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Flashcards in Eight Deck (15)
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Describe Leads I,II,III. What do they represent in the heart?

• Leads I, II, and III are bipolar leads.
• The voltage recorded in lead I is the difference
between the voltage in the left arm electrode minus
the right arm electrode.
• Any electrical impulse traveling to the left arm will
be positive in lead I.

• The left ventricular

inferior wall is

represented by leads II

and III.

• The lateral wall is

represented by lead I.


Describe the unipolar leads? Waht do they represent? What degrees are the bipolar and unipolar leads at?

• Leads aVR, aVL, and aVF are unipolar leads.

• The voltage recorded in lead aVR is the difference

between the voltage recorded in the right arm

electrode minus the voltage recorded at a point in

the center of the chest.

• The left ventricular

inferior wall is

represented by lead aVF.

• The lateral wall is

represented by aVL

• By overlaying these six

leads, an axial reference

is established (top and


• Lead I points to 0°.

• Lead II points to 60°.

Lead III points to 120

aVL points to -30

aVR points to -150

AVF points to 90 degrees.


Where is a normal QRS axis located (in what direction is it heading)? What implications does this have with the different leads?

• A normal QRS axis is between – 30 degrees
and 90 degrees.

Positive Leads I/II


What is left axis deviation? What implications does this have on the leads? What can cause it?

• Left axis deviation,

when the QRS axis is < - 30°.

• Can be due to:


– Prior inferior MI


Positive lead 1, negative lead II


What is right axis deviation? What implications does this have on the leads? What can cause it?

• Right axis deviation,

when the QRS axis is >


• Can be due to:


– Prior lateral MI


Negative lead I, positive lead II


Describe what the six precordial leads measure. What parts of the heart does each represent?

• The six precordial leads measure the electrical

impulse in the horizontal plane (front and back).

• Any impulse traveling towards these leads (i.e. from

the mitral annulus) is positive in all precordial leads.

• Any impulse traveling away from these leads (i.e.

from the apex) is negative in all leads.

• The LV septum is represented by leads V1-2.

• The anterior LV is represented by V1-4.

• The lateral LV is represented by V5-6.


How many msec and mV do each small and large box represent on ECG paper?

• The ECG paper speed is

25 mm/sec.

• Each small box

represents 40 msec.

• One heavy box

represents 200 msec.

• Each small box

represents 0.1 mV.

• One heavy box

represents 0.5 mV.


How do you calculate heart rate on ECG paper?

• To estimate heart rate, count the number of large

boxes between P waves (atrial rate) or R waves

(ventricular rate).

• Beats per minute = 300 divided by the number of

large boxes.


What do p waves represent? What are the leads like for p waves during sinus rhythm? Waht does the QRS complex represent? A QRS interval of what is markedly prolonged?

• The P wave represents the electrical forces generated

from atrial activation.

• During sinus rhythm, P waves are positive in leads I and II

and negative in lead aVR.

• The QRS complex represents depolarization of the


• QRS interval > 120 msec is markedly prolonged and

consistent with BBB.


What does the PR interval represent? What is normal? What happens when its above that?

• The PR interval

represents the time

from the onset of atrial

depolarization to the

onset of ventricular


• A normal PR interval is

< 200 msec.

• First degree AV block

occurs when the PR

interval is > 200 msec.


Waht is AV block? Second Degree? third degree? Complete block? What happens in complete block?

• During AV block, the

electrical impulse

from the atrium

cannot reach the


• During second degree

AV block, every

second or third beat

reaches the ventricle.

• During complete

AV block, no atrial

electrical impulse

reaches the ventricle.

• Heart rate dependent

on unreliable escape

pacemaker sites.

• Escape rhythms often

40 bpm or slower.


What does the QT interval represent? QRS? ST?

• The QT interval

represents the total

duration of ventricular


• The QRS represents



• The ST interval




What is a normal QT interval in men? In women? What can trigger its prolongation?

• A normal QT interval in

men is < 440 msec.

• A normal QT interval in

women is < 460 msec.

• QT prolongation and

Torsades can be triggered


– Electrolyte abnormalities

• Hypokalemia

• Hypomagnesemia

– Medications

• Methadone

• Azithromycin


What are Q waves? When are they present? What makes them pathological? How can they diagnose MI?

• A Q wave is present

when the first

deflection of the QRS is


• If Q waves are present

in two contiguous leads,

a myocardial infarction

is diagnosed.

• Pathological Q waves

must be 1 mm in width

and depth.


What does the ST segment represent? What might its elevation represent? Its depression?

• The ST segment represents the interval between the

end of ventricular depolarization and the beginning of


• ST segment elevation may represent acute myocardial


• ST segment depression may represent myocardial