Orientation and mechanical properties of human trachea : a CT scan study

Introduction We wished to determine orientation and mechanical properties of the human trachea. Materials and Methods We retrospectively analyzed a previously published database of 68 ALI/ARDS patients who underwent whole lung CT scan at PEEP 5, 15 and 45 cmH2O [Gattinoni, NEJM, 2006]. We manually draw the inner border of the trachea from below the glottis to the carina. Tracheal orientation was estimated computing the baricentrum of each tracheal section. Tracheal compliance and shape was measured at increasing PEEP level. The compliance was calculated as the ratio between the increase in tracheal gas volume (ml) and the increase in pressure (cmH2O). The shape was determined by the coronal and sagittal diameters measured at mid-trachea. Results We studied 84.4±9.6 mm of trachea length. Orientation: trachea showed a 4.1±5.2° angel toward left and 20.8±6.7° angel downward toward the back. Compliance was not linear and had a median 0.27 ml/cmH2O [IQ range 0.18 – 0.39] going from PEEP 5 cmH2O to PEEP 15 cmH2O and 0.12 [IQ range 0.07 – 0.18] from PEEP 15 cmH2O to PEEP 45 cmH2O. Shape: diameter on the coronal plane was not modified with PEEP (20.6±3.5, 20.6±3.4 and 20.6±3.3 mm at PEEP 5, 15 and 45 cmH2O, respectively) while diameter on sagittal plane increased of approximately 4% (24±5.2, 24.8±5.2 and 25±5.1 mm at PEEP 5, 15 and 45 cmH2O, respectively). Conclusion The exact determination of tracheal orientation, in particular the displacement on the sagittal plane, may be of interest to enhance postural drainage of secretions. The mechanical properties of the trachea may be of interest during “protective” mechanical ventilation and when designing endotracheal tube cuffs.