Correlation between normally aerated lung and respiratory system compliance at clinical high positive end-expiratory pressure in patients with COVID-19

Abstract

Normally aerated lung tissue on computed tomography (CT) is correlated with static respiratory system compliance (C_rs) at zero end-expiratory pressure. In clinical practice, however, patients with acute respiratory failure are often managed using elevated PEEP levels. No study has validated the relationship between lung volume and tissue and C_rs at the applied positive end-expiratory pressure (PEEP). Therefore, this study aimed to demonstrate the relationship between lung volume and tissue on CT and C_rs during the application of PEEP for the clinical management of patients with acute respiratory distress syndrome due to COVID-19. Additionally, as a secondary outcome, the study aimed to evaluate the relationship between CT characteristics and C_rs, considering recruitability using the recruitment-to-inflation ratio (R/I ratio). We analyzed the CT and respiratory mechanics data of 30 patients with COVID-19 who were mechanically ventilated. The CT images were acquired during mechanical ventilation at PEEP level of 15 cmH₂O and were quantitatively analyzed using Synapse Vincent system version 6.4 (Fujifilm Corporation, Tokyo, Japan). Recruitability was stratified into two groups, high and low recruitability, based on the median R/I ratio of our study population. Thirty patients were included in the analysis with the median R/I ratio of 0.71. A significant correlation was observed between C_rs at the applied PEEP (median 15 [interquartile range (IQR) 12.2, 15.8]) and the normally aerated lung volume (r = 0.70 [95% CI 0.46-0.85], P < 0.001) and tissue (r = 0.70 [95% CI 0.46-0.85], P < 0.001). Multivariable linear regression revealed that recruitability (Coefficient = - 390.9 [95% CI - 725.0 to - 56.8], P = 0.024) and C_rs (Coefficient = 48.9 [95% CI 32.6-65.2], P < 0.001) were significantly associated with normally aerated lung volume (R-squared: 0.58). In this study, C_rs at the applied PEEP was significantly correlated with normally aerated lung volume and tissue on CT. Moreover, recruitability indicated by the R/I ratio and C_rs were significantly associated with the normally aerated lung volume. This research underscores the significance of C_rs at the applied PEEP as a bedside-measurable parameter and sheds new light on the link between recruitability and normally aerated lung.

Keywords: Computed tomography; Coronavirus disease 2019; Mechanical ventilation; Recruitability; Respiratory distress syndrome; Respiratory system compliance.

Figure 1

Flow diagram of the study patients. COVID-19, coronavirus disease 2019; R/I, recruitment-to-inflation; CT, computed tomography; PEEP, positive end-expiratory pressure.

Figure 2

Scatter diagrams of the relationship between the C_rs at the clinical setting of PEEP and lung volume on CT. C_rs at the clinical setting of PEEP was significantly correlated with normally aerated lung volume (r = 0.70, P < 0.001, Fig. 2A). Linear regression analysis indicated that for every unit increase in C_rs, the normally aerated lung volume increased by 42.2 mL, with a y-intercept of 166.0 m. C_rs at the clinical setting of PEEP did not correlate with the poorly aerated and hyperinflated air volume (Fig. 2B and C). C_rs, static respiratory system compliance; CT, computed tomography; PEEP, positive end-expiratory pressure.

Figure 3

Scatter diagrams of the relationship between the C_rs at the clinical setting of PEEP and lung tissue on CT. C_rs at the clinical setting of PEEP was significantly correlated with normally aerated tissue (r = 0.67, P < 0.001, Fig. 3A). Linear regression analysis indicated that for every unit increase in C_rs, the normally aerated tissue increased by 12.1 g, with a y-intercept of 63.9 g. C_rs at the clinical setting of PEEP did not correlate with the poorly aerated and nonaerated tissue (Fig. 3B,C). C_rs, static respiratory system compliance; CT, computed tomography; PEEP, positive end-expiratory pressure.

Figure 4

Correlation of the C_rs at the clinical setting of PEEP with normally aerated lung volume and tissue when divided into two groups according to recruitability indicated by the median R/I ratio of 0.71. The relationship between C_rs at the clinical setting of PEEP and the normally aerated lung volume and tissue was more strongly correlated when divided by the median R/I ratio of 0.71. For lung volume, the high recruitability group had a correlation coefficient (r) of 0.73 (P = 0.0021), and the low recruitability group had an r of 0.81 (P < 0.001) (Fig. 4A,B). For tissue, the high recruitability group had an r of 0.75 (P = 0.0014), and the low recruitability group had an r of 0.71 (P = 0.0028) (Fig. 4C,D). C_rs, static respiratory system compliance; PEEP, positive end-expiratory pressure; R/I, recruitment-to-inflation ratio.

Figure 5

Correlation between the C_rs at the applied PEEP and residual inflated lung tissue (r = 0.56, P = 0.0013). C_rs, static respiratory system compliance; PEEP, positive end-expiratory pressure.