ECG trace ECG P-wave: represents atrial depolarization - the spread of electrical impulse through the atrial musculature, preceeding atrial contraction.
P-R interval: time taken for electrical activity to be conducted down to the ventricles. In "heart blocks", this interval can be long.
Q,R,S: represents ventricular depolarization - the spread of electrical impulse through the ventricular musculature, preceeding ventricular contraction.
Q-R-S interval: time for conduction within and between ventricles.
T-wave: reflects the phase of rapid repolarization of the ventricles - the period of recovery for the ventricles.

Asystole Asystole Failure of the myocardium to contract. Absence of heartbeat; cardiac arrest. The trace reflects the small amount of electrical activity present in the heart.

Ventricular Fibrillation VFVF: Heart rhythm is chaotic - the heart is in a state of electrical chaos. No blood is pumped from the heart, and the patient has no pulse. The ECG trace lacks P-waves, QRS complexes, or T-waves. Action is electrical defibrillation.

Ventricular Tachycardia VT VT: An arryhthmia characterised by fast beating of the ventricles (120 beats/min), which can lead to ventricular fibrillation. Action for pulseless VT is electrical defibrillation.
Arrhythmia reference