ECG: A Primer

Electrocardiography: the basics

The content in this post was written by me a few years ago. It will act as an introductory ECG for Undergraduates and Postgraduates.

Lead placement

Bipolar leads

Unipolar Augmented Limb Leads

• Voltage increased by 50%
• aVR: all waves are negative
• aVL: Lateral wall
• aVF: Inferior wall

Unipolar Precordial Leads (Chest Leads)

• V1: 4th intercostal space (right sternal level) (right ventricle)
• V2: 4th intercostal space (left sternal level) (Septum)
• V3: between V2 and V4 (Septum)
• V4: 5th intercostal space in mid clavicular line (Septum)
• V5: 5th intercostal space in anterior axillary line (Apical)
• V6: 5th intercostal space in mid axillary line (Apical)
• V7: 5th intercostal space in posterior axillary line (Posterior)
• V8: 5th intercostal space below lower angle of scapula (Posterior)

Right sided chest leads (for right ventricle)

• V3R and V4R: same as V3 and V4, on right side of chest

Always get it done in case of inferior wall myocardial infarction to detect right ventricular myocardial infarction

Orientation of limb leads

ECG paper

• 1mm (1 small square) = 0.04 sec on horizontal axis (Duration)
• 1mm (1 small square) = 0.1 mV on vertical axis (Amplitude)
• 5mm = 0.2 sec on horizontal axis
• In 1 minute, ECG paper moves by 300 thick lines (1500mm), if speed of paper is 25mm/sec

Calibration of machine

1. 1mV produces deflection of 10mm vertically
2. Patient and machine properly grounded
3. 25mm/sec speed

Normal wave form

Criteria for normal P wave

1. Duration < 0.12 sec (3mm)
2. Amplitude < 0.25mV(2.5mm)
3. Upright in all leads except aVR
4. Smooth and rounded
5. May be upright or biphasic in leads V1 and V2, if biphasic terminal negative deflection (width x depth) < 0.03mmsec.

Normal QRS complex

Limb leads

1. Size of Q wave a. in lead aVL or aVF must have depth < ¼th of the height of ensuing R wave b. must be < 0.04 sec (1mm wide)
2. Any Q wave in lead III or aVR should be ignored
3. R wave in aVL must be < 13mm

Precordial leads

1. Steady progression from V1 (rS) to V6 (qR, qRs)
2. Transition zone usually between V2 and V4
3. Minimum voltage: atleast one R wave should be > 8mm
4. Maximum voltage: tallest R wave should be < 27mm and deepest S wave < 30mm
5. q wave should be < 0.04 sec

Normal T wave

Limb leads

1. upright if QRS complex is positive & inverted if QRS complex is negative
2. always upright in Lead I & II
3. always negative in aVR

Precordial leads

1. V1 and V2: normally upright, may be inverted
2. must be upright in lead V3 to V6

Heart rate

Heart rate = 1500 / No of small squares between adjacent R-R interval

PR interval

• Normal PR interval ranges from 0.12 to 0.20 sec
• Start of P wave to start of QRS complex

Abnormal ST segment

1. Inverted tick mark sign: Digitalis effect
2. Depressed or horizontal: ischemia
3. Raised with convexity upwards: Myocardial infarction
4. Raised with concavity upwards: Pericarditis

QRS width

• Upper limit: 0.10 sec
• > 0.12 sec: BBB or intraventriclar conduction defect
• VAT (Ventricular Activation Time): Max 0.03 sec in V1 and V2, 0.05 sec in V5 and V6

ST segment

• Must not deviate > 1mm above or below isoelectric line in any lead

QRS axis

• If QRS complexes reach for each other in leads I and aVF, it is right axis deviation
• If QRS complexes leave each other in leads I and aVF, it is left axis deviation
• If QRS complexes are positive in leads I and aVF, it is normal axis
• RAD is seen in normal individual, right ventricular hypertrophy, isolated RBBB and left posterior hemiblock
• LAD is seen in obese individuals, left ventricular hypertrophy and left anterior hemiblock

Myocardial Infarction: ECG changes

Regions of myocardial damage & corresponding ECG changes

Conventional electrodes cover wide area and hence record all the three regions

Abnormal Q waves should be

1. > 0.04sec duration
2. ≥ 25% deeper than height of ensuing R wave (which must exceed 5mm)
3. > 4mm deep
4. Present in leads V5 and V6 when associated with LBBB. In LBBB the normal septal Q wave disappears in lead V5 and V6, hence even a small Q wave in these leads suggest infarction of the interventricular septum.
5. Poor progression of R wave in chest leads also suggest abnormality in QRS complex suggestive of anterior wall myocardial necrosis

Regression of changes

1. ST segment is the first to return to the isoelectric line
2. An abnormal inverted T wave develops after a few days and lasts for a week
3. Abnormal Q wave persists

Site of infarction

Thickness of wall involved

• Subendocardial injury: ST depression
• Transmural injury: ST elevation

Region of ventricle invovled

Area	ST elevation
Anteroseptal	V1-V4
Anterolateral	I, aVL
Apical	V5, V6
Inferior	II, III, aVF
Posterior	V8, V9
	Tall R wave, ST depression, tall T wave in V1
Right ventricle	V1 (not in V2, V3)
	V3R, V4R

Conditions simulating inferior wall MI

In normal persons, Q wave may be present and T wave may be inverted in lead III. But, they revert to normal in deep inspiration. Hence, they must be present in II, III and aVF, and persist even after deep inspiration to be significant.

Criteria to diagnose MI in presence of RBBB

1. ST segment is displaced in the same direction as the terminal QRS deflection in right precordial leads
2. ST segment becomes coved convex upwards
3. T wave becomes symmetrical, inverted and arrow headed
4. In septal infarction, Q wave may appear in leads V1-V3

Criteria to diagnose MI in presence of LBBB

1. Q wave present in lead I, aVL, V5 and V6 with ↓ R wave in V5 and V6
2. ST elevation > 1mm in leads with positive terminal QRS complex
3. ST segment elevation > 5mm in leads with negative terminal QRS complex
4. ST segment depression or elevation < 1mm in V1, V2
5. Initial slurring of QRS complex in V1

Conclusion

I finish with the above post. There are many new aspects about ECG analysis which have been developed in last few years. I have not touched upon arrhythmia diagnosis. You are welcome to suggest corrections, modifications and additions.