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ELECTROCARDIOGRAM

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					ELECTROCARDIOGRAM (ECG) MONITORING
CHOOSING A LEAD
Three-Lead System

1.       Lead II (Figure 6-7): this is a common lead used in cardiac monitoring. An advantage of this
lead is that it allows observation of QRS axis changes associated with left anterior hemiblock.

2.      Modified chest lead (MCL1) (Figure 6-8): this lead is a preferred monitoring lead for
identifying type of bundle branch block (BBB) and differentiating ventricular ectopy from aberrant
ventricular conduction.

3.      Lewis lead: this lead is especially useful for identification of P waves. Positive electrode is at
fourth intercostal space, right of sternum; negative electrode is at second intercostal space, right of
sternum.

4.      MCL6 lead: the use of this lead enables the clinician to switch from viewing MCL1 to MCL6
(V6) by moving only the positive electrode. It is a useful lead in those patients with a median
sternotomy.
Five-Lead System

The five-lead system is shown in Figure 6-9 and allows the clinician to place the chest lead on select
sites on the chest for ECG monitoring. More complete information can be obtained from the 12-lead
ECG, which includes the six limb leads and six precordial leads (Figure 6-10).
Figure 6-7      Lead II. Positive electrode—left leg; negative electrode—right arm. (From Stillwell S,
Randall E: Pocket guide to cardiovascular care, ed 2, St Louis, 1994, Mosby.)




Figure 6-8      Lead MCL1. Positive electrode—fourth intercostal space, right sternum; negative
electrode—beneath left midclavicle. (From Stillwell S, Randall E: Pocket guide to cardiovascular
care, ed 2, St Louis, 1994, Mosby.)
RHYTHM STRIP ANALYSIS

Heart Rate (HR) Determination

Standard ECG paper is made up of a series of 1-mm squares, with each millimeter equal to 0.04
second. Each group of five small squares is marked by a darker line, so that one large square (5 mm)
equals 0.20 second (Figure 6-11).

Figures 6-12 and 6-13 illustrate how to determine HR.

Rhythm Determination

To determine whether the rhythm is regular, measure the R-R or P-P intervals and determine whether
the length of the intervals is constant. The rhythm is regular if the length of the shortest and longest
interval varies by ≤0.12 second. If the rhythm is irregular, it should be determined whether there is any
kind of pattern to the irregularity or if it is totally erratic.
Figure 6-9      Five-lead system. RA electrode—below right clavicle, midclavicular line. LA
electrode—below left clavicle, midclavicular line. RL electrode—right abdomen, midclavicular line.
LL electrode—left abdomen, midclavicular line. (From Stillwell S, Randall E: Pocket guide to
cardiovascular care, ed 2, St Louis, 1994, Mosby.)
Figure 6-10 Precordial leads. V lead (positive) chest lead can be placed at identified locations to
record V1, V2, V3, V4, V5, and V6. (From Stillwell S, Randall E: Pocket guide to cardiovascular care,
ed 2, St Louis, 1994, Mosby.)




Figure 6-11   ECG paper.
Figure 6-12 Heart rate determination with regular rhythm. Rate can be determined by dividing 300
by the number of large squares between cardiac cycles (300/4) or by dividing the number of small
squares between cardiac cycles into 1500 (1500/20 = 75 beats/min). (From Stillwell S, Randall E:
Pocket guide to cardiovascular care, ed 2, St Louis, 1994, Mosby.)




In addition to the rate and regularity of the rhythm, the PR and QRS intervals must be determined, as
well as the relationship of atrial activity (P waves) to ventricular activity (QRS complex). Normal PR
interval is 0.12 to 0.20 second and normal QRS interval is up to 0.10 second. QT interval is usually
less than half the preceding R-R interval (Table 6-3).
Figure 6-13 Heart rate determination with irregular rhythm. Heart rate can be approximated by
multiplying the number of cardiac cycles in a 6-second period by 10; the heart rate in this example is
approximately 80 beats/min. (From Stillwell S, Randall E: Pocket guide to cardiovascular care, ed 2,
St Louis, 1994, Mosby.)
Table 6-3   COMPONENTS OF NORMAL ELECTROCARDIOGRAM (ECG)
DYSRHYTHMIAS

Sinus Bradycardia (Figure 6-14)

