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

Abstract

In patients with critically ill COVID-19 pneumonia, lower airways are filled with plenty of highly viscous exudates or mucus, leading to airway occlusion. The estimation of airway opening pressures and effective mucus clearance are therefore two issues that clinicians are most concerned about during mechanical ventilation. In this study we retrospectively analyzed respiratory data from 24 critically ill patients with COVID-19 who received invasive mechanical ventilation and recruitment maneuver at Jinyintan Hospital in Wuhan, China. Among 24 patients, the mean inspiratory plateau pressure was 52.4 ± 4.4 cmH₂O (mean ± [SD]). Particularly, the capnograms presented an upward slope during the expiratory plateau, indicting the existence of airway obstruction. A computational model of airway opening was subsequently introduced to investigate possible fluid dynamic mechanisms for the extraordinarily high inspiratory plateau pressures among these patients. Our simulation results showed that the predicted airway opening pressures could be as high as 40-50 cmH₂O and the suction pressure could exceed 20 kPa as the surface tension and viscosity of secretion simulants markedly increased, likely causing the closures of the distal airways. We concluded that, in some 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.

Keywords: Airway mucus; Airway opening pressure; Coronavirus disease 2019; Endotracheal suctioning; Respiratory mechanics.

Figure 1

Airway reopening model. R is the radius of the flexible-walled tube. H is the thickness of upstream film, and T is imposed axial wall tension. Positive pressure $P_{\text{total}}^{\ast }$ is applied to gas phase, peeling apart the tube at velocity U and opening angle $\theta$.

Figure 2

Graphic recording of end-tidal CO₂ curves in six critically ill patients with COVID-19.

Figure 3

Effects of surface tension and viscosity of Newtonian lining liquids on airway reopening pressures. It is assumed that in a normal lung the equilibrium surface tension and viscosity of airway lining fluid are 25 dyn/cm and 0.01 poise, respectively. In a diseased lung, the values of viscosity are assumed to increase to 0.1, 1 or 10 poise. Note that the airway reopening pressures of generations 8–14 are less than 5 cmH₂O in a normal lung and that the airway reopening pressure of generation 8 may rise to as high as 36 cmH₂O if the value of viscosity increases to 10 poise in a diseased lung. $1\text{poise}=0.1\text{Pa}\text{s}=1{\text{dyn/cm}}^2\text{s}$; $1\text{dyn}={10}^{-5}\text{N}$.

Figure 4

Effects of surface tension and viscosity of Newtonian lining liquids on airway reopening pressures. It is assumed that in a diseased lung the equilibrium surface tension airway lining fluid increases to 50 dyn/cm and that the viscosity increase from a normal value of 0.01 poise to 0.1, 1 or 10 poise respectively. Note that as compared with Fig. 3, an increased surface tension leads to an increasing in the airway reopening pressures. As the surface tension and viscosity elevates to 50 dyn/cm and 10 poise respectively, the airway reopening pressure of generation 8 may reach 41.8 cmH₂O. $1\text{poise}=0.1\text{Pa}\text{s}=1{\text{dyn/cm}}^2\text{s}$; $1\text{dyn}={10}^{-5}\text{N}$.

Figure 5

The airway reopening pressures of generations 8–14 for non-Newtonian lining liquids (pseudoplastic fluids). Solid line and dash line show the thickness of lining liquids $H=10\text{and}50\mu \text{m}$. Of note, although the values of surface tension and consistency of three pseudoplastic fluids were all greater than those of simulated fluid ④ (Table 2), the airway reopening pressures are less than 13 cmH₂O, which are appreciably lower than those for simulated fluid ④ (> 35 cmH₂O, inverted triangles in Fig. 4), indicating that shear thinning effect may reduce the airway reopening pressure.

Figure 6

The airway reopening pressures of generations 8–14 for non-Newtonian lining liquids (yield pseudoplastic fluids). Solid line and dash line show the thickness of lining liquids $\text{H}=10\text{and}50\mu \text{m}$. Note that the airway reopening pressures may be as high as 48.9 cmH₂O when the airway is filled with peanut butter-like liquid ($m=130.8{\text{dyn/cm}}^2{\text{s}}^{\text{n}},{\tau }_{\text{y}}=400{\text{dyn/cm}}^2$, and $\text{n}=0.705$, Table 2). Although mayonnaise has the highest yield stress (${\tau }_{\text{y}}=600{\text{dyn/cm}}^2$, Table 2), the corresponding airway reopening pressures are lower than 6.5 cmH₂O due to its strong shear thinning effect ($\text{n}=0.471$, Table 2).

Figure 7

The yield pressures of 8 to 14 generations of airways with surface tension of occlusion fluids $\gamma =25\text{or}50\text{dyn/cm}$. The dash line indicates the critical transmural pressure $P_{\text{trans}}=7.5{\text{cmH}}_2\text{O}$ needed to prevent compliant collapse of airways.