The electrocardiogram (ECG) is a graphic display of the heart ’s electrical activity. When electrodes are attached to the patient ’s limbs or chest and connected by cables to an ECG machine, the ECG machine functions as a voltmeter, detecting and recording the changes in voltage (ie, action poten-tials) generated by depolarization and repolarization of the heart ’s cells. The voltage changes are displayed as specific waveforms and complexes (Fig. 3.6). Practice standards for ECG monitoring are shown in Box 3.1.
TABLE 3.1 Summary of the Conduction System
Structure Function
Intrinsic
Pacemaker Rate (beats/min) Sinoatrial (SA) node Primary pacemaker; initiates impulse that is normally
conducted throughout the left and right atria
60 to 100 Atrioventricular (AV) node Receives impulse from SA node and delays relay of
the impulse to the bundle of His, allowing time for the atria to empty their contents into the ventricles before the onset of ventricular contraction.
Purkinje fibers Receives impulse from bundle branches and relays it to ventricular myocardium
Fig. 3.6 Schematic drawing of the conducting system of the heart. An impulse normally is generated in the SA node and travels through the atria to the atrioventricular (AV) node, down the bundle of His and Purkinje fibers, and to the ventricular myocardium. Recording of the depolarizing and repolarizing currents in the heart with electrodes on the surface of the body produces characteristic waveforms. (From Copstead-Kirkhorn LE, Banasik JL: Pathophysiology , ed 5, St Louis, 2013, Saunders.)
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CHAPTER 3 Cardiac Anatomy and Electrophysiology
Electrodes
Electrode refers to an adhesive pad containing a conductive substance in the center that is applied to the patient ’s skin (Fig. 3.7). The conductive media of the electrode conducts skin surface voltage changes through wires to a cardiac monitor (ie, electrocardiograph). Electrodes are applied at specific locations on the patient ’s chest wall and extremities to view the heart ’s electrical activity from different angles and planes.
One end of a monitoring cable, which is also called a lead wire, is attached to the electrode and the other end to an ECG machine. The cable conducts current back to the cardiac monitor. Three-lead wire systems are often used with portable monitor defibrillators. Five-lead wire systems allow viewing of the six limb leads (ie, I, II, III, aVR, aVL, and aVF) and one chest lead.
Leads [Objective 6]
A lead is a record (ie, tracing) of electrical activity between two electrodes. Each lead records the average current flow at a specific time in a portion of the heart. A 12-lead ECG provides views of the heart in both the frontal and horizontal planes and views the surfaces of the left ventricle from 12 different angles.
From this, ischemia, injury, and infarction affecting areas of the heart can be identified. The 12-lead ECG is an essential part of the diagnostic workup of patients with a suspected ACS.
Fig. 3.7 Electrodes are adhesive pads applied at specific locations on the patient ’s chest wall and limbs. (Courtesy Bruce R.
Shade, EMT-P, EMS-I, AAS.)
BOX 3.1 Practice Standards for Cardiac Monitoring
Cardiac monitoring is indicated in most, if not all, of the following:
• Patients resuscitated from sudden cardiac death
• Patients in the early phase of ACSs
• Patients with unstable coronary syndromes and newly diagnosed high-risk coronary lesions
• Adults and children who have undergone cardiac surgery
• Patients who have undergone nonurgent per-cutaneous coronary intervention with
complications
• Patients who have undergone implantation of an automated defibrillator lead or
a pacemaker lead and who are considered pacemaker dependent
• Patients with a temporary pacemaker or transcutaneous pacing pads
• Patients with AV block
• Patients with arrhythmias and Wolff-Parkinson-White syndrome
• Patients with long-QT syndrome and arrhythmias
• Patients with intra-aortic balloon pumps
• Patients with acute heart failure
• Patients with indications for intensive care
• Patients undergoing conscious sedation
• Patients with unstable arrhythmias
• Pediatric patients with symptoms of arrhythmia
(Drew, et al., 2004)
Frontal Plane Leads [Objectives 6, 7]
Six leads view the heart in the frontal plane. Leads I, II, and III are called standard limb leads. Leads aVR, aVL, and aVF are called augmented limb leads.
