ECG Interpretation: All you need to know

Table of Contents

Approach all EKGs systemically. Always note: rate, rhythm, QRS axis, complexes and intervals, chamber enlargement, ischemia/infarction, compare with prior EKG.

Rate (atrial, ventricular)

If the rhythm is regular, use the counting method (300 / # large boxes).

300 Method for Determining Heart Rate on ECG

If the rhythm is irregular, count R waves in the rhythm strip and multiply by 6 (EKG printoutsrecord 10 seconds)
Normal 60-100bpm; <60bpm is bradycardia, >100bpm is tachycardia

Rhythm (regular or irregular; sinus vs. non-sinus)

Sinus rhythm defined as: P before every QRS, regular w/ rate 60-100, P wave upright in Leads I, II, aVF, V5-6

P waves/morphology: Determine :
- If a P wave is present (best leads to visualize P wave are II and V1)
- The atrial rate (100-180 : sinustachycardia; 140-220 : atrial tachycardia, AVNRT, AVRT; 260-320 : atrial flutter)
- Axis (P wave upright in II and biphasic in V1)

QRS morphology:
- Narrow (<120ms) – supraventricular rhythm.
- Wide (>120ms) – aberrant supraventricular conduction or ventricular origin

P wave/QRS complex association:
- If not 1:1 association, determine if number of P>QRS (AV block) or P<QRS (accelerated junctional or ventricular rhythm).
- If P precedes QRS, evaluate the PR interval. If P after QRS, evaluate the RP interval and determine if PR or RP interval is fixed or variable

QRS Axis

Normal axis (-30 to 90º): QRS complex is positive (upright) in leads I and II

Leftward Axis – Left Axis Deviation(LAD) (-30 to -90º): QRS complex is positive in lead I but negative in lead II
- Differential Diagnosis of Leftward Axis (Left Axis Deviation):
  - normal variant
  - mechanical shifts
  - LVH (left ventricular hypertrophy)
  - LBBB (Left bundle branch block)
  - LAFB (Left anterior fascicular block)
  - congenital heart disease
  - emphysema
  - hyperKalemia
  - ventricular ectopic rhythms
  - WPW (Wolff-Parkinson-White)
  - inferior MI

Rightward axis – Right Axis Deviation (RAD) (90 to 180º): QRS complex is negative in lead I and positive in leads II, aVF
- Differential Diagnosis of Rightward Axis Right Axis Deviation (RAD):
  - normal variant
  - mechanical shifts
  - RVH (Right Ventricular Hypertrophy)
  - LPFB (Left Posterior Fascicular Block)
  - dextrocardia
  - ventricular ectopic rhythms
  - WPW
  - lateral MI
  - (NB: RBBB rarely causes RAD)

Extreme axis deviation/northwest axis (180 to -90º): QRS complexes negative in both I and II
- Differential Diagnosis of Extreme axis deviation/northwest axis:
  - Lead transposition
  - ventricular ectopic rhythms
  - hyperkalemia
  - artificial pacing
  - severe RVH (Right Ventricular Hypertrophy)

Clockwise/counterclockwise rotation (i.e. “R wave progression”): R wave amplitude typically increases from V1 to V5, with transition of R>S in amplitude at V3 or V4.
- Counterclockwise transition occurs prior to V3 due to:
  - RVH (Right Ventricular Hypertrophy)
  - WPW
  - LAFB
  - posterior MI
- Clockwise transition occurs after V4 due to :
  - cardiomyopathy
  - LVH
  - LBBB
  - anterior MI
- Both Clockwise and counterclockwise rotation are nonspecific and can be normal (Am Heart J 2004;148:80)

Low voltage: Average QRS amplitude < 5 mm in I, II, III and < 10 mm in precordial leads
- Differential Diagnosis of Low voltage QRS complex:
  - obesity
  - pericardial effusion
  - pneumothorax
  - COPD
  - restrictive or infiltrative CM (particularly amyloidosis)
  - severe hypothyroidism
  - anasarca

Complexes and Intervals

P wave: Right and left atrial depolarization; normal duration <120ms

PR interval:

Atrial depolarization, AV node and His-Purkinje conduction.
- Normally 140-200ms, changes with rate (shortened at faster rates, longer at lower rates – due to autonomic effects on AV nodal conduction)

QRS:

Ventricular depolarization.
Normal duration 60-100ms, not influenced by Heart Rate.
QRS 100-120ms is seen with incomplete BBB or interventricular conduction delay (IVCD);
QRS > 120ms represents:
- BBB (Bundle Branch Block)
- ventricular activation (PVC, VT, fusion beats, WPW, paced beats)
- hyperKalemia
- Na channel poisoning
- aberrancy
- hypothermia
RBBB:
- QRS > 120
- V1 with rSR’
- V6 with wide qRS
- Lead I will show a shallow broad S

