Ventilation heterogeneity in smokers: role of unequal lung expansion and peripheral lung structure

Structure-function in smokers: when a small airways test really reflects the small airways

If multiple-breath washout (MBW)-derived acinar ventilation heterogeneity (Sacin) really represents peripheral units, the N2 phase-III of the first MBW exhalation should be curvilinear. This is essentially due to the superposed effect of gas diffusion and convection resulting in an equilibration of N2 concentrations between neighboring lung units throughout exhalation. We investigated this in smokers with computed tomography (CT)-proven functional small airway disease. Instantaneous N2-slopes were computed over 40-ms intervals throughout phase-III and normalized by mean phase-III N2 concentration. N2 phase-III (concave) curvilinearity was quantified as the rate at which the instantaneous N2-slope decreases past the phase-II peak over a 1-s interval; for a linear N2 phase-III unaffected by diffusion, this rate would amount to 0 L-1/s. N2 phase-III curvilinearity was obtained on the experimental curves and on existing model simulations of N2 curves from a normal peripheral lung model and one with missing terminal bronchioles (either 50% or 30% TB left). In 46 smokers [66 (±8) yr; 49 (±26) pack·yr] with CT-based evidence of peripheral lung destruction, instantaneous N2-slope decrease was compared between those with (fSAD+fEmphys) > 20% [-0.26 ± 0.14 (SD) L-1/s; n = 24] and those with (fSAD+fEmphys) < 20% [-0.16 ± 0.12 (SD) L-1/s; n = 22] (P = 0.014). Experimental values fell in the range predicted by a realistic peripheral lung model with progressive reduction of terminal bronchioles: values of instantaneous N2-slope decrease obtained from model simulations were -0.09 L-1/s (normal lung; 100% TB left), -0.17 L-1/s (normal lung 50% TB left), and -0.29 L-1/s (30% TB left). In smokers with CT-based evidence of functional small airway alterations, it is possible to demonstrate that Sacin really does represent the most peripheral airspaces.NEW & NOTEWORTHY In smokers with computed tomography-based evidence of functional small airway alterations by parametric response mapping, it is possible to demonstrate that the multiple-breath washout-derived Sacin, an index of acinar ventilation heterogeneity, actually does represent the most peripheral airspaces. This is done by verifying on experimental N2 washout curves of the first breath, N2 phase-III concavity predicted by the diffusion-convection interdependence model.

Quantifying heterogeneity in an animal model of acute respiratory distress syndrome, a comparison of inspired sinewave technique to computed tomography

The inspired sinewave technique (IST) is a non-invasive method to measure lung heterogeneity indices (including both uneven ventilation and perfusion or heterogeneity), which reveal multiple conditions of the lung and lung injury. To evaluate the reproducibility and predicted clinical outcomes of IST heterogeneity values, a comparison with a quantitative lung computed tomography (CT) scan is performed. Six anaesthetised pigs were studied after surfactant depletion by saline-lavage. Paired measurements of lung heterogeneity were then taken with both the IST and CT. Lung heterogeneity measured by the IST was calculated by (a) the ratio of tracer gas outputs measured at oscillation periods of 180 s and 60 s, and (b) by the standard deviation of the modelled log-normal distribution of ventilations and perfusions in the simulation lung. In the CT images, lungs were manually segmented and divided into different regions according to voxel density. A quantitative CT method to calculate the heterogeneity (the Cressoni method) was applied. The IST and CT show good Pearson correlation coefficients in lung heterogeneity measurements (ventilation: 0.71, and perfusion, 0.60, p < 0.001). Within individual animals, the coefficients of determination average ventilation (R2 = 0.53) and perfusion (R2 = 0.68) heterogeneity. Strong concordance rates of 98% in ventilation and 89% when the heterogeneity changes were reported in pairs measured by CT scanning and IST methods. This quantitative method to identify heterogeneity has the potential to replicate CT lung heterogeneity, and to aid individualised care in ARDS.