A bipolar lead is an ECG lead that has a positive and negative electrode. Each lead records the dif-ference in electrical potential (ie, voltage) between two selected electrodes. Although all ECG leads are technically bipolar, leads I, II, and III use two distinct electrodes, one of which is connected to the pos-itive input of the ECG machine and the other to the negative input ( Wagner, et al., 2009).
Leads I, II, and III make up the standard limb leads. If an electrode is placed on the right arm, left arm, and left leg, three leads are formed (Fig. 3.8). The positive electrode is located at the left wrist in lead I, while leads II and III both have the positive electrode located at the left foot. The difference in electrical
potential between the positive pole and its corresponding negative pole is measured by each lead.
Leads aVR, aVL, and aVF are limb leads that record measurements at a specific electrode with respect to a reference electrode (see Fig. 3.8). The“a ”in aVR, aVL, and aVF refers to augmented. The“V ” refers to voltage, and the last letter refers to the position of the positive electrode. The“R ”refers to the rightarm, the“L” to left arm, and the“F ” to left foot (ie, leg). A summary of the limb leads appears in Table 3.2.
Horizontal Plane Leads [Objectives 6, 7]
Six chest (ie, precordial or “V ”) leadsview the heart in the horizontal plane. This allows a view of the front and left side of the heart. The chest leads are identified as V 1, V 2, V 3, V 4, V 5, and V 6. Each electrode placed in a “V ” position is a positive electrode (Fig. 3.9). A summary of the chest leads can be found in Table 3.3.
ACLS Pearl
Lead V 1 is particularly useful for analyzing dysrhythmias that have a wide QRS complex (eg, bundle branch blocks, ventricular pacemaker rhythms, wide-QRS tachycardias).
Fig. 3.8 View of the standard limb leads and augmented leads. LA, left arm; LL, left leg; RA, right arm. (From Boron WF:
Medical physiology, ed 2 updated edition, Philadelphia, 2011, Saunders.)
TABLE 3.2 Limb Leads
Lead Positive Electrode Position Negative Electrode Position Heart Surface Viewed
I Left arm Right arm Lateral
II Left leg Right arm Inferior
III Left leg Left arm Inferior
aVR Right arm Reference electrode None
aVL Left arm Reference electrode Lateral
aVF Left foot (ie, leg) Reference electrode Inferior
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CHAPTER 3 Cardiac Anatomy and Electrophysiology
Right chest leads are used to evaluate the right ventricle(Fig. 3.10). The placement of right chest leads is identical to the placement of the standard chestleads except that it is done on the rightside of the chest.
If time does not permit obtaining all of the right chest leads, the lead of choice is V 4R. A summary of the right chest leads can be found in Table 3.4.
Leads V 7, V 8, and V 9 permit viewing of the posterior surface of the heart ( Fig. 3.11). All of the leads are placed on the same horizontal line as V 4to V 6.LeadV 7is placed at the posterior axillary line. Lead V 8
is placed at the angle of the scapula (ie, the posterior scapular line), and lead V 9 is placed over the left border of the spine.
ACLS Pearl
Multiple-lead ECGs are used to help spot infarctions of the right ventricle and the posterior wall of the left ventricle. The 15-lead ECG uses all of the leads of a standard 12-leadECG plus leads V 4R, V 8, and V 9 or a standard 12-lead plus posterior leads V 7, V 8, and V 9. A 16-lead ECG machine allows recording of a standard 12-lead plus leads V 3R, V 4R, V 5R, and V 6R. An 18-lead ECG uses all of the leads of a standard 12-lead ECG plus leads V 4R, V 5R, V 6R, V 7. V 8, and V 9.
X
X X X X
X
V1 V3 V5
V2 V4 V6
Midaxillary line
Anterior axillary line
Midclavicular line
Fig. 3.9 Chest (ie, precordial) leads V 1 through V 6. (From Copstead-Kirkhorn LE, Banasik JL: Pathophysiology , ed 5, St Louis, 2013, Saunders.)
