Impact of Positive End-Expiratory Pressure and FiO₂ 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.