QRS complex

The QRS complex is the electrocardiographic expression of the electrical activation of the ventricular myocardium.

For a wave to be defined as a Q wave it must fulfill the following criteria:

1. It must be a negative deflection.
2. It must be the first deflection of the QRS complex.
3. It must be followed by an R wave.

Thus there can be only one Q wave per QRS complex.
For a wave to be defined as an R wave it must fulfill the following criteria:

1. It must be a positive deflection.
2. All positive deflections are R waves and only R waves (within the QRS complex).

There can be several R waves per QRS complex.
For a wave to be defined as an S wave it must fulfill the following criteria:

1. It must be a negative deflection.
2. It must follow an R wave.

There can be several S waves per QRS complex.
For a wave to be defined as a QS wave it must fulfill the following criteria:

1. It must be a negative deflection.
2. There must be no R wave in the QRS complex.

Therefore it describes a QRS complex with only one deflection and this deflection is negative.