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The P-wave, QRS complex and T-wave
The 3 Main ECG waves

The P, QRS and T-waves form the basis of the waveforms of the ECG. P-wave = atrial depolariation, QRS complex = ventricular depolarisation and the T-wave = ventricular repolarisation. Recognising these three waves is the first step to ECG interpretation. The intervals that occur between them and the durations of each of these waves is also important. Remember, the heart runs like a rhythmical timer. Everything needs to be synced up properly to produce an effective and stable heartbeat.

ECG Waves

P, QRS, and T-waves
p-wave

The SA node (SAN) discharges an action potential which flows through conducting fibres and causes the right and left atria to depolarise or contract

The atrial depolarisation forms the P-wave of the ECG

P-wave
P-waveDr Gemma
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pR interval

  • The PR interval is measured from the start of the P-wave to start of the QRS complex

  • Normal PR interval is between 0.12-0.2s (approx one large (5x5) square)

PR interval
PR intervalDr Gemma
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QRS Complex

  • The action potential spreads through a network of fibres to the ventricles

  • This happens very quickly - normal QRS lasts <0.12s; fewer than 3 small squares

  • The pattern of ventricular depolarisation is called the QRS complex

  • At the same time, the atria are re-polarising. This can't be seen on the ECG  because it is obscured by the QRS complex.

QRS complex
QRS complexDr Gemma
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ST Segment

  • The ST segment is a short period of net zero electrical activity

  • In a healthy person, this should sit at the electrical baseline

  • The J-point is the junction between the end of the QRS complex and Start of the ST segment

  • Elevations or depressions of the ST segment are seen in many cardiac pathologies such as myocardial infarctions

ST segment
ST segmentDr Gemma
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T-wave

  • Having depolarised and contracted, the ventricles move on to repolarising

  • Ventricular repolarisation is represented by the  T-wave (a smooth rounded wave that follows a QRS complex)

T-wave
T-waveDr Gemma
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QT Interval

  • So far, we have considered ventricular depolarisation and ventricular re-polarisation as separate entities

  • It is sometimes helpful  to combine the two and then consider ventricular activity as a whole. We use the QT interval to do this. The difficulty is that the QT interval will vary with heart rate,

  • This can be corrected for heart rate (giving us a QTc measurement) by using a mathematical formula called the Bazette formula

  • The normal QTc for women is <0.46s and <0.44s for men

  • Normally we worry about prolonged QTc durations but shorter QTc intervals < 0.33s can initiate arrhythmias as well 

QTc
QTcDr Gemma
00:00 / 00:17
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