Elena A, Solca M, Croci M
Istituto di Anestesia e Rianimazione, Università degli Studi, Milano.
Minerva Anestesiol. 1995 Sep;61(9):359-65.
During assist-control mode ventilation (AMV), patients perform external excess work (Wex) to activate the mechanical ventilator itself. Aim of the study was to quantitate such a Wex and evaluate the best ventilator setting in order to minimize it.
In vitro study, connecting several commercially available mechanical ventilators, with different settings, to an active model lung, developed in our department.
Gas flow and volume, and airway pressure were measured and digitally recorded; Wex was computed by pressure-volume loops. The maximum negative pressure attained in the circuit (delta Pmax), before the endotracheal tube, the time necessary to reach such a pressure [t(delta Pmax)], from the start of the model lung inspiration, and the time to return to atmospheric pressure [t(P = 0)] after mechanical ventilator activation were also recorded. Different inspiratory flow waveforms (sinusoidal, square and inverted ramp) were tested.
Wex was not different between ventilators. However, it was significantly (F8,24 = 2.697, p < 0.05) affected by different flow waveforms: Wex was markedly higher (p < 0.01) with sinusoidal (16.50 +/- 12.12 mJ) than with either square (1.17 +/- 0.96 mJ) or inverted ramp (0.60 +/- 0.70 mJ) inspiratory flow curve. Furthermore, Wex was significantly correlated (p < 0.001) with delta Pmax, t(delta Pmax) and t(p = 0). Those variables are dependent, although not uniquely, upon the trigger mechanism's sensitivity and the ventilator's response time; thus, Wex can be loosely correlated to them as well.
Since AMV is targeted at farily critical patients, whose oxygen consumption might be marginal, every excess work has to be minimized, in order to avoid respiratory fatigue appearance, and worsening of respiratory failure. Prerequisites of a correct AMV application are thus sensitive trigger mechanism, rapid ventilator response to patient's inspiratory effort, and high initial inspiratory flow (inverted ramp or square waveform).
在辅助控制模式通气（AMV）期间，患者需进行额外的外部功（Wex）来触发机械通气本身。本研究的目的是对这种Wex进行定量，并评估最佳的通气设置以将其降至最低。
体外研究，将几种具有不同设置的市售机械通气机连接到我们科室开发的主动模拟肺上。
测量并数字记录气体流量、容积和气道压力；通过压力 - 容积环计算Wex。还记录了气管插管前回路中达到的最大负压（ΔPmax）、从模拟肺吸气开始到达到该压力所需的时间[t(ΔPmax)]以及机械通气激活后恢复到大气压的时间[t(P = 0)]。测试了不同的吸气流量波形（正弦波、方波和倒梯形波）。
不同通气机之间的Wex没有差异。然而，它受到不同流量波形的显著影响（F8,24 = 2.697，p < 0.05）：正弦波吸气流量曲线时的Wex（16.50±12.12 mJ）明显高于方波（1.17±0.96 mJ）或倒梯形波（0.60±0.70 mJ）（p < 0.01）。此外，Wex与ΔPmax、t(ΔPmax)和t(p = 0)显著相关（p < 0.001）。这些变量虽然不是唯一地，但依赖于触发机制的灵敏度和通气机的响应时间；因此，Wex也可以与之松散相关。
由于AMV针对的是病情相当危急、氧耗可能处于边缘状态的患者，必须将每一项额外的功降至最低，以避免出现呼吸疲劳和呼吸衰竭恶化。因此，正确应用AMV的前提是灵敏的触发机制、通气机对患者吸气努力的快速响应以及高初始吸气流量（倒梯形波或方波波形）。
