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iagnostic ECGThe 12-Lead (Clinical Essentials) (Paramedicare) Part 6
2-Lead ECG Interpretation
ke the approach to a 12-lead ECG analysis, the approach to the 12-lead ECG interpretation
ust likewise be disciplined. First, the Paramedic should assemble the list of abnormalities (i.e.,
esence and location of Q waves, R wave progression, ST changes, and T wave abnormalities).
eflecting on these changes, the Paramedic should assess for lead groupings. Lead groupings are ECG
anges in contiguous leads that are suggestive of involvement of a specific ventricular wall.
rmed with this information,the Paramedic can attempt to identify the culprit artery that is
volved. Understanding coronary artery involvement can help the Paramedic predict the progression
the acute coronary event and prepare for these predictable events. For example, the right coronary
tery (RCA) supplies the AV node in the vast majority of patients.44 ECG changes suggestive of an
ferior wall myocardial infarction (IWMI) implicate the right coronary artery (RCA) and vis-a-vis the
V node ischemia. This AV node ischemia can manifest as type I heart block. The first indication of a
pe I heart block is a prolonged PR interval (PRI). Therefore, a Paramedic confronted with a possible
WMI would monitor the PRI in an IWMI for a possible heart block.
nally,the Paramedic should consider the 12-lead ECG as a whole. ECG changes in adjoining walls
ay be suggestive of the extent and the evolution of the AMI. For example, ST changes and T wave
normalities across all of the pre-cordial leads, from V1 to V6, are suggestive of an extensive AMI.
uch a pattern of ischemia could be suggestive of a left main coronary artery occlusion.45 The
mplications of left main coronary artery occlusion include acute pulmonary edema (backward failure),
rdiogenic shock (forward failure), and sudden cardiac death (cardiac arrest).
combination of Q waves,ST changes, and T wave abnormalities across one or more ventricular
alls may be suggestive of an AMI later in its evolution. While every STEMI has the potential for
versal, the prognosis in a late evolution AMI is poorer and the morbidity higher.
aramedic Prognosis
the AV node is suffering froma lack of oxygenated blood, it will malfunction. As noted in
evious topics, the AV node is responsible for delaying the impulse and allowing the atria to contract
d push blood into the ventricles. The node is also the electrical connection between the atria and the
ntricles. The artery that serves the AV node is the right coronary artery. If ischemic or injury patterns
the ECG leads which look at the area served by the RCA occur, the Paramedic can anticipate
nduction abnormalities in the monitoring strip. The conduction abnormalities may lead to a
crease in coronary output sufficient to decrease preload and drop the blood pressure. Concurrently,
ood may back up into the venous system, leading to distention in the neck veins. Also associated with
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CA occlusions are bradycardias.
With LAD occlusions,the conduction is affected at the bundle of His, making for more serious
nduction abnormalities and unreliable escape mechanisms. The anterior wall is the largest portion of
e left ventricle and is responsible for ejecting blood into the high pressure system. Damage to the
terior wall may lead to the inability to eject the blood delivered to it and the backup of blood to the
ngs. Anterior wall damage caused by occlusion of the LAD may lead to pump failure. Treatment
tions for anterior wall damage include anticipation of cardiogenic shock, gross irritability of the
uscle cells leading to ventricular fibrillation, and reduction of heart rate and workload, leading to a
duction in myo-cardial oxygen demand.
urther 12-Lead ECG Interpretation
s the hearts muscle depolarizes,the energy moves down the electrical pathway from the
noatrial node (SA node) to the atrioventricular node (AV node) as a wave front. The electrical wave
ont then moves across the septum in a left-to-right fashion, then to the bundle branches, and finally
e wave front radiates outward across the ventricular mass. Each of these electrical events can be
corded, over time, on an ECG. The graphic representation of these events is the traditional PQRS
mplexes seen on an ECG.
here is another way to look at the electrical event. Instead of looking at depolarization in
agments of P, Q, R, and S, the Paramedic could look at the sum of these events. The sum of these
ectrical events would be the common direction of the electrical wave front called the mean electrical
ctor (Figure 34-23).
o explain vectorography in another way, these electrical events could be likened to a battle
ont during a war. While an army may send out many patrols, some going in different directions, the
ain objective of the army is to move the front forward. This common direction would be the armys
ctor. Similarly, while there may be minor deflections on the ECG, the major direction of the energy
uring depolarization is toward the apex of the heart. This common direction, or vector, of the energydepolarization is called the hearts electrical axis. Any aberration from a normal electrical axis could
indicative of disease (which is explained in more detail shortly).
