Normal Sinus Rhythm
It refers to the rhythm that originates in the SA node.
- P wave precedes every QRS complex.
- Regular rhythm, but varies slightly during respiration
- Rate: 60–100 bpm
- The maximum amplitude of P waves is 2.5 mm in leads II and/or III.
- The P wave is positive in leads I and II and biphasic in lead V1.
- It is a small upward deflection representing atrial depolarization.
- It should be upright in leads I, II, aVF, and V2-V6 and inverted in aVR.
- P wave is normally upright, biphasic, flat, or inverted in V1, occassionally in lead V2.
- Q wave is a small downward deflection that follows P wave and represents depolarization of the interventricular septum.
- R wave is an upward deflection that usually follows Q wave.
- S wave is a small downward deflection that follows R wave and represents ventricular depolarization at the base of the heart.
- QRS is positive in all leads except aVR.
- It is an upward wave that follows the QRS complex.
- It represents ventricular repolarization.
- It is upright in all leads except aVR and V1.
- It is a small wave that follows T wave in the same direction.
- They are presumed to represent repolarization of the papillary muscles or Purkinje fibers.
- It is best seen in lead V2 and V3.
|Waves in ECG||Normal Duration||Normal Height|
|P wave||<120 ms||<2.5 mm in limb leads, <1.5 mm in precordial leads|
|QRS complex||40–100 ms||<25 mm in any lead|
|T wave||160 ms||<5 mm in limb leads, <15 mm in precordial leads|
|U wave||170–200 ms||Maximum normal 1–2 mm|
Normal Intervals and Segments in ECG
- The time from the beginning of the P wave to the beginning of the QRS complex.
- Normal range ~ 120–200 ms.
- From the end of the P wave to the beginning of the QRS complex.
- It is isoelectric and acts as a baseline to evaluate depression or elevation of the ST segment.
- Normal range ~ 50–120 ms.
- Measured from the beginning of the QRS complex to the end of the T wave in leads II and V5-V6.
- It indicates the time taken for ventricular depolarization and repolarization.
- It is inversely proportional to the heart rate.
QTc interval/Corrected QT interval
- It estimates the QT interval at a heart rate of 60 bpm.
- This correction allows comparing QT intervals over time at different heart rates and also improves detection of patients at risk of ventricular arrhythmia.
- The most frequently used formula to estimate QTc interval is Bazett’s formula.
- Bazett’s formula states QTc = QT / √RR where QTc = Corrected QT interval, QT = Uncorrected QT interval, and RR = RR interval.
- The normal QTc interval is 390–450 ms in men and 390–460 ms in women.
- It is the distance from the end of the S wave to the beginning of the T wave.
- It is usually isoelectric.
- Normal range: ~ 80–120 ms.
- It is the point at which S wave joins the baseline.
- It is important in the evaluation of ST segment deviation like ST elevation or depression.
- The portion between the end of T wave and the beginning of next P wave.
- At normal heart rate, it is usually isoelectric and used as a baseline for determination of deviation of the ST segment.
Ventricular activation time
- It is measured from the beginning of the Q wave to top of R wave.
- It should be <30 ms in leads V1-V2 and <50 ms in leads V5-V6.
Calculation of Heart Rates through ECG
The heart rate calculated through an ECG is typically the ventricular rate.
If atrial and ventricular rates differ, as in a third-degree block, we should measure both rates.
- 1500 divided by the number of small squares between two R waves.
- 300 divided by the number of large squares between two R waves.
- Count the number of QRS complexes that fit into 3 s and multiply this number by 20 to determine the number of beats per minute (bpm).
- It is the average direction of the spread of depolarization wave through the ventricles as observed from front.
- The direction of axis can be derived easily from the QRS complexes in leads I, III, and aVF.
- In adults, the normal QRS axis is considered to be within -30° and +90°.
Interpretation of Cardiac Axis through ECG
|Lead I||Lead aVF||Axis|
|Positive||Positive||Normal (00 to +900)|
|Positive||Negative||Possible left axis deviation.
Is lead II positive?
Yes – Normal (00 to -300)
No – Left axis deviation (-300 to -900)
|Negative||Positive||Right axis deviation (+900 to +1800)|
|Negative||Negative||Extreme axis deviation (-900 to +1800)|
Rotation of Heart
- It is reflected in precordial leads.
- The QRS complex is generally isoelectric in leads V3 and V4.
- As a general rule, heart rotates towards hypertrophy and away from infarction.