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Problems with tracheal intubation and mechanical ventilation are potentially important causes of perioperative morbidity and mortality. We have developed a method of monitoring the ventilation of both lungs during general anesthesia that is an advanced digital version of a more primitive analog technique developed over two decades ago. We used two miniature electret microphones connected to regular chest pieces, placing the assemblies on the anterior chest wall about 4 inches below the clavicle in the midclavicular line. After amplification and 16-bit analog-to-digital conversion, the digital signal processing software package Goldwave (Version 5.12, www.goldwave. com) was used to produce real-time X-Y plots of the signals, with sounds from the right side plotted on the horizontal axis and sounds from the left side plotted on the vertical axis. Recognizing that when two signals are identical, their X-Y plot should form a 45 degree line, we hypothesized that X-Y plots obtained under endobronchial conditions would be recognizably different to plots reflecting normal bilateral lung ventilation. We also hypothesized that as a result of noise and anatomical variations that under conditions of bilateral mechanical ventilation the obtained plot would be very different from a simple 45 degree line. The data obtained supports these hypotheses. This preliminary study suggests that our technique may help provide a practical real-time warning system for detecting endotracheal tube malpositions, and may help build on the work of other investigators.