12-Lead System, Waves, & Rhythm Interpretation (AHS 221 Lecture 2)

Uses of ECG

Determine heart rate and rhythm; assess conduction patterns; evaluate disease severity and myocardial damage

Single lead ECG use

Determines rate and rhythm

12-lead ECG use

Determines conduction patterns and localizes pathology

12-lead ECG

Provides 12 views of the heart

Frontal plane leads

I; II; III; aVR; aVL; aVF

Precordial (chest) leads

V1; V2; V3; V4; V5; V6

V1 placement

4th intercostal space right of sternum

V2 placement

4th intercostal space left of sternum

V3 placement

Between V2 and V4

V4 placement

5th intercostal space midclavicular line

V5 placement

Between V4 and V6

V6 placement

5th intercostal space midaxillary line

P wave origin

Starts in SA node in right atrium and spreads to left atrium

P wave direction

Right to left and slightly downward

P wave positive leads (frontal)

I; aVL; II; aVF

P wave biphasic lead

Lead III

P wave negative lead

aVR

P wave positive leads (horizontal)

V5; V6

P wave biphasic in V1

Yes

P wave variable leads

V2 to V4

QRS complex initial depolarization

Septum depolarizes left to right

Septal Q wave seen in leads

I; aVL; V5; V6

Normal septal Q wave amplitude

≤0.1 mV

Main QRS vector direction

Leftward due to large left ventricle

Leads with large R waves

I; II; III; aVF; V5; V6

Lead with deep S wave

aVR and V1

Transition zone leads

V3 and V4

R wave progression

Increases from V1 to V6 (right to left)

Why QRS is larger than P wave

Ventricles have more muscle mass than atria

T wave represents

Ventricular repolarization

T wave characteristics

Variable; requires energy; opposite direction of depolarization

T wave positivity rule

Positive in same leads as R waves

Einthoven's triangle

Relationship between limb leads I; II; III

aVR P wave

Always negative

Early ventricular depolarization Q waves seen in

I; aVL; sometimes II

Main ventricular depolarization R waves seen in

I; II; III; aVF

Rhythm definition

Predictable repeating pattern of electrical activity

Supraventricular rhythms

Originate above ventricles (SA node; atria; AV node)

Ventricular rhythms

Originate in ventricles; often life-threatening

Conduction blocks

Delays or interruptions in electrical conduction

Ectopic beats definition

Abnormal beats arising outside normal rhythm

PAC definition

Premature atrial contraction

PJC definition

Premature junctional contraction

PVC definition

Premature ventricular contraction

Step 1 ECG interpretation

Determine heart rate

Step 2 ECG interpretation

Assess regularity (regular vs irregular)

Step 3 ECG interpretation

Evaluate P waves (presence and ratio)

Step 4 ECG interpretation

Check PR interval (0.12-0.20 sec)

Step 5 ECG interpretation

Evaluate QRS duration (≤0.11 sec)

Step 6 ECG interpretation

Look for ectopic beats

One P wave per QRS meaning

Normal conduction

One QRS per P wave meaning

Normal conduction

Narrow QRS definition

≤0.11 seconds