This document provides an overview of basics of ECG interpretation. It discusses the history and development of ECG, conduction pathways, standardization, waves and intervals including P wave, PR interval, QRS complex, QT interval, axis determination, and abnormalities indicative of conditions like LVH and RVH. Pediatric ECG variations from adult ECG are also summarized. The document is intended as an educational guide on ECG interpretation for medical professionals.

1. Basics of ECG
Presenter- Dr.Pranay
Moderator-Dr.Sivamurukan

2. Outline
• Introduction
• Approach to an ECG
• Waves and intervals
• RVH,LVH

3. Electrocardiogram
• It is a recording - of time versus voltage – of the
electrical activity of the heart using electrodes placed
on the skin.
• 1887 – Augustus D Waller defined the term and
demonstrated the first ‘electrocardiogram‘, a tracing from
the intact human heart recorded using a mercury
capillary electrometer
• 1895 – Einthoven derived a correction formula that
compensated for the inertia and friction
• Einthoven awarded the Nobel Prize in Physiology or
Medicine 1924 for his ‘discovery of the mechanism of
the electrocardiogram‘.

4. ECG

5. ECG-Conduction pathways

6. INTERPRETATION OF AN ECG
• Standardization
• Heart rate
• Rhythm
• The QRS axis
• Intervals: PR, QRS, and QT
• The P-wave amplitude and duration
• The QRS wave
• ST-segment and T-wave abnormalities

7. Standardization
• The standard paper speed of ECG is
25mm/sec
• 1mm (small square) = 0.04 sec
(40ms)
• 5mm ( large square) = 0.20 sec
(200ms)
• 2 large squares(10 mm)=1mV
(full standardization)

8. Heart rate
HR= 1500/No of small squares btn
two consecutive RR waves
=300/No of large squares btn
two consecutive RR waves
=50 x No.of QRS within six large
boxes (1.2 ms)

9. RHYTHM
• Sinus rhythm:
i. P wave is positive in leads II
and aVF but negative in aVR
ii. a P wave always precedes each
QRS complex.
iii. PR interval must be constant
• Sinus arrhythmia-heart rate rises
and falls with inspiration and
expiration

10. Axis

11. AXIS
• Normal Axis = QRS axis
between -30 and +100 degrees.
• Left Axis Deviation = QRS axis
less than -30 degrees.
• Right Axis Deviation = QRS axis
greater than +100 degrees.
• Extreme Axis Deviation = QRS
axis between -90 and 180
degrees

12. P WAVE-atrial depolarization
• P wave is the first positive
deflection on the ECG
• Duration-< 0.08 s in infants or
<0.1 s in older children
• Morphology- Monophasic in
lead II, Biphasic in V1
• Axis- between 0° and +75°
• P waves should be upright in
leads I and II, inverted in aVR

13. P WAVE
• Tall (>2.5 mm), narrow, and spiked P
waves are indicative of right atrial
enlargement
• Broad, bifid and sometimes biphasic , are
indicative of left atrial enlargement
• Absent P waves- sinus arrest, AF
• P wave inversion-ectopic atrial and
junctional rhythms
• Presence of ≥ 3 different P wave
morphologies- multifocal atrial rhythm

14. P WAVE

15. PR INTERVAL
• START OF P WAVE TO START OF QRS COMPLEX
• Duration- 0.12-0.20 seconds
• PR interval > 200 ms - Heart block
• PR interval < 120 ms suggests pre-excitation or AV nodal (junctional)
rhythm.

