Effect of expiratory flow limitation on respiratory mechanical impedance: a model study

Non-interventional monitoring of expiratory flow limitation during experimental mechanical ventilation

Background

Expiratory flow limitation (EFL) is common among patients in the intensive care unit under mechanical ventilation (MV) and may have significant clinical consequences. In the present study, we examine the possibility of non-interventional detection of EFL during experimental MV.

Methods

Eight artificially ventilated New Zealand rabbits were included in the experiments. EFL was induced during MV by application of negative expiratory pressure (−5, −8 and −10 hPa) and detected by the negative expiratory pressure technique. Airway pressure (P_aw) and gas flow (V′) were digitally recorded and processed off-line for the evaluation of respiratory mechanics. The method is based on the computation and monitoring of instantaneous respiratory resistance R_rs(t). The resistive pressure (P_aw,res(t)) is calculated by subtracting from P_aw its elastic component and the end-expiratory pressure, as assessed by linear regression. Then, R_rs(t) is computed as the instant ratio P_aw,res(t)/V′(t).

Results

Two completely different patterns of expiratory R_rs(t) separate the cases with EFL from those without EFL. Small and random fluctuations are noticed when EFL is absent, whereas the onset of EFL is accompanied by an abrupt and continuous rise in R_rs(t), towards the end of expiration. Thus, EFL is not only detected but may also be quantified from the volume still to be expired at the time EFL occurs.

Conclusion

The proposed technique is a simple, accurate and non-interventional tool for EFL monitoring during MV.

Respiratory system resistance in expiratory flow limitation during mechanical ventilationhttps://bit.ly/34hU6Bv

Airway closure is the predominant physiological mechanism of low ventilation seen on hyperpolarized helium-3 MRI lung scans

Hyperpolarized helium-3 MRI (³He MRI) provides detailed visualization of low- (hypo- and non-) ventilated lungs. Physiological measures of gas mixing may be assessed by multiple breath nitrogen washout (MBNW) and of airway closure by a forced oscillation technique (FOT). We hypothesize that in patients with asthma, areas of low-ventilated lung on ³He MRI are the result of airway closure. Ten control subjects, ten asthma subjects with normal spirometry (non-obstructed), and ten asthmatic subjects with reduced baseline lung function (obstructed) attended two testing sessions. On visit one, baseline plethysmography was performed followed by spirometry, MBNW, and FOT assessment pre and post methacholine challenge. On visit two, ³He MRI scans were conducted pre and post methacholine challenge. Post methacholine the volume of low-ventilated lung increased from 8.3% to 13.8% in the non-obstructed group (P = 0.012) and from 13.0% to 23.1% in the obstructed group (P = 0.001). For all subjects, the volume of low ventilation from ³He MRI correlated with a marker of airway closure in obstructive subjects, Xrs (6 Hz) and the marker of ventilation heterogeneity Scond with r² values of 0.61 (P < 0.001) and 0.56 (P < 0.001), respectively. The change in Xrs (6 Hz) correlated well (r² = 0.45, p < 0.001), whereas the change in Scond was largely independent of the change in low ventilation volume (r² = 0.13, P < 0.01). The only significant predictor of low ventilation volume from the multi-variate analysis was Xrs (6 Hz). This is consistent with the concept that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.NEW & NOTEWORTHY This study introduces a novel technique of generating high-resolution 3D ventilation maps from hyperpolarized helium-3 MRI. It is the first study to demonstrate that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.

