Effects of the Lower Airway Secretions on Airway Opening Pressures and Suction Pressures in Critically Ill COVID-19 Patients: A Computational Simulation

Background: In patients with critically ill COVID-19 pneumonia, alveoli or small airways are filled with a large amount of highly viscous exudates or mucus, leading to airway occlusion. This is currently a difficult point in mechanical ventilation treatment of these patients. The estimation of airway opening pressures and effective mucus clearance are therefore two issues that clinicians are most concerned about. Methods: We retrospectively analyzed respiratory data from 24 critical ill patients with confirmed COVID-19 who received invasive mechanical ventilation and recruitment maneuver at Jinyintan Hospital in Wuhan, China, and found the extraordinarily high inspiratory plateau pressures. A computational model of airway opening was subsequently introduced to investigate possible fluid dynamic mechanisms. Using various types of fluids as lower airway secretions simulants, we further predicted airway opening pressures and suction pressures in critically ill patients with COVID-19. Findings: Among 24 critically ill patients in this retrospective study, the inspiratory plateau pressures ranged from 45 cmH2O to 60 cmH2O, and the mean was 52.4±4.4 cmH2O (mean±[SD]). Patients had low static compliance (23.1±7.3 ml/cmH2O), high alveolar dead space fraction (45±9 percent of tidal volume). Particularly, the capnograms presented a spike-like appearance with an upward slope during the expiratory plateau, indicting the existence of airway obstruction. Our simulation results showed that properties of airway secretions affected the airway opening pressures and suction pressures. When secretions simulants had viscosity similar to that of medicinal glycerin at room temperature or presented rheological properties like peanut butter, the airway opening pressures could be as high as 40~50 cmH2O; the negative pressure needed for secretions suctioning could exceed 20 kPa, likely causing the closures of the distal airways. Interpretation: In a proportion of critically ill patients with COVID-19, limiting plateau pressure to 30 cmH2O may not guarantee the opening of airways due to the presence of highly viscous lower airway secretions, not to mention spontaneous inspiratory efforts. Active airway humidification and effective expectorant drugs are therefore strongly recommended during airway management. Funding Statement: This work was supported by the National Natural Science Foundation of China (81971807, 11532003, 31670950) and Special Project of the National Natural Science Foundation of China (82041003). Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The Ethics Committee of Jinyintan Hospital approved the study (KY-2020-56.01). Written informed consent was waived due to the retrospective manner of this study.