The bundle branches are high-speed conducting fibers that run down the intraventricular septum and transmit the cardiac impulse to the Purkinje fibers, which form a complex network that mingles with ventricular myocardial cells. The function of the Purkinje fibers is to rapidly stimulate ventricular muscle fibers, resulting in the next major event in the cardiac cycle: ventricular depolarization.
Ventricular depolarization generates the QRS complex, the electrical equivalent of ventricular systole. (Remember that electrical activity precedes mechanical activity, and the ECG shows only electrical activity.) If you palpate a carotid or radial pulse while looking at a cardiac monitor, you should feel a pulse with each QRS complex on the monitor.
The QRS complex normally has a duration of 0.06 to 0.1 second. A duration of 0.12 second or more usually indicates prolonged ventricular conduction caused by a bundle-branch block. The QRS complex is variable in appearance and may have a different shape (morphology) in different patients or even look different in various ECG leads in the same patient. The QRS complex may have one, two, or three wave components, depending on the lead and your patient's clinical status.
The last major wave component of the ECG is the T wave, which is larger than the P wave and rounded or slightly peaked. Immediately following the QRS complex, it represents ventricular repolarization or a metabolic rest period between heartbeats. Depolarization and repolarization are caused by the movement of cations, including sodium, potassium, and calcium, across the myocardial cell membrane.
Besides the three waveforms, the normal ECG cardiac cycle tracing has two important segments, or flat (isoelectric) parts of the tracing between the waveforms: the PR interval and the ST segment.
The PR interval is the period from the beginning of the P wave to the beginning of the QRS complex. It consists of the P wave plus the short isoelectric segment that terminates at the start of the QRS complex. The normal PR interval lasts 0.12 to 0.2 second; this represents the time from SA node depolarization to ventricular depolarization. If the PR interval is less than 0.12 second, then the cardiac impulse didn't follow the normal conduction pathway. If the PR interval is longer than 0.2 second, then a disease process may be affecting the cardiac conduction pathway, keeping it from functioning properly.
The ST segment consists of the isoelectric line between the end of the QRS complex and the beginning of the T wave. The ST segment reveals information about the heart's oxygenation status. For example, myocardial ischemia (a temporary, reversible decrease in oxygenation) often results in an ST segment below the baseline of the ECG tracing. When myocardial cells are injured (reversible physical damage from lack of oxygen), the ST segment often is elevated above the baseline. So ST-segment elevation is a key indicator of myocardial infarction. For tips on how to use the ECG to calculate heart rates and more, see Paper training.
Catching the wave
Examination of a 12-lead ECG reveals that some QRS complexes have upward (positive) deflections and others have downward (negative) deflections. Here's why.
Each ECG lead has a positive (or sensing) electrode and a negative electrode, which acts as an anchor. The positive electrode looks toward its negative electrode and senses whether electrical energy is being directed toward or away from the positive electrode.
When electrical energy is directed toward the positive monitoring electrode, the QRS complex has an upward deflection. When the electrical energy is directed away from the positive monitoring electrode, the QRS complex has a downward deflection. The more directly aligned the direction of the electrical energy with the positive electrode, the more upright the complex. If the electrical energy approaches the positive monitoring electrode at a slight angle, the complex will still be upright, but less upright than if the energy were directly aligned with the positive electrode.
Energy arriving at a perpendicular angle to the positive electrode results in either a waveform with little deflection (isoelectric) or equal amounts of positive and negative deflection.
As energy is directed away from the positive electrode, the QRS complex becomes progressively more negative. When energy flow is directed totally away from the positive electrode, the QRS complex is deflected directly downward.
Going with the flow: A look at vectors
All cardiac cells are electrochemical, meaning they generate electrical energy during depolarization. This electrical energy, called a vector, has strength (measured in millivolts) and direction (measured in degrees from an arbitrary zero point called the electrical axis). Each cardiac cell generates its own microvector. The mathematical average of these microvectors is the mean QRS vector or mean vector, which follows the normal conduction pathway of the heart—downward and to the left. The mean vector flows slightly to the left of the ventricular septum because the left ventricle has more and larger cardiac cells.
Generally, each person has a unique mean vector direction, which remains constant unless his cardiac status changes. For example, left ventricular hypertrophy secondary to heart failure pulls the mean vector even more sharply to the left side.1 A patient who has a mean vector in an abnormal direction is said to have an axis deviation. (For details, see Axis deviation: As easy as pie [charts].)
The mean vector is a representation of the heart's overall electrical properties. A 12-lead ECG is the electrical record of the mean vector from 12 different monitoring sites (leads) on the surface of the body. As when you look at any object, you need to see all the angles to get a complete picture.
Looking at limb leads
The first six leads of the 12-lead ECG come from four electrodes placed on the patient's arms and legs; the right lower leg electrode is the ground electrode. The limb leads record the mean vector in the up-down and left-right direction along the body's frontal plane. Because they use separate positive and negative electrodes, they're called bipolar or standard leads.
