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
bottom).
- 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:
– LVH
– Prior inferior MI
– LAFB
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 >
90°.
• Can be due to:
– RVH
– Prior lateral MI
– LPFB
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
ventricles.
• 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
depolarization.
• 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
ventricle.
• 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
systole.
• The QRS represents
ventricular
depolarization.
• The ST interval
represents
repolarization.
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
by:
– 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
negative.
• 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
repolarization.
• ST segment elevation may represent acute myocardial
infarction.
• ST segment depression may represent myocardial
ischemia.