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gure 34-23 Electrical vector.
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gure 34-24 Hexaxial reference system.
o help conceptualize the hearts normal axis,and to help determine if there is any axis
viation, an artificial construct called the hexaxial reference system was created. To create the
xaxial system, the limb leads were drawn around the heart and Lead I, the lead that is horizontal and
n the right side, was assigned zero degrees and the left side 180 degrees. As the limb leads are part of
nthovens Triangle (an equilateral triangle), then Lead II would be at 60 degrees and negative 120
grees and Lead III would be at 120 degrees and negative. The three axes are then all drawn into the
iddle of the heart and the three augmented leads overlaid with aVF at 90 degrees, aVL at negative 30
grees, and aVR at 30 degrees and negative 150 degrees. The resulting construct shows the heart
vided into equal 30-degree segments (Figure 34-24).
he traditional method of calculating the mean electricalaxis was to find the most equiphasic
ad of the frontal leads (I, II, III, aVR, aVL, and aVF). An equiphasic lead is an QRS complex with an R
ave that is equal in height to the depth of the S wave. An equiphasic wave would be neither going
ward the vector nor away from it, but would be perpendicular to it. Using that lead, the Paramedicould plot it on the hexaxial reference system (Figure 34-25). The lead represented on the
rpendicular spoke would be the hearts mean electrical axis in degrees. For example, if the equipha-
c QRS was Lead I, then the perpendicular axis would be 90 degrees.
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gure 34-25 Axis determination using the hexaxial reference system.
his method of axis determination, while very accurate, is cumbersome in the field. An
ceptable alternative is the Grant method. With the Grant method, the Paramedic would refer to Lead
and Lead II only (Figure 34-26). If both leads are upright, then there is a normal axis deviation. If
ad I is upright but Lead II is primarily downward in deflection, then a left axis deviation is assumed.
ternatively, if Lead I is primarily downward but the QRS in Lead II is upright, then it can be assumed
is a right axis deviation. If the QRS for both Lead I and Lead II is negatively deflected, then the axis is
lled an extreme left axis deviation; nicknamed "no mans land" because it represents extreme
normal depolarization.
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gure 34-26 Determination of axis using Lead I and Lead II.
TREET SMART
any 12-lead ECGs provide a reading of the axis, listed as P-R-T axes. The Paramedic need only read
e R axis and compare it to the hexaxial reference to determine the axis.
o reduce confusion, some Paramedics use their thumbs to represent the QRS deflection-Lead I on
e right hand and Lead II on the left hand. If Lead I is upright (i.e., the right thumb is up and the left
umb is down), then there is a left axis deviation. If both Lead I and Lead II are negative, then both
umbs are down.
xis Deviation
xis deviation is any time the hearts axis is not normal. Determining axis deviation is another means
observing many pathological conditions. Coupled with other physical findings, axis determination
n help the Paramedic establish a diagnosis. For example, a right axis deviation which is abnormal
n often suggest pulmonary pathologies such as pulmonary embolism and chronic obstructive
ulmonary disease.46
slight left axis deviation, from 0 to (-) 30 degrees, may be physiologic and seen in obese patients
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women who are in their third trimester of pregnancy. A larger left axis deviation, from (-) 30 to (-)
0 degrees, is often associated with left ventricular hypertrophy, secondary to heart failure, inferior
all MI, or (in some cases) Wolff Parkinson White syndrome.47
f greater concern to the Paramedic is an extremeleft axis deviation (>180 degrees) into "no
ans land." While conditions such as congenital transposition of the great vessels and dextrocardia
n produce this, extreme left axis deviation in the normal heart is suggestive of ventricular
chycardia. During ventricular tachycardia, the electrical source is in the ventricle and the wave front
ns backward through the conduction system.
ifferentiating VT from SVT with Aberrant Conduction
aramedics (and other practitioners)often have difficulty determining whether a fast rhythm
th a wide QRS complex is ventricular tachycardia or supraventricular tachycardia with aberrant
nduction. Some patients can tolerate a sustained monomorphic ventricular tachycardia for a
olonged period of time, despite opinion that patients cannot tolerate ventricular tachycardia (VT).