1.     Determinants

Rhythm: regular

Rate: <60

P waves: present; same morphology

PR interval: 0.12 to 0.20 second

QRS: ≤0.10 second; same morphology

2.     Treatment: if the patient is asymptomatic, none. If symptomatic, atropine, pacemaker,
epinephrine, dopamine.

Sinus Tachycardia (Figure 6-15)

1.     Determinants

Rhythm: regular

Rate: >100

P waves: present; same morphology

PR interval: 0.12 to 0.20 second

QRS: ≤0.10 second; same morphology

2.     Treatment: treat the cause (e.g., stress/anxiety, fever, heart failure, pain, hypoxia,
hyperthyroidism)
Figure 6-14 Sinus bradycardia. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)




Figure 6-15 Sinus tachycardia. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)




Sinus Arrhythmia (Figure 6-16)
1.     Determinants

Rhythm: irregular; varies by >0.12 second

Rate: variable; increases with inspiration, decreases with expiration

P waves: present; same morphology

PR interval: 0.12 to 0.20 second

QRS: ≤0.10 second; same morphology

2.     Treatment: none

Premature Atrial Complex (Figure 6-17)

1.     Determinants

Rhythm: irregular because of ectopic beats

Rate: that of underlying rhythm

P waves: present; same morphology except for ectopic beat

PR interval: 0.12 to 0.20 second (PR interval of the ectopic beat may vary from the others)

QRS: ≤0.10 second; same morphology

2.     Treatment: usually none

Atrial Tachycardia (Figure 6-18)

1.     Determinants

Rhythm: regular

Rate: atrial rate 130 to 250; ventricular rate may be less than atrial rate as a result of lack of conduction
through the AV node

P waves: present; same morphology

PR interval: 0.12 to 0.20 second

QRS: ≤0.10 second, same morphology

2.     Treatment: identify and treat the cause (e.g., if caused by digitalis toxicity, discontinue digoxin)
Figure 6-16 Sinus arrhythmia. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)




Figure 6-17 Premature atrial complex. (From Conover MB: Pocket guide to electrocardiography, ed
5, St Louis, 2004, Mosby.)
Figure 6-18 Atrial tachycardia. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)




Atrial Flutter (Figure 6-19)

1.      Determinants

Rhythm: atrial rhythm is regular; ventricular rhythm regular or irregular, depending on the AV
conduction pattern

Rate: atrial rate 230 to 350; ventricular rate usually <150 (depends on AV conduction)

P waves: absent; replaced by flutter waves, which have a sawtooth appearance

PR interval: none

QRS: ≤0.10 second; same morphology

2.     Treatment: if hemodynamically unstable, cardioversion
Figure 6-19 Atrial flutter. (From Conover MB: Pocket guide to electrocardiography, ed 5, St Louis,
2004, Mosby.)




Atrial Fibrillation (Figure 6-20)

1.      Determinants

Rhythm: irregular

Rate: atrial rate >350; ventricular rate is variable

P waves: none; fibrillatory waves create a wavy, undulating baseline
PR interval: none

QRS: ≤0.10 second; normal morphology

2.     Treatment: if hemodynamically unstable, cardioversion

Paroxysmal Supraventricular Tachycardia (Figure 6-21)

1.     Determinants

Rhythm: regular; repeated episodes of tachycardia with an abrupt onset lasting from a few seconds to
many hours

Rate: 150 to 250

P waves: cannot be clearly identified; may distort the preceding T wave

PR interval: none measurable

QRS: ≤0.10 second; same morphology

Ratio: unable to determine

2.     Treatment: treat underlying cause; if hemodynamically unstable, cardioversion
Figure 6-20 Atrial fibrillation. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)




Figure 6-21 Paroxysmal supraventricular tachycardia. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




Junctional Rhythm (Figure 6-22)

1.     Determinants

Rhythm: regular

Rate: escape 40 to 60
accelerated 62 to 99

tachycardia 100 to 140

P waves: absent, negative in II, III, aVF; may be dissociated from QRS

PR: not applicable (NA)

QRS: ≤0.10 second; same morphology

2.    Treatment: if asymptomatic, none. If symptomatic because of decreased HR, atropine or
pacemaker may be indicated.

Premature Ventricular Complex (PVC) (Figure 6-23)

