Cardiac arrest is most commonly encountered as the end result of severe derangements in metabolic homeostasis. These derangements may include toxic, ischemic, or electrophy siological causes of circulatory and/or respiratory arrests. Occasionally, mechanical obstruction or impediment to circulation may cause cardiac arrest, and forms of resuscitation other than closed chest massage may be required to remove or relieve obstruction. Two cases of cardiac arrest due to mechanical impairment of blood flow are presented. Both occurred during thoracic invasive procedures, and in both cases, the usual externally applied closed chest resuscitative measures were not, and could not have been effective. Emergency thoracotomy promptly restored functional hemodynamics, and allowed the potential for definitive therapy.