ecause the patient is tolerating what appears to be a wide complex tachycardia of unknown etiology,
e assumption is it must be supraventricular tachycardia (SVT) with aberrant conduction. Some
tients do develop a rate-related bundle branch block.
he determination is importantas treatments for SVT, such as calcium channel blockers, can lead
rapid patient deterioration if the rhythm is actually VT. Instead of trialing a medication to "see if it
orks," at the risk of patient discomfort and wasted time, a 12-lead ECG can provide the necessary
formation.
entricular tachycardia occurs most often in patients with acute cardiac ischemia or those with a
rdiac history. The Paramedic should first obtain a quick patient history, paying attention to
tiarrythmic medications that indicate a previous history of cardiac dysrhythmia or medications that
ay predispose the patient to arrhythmias (proarrhythmic medications).
ternatively,supraventricular tachycardias often occur in otherwise healthy individuals. Some of
ese patients may have a history of SVT or a diagnosis of WPW or LGL syndromes.
ext,the Paramedic should obtain a 12-lead ECG, paying particular attention to axis deviation and R
ave progression. The first step is to determine if the rhythm is regular. Ventricular tachycardia is
ually very regular. SVT with aberrancy is also usually regular unless the underlying cause is an atrial
brillation with a rapid ventricular response. If the rhythm is atrial fibrillation, then the ventricular
sponse will be irregularly irregular. While regularity will not help differentiate an interpretation ofther VT or SVT, an irregularly irregular rhythm is suggestive of atrial fibrillation.48
ext,the Paramedic should examine the QRS morphology in V1. In ventricular tachycardia, the V1
ad will be an R wave, where typically there is no R wave. Looking across the chest leads, the
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aramedic may also observe an S wave where typically there is no S wave.
n fact, if all of the QRS complexes in the chest Leads V1 through V6 are in the same direction (a
henomena called concordance), the ECG interpretation favors VT. The direction of the QRS (the
larity) can be either positive or negative but should be in the same direction.
ext, the Paramedic should look at Lead I and Lead II. If both leads are negative, or the R vector on the
-lead ECG reads between (-) 90 degrees and (-) 180 degrees (i.e., extreme left axis deviation), thene interpretation of VT is supported.
able 34-8 Comparison of VT vs. SVT with Aberrancy
Ventricular Tachycardia Supraventricular Tachycardia
History of ischemia Healthy individual
Proarrythmic medications History of SVT
Regular or irregular rhythm Regular or irregular rhythmDissociated P wave activity P waves before each QRS
Concordance in the chest leads R wave progression
In V1 ( MCL1), R wave, Rr, QR, RS In V1 (MCL1), rSR
In V6 (MCL6), rS, QS, QR In V6 (MCL6), qRs
QRS duration of 0.16 sec or more QRS duration > 0.12 but < 0.16 sec
Initial notching or slurring of QRS Absent or ending slurring of QRS
Axis of -90 to -180 degrees Axis of -90 to +180
nally,the Paramedic should observe the 12-lead ECG for the presence of P waves. Atrial
polarization still occurs in VT, independent of the ectopic ventricular pacemaker. Because of the
dependent atrial and ventricular activity (i.e., atrial-ventricular dissociation), P waves will randomly
pear throughout the 12-lead ECG. P waves that appear regularly in front of a QRS suggest a
praventricular ectopic pacemaker (Table 34-8).
iscellaneous Effects on the ECG
ectrolyte abnormalities,particularly potassium, can cause changes in the appearance of the 12-
ad ECG. While the Paramedic does not usually have access to lab results, the patients history may
ggest the potential for electrolyte disturbances. For example, patients in end-stage renal disease may
perience elevation of potassium levels while those patients receiving diuretics may have a decreased
vel of potassium unless they receive potassium supplementation.
normal potassium level, between 3.5 mEq/L and 4.5 mEq/L, is important for optimum cardiacll function. If the patient is hypokalemic (i.e., serum potassium less than 3.5 mEq/L), then the
tient may be prone to decreased inotropy. This can lead to generalized weakness or malaise, and/or
s-rhythmias such as atrial flutter and bradycardia.
