Use of abdominal percussion for pneumoperitoneum detection.

Pneumoperitoneum refers to free air within the abdominal cavity that typically signifies serious abdominal pathology such as a perforated gut. The principal hypothesis of the study was that abdominal structure alterations due to pneumoperitoneum cause diagnostic changes in the sounds induced by abdominal percussion. The current pilot study investigated these changes in a mongrel dog model. Abdominal percussion was performed at baseline and after creation of pneumoperitoneum states. The resulting acoustic events were acquired, digitised and analysed. The event attack and decay rates and dominant frequencies during decay decreased with pneumoperitoneum (p = 0.084, 0.014 and 0.004, respectively; Wilcoxon signed-rank test). Simple theoretical models were constructed and predicted the observed decrease in resonant frequencies with increasing air pocket size. The results suggested that the normal and the 1,000 ml pneumoperitoneum states can be separated using thresholds of the attack and decay rates and resonant frequency (specificity = 80%, 100% and 100%, and sensitivity = 100%, 100% and 100%, respectively). Separating the control and the 500 ml pneumoperitoneum cases may be also possible (specificity = 80%, 100%, 100% and sensitivity = 50%, 70% and 90%, respectively), but separating the two levels of pneumoperitoneum was not feasible using the current approach. Therefore analysis of abdominal percussion sounds may prove useful for pneumoperitoneum detection, but not for distinguishing different levels of that condition.