1.     Determinants

Rhythm: irregular because of the PVC

Rate: varies with the underlying rhythm

P waves: no P wave with the PVC

QRS: PVC is wide and bizarre morphology

2.      Treatment: none if benign
Figure 6-22 Accelerated idiojunctional rhythm. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




Figure 6-23 Premature ventricular complex. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)
Accelerated Idioventricular Rhythm (Figure 6-24)

1.     Determinants

Rhythm: regular

Rate: 40 to 100 beats/min

P waves: absent

PR interval: none

QRS: wide (>0.12 second); bizarre appearance with same morphology

2.      Treatment: none unless hemodynamically unstable, then treat as with other bradydysrhythmias
(atropine and/or pacemaker)

Ventricular Tachycardia (Figure 6-25)

1.     Determinants

Rhythm: regular or slightly irregular

Rate: 100 to 250

P waves: usually not seen; if present, will be dissociated from ventricular rhythm

PR interval: none measurable

QRS: wide (>0.12 second) and bizarre morphology

2.      Treatment: defibrillation, CPR, vasopressor (i.e., vasopressin), epinephrine, amiodarone,
lidocaine
Figure 6-24 Accelerated idioventricular rhythm. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




Figure 6-25 Ventricular tachycardia. (From Conover MB: Pocket guide to electrocardiography, ed
5, St Louis, 2004, Mosby.)
Ventricular Fibrillation (Figure 6-26)

1.     Determinants

Rhythm: irregular, chaotic baseline

Rate: unable to measure

P waves: none

PR interval: none

QRS: none

2.      Treatment: defibrillation, CPR, vasopressor (i.e., vasopressin), epinephrine, amiodarone,
lidocaine

Torsades de Pointes (Figure 6-27)

1.     Determinants

Rhythm: regular or slightly irregular

Rate: 200 to 250

P waves: usually not seen; if present, will be dissociated from ventricular rhythm

PR interval: none measurable

QRS: wide (>0.12 second) and bizarre morphology; QRS complexes appear to be constantly changing
and twist in a spiral pattern around the baseline

2.     Treatment: if hemodynamically unstable, electrical therapy, magnesium, or overdrive pacing
Figure 6-26 Ventricular fibrillation. (From Conover MB: Pocket guide to electrocardiography, ed 5,
St Louis, 2004, Mosby.)




Figure 6-27 Torsades de pointes. (From Conover MB: Pocket guide to electrocardiography, ed 5, St
Louis, 2004, Mosby.)
First-Degree Atrioventricular (AV) Block (Figure 6-28)

1.     Determinants

Rhythm: regular

Rate: 60 to 100

P waves: normal; same morphology

PR interval: >0.20 second

QRS: ≤0.10 second; same morphology

2.     Treatment: usually none unless associated with symptomatic bradycardia

Second-Degree Atrioventricular (AV) Block (Type I, Wenckebach) (Figure 6-29)

1.     Determinants

Rhythm: irregular

Rate: atrial rate 60 to 100; ventricular rate is slower as a result of dropped beats

P waves: normal, same morphology

PR interval: progressive lengthening with each beat until a QRS is dropped; the PR interval is reset to
normal with the dropped beat and the cycle of PR lengthening begins again

QRS: ≤0.10 second; same morphology

2.      Treatment: usually none
Figure 6-28 First-degree atrioventricular (AV) block. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)
Figure 6-29 Second-degree atrioventricular (AV) block, type I. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




Second-Degree Atrioventricular (AV) Block (Type II) (Figure 6-30)

1.     Determinants

Rhythm: irregular

Rate: atrial 60 to 100; ventricular rate is slower as a result of dropped beats

P waves: normal; same morphology

PR interval: 0.12 to 0.20 second and fixed, except where beat is dropped

QRS: wide; same morphology

2.      Treatment: pacemaker; there is a high tendency to progress to complete heart block
Figure 6-30 Second-degree atrioventricular (AV) block, type II. (From Conover MB: Pocket guide
to electrocardiography, ed 5, St Louis, 2004, Mosby.)




