Risk factors and prognostic impact of decreased breathing variability in mechanically ventilated patients. A prospective study

Abstract

Rational: Mechanical ventilation decreases the natural variability of major descriptors of the breathing pattern such as tidal volume (VT), inspiratory time (Ti), respiratory rate (RR) and mean inspiratory flow (Vt/Ti). Few studies have investigated the risk factors and the impact of a decreased variability on the outcome. Study aims: to identify risk factors of decreased breathing variability,to describe the consequences of a decreased breathing variability on the outcome. Methods: Ancillary study of a randomized controlled trial comparing neurally ventilator adjusted assist to pressure support ventilation in 127 patients. VT, RR and Vt/Ti were computed from the flow signal recordings and their respective coefficients of variation (CV) were calculated. Subjects were classified as having high or low variability pattern according to a cut-off determined by the respective median of each CV. Results: Mean coefficient of variation for RR, VT, VT/Ti was respectively 0.22 ± 0.09, 0.21 ± 0.10 and 0.17 ± 0,12. Age was significantly lower in patients with a high CV for RR compared to those with a low CV (65±13 vs.70±11, p= 0.038). No other significant difference was observed in term of gender, severity score (SAPS II), dyspnea, comfort and mechanical ventilation duration before inclusion between patients with a high or low CV. No significant difference was found between patients with high and low variability in term of on D28 mortality, duration of mechanical ventilation and length of stay. Conclusion: Among all factors, only age was associated with a decrease of breathing variability. Decrease of breathing variability was not associated with an alteration of the outcome.