Its not about pulse, its about the beating of the heart. Even if
no pulse is detected, the heart might still be beating, just that
the rhythm is unable to pump blood to a point where the pulse can
be felt.
Defibs will not have any effect if the heart has completely
stopped. A lot of people have a wrong understanding about
defibrillation, no thanks to the movies.
Defibs are used for getting the heart rhythm from a irregular
rhythm back to a regular rhythm. Defib in itself cannot "restart" a
heart that has stopped completely.
Regarding the last question, there are many answers as to why defib
is not given. If the Defib on scene is an AED, chances are that the
AED will not allow the shock as the rhythm is non-shockable, in
this case no heartbeat at all. If a manual defib is on scene, i.e.
Lifepak12, the paramedic will analyse the ECG and deem that defib
will not be of use due to the rhythm.
The SAF uses the AED on medical cover, only SCDF will bring out a
manual defib to location.