Early onset of airway derecruitment assessed using the forced oscillation technique in subjects with asthma

Derecruitment of air spaces in the lung occurs when airways close during exhalation and is related to ventilation heterogeneity and symptoms in asthma. The forced oscillation technique has been used to identify surrogate measures of airway closure via the reactance (Xrs) versus lung volume relationship. This study used a new algorithm to identify derecruitment from the Xrs versus lung volume relationship from a slow vital capacity maneuver. We aimed to compare two derecruitment markers on the Xrs versus volume curve, the onset reduction of Xrs (DR1_vol) and the onset of more rapid reduction of Xrs (DR2_vol), between control and asthmatic subjects. We hypothesized that the onset of DR1_vol and DR2_vol occurred at higher lung volume in asthmatic subjects. DR1_vol and DR2_vol were measured in 18 subjects with asthma and 18 healthy controls, and their relationships with age and height were examined using linear regression. In the control group, DR1_vol and DR2_vol increased with age (r² = 0.68, P < 0.001 and r² = 0.71, P < 0.001, respectively). DR1_vol and DR2_vol in subjects with asthma [76.58% of total lung capacity (TLC) and 56.79%TLC, respectively] were at higher lung volume compared with control subjects (46.1 and 37.69%TLC, respectively) (P < 0.001). DR2_vol correlated with predicted values of closing capacity (r = 0.94, P < 0.001). This study demonstrates that derecruitment occurs at two points along the Xrs-volume relationship. Both derecruitment points occurred at significantly higher lung volumes in subjects with asthma compared with healthy control subjects. This technique offers a novel way to measure the effects of changes in airways/lung mechanics. NEW & NOTEWORTHY This study demonstrates that the forced oscillation technique can be used to identify two lung volume points where lung derecruitment occurs: 1) where derecruitment is initiated and 2) where onset of rapid derecruitment commences. Measurements of derecruitment increase with age. The onset of rapid derecruitment was highly correlated with predicted closing capacity. Also, the initiation and rate of derecruitment are significantly altered in subjects with asthma.

Comparison of two methods of determining lung de-recruitment, using the forced oscillation technique

Airway closure has proved to be important in a number of respiratory diseases and may be the primary functional defect in asthma. A surrogate measure of closing volume can be identified using the forced oscillation technique (FOT), by performing a deflation maneuver and examining the resultant reactance (Xrs) lung volume relationship. This study aims to determine if a slow vital capacity maneuver can be used instead of this deflation maneuver and compare it to existing more complex techniques. Three subject groups were included in the study; healthy (n = 29), asthmatic (n = 18), and COPD (n = 10) for a total of 57 subjects. Reactance lung volume curves were generated via FOT recordings during two different breathing manoeuvres (both pre and post bronchodilator). The correlation and agreement between surrogate closing volume (Vol_crit) and reactance (Xrs_crit) at this volume was analysed. The changes in Vol_crit and Xrs_crit pre and post bronchodilator were also analysed. Across all three subject groups, the two different measures of Vol_crit were shown to be statistically equivalent (p > 0.05) and demonstrated a strong fit to the data (R² = 0.49, 0.78, 0.59, for asthmatic, COPD and healthy subject groups, respectively). A bias was evident between the two measurements of Xrs_crit with statistically different means (p < 0.05). However, the two measurements of Xrs_crit displayed the same trends. In conclusion, we have developed an alternative technique for measuring airway closure from FOT recordings. The technique delivers equivalent and possibly more sensitive results to previous methods while being simple and easily performed by the patient.

Chronic obstructive pulmonary disease

COPD is characterized by airflow limitation that is not fully reversible. The morphological basis for airflow obstruction results from a varying combination of obstructive changes in peripheral conducting airways and destructive changes in respiratory bronchioles, alveolar ducts, and alveoli. A reduction of vascularity within the alveolar septa has been reported in emphysema. Typical physiological changes reflect these structural abnormalities. Spirometry documents airflow obstruction when the FEV1/FVC ratio is reduced below the lower limit of normality, although in early disease stages FEV1 and airway conductance are not affected. Current guidelines recommend testing for bronchoreversibility at least once and the postbronchodilator FEV1/FVC be used for COPD diagnosis; the nature of bronchodilator response remains controversial, however. One major functional consequence of altered lung mechanics is lung hyperinflation. FRC may increase as a result of static or dynamic mechanisms, or both. The link between dynamic lung hyperinflation and expiratory flow limitation during tidal breathing has been demonstrated. Hyperinflation may increase the load on inspiratory muscles, with resulting length adaptation of diaphragm. Reduction of exercise tolerance is frequently noted, with compelling evidence that breathlessness and altered lung mechanics play a major role. Lung function measurements have been traditionally used as prognostic indices and to monitor disease progression; FEV1 has been most widely used. An increase in FVC is also considered as proof of bronchodilatation. Decades of work has provided insight into the histological, functional, and biological features of COPD. This has provided a clearer understanding of important pathobiological processes and has provided additional therapeutic options.