Third-Degree Heart Block (Complete Heart Block) (Figure 6-31)

1.     Determinants

Rhythm: regular

Rate: atrial 60 to 100; ventricular depends on site of escape rhythm, usually 20 to 60

P waves: normal; same morphology

PR interval: not measurable; no association between atrial rhythm and ventricular rhythm
QRS: normal (≤0.10 second) if from AV junction; widened (>0.12 second) if from below the bundle of
His

2.     Treatment: usually pacing is required

ELECTROCARDIOGRAPHY: TWELVE-LEAD

Clinical Brief

Twelve-lead ECG is used as a diagnostic tool in determining overall electrical functioning of the heart
and can aid in identifying pathologic conditions. Normal and abnormal activity, as evidenced by
examining individual waves, deflections, intervals, and segments, can be evaluated.

Twelve Leads

The 12 leads are either bipolar or unipolar. The precordial leads (V1-V6) are unipolar and provide
information about anterior, posterior, right, and left electrical forces. The bipolar limb leads (I, II, III)
consist of a positive and a negative electrode and compose Einthoven's triangle. Leads aVR, aVL, and
aVF are unipolar limb leads representing augmented vector right, left, and foot. The limb leads provide
information about vertical electrical forces, as well as left and right forces.
Figure 6-31 Third-degree atrioventricular (AV) block (complete heart block). (From Conover MB:
Pocket guide to electrocardiography, ed 5, St Louis, 2004, Mosby.)




Deflections

Deflections signify individual cardiac cycle events and their electrical direction in relation to a positive
electrode. When the electrical current moves in the general direction of a positive electrode, an upward
or positive deflection is recorded. Conversely, a downward or negative deflection signifies movement
away from the positive electrode. Major deflections are referred to as the P, Q, R, S, T, and U waves.

Waves, Intervals, and Segments

See Table 6-3 for components of a normal ECG.

Electrical Axis

An imaginary line drawn between two electrodes is called the axis of the lead. A vector signifies a
quantity of electrical force that has both a given magnitude and direction. When the cardiac vector is
parallel to the axis of the lead recording it, the ECG deflection is either the most upright or the most
negative (Figure 6-32). When the direction of the electrical activity is perpendicular to the axis of the
lead recording it, an equiphasic deflection will be recorded.
Figure 6-32 Axis. When a mean vector is parallel to the axis of a lead, the tallest (electrical current
flowing toward the positive lead) or the deepest (electrical current flowing away from the positive
lead) complex will result in that lead.




Hexaxial Reference System

The mean vector, or axis of the heart, can be measured in degrees using the hexaxial reference system.
A normal QRS vector should lie between −30 and +110. Left axis deviation can be caused by left
anterior hemiblock, left bundle branch block (LBBB), left ventricular hypertrophy, obesity, or inferior
myocardial infarction (MI). Right axis deviation can be caused by left posterior hemiblock, RV
hypertrophy, limb lead reversal, dextrocardia, or lateral MI.

Quick Method to Axis Determination

Figure 6-33 illustrates a quick method to determine axis.
Figure 6-33 Estimating axis using leads I and II. A, If the QRS is upright in both I and II, the axis is
normal. B, If the QRS is upright in I and down in II, left axis deviation is present. C, If the QRS is
down in both I and II, indeterminate axis is present. D, If the QRS is down in I and upright in II, right
axis deviation is present.
Electrocardiogram (ECG) Pattern Associated with Ischemia

Reduced blood supply is characterized by inverted T waves, transient ST depression during anginal
episodes because of a fixed lesion, and transient ST elevation during anginal episodes that are due to
vasospasm (Figure 6-34).

Electrocardiogram (ECG) Pattern Associated with Injury

Acuteness of an infarction is represented by ST segment elevation. ST segment elevation is one of the
earliest changes characteristic of infarction. ST segment depression is characteristic of non–Q-wave
infarction.

Electrocardiogram (ECG) Pattern Associated with Q-Wave Infarction

Indicative changes, significant Q waves, ST elevation, and T-wave inversion can be found in leads
over infarcted myocardium.

Anterior MI—leads V1-V4

Lateral MI—leads I, aVL, and V5 and V6

Inferior MI—leads, II, III, and aVF

Posterior MI—reciprocal changes in anterior leads (e.g., tall R wave, ST depression, and tall
symmetric T wave in V1-V4)

RV infarction is represented by lead V4R and will exhibit ST elevation >1 mm.
Wellen's Syndrome—Critical Stenosis of Proximal Left Anterior Descending (LAD) Artery

Signs on ECG during pain-free period in patients with unstable angina indicating critical stenosis of
the proximal LAD artery and impending infarction. These include little or no ST elevation in V2 and
V3, ST segment turning down into a deeply inverted and symmetric T wave in V2 and V3, and no
significant Q waves in precordial leads (Figure 6-35).
Figure 6-34 ECG changes with infarction.




