Impact of Positive End-Expiratory Pressure and FiO 2 on Lung Mechanics and Intrapulmonary Shunt in Mechanically Ventilated Patients with ARDS Due to COVID-19 Pneumonia.

Purpose: This study aimed to investigate the effects of inspired oxygen fraction (FiO 2 ) and positive end-expiratory pressure (PEEP) on gas exchange in mechanically ventilated patients with COVID-19. Methods: Two FiO 2 (100%, 40%) were tested at 3 decreasing levels of PEEP (15, 10, and 5 cmH 2 O). At each FiO 2 and PEEP, gas exchange, respiratory mechanics, hemodynamics, and the distribution of ventilation and perfusion were assessed with electrical impedance tomography. The impact of FiO 2 on the intrapulmonary shunt (delta shunt) was analyzed as the difference between the calculated shunt at FiO 2 100% (shunt) and venous admixture at FiO 2 40% (venous admixture). Results: Fourteen patients were studied. Decreasing PEEP from 15 to 10 cmH 2 O did not change shunt (24 [21-28] vs 27 [24-29]%) or venous admixture (18 [15-26] vs 23 [18-34]%) while partial pressure of arterial oxygen (FiO 2 100%) was higher at PEEP 15 (262 [198-338] vs 256 [147-315] mmHg; P  < .05). Instead when PEEP was decreased from 10 to 5 cmH 2 O, shunt increased to 36 [30-39]% ( P  < .05) and venous admixture increased to 33 [30-43]% ( P  < .05) and partial pressure of arterial oxygen (100%) decreased to 109 [76-177] mmHg ( P  < .05). At PEEP 15, administration of 100% FiO 2 resulted in a shunt greater than venous admixture at 40% FiO 2, ((24 [21-28] vs 18 [15-26]%, P  = .005), delta shunt 5.5% (2.3-8.8)). Compared to PEEP 10, PEEP of 5 and 15 cmH 2 O resulted in decreased global and pixel-level compliance. Cardiac output at FiO 2 100% resulted higher at PEEP 5 (5.4 [4.4-6.5]) compared to PEEP 10 (4.8 [4.1-5.5], P  < .05) and PEEP 15 cmH 2 O (4.7 [4.5-5.4], P  < .05). Conclusion: In this study, PEEP of 15 cmH 2 O, despite resulting in the highest oxygenation, was associated with overdistension. PEEP of 5 cmH 2 O was associated with increased shunt and alveolar collapse. Administration of 100% FiO 2 was associated with an increase in intrapulmonary shunt in the setting of high PEEP. Trial registration: NCT05132933.