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auses of hypokalemiaare numerous and include vomiting, aggressive gastric suctioning, diarrhea
econdary to infectious diseases), or abuse of potassium-wasting diuretics such as furosemide. With
pokalemia, the 12-lead ECG may show T wave flattening, ST-segment depression, and/or U wave
velopment.49,50
ypokalemia is often associatedwith low magnesium levels or hypomagnesemia (Figure 34-27).
ypomagnesemia may predispose the patient to a form of polymorphic ventricular tachycardia called
rsades de pointes.51
buterol is a bronchodilator but alsodrives potassium into the cells. Aggressive use of albuterol
e., stacked treatments) may cause changes in cellular uptake of potassium, putting the patient at risk
r low potassium levels and dysrhythmias.
erhaps more problematic for theParamedic may be hyperkalemia. A serum potassium level
ove 4.5 mEq/L is considered hyperkalemia. One of the most common causes of hyperkalemia is
dney failure. Patients who are on kidney dialysis are at obvious risk of hyperkalemia prior to dialysis.
her at-risk patients include patients with diabetes who are experiencing diabetic ketoacidosis,
tients with severe burns, patients with crush injury, and those patients with acute tubular necrosis
condary to shock.
gure 34-27 ECG changes associated with hypokalemia and hyperkalemia.
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he common ECG alterations seen in hyperkalemiaare changes in the T wave. At potassium
vels greater than 4.5 mEq/L but less than 6.5 mEq/L, the T wave appears tall and peaked and is best
en in inferior leads (Lead II and Lead III). As the potassium level continues to climb toward 8 Eq/L,
e QRS starts to widen and a left axis deviation may be appreciated. Finally, as the potassium level
mbs above 8 mEq/L, the P waves all but disappear and the QRS starts to flatten into a sine wave
nfiguration. It is at this time the patients cardiac output has dropped precipitously and the patient is
risk for ventricular fibrillation or asystole.
rrhythmias caused by hyperkalemia are very difficultto treat with defibrillation or the usual
mergency drugs without lowering the serum potassium level. Calcium chloride, calcium gluconate, or
dium bicarbonate, all competitive electrolytes, may be used to lower potassium levels. Alternatively,
rial treatments with Albuterol may help to treat mild to moderate hyperkalemia. In severe cases, it
ay be necessary to administer 50% dextrose with short-acting insulin.52 The insulin helps to drive
th glucose and potassium into the cells.
TREET SMART
alcium is needed for regular cell function. Loss of calcium (serum calcium levels less than 8.5 mg/dL)
hypocalcemia is rare. Typical causes of calcium disturbances are chronic diseases. The effect of
lcium is seen on the QT interval. Hypocalcemia causes a widened QT interval whereas an elevated
rum calcium causes a short QT interval. To remember that calcium is related to QT, the Paramedic
ed only remember that QT interval is corrected for heart rate and recorded on the 12-lead ECG as
ch (i.e., QTc). The little c could represent calcium, to remind the Paramedic of other causes of
olonged/shortened QT intervals.
xtra-Cardiac Causes of ECG Changes
otentially devastating extra-cardiac pathologies, such as intracranial hemorrhage,
pothermia, and pericarditis, can also cause changes on the 12-lead ECG. While not pathognomonic
r these pathologies, they are another sign to be added to the symptom complex for diagnosis.
n acute rise in intracranial pressure secondary to sub-arachnoid hemorrhage, intracerebral bleed, or
epidural bleed may lead to wide and deeply inverted T waves in the chest leads.53-55 The
aramedics attention is likely focused on other more urgent matters during one of these events.
owever, 12-lead ECG evidence, if obtained, may be useful at the emergency department.
ypothermia affects all cellular functionsand can also cause changes in the ECG. When a
tient is hypothermic, all of the interval durations (i.e., PR, QRS, and QT) lengthen and positiveflections at the J point, or point where the ventricular complex ends and the ST segment begins,
come noticeable. These deflections are in the same direction (polarity) as the QRS and are termed
sborn waves (sometimes called the camel-hump sign). The Osborn wave is seen in all leads, but is
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ore prominent in the inferior limb leads. The size of the Osborn waves correlates directly with the
gree of hypothermia. Osborn waves are often difficult to discern because of artifact from muscle
emors (Figure 34-28), but are seen in 80% of patients with hypothermia (below 33C/91.4F).56
nally, pericarditis,an inflammation between the pericardium and the epicardium, can cause
est pain and 12-lead ECG abnormalities. Initially, the Paramedic may be led to believe that the chest
in is secondary to acute coronary syndrome. However, nitrates are not useful in treating the pain of
ricarditis, so it is important for the Paramedic to seek historical clues to the diagnosis of pericarditis
e., fevers, etc.) as well as ECG evidence.