TABLE 3.3 Chest Leads
Lead Positive Electrode Position Heart Area Viewed
V 1 Right side of sternum, fourth intercostal space Interventricular septum V 2 Left side of sternum, fourth intercostal space Interventricular septum
V 3 Midway between V 2 and V 4 Anterior surface
V 4 Left midclavicular line, fifth intercostal space Anterior surface V 5 Left anterior axillary line; same level as V 4 Lateral surface V 6 Left midaxillary line, fifth intercostal space Lateral surface
8
V2R: Fourth ICS at right sternal border (same as V1)
V3R: Halfway between V2R and V4R
V5R: Right anterior axillary line at the fifth ICS
V6R: Right midaxillary line at the fifth ICS V4R: Right midclavicular line in the fifth ICS 1
Fig. 3.10 Electrode locations for recording a right chest electrocardiogram (ECG). Right chest leads are not part of a standard 12-lead ECG but are used when a right ventricular infarction is suspected. (From Drew BJ, Ide B: Right ventricular infarction, Prog Cardiovascular Nurs 10:46, 1195.)
Fig. 3.11 Posterior chest lead placement. (From Drew BJ, Ide B: Right ventricular infarction, Prog Cardiovascular Nurs 10:46, 1195.)
TABLE 3.4 Right Chest Leads and Their Placement
Lead Placement
CHAPTER 3 Cardiac Anatomy and Electrophysiology
Electrocardiography Paper
ECG paper is graph paper made up of small and large boxes measured in millimeters (mms). The smallest boxes are 1 mm wide and 1 mm high (Fig. 3.12). The horizontal axis of the paper corresponds with time, which is stated in seconds. ECG paper normally records at a constant speed of 25 mm/second.
Thus each horizontal 1 mm box represents 0.04 second (25 mm/sec0.04 second¼1 mm). The lines after every five small boxes on the paper are heavier. The heavier lines indicate one large box, which represents 0.20 second.
The vertical axis of the graph paper represents the voltage or amplitude of theECG waveformsor deflec-tions. Voltage is measured in mV. Amplitude is measured in mm. When properly calibrated, a small box is 1 mm high (ie, 0.1 mV), and a large box, which is equal to five small boxes, is 5 mm high (ie, 0.5 mV).
Waveforms and Complexes [Objective 8]
An ECG waveform (ie, a deflection) is movement away from the baseline (ie, isoelectric line) in either a positive (ie, upward) or negative (ie, downward) direction. Waveforms are named alphabetically, begin-ning with P, QRS, and T (Fig. 3.13).
The P wave is the first waveform in the cardiac cycle and represents atrial depolarization and the spread of the electrical impulse throughout the right and left atria. A P wave is normally positive (ie, upright) in standard leads and precedes each QRS complex.
A m p
Fig. 3.12 ECG strip showing the markings for measuring amplitude and duration of waveforms, using a standard recording speed of 25 mm/sec. (From Copstead-Kirkhorn LE, Banasik JL: Pathophysiology , ed 5, St Louis, 2013, Saunders.)
0 0.2 0.4 0.6 0.8 1.0
Fig. 3.13 Components of the ECG recording. AV, atrioventricular; SA, sinoatrial. (From Boron WF: Medical physiology , ed 2 updated edition, Philadelphia, 2011, Saunders.)
The QRS complex consists of the Q wave, R wave, and S wave. It represents the spread of the elec-trical impulse through the ventricles (ie, ventricular depolarization). A QRS complex normally follows each P wave. In adults, the normal duration of the QRS complex is 0.11 second or less (Surawicz, et al., 2009). When viewing the chest leads in a normal heart, the R wave becomes taller (ie, increases in ampli-tude) and the S wave becomes smaller as the electrode is moved from right to left. This pattern is called R-wave progression. The transition zone is the area at which the amplitude of the R wave begins to exceed the amplitude of the S wave (Ganz, 2012). This usually occurs in the area of leads V 3 and V 4. Poor R-wave progression is a phrase used to describe R waves that decrease in size from V 1 to V 4. Possible causes include right or left ventricular hypertrophy and left bundle branch block, among other causes.
