She credits the pharmacy staff and doctors for keeping her alive until a St John ambulance arrived.
Mrs Wrenn said if she had collapsed somewhere else, her family may have been holding a funeral for her.
She is now encouraging members of the public to learn CPR because you may save a life.
During my time in the ambulance service, I was always grateful when going to a cardiac arrest to see members of the public performing CPR. Some were doing it that well, it was better to get them to continue while I got the defibrillator and other equipment ready.
A common fear of people who are reluctant to learn CPR was "what if I get it wrong?" My response was always, the only thing wrong would be not trying to do something at all.
Many people say they may panic and forget how many compressions to breaths should be done. Again, the same answer applies: having some air going to the brain and other vital organs is better than none at all.
If you are interested in learning CPR, go to St John's website: stjohn.org.nz