- Lead I puts the positive electrode on the left arm and looks toward the negative electrode on the right arm for electrical energy. Because the mean vector travels from upper right to lower left, energy flows toward the positive electrode of lead I, resulting in an upward deflection of the QRS. And because the mean vector doesn't flow directly toward lead I but approaches it at a somewhat broad angle, the upward deflection of the QRS complex is moderate.
- In lead II, the positive electrode is on the left foot and the negative electrode is on the right arm. Because the mean vector flows directly at the positive lead II electrode, this lead usually has the most upright QRS complexes and the most prominent P waves of the entire 12-lead ECG. That's why lead II is a favorite monitoring lead in many intensive care and telemetry units.
- Lead III puts the positive electrode on the left foot and the negative one on the left arm. The mean vector flow approaches lead III downward from the right, again producing an upward QRS deflection. Because the angle is narrower than the angle between the mean vector and lead I, the lead III QRS complex is more upright than the lead I QRS complex.
The second set of limb leads are called the augmented or unipolar leads and use a single positive monitoring electrode. The negative electrode is an electrically calculated location at the center of the heart.2
- Lead aVR is the only limb lead on the right side of the body. Its positive monitoring electrode is located on the right arm and looks downward and to the left. The mean vector also flows downward and to the left, directly away from lead aVR, resulting in a negative deflection for all waveforms. In a normal ECG, aVR is the only limb lead with a downwardly deflected QRS.
- Lead aVL positions a positive electrode on the left arm and looks to the right and downward toward the center of the heart (in contrast to lead I, which looks strictly to the right). The mean vector approaches aVL at a very broad angle, producing the least upright QRS complex among the limb leads.
- Lead aVF has its positive monitoring lead on the left leg and looks straight up to the center of the chest. The mean vector approaches aVF at a fairly direct angle, although not as directly as lead II, so lead aVF has very upright QRS complexes with prominent P waves. Leads II, III, and aVF all look upward at the oncoming mean vector, so their waveforms share many qualities, such as highly positive QRS complexes and prominent P waves. Because these leads look upward at the bottom or inferior ventricular wall of the heart, they're known as the inferior leads.
Six chest leads weigh in
The six chest or precordial leads lie across the anterior chest and measure the mean vector in the horizontal plane.
- Lead V1 is located at the right sternal border, fourth intercostal space, and lies above the right ventricle and septum.
- Lead V2 is at the left side of the sternum, fourth intercostal space.
- Lead V3 is midway between leads V2 and V4.
- Lead V4 is at the midclavicular line in the fifth intercostal space.
- Lead V5 is at the anterior axillary line in the fifth intercostal space.
- Lead V6 is at the midaxillary line, fifth intercostal space, and is positioned above the lateral wall of the left ventricle.
The mean vector in the horizontal plane is influenced by the overwhelming power of the left ventricle and can be thought of as flowing toward the left side. Because the mean vector flows away from lead V1, this lead has a downward QRS deflection; the QRS is almost totally upright in leads V5 and V6 because the mean vector flows directly at these leads. The QRS complex becomes progressively more upright across the chest wall from V1 to V6, a change known as R-wave progression (see R-wave ups and downs).3 This is another characteristic of a normal ECG.
Putting it all together
With this knowledge of 12-lead ECGs in mind, Mr. S's nurse examines his 12-lead ECG. His heart rate is normal, with clear P waves, QRS complexes, and T waves. The PR interval is 0.14 second, which falls within the normal range. The QRS complex should be less than 0.12 second; Mr. S's QRS complexes are 0.08 second wide. The T waves are upright and normal looking. Finally, the ST segment is level with the baseline (isoelectric).
Mr. S's limb leads are all upright with the exception of aVR, which is normal. Lead II is the most upright and aVL is the least upright. The chest leads demonstrate downward lead V1 and upright leads V5 and V6 with normal R-wave progression.
The nurse concludes that Mr. S has a normal 12-lead ECG, indicating no electrical abnormalities. However, he's not out of the woods yet. Some types of myocardial ischemia aren't apparent on a standard 12-lead ECG, so the healthcare provider may consider following up with a cardiac stress test.4
Mr. S's normal ECG, negative serum cardiac biomarkers, and benign patient history lead the medical team to rule out a cardiac source for his discomfort. He's discharged home the next day and scheduled for outpatient cardiac stress testing.
Use the markings on ECG paper to calculate events within the cardiac cycle. The ECG paper is a grid of large and small blocks. On the horizontal axis, a large block is equal to 0.2 second and a small block is equal to 0.04 second. The vertical axis represents voltage or electrical energy, with each vertical millimeter (small block) being 0.1 millivolt of electrical energy. However, in practice, deflections are typically described in millimeters, not millivolts.