LBBB:
- QRS > 120 mm
- V1 with wide negative QS
- V6 with wide tall R

LAFB:
- Left axis deviation (-45 to -90) w/ QRS < 120
- qR in I, aVL
- rS in II, III, aVF
- Common, nonspecific
LPFB:
- Right axis deviation (0 to +90) w/ QRS < 120
- No alternate reason (e.g., RVH, emphysema, lateral MI, PE)
- Rare to see in isolation, usually occurs with RBBB
Bifascicular block: RBBB with either LAFB or LPFB

ST segment: Represents a time of electrical silence. See “Ischemia/Infarction”

T-wave:

Ventricular repolarization, with a slow upstroke and a rapid return to the isoelectric line after peaking.
Usually asymmetric and in the same direction as the QRS. Should have smooth contours (bumps in T are usually buried P waves)

U wave:

occurs in the same direction as T wave
rate-dependent (shorter at faster rates)
differential diagnosis: bradycardia, hypoKalemia / Mg /Ca , hypothermia

QT interval:

Ventricular depolarization and repolarization
Excludes U-wave unless fused with the T wave
Rate-dependent (shortened at faster rates)
Normal <440ms in men, <460ms in women

Chamber Enlargement

LVH:
- Sokolow-Lyon criteria: S in V1+ R in V5 or V6 ≥35mm OR R in aVL ≥11mm.
- Cornell criteria: S in V3+ R in aVL > 28mm (men) or 20mm (women)
RVH: R>S or R ≥7mm in V1, S ≥7mm in V5 or V6
LAE: negative p wave in V1 >1mm wide and deep, total p wave duration >110ms in II
RAE: p wave >2.5mm in lead II

Frequent ECG findings in right and left ventricular hypertrophy (LVH)

Major ECG findings induced by left ventricular hypertrophy (LVH) — Frequent ECG findings in right and left ventricular hypertrophy (LVH)

READ MORE about Left Ventricular Hypertrophy (LVH): How to Recognize it on ECG [With Examples]

Ischemia/Infarction

Analyze abnormalities along the vectors of ventricular depolarization and repolarization (QRS-ST-T)
T-wave abnormalities:
- Hyperacute, symmetric T-waves can be found within minutes; followed by T wave inversions (≥0.1 mV in 2 contiguous leads)

ST depression:
- Suggests subendocardial injury;
- ≥0.05 mV below the baseline (PR segment), measured at the J point, in two contiguous leads, downsloping or horizontal = more ominous.
- ST depressions do not localize to territories (Circ Res 1998;82;957) NB: always look for ST elevations to rule out reciprocal ST depression.
- Digoxin toxicity: scooping ST depressions.

ST elevation:
- Suggests transmural ischemia;
- ≥0.1 mV, except for leads V2 to V3 (≥0.2 mV in men ≥40yo and ≥0.15 mV in women), measured at the J point.
- PR segment is the isoelectric interval on the ECG and can be used to assess ST segment elevation/depression.

Coronary Artery Territory Supply and ECG lead Changes

ST Segment Elevation - Differential Diagnosis — ST Segment Elevation – Differential Diagnosis

Q-wave:
- Usually a marker of scar;
- Must be deep (>1 mm) and broad (>0.04 seconds), more likely prior MI if inverted T wave in same lead.
- Pathologic Q wave defined by
  - 40ms duration (1 box wide),
  - 25% height of QRS.
- “Isolated Q in III is free” (non-pathologic).

Sgarbossa Criteria:
- Used to diagnose acute MI in presence of LBBB. Score of 3 = 90% Sp
- Concordant STE > 1mm in any lead = 5 points;
- Discordant STE > 5 mm in any lead = 2 points;
- ST depression > 1 mm in V1- V3 = 3 points.

Wellens’ Syndrome:
- Sign of critical prox LM (Left Main) or proximal LAD lesion, 75% MI in <2wks.
- Patient often pain free with history of angina.
- Normal/slightly elevated troponin.
- Symmetric, deeply inverted T waves or biphasic T waves in V2 + V3.
- Isoelectric or minimally elevated (<1mm) ST segment.
- No precordial Q waves. (Am J Emerg Med 2002;20:7)

Electrolyte Abnormalities

Characteristic ECG Findings in Electolyte Abnormalities

J-point Elevation Syndromes

Early repolarization:

ST segment elevation in absence of chest pain, terminal QRS slur, or terminal QRS notch
Features suspicious for malignant forms of ER:
- 1) Family history of sudden cardiac arrest or early unexplained death.
- 2) evalution and workup suggestive of a channelopathy.
- 3) history of unheralded syncope suggestive of an arrhthmogenic pathogensis (Circ 2016; 133:1520)

Brugada Syndrome (Circ Arrhythm Electrophys 2012;5:606)