Background and Goal of Study:Variable volume controlled ventilation (VV) has the potential to reverse atelectasis. We hypothesized that VV is superior to a conventional stepwise recruitment maneuver (RM) to open atelectatic lung tissue.

Materials and Methods:In total, 9 pigs were anesthetized and ventilated in airway pressure release ventilation (APRV) mode with positive end-expiratory pressure (PEEP) and driving pressure of 15 cmH2O, as well as respiratory rate (RR)=20/min. Four saline lung lavages were performed and lungs recruited in APRV by stepwise increase of PEEP to 45 cmH2O (10 cmH2O increments, RR=20/min, driving pressure=15 cmH2O) (stepwise RM). Then, a decremental PEEP trial was conducted in volume-controlled ventilation with VT=6 mL/kg and RR=30/min to determine the PEEP that resulted in the highest compliance (optimal PEEP). Animals were submitted to two recruitment strategies (random sequence): 1) stepwise RM followed by 30 min of volume-controlled ventilation with VT=6 mL/kg, RR=30/min and optimal PEEP; and 2) VV, consisting of 30 min volume-controlled ventilation with random variation of VT (mean=6mL/kg, normal distribution, coefficient of variation of 30%), RR=30/min and optimal PEEP. Before recruitment strategies, animals were disconnected from the ventilator for 30 seconds and volume-controlled mechanical ventilation resumed during 5 minutes with constant VT=6 mL/kg, RR=30/min and optimal PEEP to reset lung history. I:E ratio and FIO2 were kept constant at 1:1 and 1.0, respectively. Before and after each recruitment strategy, lung aeration was assessed by computed tomography, and the center of aeration along the ventral-dorsal gradient was determined.

Results and Discussion:Both recruitment strategies increased the amount of normally aerated tissue and decreased the amount of poorly and non-aerated lung tissue (Table 1). The reduction of non-aerated lung tissue was greater after VV than stepwise RM. The stepwise RM, but not VV, resulted in significant reduction of mean arterial pressure (76 [71...84] to 42 [38...43] mmHg, P<0.001) and increase of heart rate (111 [80...111] to 121 [106…131] bpm, P=0.004, median [1st …3rd quartile]).

Conclusion(s):In this model of ARDS, VV more effectively reduced non-aerated lung tissue, with less effects on hemodynamics as compared to a stepwise RM.

09 / Comparative effects of variable ventilation and a stepwise recruitment maneuver on atelectasis and hemodynamics in an experimental model of ARDS.