I am sorry to hear that your husband had a cardiac arrest. According to my knowledge, patients can only arrest for a few minutes and survive if they are brought back without having any interventions. I believe that 3-4 mins is the maximum that the brain can survive without any oxygen. If he was arrested for 20 mins and survived this most likely means that medical staff were trying different things throughout this time such as CPR and they were providing him with oxygen through this. This case can be common. However, there are still risks than need to be monitored. Obviously the medical staff will look in to this thoroughly.
I would advise you that when your husband is discharged from hospital he could check whether the hospital offers any cardiac rehabilitation programs that are very beneficial and speed up recovery, physical and emotional. Having a cardiac arrest is usually perceived as a big shock from the patient and family. Having as much input as he can would help him accept the condition and adapt easier to any changes he needs to make in his life style. Some cardiac rehabilitation team also have Psychology service, which he could possible use to process the trauma of his experience.
Hope this helps
All the best