When an airplane is about to takeoff (after leaving the gate), how should the flight attendants respond to an emergency with a passenger which requires medical immediate assistance?
If the pilot knows before committing to take-off and reaching the point of no return then he will abort the takeoff and ask the tower to send an ambulance and stairs.
If he is committed to take off then he will get into the air and then tell the tower of the emergency; circle around (possibly dump fuel depending on the severity) and land again to get the passenger off board on the ambulance waiting near the runway.
Whether the plane can then top up the tanks and take off again will depend on whether the landing was overweight or not. If it was then the plane needs to be inspected before it can be put back into use and the other passengers will need to leave the plane and wait until the next plane can ferry them on.
The cabin crew will apply first aid until the ambulance crew can take the sick passenger.