Expiratory flow limitation

Expiratory flow limitation occurs when flow ceases to increase with increasing expiratory effort. The equal pressure point concept has been largely successful in providing intuitive understanding of the phenomenon, wherein maximal flows are determined by lung recoil and resistance upstream of the site where bronchial transmural pressure is zero (the EPP). Subsequent work on the fluid dynamical foundations led to the wave-speed theory of flow limitation, where flow is limited at a site when the local gas velocity is equal to speed of propagation of pressure waves. Each is a local theory; full predictions require knowledge of both density-dependent Bernoulli pressure drops and viscosity-dependent pressure losses due to dissipation. The former is dominant at mid to high lung volumes, whereas the latter is more important at low lung volumes as the flow-limiting site moves peripherally. The observation of relative effort independence of the maximal flow versus volume curves is important clinically insofar as such maneuvers, when carefully performed, offer a unique window into the mechanics of the lung itself, with little confounding effects. In particular, the important contributions of lung recoil and airways resistance can often be assessed, with implications and applications to diagnosis and management of pulmonary disease.

Oscillation mechanics of the respiratory system: applications to lung disease

Since its introduction in the 1950s, the forced oscillation technique (FOT) and the measurement of respiratory impedance have evolved into powerful tools for the assessment of various mechanical phenomena in the mammalian lung during health and disease. In this review, we highlight the most recent developments in instrumentation, signal processing, and modeling relevant to FOT measurements. We demonstrate how FOT provides unparalleled information on the mechanical status of the respiratory system compared to more widely used pulmonary function tests. The concept of mechanical impedance is reviewed, as well as the various measurement techniques used to acquire such data. Emphasis is placed on the analysis of lower, physiologic frequency ranges (typically less than 10 Hz) that are most sensitive to normal physical processes as well as pathologic structural alterations. Various inverse modeling approaches used to interpret alterations in impedance are also discussed, specifically in the context of three common respiratory diseases: asthma, chronic obstructive pulmonary disease, and acute lung injury. Finally, we speculate on the potential role for FOT in the clinical arena.

Respiratory impedance and response to salbutamol in asthmatic Vietnamese children

There is a high incidence of pediatric asthma in Vietnam, but little lung function data are available. The aim of the study was to compare respiratory resistance (Rrs), reactance (Xrs), and responses to salbutamol between asthmatic and healthy primary school children in Hanoi. Because respiratory mechanics vary along the breathing cycle, measurements were assessed separately in inspiration (Rrsi, Xrsi) and expiration (Rrse, Xrse).Inpatients with doctor-diagnosed asthma were measured 2-3 days following admission using the forced oscillation technique (FOT) at a single frequency (8 Hz). Z-scores and responses to salbutamol were compared between 102 asthmatics and 98 controls, and accuracy of group classification by FOT parameters was assessed by Youden index, an indicator to the proportion of subjects correctly classified in each group.In asthmatics versus controls, Rrsi-but not Rrse-was significantly larger and both Xrsi and Xrse were significantly more negative (P < 0.01). Both Rrs and Xrs responses to salbutamol were significantly larger in asthmatics than controls (P < 0.001). Youden indexes indicated response to salbutamol generally had better diagnostic values than Z-scores and was best discriminative first with Rrsi, then with Xrse.It is concluded that different FOT characteristics may be described in asthmatic and healthy Vietnamese children. The diagnostic value of each parameter depends upon the breathing cycle. Most useful in practice probably is the response to salbutamol measured by Rrsi.

