ECG Intervals
Features on an ECG
Rate (regularity)
R-R interval represents the time between 2 successive ventricular depolarisations
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Classed as either
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Regular
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Irregular
Measured from peak R wave to next peak R wave on any lead
Determining heart rate
6 second count
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Counting number of QRS complexes in 6s and multiplying by 10 (60 seconds)
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The triplicate method
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Less accurate
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Divide 300 by the number of big squares. E.g., 300/3 squares = 100bpm
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P wave
Normal P wave represents normal depolarisation of the atria arising from the sinus node.
Duration: 80-100 ms
Amplitude: 0.5-2.5 mm (in lead II)
Easiest to identify in lead II or V1
Upright in most leads (inverted p waves may suggest abnormal focus)
Normal P wave
Abnormal P wave
PR interval
PR interval represents the time of conduction from the atrium to the ventricles (conduction through AV node).
PR interval is measured from the start of the P wave to the start of the QRS complex.
Duration: 120-200 ms
A P wave followed by a QRS suggests normal conduction between the atrium & the ventricle.
Prolonged PR interval or blocked P waves (P wave followed by no QRS) suggests AV node disease.
QRS complex
QRS complex represents depolarisation of the ventricle.
QRS duration is measured from the start of the QRS to the end of the QRS
Duration: 60-120ms
Amplitude: depends on lead
Prolonged QRS duration may suggest intra-ventricular conduction delay (such as BBB) or that the rhythm is arising from the ventricle (as opposed to the atrium).
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QRS axis
General direction in which the ventricular depolarisation is spreading
Easiest way to assess is to look at leads I and aVF
QT/QTc interval
QT interval represents the time between depolarisation onset & the end of repolarisation of the ventricles.
QT is measured from the start of the QRS to the end of the T wave.
Corrected QT (QTc) takes heart rate into account when evaluating the QT interval, as QT generally lengthens and slower rates and shortens at faster rates.
Most common calculation uses Bazettes Formula: QT(sec)/square root RR interval(sec)
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QT Duration: 380-440 ms
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QTc (females): 360-460 ms
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QTc (males): 350-450 ms
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ST segment
ST segment represents the early part of repolarisation of the ventricles.
ST segment includes the segment from the end of the QRS to the start of the T wave
Duration: 200ms or less
Amplitude: <1mm above or below baseline
Various factors may effect the ST segment. Most serious would be ST elevation in the setting of myocardial infarction (STEMI)
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T wave
T wave represents ventricular repolarisation.
Measured from the start to end of T wave
Duration: 100-250ms
Amplitude: usually <5mm (usually <2/3rds of the R wave)
Upright in all leads except for V1 and aVR
Is generally rounded rather than peaked
U wave
Source of the U wave is unknown
Probably represents the final stage of repolarisation of the ventricles.
Not always present
Occurs immediately after the T wave
Generally are <2mm
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