Basics of Interpretation
Limb leads
Precordial leads
Atrial and ventricular depolarization and contraction in relation to the EKG waveforms
Rate
Evaluate the distance between R-R intervals.
1 small box = 0.04s
1 large box = 0.2s
Counting based off of large boxes:
Irregular rhythms (or very slow/fast):
6 second strip: # cycles × 10 = rate
10 second strip: # cycles × 6 = rate
Rate > 100 = tachycardia
Rate < 60 = bradycardia
Rhythm
Identify Basic Rhythm
Evaluated for pauses, premature beats, irregularity, abnormal waves
Always check for:
P waves before each QRS complexes
PR intervals to determine AV blocks
QRS intervals to determine bundle branch blocks
QT intervals to evaluate for risk for Torsade's and to limit / discontinue any QT prolonging medications
Axis
Normal Sinus Rhythm
Regular rate - 60-100bpm
P wave before each QRS
Normal P wave Axis
PR interval constant
QRS <120ms
Sinus Bradycardia
Sinus rhythm criteria with rate < 60 bpm
Sinus Tachycardia
Sinus rhythm criteria with rate > 100 bpm
SVT
Any tachycardia above bundle of His, but commonly used to describe AVNRT
Regular Rate ~140-280
Narrow QRS (< 120 ms)
P waves can have retrograde conduction or inversion
Atrial Fibrillation
Irregularly irregular
No P waves
Variable ventricular rate
QRS usually narrow, < 120 ms unless other BBB, accessory, aberrancy
Fibrillatory waves
Atrial Flutter
Atrial activity ~300 bpm
Narrow QRS < 120 ms
“Saw tooth” inverted flutter waves in inferior leads.
Ventricular rate a fraction of atrial rate
2:1 block = 150 bpm
3:1 block = 100 bpm
4:1 block = 75 bpm
Ventricular Tachycardia
Wide and broad complex tachycardia
Uniform QRS complexes in each lead
Ventricular Fibrillation
Chaotic irregular deflections
No P waves, QRS, T
Amplitude rate 150-500
Amplitude decrease with duration (coarse to fine)
AV Blocks
First Degree Block
Fixed PR interval elongation (>200ms)
Second Degree block Mobitz Type I (Wenckebach)
Progressive prolongation of the PR interval until a non-conducted P wave
Second Degree block Mobitz Type II
Intermittent non-conducted P waves without progression of PR interval
Third Degree block
AV dissociation
Independent atrial and ventricular rates
Myocardial injury morphologies
Ischemia
Inverted T waves (symmetrical inversion)
Usually in same lead that demonstrates acute infarction
ST depression can represent subendocardial ischemia
Injury
ST elevation
Represents an acute process (normalizes over time)
Acute or recent infarct can presents with significant Q waves
Infarct / Necrosis
Significant Q wave
> 1 mm wide or 1/3 amplitude of QRS
Significant Q waves can remain over time (permanent)
Advanced ECG Interpretations
sinus arrhythmia, wandering pacemaker, MAT, torsades
Ectopy
Ventricular Hypertrophy
Atrial enlargement
Electrolyte abnormalities
Sinusoidal waves
hyperacute T waves
u waves
Fascicular blocks/hemi-blocks
Aberrant conduction patterns
Brugada criteria
Wolf-parkinson-white
Epsilon waves
OMI (occlusive myocardial infarctions) vs. STEMI paradigm shift
Sgarbossa criteria
Wellen's waves
DeWinter T waves
RV infarction
Posterior MI
Diffuse ST depression
Wide based peaked T waves
Medication effects
TCAs
Digoxin
References
Cadogan, Mike. “ECG Lead Positioning.” Life in the Fast Lane • LITFL, 30 Jan. 2022, litfl.com/ecg-lead-positioning/.
Cadogan, Mike. “ECG Library.” Life in the Fast Lane • LITFL, 14 Nov. 2021, litfl.com/ecg-library/.
Cummins, Richard O., et al. ACLS Provider Manual. American Heart Association, 2004.
Dubin, Dale. Rapid Interpretation of EKG’s: Dr Dubin’s Classic, Simplified Methodology for Understanding EKG’s. Cover Publishing, 2016.
John Coacci, DO