Figure 6-35 ECG changes associated with Wellen's syndrome. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




Differentiating Bundle Branch Block (BBB)

LBBB characteristics include a mainly negative QRS in V1 and an R wave with no Q or S wave in
leads I, aVL, and V6. Right bundle branch block (RBBB) characteristics include an upright QRS in V1;
intrinsicoid deflection 0.07 second or later in V1; and small Q wave and broad S wave in leads I, aVL,
and V6. Figure 6-36 illustrates a quick identification of BBB.

Chamber Enlargement

Table 6-4 outlines changes on ECG indicative of chamber enlargement.

Ventricular Ectopy versus Aberrant Ventricular Conduction

Wide QRS complexes with bizarre morphology generally signify either ventricular ectopy or aberrant
ventricular conduction. Ventricular ectopy has its origin in the ventricle and indicates increased
ventricular irritability. Aberrant ventricle conduction is caused by a supraventricular impulse (e.g.,
premature atrial contraction [PAC]) arriving at the ventricle too early; the ventricles are not totally
repolarized so the impulse is conducted abnormally or aberrantly.
Figure 6-36 Right bundle branch block (RBBB) and left bundle branch block (LBBB). A, Check
right precordial leads (V1, V2) for RR′, suggesting RBBB. B, Check left precordial leads (V5, V6) for
RR′, suggesting LBBB.
Table 6-4      CHAMBER ENLARGEMENT




       *4 points, left ventricular hypertrophy (LVH) likely; 5 points, SSSLVH present.

Both ventricular ectopy and aberrant conduction present as a wide QRS. Differential diagnosis is
important because treatment is different.

Characteristics favoring aberrancy include QRS 0.12 to 0.14 second, and P waves (if identifiable) are
associated with the QRS. Characteristics favoring ectopy include QRS >0.14 second and P waves (if
identifiable) independent of the QRS. Additional diagnostic clues are seen in Figure 6-37.

Wolff-Parkinson-White Syndrome (WPW)

WPW is a form of ventricular preexcitation characterized by a short PR and a prolonged QRS and a
delta wave (initial slurring of the QRS). An anomalous or accessory pathway between the atrium and
ventricle allows the atrial impulse to bypass the AV node and therefore reach the ventricle earlier than
normal. The patient with WPW is predisposed to paroxysmal supraventricular ventricular tachycardia
(PSVT) and atrial fibrillation. The tachydysrhythmias usually have a narrow QRS (orthodromic);
however, a wide QRS tachycardia (antidromic) may occur and be confused with ventricular
tachycardia (VT).
Figure 6-37 Differential diagnosis in wide QRS tachycardia. (From Conover MB: Pocket guide to
electrocardiography, ed 5, St Louis, 2004, Mosby.)




SYSTEMATIC APPROACH TO TWELVE-LEAD ELECTROCARDIOGRAM (ECG)

Review all 12 leads to determine (1) underlying rhythm; (2) patterns of ischemia, injury, or infarction;
(3) chamber enlargement; and (4) ventricular axis.

1.     Determine rate.

2.     Examine P-P and R-R intervals for regularity in rhythm.

3.     Analyze P waves in each lead.

4.     Measure PR, QRS, and QT intervals.

5.     Analyze QRS complex.

6.     Identify leads having significant Q waves.
7.     Determine presence of R-wave progression and identify lead associated with transition.

8.     Measure intrinsicoid deflection.

9.     Determine axis.

10.    Identify leads displaying ST segment elevation or depression (Figure 6-38).

11.    Analyze T wave for increased amplitude.

12.     Identify presence of U wave.
Figure 6-38 ST measurement points: PR point, J point, and ST point. The PR segment is used to
identify the isoelectric line. The ST segment begins at the J point, which is the end of the QRS
complex. The ST segment measurement point can be measured at 60 or 80 ms past the J point. (From
Mims et al: Critical care skills: a clinical handbook, ed 2, St Louis, 2004, Saunders.)

				
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