16. Prolonged PR Interval
First degree AV block
Second degree heart block, Mobitz type I (Wenckebach phenomenon).

17. Short PR interval (<120ms)
1. Preexcitation syndromes
• Wolff-Parkinson-White syndrome
are a short PR interval (<120ms),
broad QRS and a slurred upstroke to
the QRS complex, the delta wave.
2. AV nodal (junctional) rhythm
• Narrow complex, regular rhythms
arising from the AV node.
• P waves are either absent or
abnormal

18. QRS Interval
• Normal QRS width is 70-100 ms
• Narrow complexes (QRS < 100
ms)- supraventricular in origin.
• Broad complexes (QRS > 100 ms)
- Ventricular in origin or due to
aberrant conduction of
supraventricular complexes

19. Narrow Complex Rhythms- supraventricular
• Sinus rhythm: Each narrow
complex is preceded by a normal
P wave.
• Junctional tachycardia: Narrow
QRS complexes with no visible P
waves.
• Atrial flutter: Narrow QRS
complexes with regular flutter
waves.

20. Broad QRS Complexes
RBBB
1. QRS duration > 120ms
2. RSR’ pattern in V1-3 (“M-shaped” QRS
complex)
3. Wide, slurred S wave in lateral leads (I,
aVL, V5-6)
LBBB
1. QRS duration > 120ms
2. Dominant S wave in V1
3. Broad monophasic R wave in lateral leads
(I, aVL, V5-6)
4. Absence of Q waves in lateral leads
5. Prolonged R wave peak time > 60ms in
leads V5-6

21. QT Interval
• Time from the start of the Q
wave to the end of the T wave
• Qt interval varies with age
• Bazett formula: QTc= QT / √ RR
• QTc > 500 is associated with an
increased risk of Ventricular
arrythmias.
• QTc is abnormally short if
< 350ms
Causes of a short QTc (<350ms)
Hypercalcaemia
Congenital short QT syndrome
Digoxin effect
Causes of a prolonged QTc (>440ms)
Hypokalaemia
Hypomagnesaemia
Hypocalcaemia
Hypothermia
Myocardial ischemia
ROSC Post-cardiac arrest
Raised intracranial pressure
Congenital long QT syndrome

22. T WAVE- Ventricular repolarisation
• The T wave is the positive deflection after each QRS complex. It
represents ventricular repolarisation.
• Upright in all leads except aVR and V1
• Amplitude < 5mm in limb leads, < 10mm in precordial leads
• Inverted T-waves in the right precordial leads (V1-3)

23. • Right bundle branch block
produces T-wave inversion in the
right precordial leads V1-3
• Left bundle branch block
produces T-wave inversion in the
lateral leads I, aVL and V5-6

24. T wave- Pottasium abnormalities

25. LVH
• Left axis deviation
• Markedly increased LV voltages: huge
precordial R and S waves that overlap
with the adjacent leads (SV1 + RV6 > 35
mm).
• R-wave peak time > 50 ms in V5-6 with
associated QRS broadening.
• LV strain pattern with ST depression and
T-wave inversions in I, aVL and V5-6.
• ST elevation in V1-3.
• Prominent U waves in V1-3.

26. RVH
• Right axis deviation
• Dominant R wave in V1 (> 7
mm tall; R/S ratio > 1)
• Dominant S wave in V6 (> 7
mm deep; R/S ratio < 1).
• Right ventricular strain
pattern with ST depression
and T-wave inversion in V1-
4.
• Voltage criteria for RVH – R
WAVE IN V 1 > 7mm

27. Pediatric vs adult ECG
• Heart rate > 100 beats/min
• Apparent right ventricular strain pattern:
• T wave inversions in V1-3 (“juvenile T-wave pattern”)
• Right axis deviation
• Dominant R wave in V1
• RSR’ pattern in V1
• Marked sinus arrhythmia
• Short PR interval (< 120ms) and QRS duration (<80ms)
• Slightly peaked P waves (< 3mm in height is normal if ≤ 6 months)
• Slightly prolonged QTc (≤ 490ms in infants ≤ 6 months)
• Q waves in the inferior and left precordial leads

28. References
• Evans, William & Acherman,
Ruben & Mayman, Gary &
Rollins, Robert & Kip, Kaan.
(2010). Simplified Pediatric
Electrocardiogram
Interpretation. Clinical
pediatrics. 49. 363-72.
10.1177/0009922809336206.
• https://litfl.com/