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This article discusses the rate, rhythm, cardiac axis, and normal waves & intervals in an ECG.
Normal Waves in ECG
- 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).
Fig 1. ECG of an RR Interval for Calculation of the Heart Rate.
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.
Fig 2. Anatomy of the Conduction System.
Fig 4. ECG showing Sinus Rhythm.
Findings: Rate: 60–80 bpm, regular rhythm, an upright P wave preceding every QRS complex.
- 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°.
Fig 5. The Hexaxial Reference System.
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)|
Fig 6. Axis Deviation.
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.