Bernstein G, Knodel E, Heldt G P
Department of Pediatrics, University of California, San Diego, School of Medicine, USA.
Crit Care Med. 1995 Oct;23(10):1739-44. doi: 10.1097/00003246-199510000-00020.
To define the spectrum of airway leak in the neonatal population and examine the occurrence rate of autocycling of three flow-triggered ventilators within the defined spectrum of airleak.
Prospective study of pulmonary function tests of intubated infants and performance of ventilators on a mechanical lung model under simulated clinical conditions.
An intensive care nursery and research laboratory at a university medical center.
Analysis of pulmonary function tests of 50 infants from our intensive care nursery, selected at random, to determine size of airleak around the endotracheal tube. The rate of autocycling of ventilators due to airleak of variable size, while connected to a test lung was subsequently studied. Ventilators were set on the assist-control mode with the control rate set at 0 breath/min. Each ventilator was studied at the maximum sensitivity setting, which was 1, 2.5, and 3.3 mL/sec for each ventilator, respectively, and also at decreased sensitivity settings to 10 mL/sec. Airleak size was varied (10% to 45%) by increasing the orifice size within the endotracheal tube adapter/connector sideport and/or the positive end-expiratory pressure level (2 to 8 cm H2O).
In the infants, airleak size was calculated during synchronous ventilator breaths as (inspiratory minus expiratory) tidal volume/expiratory tidal volume x 100% (n = 25 +/- 11 breaths/patient). Mean +/- SD leak size in the infants was 15.6 +/- 11%. A minimal leak size of 0 to 10% was present in 15 (30%) infants, leak size of 10% to 20% in 24 (48%), leak size of 20% to 30% in seven (14%), and leak size > 30% in four (8%) infants. The relative tendency of the three ventilators to autocycle is a function of the maximum sensitivity setting, which varies with each ventilator. The ventilator with the maximum sensitivity set at 1 mL/sec autocycled rapidly (> or = 40 breaths/min) at leak size of > 10%; the ventilator set at 2.5 mL/sec autocycled rapidly at leak size of > or = 20%; and the ventilator set at 3.3 mL/sec autocycled rapidly at leak size of > or = 30%. In all ventilators, the rate of autocycling increased with increased leak size, and decreased with decreased sensitivity setting.
Flow-triggered ventilators are susceptible to autocycling due to flow compensation to maintain positive end-expiratory pressure levels in the presence of an airway leak. The difference in autocycling is due to the maximum sensitivity setting of each ventilator, and not to intrinsic ventilator flowsensing or other software mechanisms. The 3.3-mL/sec setting was the least prone to autocycling and seems appropriate. The ventilator set at 2.5 mL/sec at the time of this study has been released instead at 4 mL/sec, due to these findings. The ventilator with the maximum setting at 1 mL/sec autocycled readily at leak size of > or = 10%. Since such a leak size was present in 70% of infants, this setting should be used with caution. Using these guidelines, autocycling of all three ventilators is likely to occur mainly in 8% of infants with leak size of > 30%. In these cases, lowering the sensitivity setting and/or positive end-expiratory pressure level may decrease autocycling, or may necessitate reintubation with a larger endotracheal tube.
明确新生儿气道漏气的范围，并在定义的漏气范围内检查三种流量触发通气机的自动循环发生率。
对插管婴儿进行肺功能测试的前瞻性研究，以及在模拟临床条件下在机械肺模型上对通气机性能的研究。
一所大学医学中心的重症监护病房和研究实验室。
分析从我们的重症监护病房随机选取的50名婴儿的肺功能测试结果，以确定气管插管周围的漏气大小。随后研究了通气机在连接测试肺时因不同大小漏气而发生自动循环的速率。通气机设置为辅助控制模式，控制频率设置为0次/分钟。每种通气机分别在最大灵敏度设置下进行研究，其最大灵敏度分别为1、2.5和3.3 mL/秒，同时也在降低至10 mL/秒的灵敏度设置下进行研究。通过增加气管插管适配器/连接器侧孔内的孔口大小和/或呼气末正压水平（2至8 cm H₂O）来改变漏气大小（10%至45%）。
在婴儿中，同步通气机呼吸期间的漏气大小计算为（吸气潮气量减去呼气潮气量）/呼气潮气量×100%（每位患者n = 25±11次呼吸）。婴儿中的平均±标准差漏气大小为15.6±11%。15名（30%）婴儿的最小漏气大小为0至10%，24名（48%）婴儿的漏气大小为10%至20%，7名（14%）婴儿的漏气大小为20%至30%，4名（8%）婴儿的漏气大小>30%。三种通气机自动循环的相对倾向是最大灵敏度设置的函数，每种通气机的最大灵敏度设置各不相同。最大灵敏度设置为1 mL/秒的通气机在漏气大小>10%时迅速自动循环（≥40次/分钟）；最大灵敏度设置为2.5 mL/秒的通气机在漏气大小≥20%时迅速自动循环；最大灵敏度设置为3.3 mL/秒的通气机在漏气大小≥30%时迅速自动循环。在所有通气机中，自动循环速率随漏气大小增加而增加，随灵敏度设置降低而降低。
在存在气道漏气的情况下，流量触发通气机由于流量补偿以维持呼气末正压水平而容易发生自动循环。自动循环的差异是由于每种通气机的最大灵敏度设置，而不是由于通气机固有的流量传感或其他软件机制。3.3 mL/秒的设置最不容易发生自动循环，似乎是合适的。由于这些研究结果，本研究中最大灵敏度设置为2.5 mL/秒的通气机现已改为4 mL/秒发布。最大灵敏度设置为1 mL/秒的通气机在漏气大小≥10%时容易自动循环。由于70%的婴儿存在这样的漏气大小，因此应谨慎使用此设置。按照这些指导原则，所有三种通气机的自动循环可能主要发生在8%的漏气大小>30%的婴儿中。在这些情况下，降低灵敏度设置和/或呼气末正压水平可能会减少自动循环，或者可能需要重新插入更大尺寸的气管插管。