Count the number of small squares and multiply by 0.04 to calculate the duration of any event in the ECG tracing. A QRS complex that's 2.5 small squares wide is 0.1 second. The ECG paper can also be used to calculate heart rates, using one of two methods. In the 6-second method, look for the markings (usually short vertical lines) at the top of the rhythm strip or ECG paper. These markings divide the ECG paper into 3-second intervals. Count the number of QRS complexes contained in two intervals (6 seconds) and multiply by 10. This method works for both regular and irregular heart rhythms.
In the division method, count the number of small squares between any two heartbeats. Use the same part in both QRS complexes—usually the peak of the complex works best. Dividing 1,500 by the number of small squares gives the heart rate in beats per minute. This method is accurate only with regular heart rates because irregular heart rhythms have a varying number of small squares between any two QRS complexes.
Axis deviation: As easy as pie (charts)
Combining assessment skills with an understanding of axis deviation can give nurses a more detailed picture of a patient's condition. The hexaxial reference system and the quadrant method can help you visualize problems with cardiac conduction.
Hexaxial reference system
The normal QRS complex (or vector) represents the average electrical signal that the heart generates during depolarization. Within the heart, the mean vector generally flows from upper right to lower left. The exact direction of that flow (called the electrical axis) can be used as an assessment tool in the 12-lead ECG because an abnormal axis can give clues about what's going wrong in the heart's electrical system.
To measure the electrical axis, imagine all six limb leads displayed simultaneously around a central point in a circle, which represents the heart (see the illustration at left). In this hexaxial system, the leads divide the circle into equal 30-degree segments.
Each lead can be assigned a number of degrees, and the mean vector's direction can be given in degrees. If the mean vector is aligned directly with lead I, its axis is 0 degrees. A mean vector directed halfway between leads II and aVF has an axis of 75 degrees. (Although a patient's electrical axis can be manually calculated, all modern 12-lead ECG machines provide this information automatically.)
The normal electrical axis of the heart falls between 0 and +90 degrees. Although this is a wide range, it's a numeric equivalent of the concept that the electrical conduction of the normal heart is right to left and top to bottom.
A left axis deviation occurs when the electrical axis of the heart is between 0 and 90 degrees. A right axis deviation occurs when the electrical axis is in the +90 to +180 degree range. A mean vector having an electrical axis within the range of 90 to 180 degrees is called an indeterminate axis or extreme right axis deviation.
To approximate axis deviation using the quadrant method, divide the circle (which represents the patient's heart) into four quadrants (see the illustration below). Only two ECG leads are required to make this assessment. Examine leads I and aVF. If lead I is upright, then the vector is flowing right to left. If lead aVF is upright, the vector is directed top to bottom. If they're both upright, the electrical axis must fall into the lower left or normal quadrant. This quadrant roughly matches the criteria for normal electrical axis, indicating a normal direction of electrical conduction.
Left axis deviation occurs when lead I is upright and lead aVF is down or negative. The electrical axis is located in the upper right quadrant. The mean vector is abnormally directed to the left side of the heart. A left axis deviation can be caused by many different pathologic conditions. Some left bundle-branch blocks will produce a left axis deviation because the cardiac vector flows abnormally from the right side of the heart to the left. Because the mean vector is not conducted by infarcted tissue and flows away from it, an inferior-wall myocardial infarction will produce a left axis deviation (due to a negative QRS in lead aVF). Many patients with pacemakers have a left axis deviation because the pacemaker leads are on the right side of the heart.
Finally, some structural body changes will produce a left axis deviation. In advanced pregnancy, the enlarged uterus may occupy so much space in the abdomen that the elevated diaphragm pushes the heart to a more horizontal or leftward-lying position, producing a left axis deviation. Similarly, morbidly obese patients or patients with ascites or an abdominal tumor may have a left axis deviation because of the heart's position in the chest.
A right axis deviation is apparent when lead I is negative and lead aVF is upright. The mean vector is abnormally directed to the right side of the heart. Causes of right axis deviation include chronic obstructive pulmonary disease and right ventricular hypertrophy. In both instances, enlargement of the right cardiac chambers pulls the mean vector to the right side. A right bundle-branch block causes the mean vector to flow from left to right, resulting in right axis deviation. Children and tall, thin adults may have a non-pathologic right axis deviation if the heart hangs down in a more vertical position.
If both leads I and aVF are negative, then the axis deviation is termed indeterminate axis or extreme right axis deviation. The mean vector is directed upward and to the right. If you find an indeterminate axis deviation on your patient's ECG, check the leads; incorrect ECG lead placement is a common cause of this finding. Other causes are some types of pacemakers, abnormal cardiac rhythms such as ventricular tachycardia, congenital heart disease, or dextrocardia (heart positioned on the right side of the chest).