According to Prashna Upanishad, at the time of death, the Prana Vayu (life force and respiration) merges with Udana Vayu (brain stem reflexes) and leaves the body.
But this does not happen immediately after clinical death which is defined as stoppage of heart and respiration. Medically the term used for clinically dead patients is sudden cardiac arrest.
As per the modern medicine, in cardiac arrest, the brain does not die for the next ten minutes and during this period, if the heart can be revived, life can be brought back.
The revival of patient during this period can be remembered by the formula of ten which is that within ten minutes of the stoppage of heart (cardiac arrest), if effective chest compressions are given for the next ten minutes with a speed of 100 per minutes (10X10), 80% of the cardiac arrest victims can be revived.
This period can be much longer in hypothermia state. If the temperature of the body is low, the soul does not leave the body till the temperature is brought back to normal. Today, this property of soul is also used as therapeutic measure where patients who cannot be revived in the first ten minutes of clinical death are put in a freezing chamber and artificial hypothermia is produced and these patients can then be transported to an advance cardiac centre where even after 24 hours resuscitation measure can be applied after re-warming the body. Many people have been revived even after 24 hours of cardiac arrest with such a technology.
There are instances in literature where a newborn with hypothermia was declared dead and got revived in the cremation ground when the heat of the atmosphere brought his temperature to normal and the pressure of the wood worked like cardiac massage.
This aspect of “life after death” is a contribution of the modern science to the Vedic science. Though in Vedic literature, it was well known phenomenon as Savitri brought life back into Satyavan even after his clinical death.
Take home message is that one should not declare a patient dead in the first ten minutes give cardiac massage and try reviving him with chest compression cardio pulmonary resuscitation.