図１では心陰影拡大と胸水（▲）を示している．気管内挿管と陽圧呼吸後に腹部膨満を認めた時の胸部X線写真（図２）で大量の横隔膜下（腹腔内）遊離ガス（白矢印）を示し，注目すべきは△の縦隔気腫である．CTでは両側胸水（※）と大量の腹腔内遊離ガス（G），また大動脈周囲にもガス像を認める（↑）．図９あたりから尾側は後腹膜腔と思われ，縦隔から後腹膜へガスが進展したことを意味する．腹部は膨満以外に腹膜刺激症状を認めなかったが，大量遊離ガスの所見から試験開腹が行われた．消化管の穿孔部位は発見されず，腹膜炎や他の異常所見を認めなかった．心不全は改善し，術後経過は原因不明の消化管出血を合併したが，食餌摂取可能となり退院できた．陽圧呼吸により縦隔気腫が発生し，後腹膜へ進展し腹腔内に漏れ，大量の遊離ガスが出現したと思われる症例である．

文献考察１）：nonsurgical pneumoperitoneum （表１，２）
Crit Care Med. 2000 Jul;28(7):2638-44.
Pneumoperitoneum: a review of nonsurgical causes.
Mularski RA, Sippel JM, Osborne ML.

OBJECTIVE: To review causes of nonsurgical pneumoperitoneum (NSP), identify nonsurgical etiologies, and guide conservative management where appropriate. DATA SOURCE: We conducted a computerized MEDLINE database search from 1970 to 1999 by using key words pneumoperitoneum and benign, nonsurgical, spontaneous, iatrogenic, barotrauma, pneumatosis, diaphragmatic defects, free air, mechanical ventilation, gynecologic, and pelvic. We identified 482 articles by using these keywords and reviewed all articles. Additional articles were identified and selectively reviewed by using key words laparotomy, laparoscopy, and complications. STUDY SELECTION: We reviewed all case reports and reviews of NSP, defined as pneumoperitoneum that was successfully managed by observation and supportive care alone or that required a nondiagnostic laparotomy. DATA SYNTHESIS: Each unique cause of nonsurgical pneumoperitoneum was recorded. When available, data on nondiagnostic exploratory laparotomies were noted. Case reports were organized by route of introduction of air into the abdominal cavity: abdominal, thoracic, gynecologic, and idiopathic. CONCLUSIONS: Most cases of NSP occurred as a procedural complication or as a complication of medical intervention. The most common abdominal etiology of NSP was retained postoperative air (prevalence 25% to 60%). NSP occurred frequently after peritoneal dialysis catheter placement (prevalence 10% to 34%) and after gastrointestinal endoscopic procedures (prevalence 0.3% to 25%, varying by procedure). The most common thoracic causes included mechanical ventilation, cardiopulmonary resuscitation, and pneumothorax. One hundred ninety-six case reports of NSP were recorded, of which 45 involved surgical exploration without evidence of perforated viscus. The clinician should maintain a high index of suspicion for nonsurgical causes of pneumoperitoneum and should recognize that conservative management may be indicated in many cases.PMID: 10921609
要旨：482の文献を考察し，頻度の高いnonsurgical pneumoperitoneum は表１に示す．開腹術後が最も多く，CTで3日目に85％に，6日目でも50％に遊離ガスを認めるが，ほとんどが2週間で消失する．原因となる胸部疾患は表２．85例中機械的人工呼吸は40例，心肺蘇生術21例，気胸16例が多くを占める．85例中23例（27％）に開腹術が施行されている．

文献考察２）：腹腔内遊離ガスの約10％はNonsurgical（手術を要しない症例）である．
Postgrad Med J. 1997 Sep;73(863):531-7.
Spontaneous pneumoperitoneum and other nonsurgical causes of intraperitoneal free gas.
Williams NM, Watkin DF.

Intraperitoneal free gas seen radiologically as air under the diaphragm nearly always indicates a perforated abdominal viscus that requires surgical intervention. Rarely, however, the presence of a pneumoperitoneum may not indicate an intra-abdominal perforation and thus may not require laparotomy. Such a situation is termed spontaneous or nonsurgical pneumoperitoneum. In this review, we explore the aetiological mechanisms and the pathophysiology of the appearance of intra-abdominal free gas. An appreciation of the condition and its likely aetiological factors should improve awareness and possibly reduce the imperative to perform an emergency laparotomy on an otherwise well patient with an unexplained pneumoperitoneum.PMID: 9373590
要旨：陽圧呼吸やpositive end-expiratory pressure（PEEP）を受けていて，肺胞が破裂すると，肺胞周囲の血管に沿って縦隔へ空気が波及し縦隔気腫が生じることがある．縦隔気腫は大動脈周囲から後腹膜へ，後腹膜が穿破し腹腔内へ漏出し気腹が生じるといわれる．PEEP治療中の4.3%に気腹が発生したとの報告がある．