Diagnostic ECG—

8/11/2019 Diagnostic ECG

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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.

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Diagnostic ECG—