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Electrocardiogram Waves

Waves of the Electrocardiogram

Waves are the different upward or downward deflections represented on the EKG tracing. They are the product of the action potentials created during the cardiac stimulation, and repeated from one heart beat to another, barring alterations.

The electrocardiographic waves are called P, Q, R, S, T, U (in that order) and they are connected to each other by an isoelectric line.

P Wave

The P Wave is the first wave of the cardiac cycle. It represents atrial depolarisation. It is the result of overlaying the electrical activity of both atria.

Its first part corresponds to Right Atrial depolarisation and its final part, to the Left Atrial depolarisation.

P Wave duration is less that 0.10 s (2.5 mm wide) and its maximum voltage is 0.25 mV (2.5 mm height). It is usually positive in all leads, except in aVR where it is negative, and in V1 where it is usually biphasic.

Atrial enlargements can bring about an increase in the P wave height or duration (See Abnormal P Wave). The P wave is absent in Atrial fibrillation.

Q Wave

Two important things about this wave:

1. If there is a minimum positive wave in the QRS complex before a negative wave, the latter is not a Q wave but an S wave, no matter how small the previous positive wave.

2. Not every Q wave means infarction. On a normal Electrocardiogram there are Q waves in certain leads without pathological significance.

Normal Q wave characteristics

  • Limb Leads: A normal Q wave is usually narrow and shallow (less that 0.04 s wide, 2 mm deep). Generally it does not exceed 25 % of the QRS complex. A relatively deep Q wave can be seen in lead III in horizontally positioned hearts; a QS pattern can be seen in aVL in vertically positioned hearts. A deep Q wave is normal in aVF.
  • Precordial Leads: Q waves are not to be seen in V1-V2. Normally a Q wave can be seen in V5-V6, usually less that 0.04 s wide, 2 mm deep and it should never exceed 15 % of the QRS complex.

QRS Complex

It consists of a collection of waves which represents the ventricular depolarisation. Its duration ranges from 0.06s and 0.10s. It can present different morphologies depending on the lead (See QRS Complex Morphology).

  • Q Wave: If the first wave of the QRS complex is negative, it is referred to as Q.
  • R Wave: It is the first positive wave in the QRS complex. It can be preceded by a negative wave. If another positive wave were present in the QRS complex, it is referred to as R’.
  • S Wave: It is the second negative wave in the QRS complex, appearing after the R wave.
  • QS Wave: When a complex is completely negative, with absence of any positive wave, it is referred to as QS pattern. It is usually a sign of necrosis.
  • R' and S' Waves: When more than a R or S wave are present, they are referred to as R’ and S’.

Remember: If in the QRS complex there’s a minimal initial positive wave, no matter how small it might be, this would be an R wave. The following negative wave should be an S, not a Q wave. Confusing the two is a common mistake.

T Wave

It represents ventricular repolarisation. In general it has a smaller amplitude than the QRS complex than precedes it.

It is positive in all leads, except in aVR. It could be negative in lead III in obese patients; in leads V1-V4 in children, young people and women.

The normal T wave is asymmetric, with an ascending portion which is slower than the descending one. Its maximum amplitud is less than 5 mm in limb leads and less than 15mm in precordial leads.

There are many pathologies which cause changes in the T wave, such as coronary artery disease or electrolyte disorders. (See Abnormal T Wave).

U Wave

It is a typically positive, low-voltage wave, which appears predominantly in precordial leads following the T wave. Its source is unknown, it could mean the repolarisation of the papillary muscles.

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