Computed cardiopulmonography and the idealized lung clearance index, iLCI2.5, in early-stage cystic fibrosis

This study explored the use of computed cardiopulmonography (CCP) to assess lung function in early-stage cystic fibrosis (CF). CCP has two components. The first is a particularly accurate technique for measuring gas exchange. The second is a computational cardiopulmonary model where patient-specific parameters can be estimated from the measurements of gas exchange. Twenty-five participants (14 healthy controls, 11 early-stage CF) were studied with CCP. They were also studied with a standard clinical protocol to measure the lung clearance index (LCI2.5). Ventilation inhomogeneity, as quantified through CCP parameter σlnCl, was significantly greater (P < 0.005) in CF than in controls, and anatomical deadspace relative to predicted functional residual capacity (DS/FRCpred) was significantly more variable (P < 0.002). Participant-specific parameters were used with the CCP model to calculate idealized values for LCI2.5 (iLCI2.5) where extrapulmonary influences on the LCI2.5, such as breathing pattern, had all been standardized. Both LCI2.5 and iLCI2.5 distinguished clearly between CF and control participants. LCI2.5 values were mostly higher than iLCI2.5 values in a manner dependent on the participant's respiratory rate (r = 0.46, P < 0.05). The within-participant reproducibility for iLCI2.5 appeared better than for LCI2.5, but this did not reach statistical significance (F ratio = 2.2, P = 0.056). Both a sensitivity analysis on iLCI2.5 and a regression analysis on LCI2.5 revealed that these depended primarily on an interactive term between CCP parameters of the form σlnCL*(DS/FRC). In conclusion, the LCI2.5 (or iLCI2.5) probably reflects an amalgam of different underlying lung changes in early-stage CF that would require a multiparameter approach, such as potentially CCP, to resolve.NEW & NOTEWORTHY Computed cardiopulmonography is a new technique comprising a highly accurate sensor for measuring respiratory gas exchange coupled with a cardiopulmonary model that is used to identify a set of patient-specific characteristics of the lung. Here, we show that this technique can improve on a standard clinical approach for lung function testing in cystic fibrosis. Most particularly, an approach incorporating multiple model parameters can potentially separate different aspects of pathological change in this disease.

Model-based Bayesian inference of the ventilation distribution in patients with cystic fibrosis from multiple breath washout, with comparison to ventilation MRI

BACKGROUND

Indices of ventilation heterogeneity (VH) from multiple breath washout (MBW) have been shown to correlate well with VH indices derived from hyperpolarised gas ventilation MRI. Here we report the prediction of ventilation distributions from MBW data using a mathematical model, and the comparison of these predictions with imaging data.

METHODS

We developed computer simulations of the ventilation distribution in the lungs to model MBW measurement with 3 parameters: σ_V, determining the extent of VH; V₀, the lung volume; and V_D, the dead-space volume. These were inferred for each individual from supine MBW data recorded from 25 patients with cystic fibrosis (CF) using approximate Bayesian computation. The fitted models were used to predict the distribution of gas imaged by ³He ventilation MRI measurements collected from the same visit.

RESULTS

The MRI indices measured (I_1/3, the fraction of pixels below one-third of the mean intensity and I_CV, the coefficient of variation of pixel intensity) correlated strongly with those predicted by the MBW model fits (r=0.93,0.88 respectively). There was also good agreement between predicted and measured MRI indices (mean bias ± limits of agreement: I_1/3:-0.003±0.118 and I_CV:-0.004±0.298). Fitted model parameters were robust to truncation of MBW data.

CONCLUSION

We have shown that the ventilation distribution in the lung can be inferred from an MBW signal, and verified this using ventilation MRI. The Bayesian method employed extracts this information with fewer breath cycles than required for LCI, reducing acquisition time required, and gives uncertainty bounds, which are important for clinical decision making.

Smoking is associated with increased risk of cardiovascular events, disease severity, and mortality among patients hospitalized for SARS-CoV-2 infections

The clinical sequalae of SARS-CoV-2 infection are in part dependent upon age and pre-existing health conditions. Although the use of tobacco products decreases cardiorespiratory fitness while increasing susceptibility to microbial infections, limited information is available on how smoking affects COVID-19 severity. Therefore, we examined whether smokers hospitalized for COVID-19 are at a greater risk for developing severe complications than non-smokers. Data were from all hospitalized adults with SARS-CoV-2 infection from the American Heart Association's Get-With-The-Guidelines COVID-19 Registry, from January 2020 to March 2021, which is a hospital-based voluntary national registry initiated in 2019 with 122 participating hospitals across the United States. Patients who reported smoking at the time of admission were classified as smokers. Severe outcome was defined as either death or the use of mechanical ventilation. Of the 31,545 patients in the cohort, 6,717 patients were 1:2 propensity matched (for age, sex, race, medical history, medications, and time-frame of hospital admission) and classified as current smokers or non-smokers according to admission data. In multivariable analyses, after adjusting for sociodemographic characteristics, medical history, medication use, and the time of hospital admission, patients self-identified as current smokers had higher adjusted odds of death (adjusted odds ratio [aOR], 1.41; 95% CI, 1.21-1.64), the use of mechanical ventilation (aOR 1.15; 95% CI 1.01-1.32), and increased risk of major adverse cardiovascular events (aOR, 1.27; 95% CI 1.05-1.52). Independent of sociodemographic characteristics and medical history, smoking was associated with a higher risk of severe COVID-19, including death.

