How large is the lung recruitability in early acute respiratory distress syndrome: a prospective case series of patients monitored by computed tomography

Abstract

Introduction: The benefits of higher positive end expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS) have been modest, but few studies have fully tested the "open-lung hypothesis". This hypothesis states that most of the collapsed lung tissue observed in ARDS can be reversed at an acceptable clinical cost, potentially resulting in better lung protection, but requiring more intensive maneuvers. The short-/middle-term efficacy of a maximum recruitment strategy (MRS) was recently described in a small physiological study. The present study extends those results, describing a case-series of non-selected patients with early, severe ARDS submitted to MRS and followed until hospital discharge or death.

Methods: MRS guided by thoracic computed tomography (CT) included two parts: a recruitment phase to calculate opening pressures (incremental steps under pressure-controlled ventilation up to maximum inspiratory pressures of 60 cmH2O, at constant driving-pressures of 15 cmH2O); and a PEEP titration phase (decremental PEEP steps from 25 to 10 cmH2O) used to estimate the minimum PEEP to keep lungs open. During all steps, we calculated the size of the non-aerated (-100 to +100 HU) compartment and the recruitability of the lungs (the percent mass of collapsed tissue re-aerated from baseline to maximum PEEP).

Results: A total of 51 severe ARDS patients, with a mean age of 50.7 years (84% primary ARDS) was studied. The opening plateau-pressure was 59.6 (±5.9 cmH2O), and the mean PEEP titrated after MRS was 24.6 (±2.9 cmH2O). Mean PaO2/FiO2 ratio increased from 125 (±43) to 300 (±103; P<0.0001) after MRS and was sustained above 300 throughout seven days. Non-aerated parenchyma decreased significantly from 53.6% (interquartile range (IQR): 42.5 to 62.4) to 12.7% (IQR: 4.9 to 24.2) (P<0.0001) after MRS. The potentially recruitable lung was estimated at 45% (IQR: 25 to 53). We did not observe major barotrauma or significant clinical complications associated with the maneuver.

Conclusions: MRS could efficiently reverse hypoxemia and most of the collapsed lung tissue during the course of ARDS, compatible with a high lung recruitability in non-selected patients with early, severe ARDS. This strategy should be tested in a prospective randomized clinical trial.

Figures

Figure 1

Design of Maximum Recruitment Strategy (MRS) protocol. The first phase, "recruitment phase", starts at baseline-PEEP (around 10 cmH2O) and goes up to maximum-PEEP (around 45 cmH2O), transiently generating plateau-inspiratory pressures around 60 cmH2O. The second phase, "PEEP titration phase", starts at PEEP = 25 cmH2O, and goes down to PEEP = 10 cmH2O. PCV: pressure controlled ventilation; ΔP: driving pressure = plateau inspiratory pressure minus PEEP. The arrows mark the timing of CT scanning during an expiratory pause at the designated PEEP.

Figure 2

Amount of non-aerated tissue observed along the protocol. Numbers represent the percentage of non-aerated tissue (in relation to the total lung tissue) within the thick lung section (7 to 8 cm thick) from the carina to the diaphragm. Dashed lines represent the 45 individual cases and boxes represent the interquartile range, with a central line representing the median value at each moment.

Figure 3

Illustrative cases of MRS. One with best relative-response (98% - left column), one with typical median response (82% - mid column), and another with the worst response (45% - right column). As can be noticed, the measurement of the potentially recruitable lung does not match this relative response (with values of 48%, 63% and 36%, respectively), since this last parameter is also strongly coupled with the amount of non-aerated tissue at baseline PEEP (49%, 78% and 80%, respectively).

Figure 4

Histogram for the amount of potentially recruitable lung observed in our sample of patients (N = 45). Dashed lines represent the equivalent histogram described by Gattinoni and cols. in a previous publication [29].

Figure 5

Individual variations of PaO2/FIO2 ratio, before and after MRS. The red dashed line represents the threshold of 200. Whereas the baseline blood gas was collected at 100% oxygen fraction, the second one, collected at the titrated-PEEP, was collected at an average FIO2 = 45% (± 17%).

Figure 6

Evolution of PaO2/FIO2 ratio, plateau pressures and PEEP levels throughout seven days. Observe the relation among the variables before, during and throughout the seven days after MRS.