The Influence Of The Muscle Effort And The Respiratory Rate Setting In The Assisted-Controlled Modes On The Tidal Volume, Alveolar Pressure And PEEP Variability In A Mechanical Model Of ARDS

Abstract

In ARDS the presence of spontaneous muscle effort by the patient can lead to increased tidal volume (TV) and induce active expiration with PEEP reduction in expiration, compromising patient-ventilator interaction in a way that may aggravate factors related to ventilator-induced lung injury (VILI). Objectives: To evaluate the influences of muscle effort and ventilator respiratory rate (Vent-RR) settings on the variability of the TV, alveolar pressure and PEEP level. Methods: In a bench study, four mechanical ventilators were tested. All were set in the A/C-mode both in the VCV and the PCV modes adjusted to deliver a tidal volume of 420ml with an inspiratory time of 1 second and 10cmH2O of PEEP. The simulated patient was configured in the ASL 5000 simulator (Ingmar, USA) with a restrictive lung compliance of 25 mL/cmH2O and a RR of 20bpm with an inspiratory muscle effort of -5cmH2O and an expiratory effort of +5cmH2O for the spontaneous breathing, or zero to simulate neuromuscular blockade as a control. The Vent-RR was set in 15bpm, lower than the patient-RR, or 25bpm, higher than the patient-RR. Results: The TV, peak alveolar pressures and PEEP level showed marked variations in both the VCV and PCV modes in all ventilators while the patient was in spontaneous breathing. This variation was even higher when the Vent-RR was set above the patient-RR. Figure 1 shows a representative result for TV variation in one ventilator. This pattern was reproducible in all other 3 ventilators. Tidal colume variation in a representative ventilator according to ventilator settings Conclusions: In patients with ARDS on MV the presence of inspiratory and expiratory muscle efforts causes large fluctuations in TV, alveolar pressures and PEEP level, even in the VCV mode. The simple setting of a Vent-RR higher than the patient-RR may even be more harmful by increasing disynchrony and amplifying this phenomenon This abstract is funded by: None Am J Respir Crit Care Med 185;2012:A1684 Internet address: www.atsjournals.org Online Abstracts Issue