In asthma positive phase III slopes result from structural heterogeneity of the bronchial tree

We have previously identified bronchial generations 5-7 as the locus of maximum contribution to the convective portion of the phase III slope in CT-based lung models of asthma patients. In the present study, we examined how exactly phase III slope is generated locally, by specifically interrogating at individual branch points, the necessary condition for a phase III slope to occur : some degree of convective flow sequencing between any two daughter branches that have a heterogeneity in gas washout concentration between them. Flow sequencing at individual branch points showed a wide range of values, including branch points where flow sequencing was such that phase III slopes were negative locally. Yet, the net effect in the 24 bronchial trees that we studied was that flow sequencing between least and best ventilated units was of the correct sign to generate a positive phase III slope in generations 5-7. By investigating the link of local flow sequencing between any two daughter branches to the corresponding heterogeneity of mechanical lung properties, heterogeneity of compliance was seen to be a major determinant of flow sequencing. In these structures bronchial structures, compliance heterogeneity was essentially brought about by volume asymmetry resulting from terminating pathways within the 3D confines of the lung contours. We conclude that the serial and parallel combination of lung mechanical properties at individual branch points in an asymmetrical branching network generate flow sequencing in mid-range conductive airways, so as to obtain positive at-mouth phase III slopes.

Simulation-based optimisation to quantify heterogeneity of specific ventilation and perfusion in the lung by the Inspired Sinewave Test

The degree of specific ventilatory heterogeneity (spatial unevenness of ventilation) of the lung is a useful marker of early structural lung changes which has the potential to detect early-onset disease. The Inspired Sinewave Test (IST) is an established noninvasive 'gas-distribution' type of respiratory test capable of measuring the cardiopulmonary parameters. We developed a simulation-based optimisation for the IST, with a simulation of a realistic heterogeneous lung, namely a lognormal distribution of spatial ventilation and perfusion. We tested this method in datasets from 13 anaesthetised pigs (pre and post-lung injury) and 104 human subjects (32 healthy and 72 COPD subjects). The 72 COPD subjects were classified into four COPD phenotypes based on 'GOLD' classification. This method allowed IST to identify and quantify heterogeneity of both ventilation and perfusion, permitting diagnostic distinction between health and disease states. In healthy volunteers, we show a linear relationship between the ventilatory heterogeneity versus age ([Formula: see text]). In a mechanically ventilated pig, IST ventilatory heterogeneity in noninjured and injured lungs was significantly different (p < 0.0001). Additionally, measured indices could accurately identify patients with COPD (area under the receiver operating characteristic curve is 0.76, p < 0.0001). The IST also could distinguish different phenotypes of COPD with 73% agreement with spirometry.

Peripheral lung effect of elexacaftor/tezacaftor/ivacaftor in adult cystic fibrosis

Despite being an important patient group, adult cystic fibrosis patients with an FEV₁ below 40%predicted have been excluded from clinical trials with elexacaftor/tezacaftor/ivacaftor. We conducted a real-life 3 months follow-up study in 14 adult CF patients (median FEV₁ 34%predicted) demonstrating significant treatment effects in terms of FEV₁ (an increase of 12%predicted at 4 weeks, remaining stable thereafter). Corresponding decreases in lung clearance index LCI (by 31%predicted, down from baseline 247%predicted) and ventilation heterogeneity in the acinar compartment (Sacin) (by 411%predicted, down from baseline 798%predicted) suggest a distinct peripheral lung effect. One patient had intermittent treatment interruptions because of drug-induced liver injury. Our real-life data confirm that treatment with elexacaftor/tezacaftor/ivacaftor is effective in severely obstructive patients, and this is the first study to show time evolution of ventilation distribution improvement, pointing to the peripheral lung as the main site of treatment effect.