Department of Pediatrics and Adolescent Medicine, Division of Neonatology/Intensive Care, University Hospital of Freiburg, Freiburg, Germany.
Physiol Meas. 2011 Sep;32(9):1439-51. doi: 10.1088/0967-3334/32/9/007. Epub 2011 Jul 28.
Resistive properties of endotracheal tubes (ETTs) are particularly relevant in newborns and small infants who are generally ventilated through ETTs with a small inner diameter. The ventilation rate is also high and the inspiratory time (ti) is short. These conditions effectuate high airway flows with excessive flow acceleration, so airway resistance and inertance play an important role. We carried out a model study to investigate the impact of varying ETT size, lung compliance and ventilator settings, such as peak inspiratory pressure (PIP), positive end expiratory pressure (PEEP) and inspiratory time (ti) on the pressure-flow characteristics with respect to the resistive and inertive properties of the ETT. Pressure at the Y piece was compared to direct measurement of intratracheal pressure (P(trach)) at the tip of the ETT, and pressure drop (ΔP(ETT)) was calculated. Applying published tube coefficients (Rohrer's constants and inertance), P(trach) was calculated from ventilator readings and compared to measured P(trach) using the root-mean-square error. The most relevant for ΔP(ETT) was the ETT size, followed by (in descending order) PIP, compliance, ti and PEEP, with gas flow velocity being the principle in common for all these parameters. Depending on the ventilator settings ΔP(ETT) exceeded 8 mbar in the smallest 2.0 mm ETT. Consideration of inertance as an additional effect in this setting yielded a better agreement of calculated versus measured P(trach) than Rohrer's constants alone. We speculate that exact tracheal pressure tracings calculated from ventilator readings by applying Rohrer's equation and the inertance determination to small size ETTs would be helpful. As an integral part of ventilator software this would (1) allow an estimate of work of breathing and implementation of an automatic tube compensation, and (2) be important for gentle ventilation in respiratory care, especially of small infants, since it enables the physician to estimate consequences of altered ventilator settings at the tracheal level.
气管内导管（ETT）的阻力特性在新生儿和小婴儿中尤为重要，他们通常通过内径较小的 ETT 进行通气。通气频率也很高，吸气时间（ti）很短。这些条件会导致气道流量过大，流速加速过度，因此气道阻力和惯性起着重要作用。我们进行了一项模型研究，以调查 ETT 尺寸、肺顺应性和呼吸机设置（如吸气峰压（PIP）、呼气末正压（PEEP）和吸气时间（ti））的变化对压力-流量特性的影响，以及 ETT 的阻力和惯性特性。Y 型件处的压力与 ETT 尖端的直接测量的气管内压力（P(trach)）进行了比较，并计算了压力降（ΔP(ETT)）。应用已发表的管系数（Rohrer 常数和惯性），从呼吸机读数计算 P(trach)，并使用均方根误差将其与使用测量的 P(trach)进行比较。对ΔP(ETT)影响最大的是 ETT 尺寸，其次是（降序排列）PIP、顺应性、ti 和 PEEP，所有这些参数的共同原理是气体流速。根据呼吸机设置，最小的 2.0mm ETT 中 ΔP(ETT)超过 8mbar。在这种情况下，将惯性视为附加影响，与单独使用 Rohrer 常数相比，计算的 P(trach)与测量的 P(trach)之间的一致性更好。我们推测，通过应用 Rohrer 方程和惯性确定值从小型 ETT 计算从呼吸机读数得出的精确气管压力轨迹将有所帮助。作为呼吸机软件的一个组成部分，这将（1）允许估计呼吸功并实施自动管补偿，（2）在呼吸护理中尤其对小婴儿的轻柔通气很重要，因为它使医生能够估计在气管水平改变呼吸机设置的后果。
