There all kinds of issues from a medical standpoint. It only takes about 4 minutes of apoxia to cause permanent brain damage. In some individuals, respiratory distress can trigger ventricular fibrillation. Physical changes can happen so fast and the margin for error in the event of a problem is so small that who ever is performing the play should be very very knowledgeable in not only the mechanics of the scene, but also the proper emergency medical response in case there is a problem. In my opinion, any of the many forms of erotic asphyxiation is probably the most dangerous and unpredictable type of play in which one can engage.