Poor R-wave progression may also be a nonspecific indicator of anterior wall infarction. Electrode place-ment in the correct intercostal space is critical when evaluating R-wave progression.
Ventricular repolarization is represented on the ECG by the ST segment (discussed later) and the T wave. The direction of the T wave is normally the same as the QRS complex that precedes it.
A U wave is a small waveform that, when seen, follows the T wave. The U wave is thought to represent repolarization of the Purkinje fibers in the papillary muscle of the ventricular myocardium.
Segments and Intervals [Objectives 8, 9]
A segment is a line between waveforms. It is named by the waveform that precedes or follows it. An interval is made up of a waveform and a segment.
The PR segment is the horizontal line between the end of the P wave and the beginning of the QRS complex. The P wave plus the PR segment equals the PR interval. The PR interval normally measures 0.12 to 0.20 second in adults.
The TP segment is the portion of the ECG tracing between the end of the T wave and the beginning of the next P wave, during which there is no electrical activity (Fig. 3.14). When the heart rate is within normal limits, the TP segment is usually isoelectric and is used as the reference point from which to estimate the position of the isoelectric line and determine ST segment displacement. With rapid heart rates, the TP segment is often unrecognizable because the P wave encroaches on the preceding T wave.
When the TP segment is unrecognizable, the PR segment is used as the reference point from which to estimate the position of the isoelectric line.
The portion of the ECG tracing between the QRS complex and the T wave is the ST segment (see Fig. 3.13). The ST segment represents the early part of repolarization of the right and left ventricles.
In the limb leads, the normal ST segment is isoelectric (ie, flat) but may normally be slightly elevated or depressed. The point where the QRS complex and the ST segment meet is called the ST junction or the J point. The ST segment is considered elevated if the segment is deviated above the baseline and is considered depressed if the segment deviates below it. Various conditions may cause the displace-ment of the ST segdisplace-ment from the isoelectric line in either a positive or a negative direction. Some displacement of the ST segment from the isoelectric line is normal and dependent on age, gender, and ECG lead.
When looking for ST segment elevation or depression, first locate the J point. Next use the TP seg-ment to estimate the position of the isoelectric line. Then compare the level of the ST segseg-ment to the isoelectric line. Deviation is measured as the number of mm of vertical ST segment displacement from the isoelectric line or from the patient ’s baseline at the J point ( Thygesen, et al., 2012). Proper machine
P
Fig. 3.14 The TP segment is used as the reference point for the isoelectric line. (From Aehlert B: ECGs made easy, ed 3, St. Louis, 2006, Mosby.)
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CHAPTER 3 Cardiac Anatomy and Electrophysiology
calibration is critical when analyzing ST segments. The ST segment criteria described here apply only when the monitor is adjusted to standard calibration.
The QT interval is the period from the beginning of the QRS complex to the end of the T wave (see Fig. 3.13). It represents total ventricular activity; this is the time from ventricular depolarization (ie, acti- vation) to repolarization (ie, recovery). The QT interval is measured from the beginning of the QRS
complex to the end of the T wave. In the absence of a Q wave, the QT interval is measured from the beginning ofthe R wave to the end of the T wave. The term QT interval is used regardless of whether the QRS complex begins with a Q wave or an R wave.
The duration of the QT interval varies in accordance with age, gender, and heart rate. As the heart rate increases, the QT interval shortens (ie, decreases). As the heart rate decreases, the QT interval lengthens (ie, increases). Because of the variability of the QT interval with the heart rate, it can be measured more accurately if it is corrected (ie, adjusted) for the patient ’s heart rate. The corrected QT interval is noted as QTc. The QT interval is considered short if it is 0.39 second or less and prolonged if it is 0.46 second or longer in women or 0.45 second or longer in men (Rautaharju, et al., 2009). A prolonged QT interval may be congenital or acquired and indicates a lengthened RRP. A QTc of more than 0.50 second in either gender has been correlated with a higher risk for life-threatening dysrhythmias (eg, torsades de pointes [TdP]). A systematic approach to rhythm analysis appears in Box 3.2.