Guttmann J, Eberhard L, Fabry B, Bertschmann W, Wolff G
Department of Surgery, University of Basel, Switzerland.
Anesthesiology. 1993 Sep;79(3):503-13. doi: 10.1097/00000542-199309000-00014.
Intratracheal pressure (Ptrach) should be the basis for analysis of lung mechanics. If measured at all, Ptrach is usually assessed by introducing a catheter into the trachea via the lumen of the endotracheal tube (ETT). The authors propose a computer-assisted method for calculating Ptrach on a point-by-point basis by subtracting the flow-dependent pressure drop delta PETT(V) across the ETT from the airway pressure (P(aw)), continuously measured at the proximal end of the ETT.
The authors measured the pressure-flow relationship of adult endotracheal tubes with different diameters (ID, 7-9 mm) at different lengths and of tracheostomy tubes (ID, 8-10 mm) in the laboratory. The coefficients of an approximation equation were fitted to the measured pressure-flow curves separately for inspiration and expiration. In 15 tracheally intubated patients under volume-controlled ventilation and spontaneous breathing, the calculated Ptrach was compared with the measured Ptrach.
The authors present the coefficients of the "nonlinear approximation": delta PETT = K1.VK2, with delta PETT being the pressure drop across the ETT and K1 and K2 being the coefficients relating V to delta PETT. An important result was an inspiration/expiration asymmetry: the pressure drop caused by the inspiratory flow exceeds that of the expiratory flow. A complete description of the pressure-flow relationship of an ETT, therefore, requires a set of four coefficients: K1I, K2I, K1E, and K2E. The reason for this asymmetry is the abrupt sectional change between ETT and trachea and the asymmetric shape of the swivel connector. Comparison of calculated and measured Ptrach in patients gives a correspondence within +/- 1 cmH2O (mean limits of agreement). The mean root-mean-square (rms) deviation is 0.55 cmH2O.
Ptrach can be monitored by combining our ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.
气管内压力（Ptrach）应作为肺力学分析的基础。如果要进行测量，Ptrach通常是通过将导管经气管内插管（ETT）管腔插入气管来评估。作者提出了一种计算机辅助方法，通过从在ETT近端连续测量的气道压力（P(aw)）中减去ETT上与流量相关的压力降ΔPETT(V)，逐点计算Ptrach。
作者在实验室中测量了不同直径（内径，7 - 9 mm）、不同长度的成人气管内插管以及气管造口插管（内径，8 - 10 mm）的压力 - 流量关系。分别针对吸气和呼气，将近似方程的系数拟合到测量的压力 - 流量曲线上。在15例接受容量控制通气和自主呼吸的气管插管患者中，将计算得到的Ptrach与测量得到的Ptrach进行比较。
作者给出了“非线性近似”的系数：ΔPETT = K1.VK2，其中ΔPETT是ETT上的压力降，K1和K2是将V与ΔPETT相关联的系数。一个重要结果是吸气/呼气不对称：吸气流量引起的压力降超过呼气流量引起的压力降。因此，对ETT压力 - 流量关系的完整描述需要一组四个系数：K1I、K2I、K1E和K2E。这种不对称的原因是ETT与气管之间的突然截面变化以及旋转接头的不对称形状。患者中计算得到的Ptrach与测量得到的数据比较，在±1 cmH2O范围内相符（平均一致性界限）。平均均方根（rms）偏差为0.55 cmH2O。
通过结合我们的ETT系数以及在ETT近端连续测量的流量和气道压力，可以监测Ptrach。
