Extension of advanced odontogenic infection from deep neck fascial spaces into the mediastinum is heralded by chest pain, dyspnea, fever, and radiographic demonstration of mediastinal widening. The critical care should be done in a team approach by multiple medical and dental departments, such as, oral & maxillofacial surgery, otolaryngology, anesthesiology, chest surgery, and infection medicine. Especially, fluid & drug therapy, adequate incision & drainage and systemic supportive psychosedation care are important. But, acute hyperventilation can be produced by several distinct causes: severe anxiety, respiratory alkalosis, increased blood catecholamine levels, and a decrease in the level of the ionized calcium in the blood. The orofacial fears about acute pain, trismus, dysphagia, swelling and oral surgical treatment lead to the severe anxiety and increased blood catecholamine level by stress. Therefore, the most dental patient should be cared gently as the stress reduction protocol. In spite of the care, hyperventilation was occurred during psychosedation and local anesthesia for incision and drainage of the masticatory fascial space abscess with deep neck infection & mediastinitis. We suggest that the dental patient with advanced odontogenic infection must be attention for the manifestation of hyperventilation, especially in the medically compromised conditions.