Sorry to hear that. Lack of oxygen is a non traumatic brain injury. From the info below, the recovery rate is 14% at the one year mark. But there are therapies that may help. I added some info from the Brain Foundation on this. And as you mention your mother was on antidepressants, that has to be taken into consideration as a cause of cardiac arrest.
Excerpt from Brain Foundation - Vegetative State (Unresponsive Wakefulness Syndrome)....
"Pharmacological therapy mainly uses activating substances such as tricyclic antidepressants or methylphenidate (Dolce et al. 2002). Surgical methods (eg. deep brain stimulation) are used infrequently due to the invasiveness of the procedures. Stimulation techniques include sensory stimulation, sensory regulation, music and musicokinetic therapy, social-tactile interaction, etc.
While not empirically validated, families have reported benefits from arousal regimes, such as those implemented by Dr Ted Freeman (eg Coma Arousal Therapy). The therapy involves family members taking the patient through a regimen of controlled auditory, visual and physical stimulation for up to six hours a day every day.
Prognosis (outlook for recovery)
Many patients emerge spontaneously from VS/UWS within a few weeks (Jennett, 2007). The chances of recovery depend on the extent of injury to the brain and age, with younger patients having a better chance of recovery than older patients. Generally, adults have a circa 50 percent chance and children a 60 percent chance of recovering consciousness from VS/UWS within the first 6 months in the case of traumatic brain injury.
For non-traumatic injuries such as strokes, the recovery rate falls to 14% at one year (Jennett, 2002). After this period the chances that VS/UWS patient will regain consciousness are very low and, of those patients who do recover consciousness, most experience significant disability. The longer a patient is in VS/UWS the more severe the resulting disabilities are likely to be."
Increased risk of cardiac arrest associated with the use of TCA and SSRI antidepressants...
"A new Danish study (1) has found an increased risk of cardiac arrest associated with treatment with TCAs (tricyclic antidepressants) or SSRIs (selective serotonin reuptake inhibitors) (2).
The study found no association between increased risk of cardiac arrest and the use of SNRI antidepressants (serotonin and norepinephrine reuptake inhibitors) or NaSSA antidepressants (noradrenergic and specific serotonergic antidepressants)."
there is always a chance. with that said i dont want to give you false hope. twenty minutes is a long time without oxygen. i lost my mother on Thanksgiving. its not an easy choice to make at any age.I am so very sorry. it is a choice you will have to make.Dont beat yourself up, you have been through enough. you know the right thing to do. I would talk to the chaplin