Changes in ventilation-perfusion during and after an COPD exacerbation: an assessment using fluid dynamic modeling

Abstract

Introduction: Severe exacerbations associated with chronic obstructive pulmonary disease (COPD) that require hospitalization significantly contribute to morbidity and mortality. Definitions for exacerbations are very broad, and it is unclear whether there is one predominant underlying mechanism that leads to them. Functional respiratory imaging (FRI) with modeling provides detailed information about airway resistance, hyperinflation, and ventilation-perfusion (V/Q) mismatch during and following an acute exacerbation.

Materials and methods: Forty-two patients with COPD participating in a multicenter study were assessed by FRI, pulmonary function tests, and self-reported outcome measures during an acute exacerbation and following resolution. Arterial blood gasses and lung function parameters were measured.

Results: A significant correlation was found between alveolar-arterial gradient and image-based V/Q (iV/Q), suggesting that iV/Q represents V/Q mismatch during an exacerbation (p<0.05).

Conclusion: Recovery of an exacerbation is due to decreased (mainly distal) airway resistance (p<0.05). Improvement in patient-reported outcomes were also associated with decreased distal airway resistance (p<0.05), but not with forced expiratory volume. FRI is, therefore, a sensitive tool to describe changes in airway caliber, ventilation, and perfusion during and after exacerbation. On the basis of the fact that FRI increased distal airway resistance seems to be the main cause of an exacerbation, therapy should mainly focus on decreasing it during and after the acute event.

Keywords: acute exacerbation; airway resistance; functional respiratory imaging; high-resolution computed tomography; ventilation–perfusion.

Figure 1

Functional respiratory imaging can describe vascular structures and blood vessel density as well as lobe and lung expansion and, therefore, indicate the image-based ventilation–perfusion ratio.

Figure 2

There is a significant correlation (Spearman rank order R=−0.47, p=0.02) between iV/Q (at visit 2) and AaDO₂ (at visit 2) indicating that iV/Q reflects ventilation–perfusion mismatch. Abbreviations: AaDO₂, alveolar–arterial oxygen gradient; iV/Q, image-based ventilation–perfusion.

Figure 3

There is a significant correlation between the improvement in lung expansion (the difference at lobar level between FRC and TLC volumes) and the improvement in iV/Q (Spearman rank order R=0.51, p=0.00) indicating that the improvement in iV/Q is due to improved ventilation at the lobar level. Abbreviations: FRC, functional residual capacity; iV/Q, image-based ventilation–perfusion; TLC, total lung capacity.

Figure 4

Correlation between the improvement in lung expansion (as a sum of lobar expansion; Spearman rank order R=−0.34, p=0.03) after recovery and distal airway resistance (measured at TLC) after recovery (at visit 2). Abbreviation: TLC, total lung capacity.

Figure 5

Correlation between lung expansion (Spearman rank order R=−0.53, p=0.00) after recovery and central airway resistance (at TLC) after recovery (at visit 2). Abbreviation: TLC, total lung capacity.

Figure 6

Correlation between the improvement in CAT score (Spearman rank order R=0.33, p=0.04) and the drop in distal airway resistance (at FRC) after recovery. Abbreviations: CAT, COPD assessment test; FRC, functional residual capacity.

Figure 7

Absence of correlation between the improvement in FEV₁ and the improvement in CAT score (Spearman rank order R=−0.25, p=0.12). Abbreviations: CAT, COPD assessment test; FEV₁, forced expiratory volume in 1 second.

Figure 8

Relationship between airway resistance and airway generation. Notes: The largest part of the airway resistance is located before the ninth generation. Reproduced with permission from The Radiological Society of North America (RSNA), De Backer JW, Vos WG, Vinchurkar SC, et al. Validation of computational fluid dynamics in CT-based airway models with SPECT/CT. Radiology. 2010;257(3):854–862. Abbreviations: FRC, functional residual capacity; TLC, total lung capacity.