ECG Interpretation

What does an ECG actually measure?

  • ECG measures small extracellular currents at the body surface
  • These dipoles are the result of, but not equal to, myocardial action potentials
    • Movement of negative extracellular charge towards (or positive away) from an electrode gives a positive deflection.
  • Dipoles are vectorial: have magnitude and direction
    • N.B. An electrical axis measuring perpendicular to a dipole will register no signal.

What do the waves represent?

P wave

Atrial Depolarisation

R wave

Ventricular Depolarisation

PR interval

AV node delay

S wave

Left Ventricular Depolarisation

T wave

Ventricular Repolarisation

Q wave

Septal Depolarisation

ST segment

Isoelectric Depolarised Ventricles

QT interval

Duration of ventricular depol + repol

RR interval

Duration of one ventricular cycle

Electrode Placement and Lead Distribution

Leads measure the potential difference (voltage) across the axis formed between two electrodes - e.g. lead I is the difference between the electrodes on the right and left arms; it ‘looks across’ the heart. A typical ECG will have 12 leads generated from 9 electrodes (+ ground) in 2 planes:

  • Frontal: 3 bipolar (I, II, III) + 3 unipolar (aVF, aVL, aVR)
    • N.B. the unipolar leads are ‘virtual’ leads calculated from the other leads
  • Transverse: 6 precordial leads (V1-V6)

This gives a 3D view of the heart which can be very helpful in localising ischaemia or an MI.

I lateral

aVR

V1 septal

V4 anterior

II inferior

aVL lateral

V2 septal

V5 lateral

III inferior

aVF inferior

V3 anterior

V6 lateral

Rhythm Strip (normally lead II)

INTERPRETING THE ECG

Clinical Context

  • Patient name, DoB, time ECG taken. Previous ECG to compare?

  • Technical details:

    • Speed (25mm/s)

    • Voltage (10mm/mV = 2 big squares)

    • Any abnormal lead placement?

(Ventricular) Rate

If regular:

  • Divide 300 by # big squares between adjacent QRS complexes

  • Divide 1500 by # small squares

If irregular:

  • Count #complexes in rhythm strip and x6

Rhythm

  • Regular or Irregular?

  • If irregular:

    • Regularly Irregular or

    • Irregularly Irregular

  • P wave before each QRS? If not what ratio?

  • Pacing spikes?

(QRS) Axis


Average direction of electrical activity during ventricular depolarisation. Deviation gives insight into chamber enlargement, conduction abnormalities and some arrhythmias.

Look at the net direction of the QRS in leads I and II:

  • Normal is positive in lead I and II

  • Left axis deviation is positive in I and negative in II (left is leaving)

    • Seen in left anterior hemiblock, LVH and inferior MI

  • Right axis deviation is positive in II and negative in I (right is reaching)

    • Seen in right heart strain and RVH

P wave


  • Normal - <3 small squares (0.12s), <2.5ss high

    • Positive in II, biphasic in V1

  • Absent + Irregularly Irregular → AFib

  • Sawtooth + 2:1 (or 3:1/4:1)ratio with QRS → Atrial Flutter

  • Morphology:

    • Bifid (= P mitrale → left atrial hypertrophy

    • Peaked (= P pulmonale → right atrial hypertrophy)

PR interval


P always followed by QRS? No = heart block:

  • 1st degree = prolonged PR interval (>3 small sq/0.12s)

  • 2nd degree

    • Mobitz type 1 (Wenckebach): progressive lengthening of PR interval eventually resulting in a dropped beat

    • Mobitz type 2: intermittent non-conductions of P waves, not preceded by PR lengthening (what’s the ratio? E.g. 2:1)

  • 3rd degree (complete) heart block: complete dissociation between P waves and QRS


QRS complex


Duration: <3 small sq (0.12s), broad = conduction abnormality:

  • Delta wave (slurred upstroke) → Wolff-Parkinson-White syndrome

  • Bundle Branch Block (WiLLiaM and MaRRoW)

    • Left BBB: QRS looks like W in V1 and M in V6

    • Right BBB: QRS looks like M in V1 and W in V6

  • Broad Complex Tachycardia (emergency!)

    • Monomorphic

    • Polymorphic (can be Torsade de Point)

Morphology

  • As above plus

  • Q waves - normal in V6 if <1 small sqwide and <2 small sq deep. Otherwise pathological → ischaemia

ST segment

  • Normal variants: benign early repolarisation (aka high take-off) most commonly seen in young people, looks like ST – elevation

  • ST elevation: myocardial infarction, pericarditis (“saddle shaped” ST segment across all waves)

  • ST depression: myocardial ischaemia, posterior infarct (ST depression in V1 and V2), drugs (digoxin → reverse tick sign)

T wave

  • Inverted: ischaemia, ventricular hypertrophy, PE, bundle branch block. 

    • Can be normal in V1-V3 in black or young people.

  • Peaked: hyperkalaemia → tall tented T waves

  • Flattened: hypokalaemia

QTc interval

Measured from start of Q to end of T: represents total time for de- and repolarization

  • Needs to be corrected for heart rate, commonly used Bazett’s formula (QTc=QTRR)

  • Should be <0.45s in♂and <0.47s in ♀, which ≈ ≤2BS+1SS

  • Prolonged by drugs (esp. psychotropic) and electrolyte derangements

U wave

  • Not normally seen, might represent repolarisation of papillary muscles or Purkinje fibres

  • Can be normal, but point it out if you see one!

An easy way to categorise these to make sure you don’t miss something is to use the rule of 4s:

  • 4 basic details: clinical context, rate, rhythm, axis
  • 4 intervals: PR, QRS, ST, QTc
  • 4 waves: P, QRS, T, U