[Assisted ventilation: experimental evaluation in vitro].
OBJECTIVE
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.


METHODS
In vitro study, connecting several commercially available mechanical ventilators, with different settings, to an active model lung, developed in our department.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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).
Elena
A
A
Istituto di Anestesia e Rianimazione, Università degli Studi, Milano.
Solca
M
M
Croci
M
M
D000399:Air Pressure; D000758:Anesthesia; D005069:Evaluation Studies as Topic; D012121:Respiration, Artificial / Q000295:instrumentation*; D014939:Work of Breathing