Why do the spikes in the electrocardiogram, such as the QRS complex, come shortly before a contraction? What would it mean if the spikes were irregular?

The spike in the electrocardiogram happens shortly before a contraction in the heart because of the depolarization of the electrical circuitry in the body which trigger the contraction of the heart itself.

After every heartbeat, in other to achieve a contraction which is required for the atria to collect blood back into the heart and transport it into the ventricles, the heart itself must be depolarized.

Depolarization in biology or physiology refers to the process or condition of being depolarized. It could also refer to the absence or loss of the electrical difference between the outside and inside of the plasma membrane of a muscle or nerve cell.

An irregular surge means there is abnormal blood flow or supply due to abnormal or irregular heartbeat.

There may or may not be symptoms for this.  If they appear at all, they may include palpitations, shortness of breath and fatigue.

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