Introduction to EKG - 3rd Year Medicine Clerkship

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Introduction to EKG
And then a little more
• To get an accurate EKG, leads must be
properly applied:
aVR: RA(-) to [LA & LL(+)]
I: RA(-) to LA(+)
II RA(-) to LL(+)
III:LA(-) to LL(+)
Precordial lead is +
aVL: LA(+) to [RA & LL(+)]
aVF: LL(+) to [RA & LA(-)]
• Normal activation
• Interpretation:
– Rhythm: look for P waves, regularity, reproducible intervals, PR
interval, shape
– Rate
– Axis
– Intervals: PR, QRS, QTc
– Conduction
– R wave progression
– ST segments and T waves
– Ectopic beats
– Q waves: where they should and should not be
– Other stuff
• Some general guidelines:
– P waves
• Best seen in lead II
• Upright or biphasic (neg component smaller) in V1-V2, upright in V4-V6
– QRS complex
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V1 shows rS, V6 shows qR
Size of r wave progressively increases, transition V3-V4
QRS duration < .120 sec
One R wave in precordial leads should be > 8mm
No R wave in precordial leads > 27mm
Sum of tallest R in left leads and S in right leads should be < 35-40mm
Precordial q waves should not exceed .04 sec nor have a depth greater than ¼
the height of the R wave following
• R wave in aVL <12-13mm
– ST segment
• Should not be more than 1mm above or below baseline. Normal minor
elevation in leads with large S waves ( V1-V3) and normal configuration is
concave up.
• T waves
– Often inverted in V1. May be inverted in V2 if already inverted in V1.
– Always upright in leads I, II, V3-V6
– Always inverted aVL
• U waves
– Amplitude usually < 1/3 T wave height in same lead
– Direction is same as T wave in that lead
• Axis
– Frontal plane lead with the sum of r wave and s wave most
closely approximates 0.
– Look at QRS in the lead perpendicular to original lead
– If QRS id positive, axis along that direction. If negative, axis
in opposite direction.
• Axis- cont
Normal axis
Left axis
Right axis
• Heart block
– Normal PR interval < .2 sec
– 1st degree AV block- prolonged PR
• Heart Block
– 2nd degree AV block- Wenchebach- Mobitz 1
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Prolonged PR until dropped QRS
1st PR interval always the shortest
1 dropped QRS only
RR intervals shorten
• 2nd Degree- 2 to 1 block
• 2nd degree type 2- mobitz 2
• Complete heart block
• Bundle branch block
– QRS > .120 sec
– RBBB- R-R’ in V1-V2, s wave in lead 1 & V6
• LBBB
– QRS >.120 sec
– Neg QRS in V1
– Lack of small q in lead 1, V5-V6

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