ECG Interpretation


ECG Interpretation

What is an ECG

• Electrocardiogram

• Traces the electrical activity of the heart

• 12 lead, 15 lead

Uses of ECG Tracing

• Ischemia/infarct

• Arrhythmias

• Ventricular and atrial enlargements

• Conduction defects

• Pericarditis

• Effects of some drugs and electrolytes


How to Conduct an ECG


Patient lies flat on back


Electrodes are placed on the body


Sites may need to be shaved or cleaned to ensure the leads will stick properly.


Patient will lie as still as possible, hold breath, or put hands under bottom to keep from moving.


The results are then printed out on paper for

MD to review.

ECG Strip

Lead Placements

• V1 - Junction of the 4th ICS, Right sternal border

• V2 - Junction of the 4th ICS, Left sternal border

• V3 - Midway between V2 and V4

• V4 - Junction of 5th ICS, Mid clavicle

• V5 - Anterior aspect of axilla, same line as V4

• V6 - Mid axilla, same line a V4

• 4 limb leads (for grounding etc)

Lead Placements Cont’

Normal Conduction Pattern

Understanding the Waves

• P

• Q

• R

• S

• T

Each wave

ECG Strip r/t heart

Understanding the Waves

• One small box = 0.04 seconds

• One large box = 0.2 seconds

• 5 large boxes = 1 second

Understanding the Waves

• Baseline (what is it?)

• P wave

– Length of time it takes the impulse to pass from the SA node to the AV node

– Should precede every QRS wave

• PR interval

– Should be no longer than 0.12 – 0.2

Understanding the Waves


– Should be no longer than 0.12

– If energy is going towards a positive electrode

(camera), the picture will show a positive QRS complex

– If energy is going away from positive electrode, the picture will show a negative QRS complex

– If energy is toward the positive electrode and then passes by it, the QRS will be biphasic

– Ventricle contracting

Understanding the Waves

• T wave

– Should always start from baseline

– Will indicate ischemia

Understanding an ECG

• Is as easy as…




1. = rate

2. = intervals

3. = rhythm

Step 1 = Rate

• Different ways to calculate a rate:

– a) ECG usually tells you

– b) Locate a QRS that is close to a big line and count to next big line: 300, 150, 100, 75, 60, 50

– c) take a 6 second strip, count QRS and multiply by 10 (hint: the middle of V3 on the lead II strip is 6 seconds)

Step 1 = Rate (cont’)

• Checking the regularity:

– map out QRS’s

Step 2 = Intervals

• We assess intervals to see where the impulse is coming from (pacemaker beat)

• Remember:

– PR interval: normal is 0.12 - 0.2

– QRS interval: normal is less than or equal to


Step 3 = Rhythm

The biggest question in relation to rhythm is…

Is this rhythm affecting my patient?

(Normal) Sinus Rhythm

• 60-80 bpm

• P preceding each QRS

• Normal intervals

Normal ECG

Other Sinus Rhythms

• Sinus bradycardia

– a sinus rhythm with a rate <60 bpm

Other Sinus Rhythms

• Sinus tachycardia

– a sinus rhythm with a rate >80 bpm

Atrial Arrhythmias

• Atrial fibrillation (A-fib)

– irregular rate

– no discernable P waves

– increased risk of strokes due to clots that might form due to fibrillation (patients are usually on anticoagulation therapy)

Atrial fibrillation

Atrial Arrhythmias Cont’

• Atrial flutter

– saw tooth in appearance

– irregular rate

Atrial flutter


• 1st degree AV block

– a PR interval that exceeds 0.20 sec

Blocks Cont’

• 2nd degree AV block (Mobitz)

– a) type I (Wenckebach)

• longer and longer PR intervals until a QRS is dropped

Blocks Cont’

– b) type II

• P waves and then suddenly a QRS is dropped

• P’s are regular

Blocks Cont’

• 3rd degree AV block

– HR <40 bpm

– a complete block of electrical activity from atria to ventricle

– P’s are regular

Ventricular Rhythms

• Ventricular fibrillation

– complete breakdown of all rhythm

– a) course

– b) fine


• What’s the first thing to do when coming up on a patient with this rhythm?

Ventricular Rhythms Cont’

• Ventricular tachycardia (V-tach)

– impulse originates in the ventricle

– always has a wide QRS complex


What’s the first thing to do when coming up on a patient with this rhythm?

• Pulseless


Coronary Arteries




• How can you tell on an ECG that your patient is having a heart attack?

• T waves

• Different views of heart will show different injured areas of heart


• T wave shouldering is classic


• How else can you tell that your patient has had an MI?

• Cardiac Markers

– CK will show in 4-6 hours (starts to come back down after 1 day)

– Troponin will show in 4-6 hours (may stay elevated for weeks)

– Others are: LDH, CK-MB, myoglobin, AST

Locating an MI





Anterior V1, V2, V3,



Right coronary



Left anterior descending

Pulm edema, hypotensive

Lateral I, aVL, V5,



I Lateral (circumflex) aVR V1 Anterior (Lt ant desc) V4 Anterior

II Inferior (Rt coronary) aVL Lateral V2 Anterior V5 Lateral

III Inferior aVF Inferior V3 Anterior V6 Lateral

ECG Tissue Damage Locations

Where is this MI?