he inflammation that occurs between the sac surrounding the heart and the epicardium leads to
welling which puts some pressure on the myocardium. The myocardium cannot repolarize as it
ormally does due to the swelling, so there are T wave changes. The T will become pointed and tall
milar to a hyperacute T wave found in an MI). However, the changes tend to occur in all leads rather
an within contiguous leads only, leading the Paramedic to suspect other causes for the chest pain,ch as pericarditis.
valuation
ne of the advantages of the 12-lead ECG is its ability to predict the clinical progression of the
tients disease if left unchecked. For example, in the case of a patient with an anterior wall
yocardial infarction (AWMI), the patient may eventually develop cardiogenic shock secondary to lost
yo-cardial function. In this case, the patient had an IWMI that could, predictably, either extend to
e mitral valve (causing mitral valve regurgitation) or extend into the right ventricle. It is estimated
at 50% of IWMI extend into the right ventricle, with a resultant loss of preload.
gure 34-28 Osborn wave secondary to hypothermia.
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gure 34-29 Lead placement for a 15-lead ECG, which is helpful in assessing the right
entricle.
TREET SMART
he right ventricle essentially primes the pump (the left ventricle). Loss of right ventricular function,
condary to myocardial injury, can lead to profound hypotension. For this reason, some Paramedics
rform a 15-lead ECG to identify right ventricular involvement before administering vasodilators such
nitroglycerin.
5-Lead ECG
n additional diagnostic test available to the Paramedic if the Paramedic suspects right ventricular
volvement is the 15-lead ECG.57 The electrode placement for a 15-lead ECG will place positive
ectrodes onto the right side of the chest and view the right ventricle.
ocations for these electrodes are the 5th rightintercostal space at the midclavicular line, 5th
ght intercostal space anterior axillary line, and 5th right intercostal space at midaxillary. The
rresponding V4 to V6 wires from the left chest electrodes are switched over to the right electrodes
d the ECG is rerecorded (Figure 34-29). The repeated ECG is marked right chest leads or V4R, V5R,
d V6R.
onclusion
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he diagnostic 12-lead electrocardiogram is a useful tool in the Paramedics assessment tool box with
e potential to improve patient outcome by early detection of cardiac abnormalities. This is especially
ue in situations where the patient presents with an acute ST elevation myocardial infarction, where
e patient can be triaged to the appropriate hospital, or in cases of dynamic changes in the ECG that
ange with treatment, uncovering underlying cardiovascular disease.
ey points:
Death from AMI remainsa national health problem.
Aggressive prehospital treatmentincluding obtaining and interpreting a 12-lead ECG can
vorably impact patient mortality and morbidity.
Paramedics must have a higherindex of suspicion with patient populations that may present
th atypical cardiac symptoms.
A regular ECG uses standard limb leads,augmented limb leads, and precordial leads.
The regular ECG allows for inferior,anterior, and lateral views of the left ventricle, as well as
mbinations.
Accurate 12-lead ECG requiresproper patient preparation including standardized electrode
acement.
A 12-lead ECG is printedin a standard configuration.
Viewing a specific combination of leads,called contiguous leads, allows correlation to specific
ntricular walls.
Based upon coronary artery anatomy, ECG changes in contiguous leads permit Paramedics to
timate damage in specific arteries.
Estimation of damage inspecific arteries permits prognosis and planning.
Understanding an acute myocardialinfarction requires an understanding of penumbra.
Additional ECG evidence,such as new onset left bundle branch block (LBBB) and reverse R wave
ogression (RRWP), are important in supporting the diagnosis of myocardial infarction.
Some 12-lead ECGs donot show acute changes. The Paramedic should focus on treating the
tient based on history.
There are numerous extra-cardiaccauses to ECG abnormalities.
12-lead ECG interpretation takesa disciplined approach that gathers all the pertinent
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formation to prevent premature interpretation.
Based on the 12-lead ECGinterpretation and the patient history, the Paramedic can make a field
agnosis.
Additional information is alsoavailable from the 12-lead ECG that can lend insight into other
alth conditions.
The 12-lead ECG can helpdifferentiate ventricular tachycardia (VT) from supraventricular
chycardia.
The addition of three right-sided leads can help identify right ventricular AMI.
Early detection of MI, via 12-lead ECG, and rapid transportation to an interventional cardiac care
nter can lead to better patient outcomes.