Skill Station 3 - ECG Interpretation

What is the 6 step rule to ECG interpretation?

1. rate

2. regularity

3. P wave

4. PR interval

5. QRS complex

6. T wave

Why is it important to interpret rate?

confirms whether the heart’s pacing is within normal range (60-100 bpm), bradycardic (under 60 bpm), or tachycardic (over 100 bpm)

Why is it important to measure regularity?

• determine if the heart is beating at a regular or irregular rhythm

• identification of premature contractions

What are the properties of a normal P wave?

• less than 0.12 seconds (120 ms or 3 small squares)

• amplitude is than 2.5 mm (0.25 mV) in the limb leads

• smooth, rounded, and typical monophasic in most leads

  • may be biphasic in lead V1

• frontal plane axis between 0-75 degrees

• upright (positive) in leads I, II, and aVF

• consistently inverted (negative) in aVR

What are the properties of a normal PR interval?

120-200 ms in duration (three to five small squares)

What are the properties of a normal QRS complex?

70-100 ms in duration

What is important to note about QRS length and its correlation to beat origin?

• narrow complexes (QRS < 100 ms) are supraventricular in origin

• broad complexes (QRS > 100 ms) may be either ventricular in origin or due to aberrant conduction of supraventricular complexes (e.g., due to bundle branch block, hyperkalaemia, or sodium-channel blockade)

What are the properties of a normal T wave?

• asymmetrical

  • initial slope is more gradual followed by a steeper downslope

• smooth

• rounded

• usually upright

• 0.10-0.25 secs

  • scales proportionately with the QT interval

• amplitude is < 5 mm in limb leads

• amplitude is < 10 mm in precordial leads

What rhythm/abnormality is seen below?

sinus rhythm

What are the diagnostic criteria for normal sinus rhythm?

• regular rhythm (60-100 bpm)

  • or age-appropriate in children

• each QRS complex is preceded by a normal P waves without progressive

• normal P wave axis

  • upright in leads I and II

  • inverted in aVR

• PR interval remains constant

• QRS complexes are < 100 ms wide

What rhythm/abnormality is seen below?

atrial fibrillation

What are is diagnostic criteria for atrial fibrillation?

• irregularly irregular rhythm

• P waves are absent

• lack of isoelectric baseline

• variable ventricular rate

• QRS complex usually < 120 ms

  • unless pre-existing BBB, accessory pathway, or rate-related aberrant conduction

• fibrillatory waves may be present (mimic P waves leading to misdiagnosis)

  • fine (amplitude < 0.5 mm)

  • coarse (amplitude > 0.5 mm)

What rhythm/abnormality is seen below?

atrial flutter

What is the diagnostic criteria for atrial flutter?

• narrow complex tachycardia

• regular atrial activity at ~300 bpm

• loss of the isoelectric baseline

• “saw-tooth” pattern of inverted flutter waves in leads II, III, and aVF

• upright flutter waves in V1

  • may resemble P waves

• ventricular rate depends on AV conduction rate

  • fixed

    • 2:1 block = 150 bpm

    • 3:1 block = 100 bpm

    • 4:1 block = 75 bpm

  • variable

    • ventricular rate is irregular and may mimic atrial fibrillation

    • may be a pattern of alternating 2:1, 3:1, and 4:1 conduction ratios

What rhythm/abnormality is seen below?

pre-ventricular complex (PVC) - bigeminy

What are the diagnostic criteria for PVCs?

• broad QRS complex with abnormal morphology

• premature

• discordant ST segment and T wave changes

• usually followed by a full compensatory pause

• retrograde capture of the atria may or may not occur

What rhythm/abnormality is seen below?

pre-atrial complex (PAC)

What are the diagnostic criteria for PACs?

• abnormal P wave usually followed by a normal QRS complex

• post-extrasystolic pauses may be present

• may or may not be conducted aberrantly or not conducted at all

  • either way, P wave will still be visible

What are post-extrasystolic pauses?

brief pauses that result after a premature/extra beat depolarizes the SA node = SA node resets with a longer than normal interval before the next sinus beat arrives

What rhythm/abnormality is seen below?

right bundle branch block (RBBB)

What is aberrant conduction?

• temporary, abnormal travel of an electrical impulse through the heart’s ventricles

• occurs when an impulse arrives before the heart’s conduction pathways (specifically the bundle branches) have fully recovered from a previous beat

  • causes a delay/block that results in a widened QRS complex

What are the diagnostic criteria for RBBBs?

• QRS duration > 120 ms

• RSR’ pattern in V1-3 (“M” shaped QRS complex)

• wide, slurred S wave in lateral leads (I, aVL, V5-6)

What rhythm/abnormality is seen below?

left bundle branch block (LBBB)

What are the diagnostic criteria for LBBBs?

• QRS duration > 120 ms

• dominant S wave in V1

• broad monophasic R wave in lateral leads (I, aVL, V5-6)

• absence of Q waves in lateral leads

• prolonged R wave peak time > 60 ms in leads V5-6

What rhythm/abnormality is seen below?

1st degree AV block

What are the diagnostic criteria for 1st degree AV block?

PR interval > 200 ms (five small squares) caused by delay without interruption in conduction from atria to ventricles

What rhythm/abnormality is seen below?

2nd degree AV block (Mobitz type I)

What are the diagnostic criteria for 2nd degree AV block (Mobitz type I)?

• progressive prolongation of the PR interval culminating in a non-conducted P waves without progressive

• PR interval is longest immediately before dropped beat

• PR interval is shortest immediately after dropped beat

What rhythm/abnormality is seen below?

2nd degree AV block (Mobitz type II)

What are the diagnostic criteria for 2nd degree AV block (Mobitz type II)?

2nd degree AV block with intermittent non-conducted P waves without progressive prolongation of the PR interval

What rhythm/abnormality is seen below?

3rd degree AV block (complete heart block)

What are the diagnostic criteria for 3rd degree AV block (complete heart block)?

• severe bradycardia due to absences of AV conduction

• complete AV dissociation with independent atrial and ventricular rates

What rhythm/abnormality is seen below?

ventricular tachycardia

What are the diagnostic criteria for ventricular tachycardia?

• regular broad complex tachycardia

• uniform QRS complexes within each lead

  • with the exception of fusion/capture beats

What rhythm/abnormality is seen below?

ventricular fibrillation

What are the diagnostic criteria for ventricular fibrillation?

• chaotic irregular deflections of varying amplitude

• no identifiable P waves, QRS complexes, or T waves

• rate of 150-500 bpm

• amplitude decreases with duration

  • coarse v-fib eventually turns into fine v-fib

What rhythm/abnormality is seen below?

supraventricular tachycardia

What are the diagnostic criteria for supraventricular tachycardia?

• any tachydysrhythmia arising from above the level of the Bundle of His

• encompasses regular atrial, irregular atrial, and regular atrioventricular tachycardias