Effects of inspiratory flow on lung stress, pendelluft, and ventilation heterogeneity in ARDS: a physiological study

Abstract

Background: High inspiratory flow might damage the lungs by mechanisms not fully understood yet. We hypothesized that increasing inspiratory flow would increase lung stress, ventilation heterogeneity, and pendelluft in ARDS patients undergoing volume-controlled ventilation with constant tidal volume and that higher PEEP levels would reduce this phenomenon.

Methods: Ten ARDS patients were studied during protective volume-controlled ventilation. Three inspiratory flows (400, 800, and 1200 ml/s) and two PEEP levels (5 and 15 cmH₂O) were applied in random order to each patient. Airway and esophageal pressures were recorded, end-inspiratory and end-expiratory holds were performed, and ventilation distribution was measured with electrical impedance tomography. Peak and plateau airway and transpulmonary pressures were recorded, together with the airway and transpulmonary pressure corresponding to the first point of zero end-inspiratory flow (P1). Ventilation heterogeneity was measured by the EIT-based global inhomogeneity (GI) index. Pendelluft was measured as the absolute difference between pixel-level inflation measured at plateau pressure minus P1.

Results: Plateau airway and transpulmonary pressure was not affected by inspiratory flow, while P1 increased at increasing inspiratory flow. The difference between P1 and plateau pressure was higher at higher flows at both PEEP levels (p < 0.001). While higher PEEP reduced heterogeneity of ventilation, higher inspiratory flow increased GI (p = 0.05), irrespective of the PEEP level. Finally, gas volume undergoing pendelluft was larger at higher inspiratory flow (p < 0.001), while PEEP had no effect.

Conclusions: The present exploratory analysis suggests that higher inspiratory flow increases additional inspiratory pressure, heterogeneity of ventilation, and pendelluft while PEEP has negligible effects on these flow-dependent phenomena. The clinical significance of these findings needs to be further clarified.

Keywords: ARDS; Electrical impedance tomography; Heterogeneity; Inspiratory flow.

Fig. 1

Method for P1 calculation. Flow–time trace (red) and airway (Paw, black), esophageal (Pes, blue), and transpulmonary (P_L, green) pressure–time traces of a representative patient during an end-inspiratory occlusion. Transpulmonary pressure trace is obtained by subtraction of esophageal pressure trace from airway pressure trace. Peak pressure (Ppeak) is the highest (airway or transpulmonary) pressure value reached during inspiration. P1 is calculated as the point on the (airway or transpulmonary) pressure–time trace corresponding to the first zero or negative flow value on the flow–time trace after end-inspiratory occlusion. Plateau pressure (Pplat) is calculated as the (airway or transpulmonary) pressure value after 3 s from end-inspiratory occlusion

Fig. 2

Flow and PEEP effect on additional inspiratory pressure and pendelluft. Box plot of additional flow-dependent pressure (measured as P1 − Pplat, a) and pendelluft (b) at the different combinations of PEEP (5 and 15 cmH₂O) and inspiratory flow (400, 800, and 1200 ml/s; white, light gray, and dark gray, respectively). a p < 0.001 for flow effect, p = 0.168 for PEEP effect; b p < 0.001 for flow effect, p = 0.676 for PEEP effect; two-way repeated-measures ANOVA. *p < 0.05 within PEEP; †p < 0.01 within PEEP

Fig. 3

Pendelluft at end-inspiration. Pendelluft occurring at end-inspiration at PEEP 5 cmH₂O (upper panels) and 15 cmH₂O (lower panels) at low (left panels) and high (right panels) inspiratory flow in a representative patient. Pendelluft was calculated from the pixel-by-pixel difference between EIT-derived aeration at Pplat and P1 (see text for details). Each pixel was color-coded based on the amount of gas in milliliters entering (white) or leaving (black) the pixel. The color bar is provided on the right-hand side of each panel