Evaluation of respiratory impedance in asthma and COPD by an impulse oscillation system

OBJECTIVE

The purpose of this study was to clarify the differences in physiological properties of the airways between asthma and COPD using an impulse oscillation system (IOS).

PATIENTS AND METHODS

Subjects comprised 95 stable COPD patients, 52 never-smoker asthma patients and 29 healthy never-smokers >60 years old, all matched for age, in whom respiratory impedance was examined by IOS.

RESULTS

In both asthma and COPD patients, a significant increase in respiratory resistance (Rrs5) and more negative value of respiratory reactance (Xrs5) at 5 Hz of oscillatory frequency with an increase in resonant frequency (fres) were observed when compared with healthy never-smokers. In asthma, a significant increase in respiratory resistance at 20 Hz (Rrs20) was also observed when compared with healthy never-smokers and COPD. The increases in Rrs5 and relative changes of Xrs5 to more negative were remarkable with increasing severity of COPD. On the other hand, among patients with asthma, these changes in Rrs5 and Xrs5 were also observed in asthmatics with normal FEV(1)/FVC. Interestingly, Xrs5 showed further changes to more negative in expiration of tidal breath in severe COPD, whereas no significant changes in Xrs5 to more negative in expiration was observed in healthy never-smokers and asthmatics with and without normal FEV(1)/FVC.

CONCLUSION

IOS may be useful for detecting pathophysiological changes of respiratory system in accordance with severity of COPD and even in asthmatics with normal FEV(1)/FVC. The larger within-breath changes of Xrs5 to more negative in severe COPD may represent easy collapsibility of small airways in expiration of tidal breath. These properties may help to analyze airway mechanics and to identify abnormalities of the airways that cannot be found by spirometry alone.

Biofluid Mechanics of the Pulmonary System

Presents an overview of leading areas of discovery in bio-fluid mechanics related to the pulmonary system, with particular reference to the airways. Areas briefly reviewed include airway gas dynamics, impedance studies, collapsible-tube studies, and airway liquid studies. Emphasis is placed on promising further directions, such as analysis of interacting fluid-mechanical or fluid-structure phenomena, multi-scale modeling across widely varying length and time scales, and integration of advanced simulations into respiratory investigation and pulmonary medicine.

Tissue heterogeneity in the mouse lung: effects of elastase treatment

We developed a network model in an attempt to characterize heterogeneity of tissue elasticity of the lung. The model includes a parallel set of pathways, each consisting of an airway resistance, an airway inertance, and a tissue element connected in series. The airway resistance, airway inertance, and the hysteresivity of the tissue elements were the same in each pathway, whereas the tissue elastance (H) followed a hyperbolic distribution between a minimum and maximum. To test the model, we measured the input impedance of the respiratory system of ventilated normal and emphysematous C57BL/6 mice in closed chest condition at four levels of positive end-expiratory pressures. Mild emphysema was developed by nebulized porcine pancreatic elastase (PPE) (30 IU/day × 6 days). Respiratory mechanics were studied 3 wk following the initial treatment. The model significantly improved the fitting error compared with a single-compartment model. The PPE treatment was associated with an increase in mean alveolar diameter and a decrease in minimum, maximum, and mean H. The coefficient of variation of H was significantly larger in emphysema (40%) than that in control (32%). These results indicate that PPE treatment resulted in increased time-constant inequalities associated with a wider distribution of H. The heterogeneity of alveolar size (diameters and area) was also larger in emphysema, suggesting that the model-based tissue elastance heterogeneity may reflect the underlying heterogeneity of the alveolar structure.

Forced oscillation assessment of respiratory mechanics in ventilated patients

The forced oscillation technique (FOT) is a method for non-invasively assessing respiratory mechanics that is applicable both in paralysed and non-paralysed patients. As the FOT requires a minimal modification of the conventional ventilation setting and does not interfere with the ventilation protocol, the technique is potentially useful to monitor patient mechanics during invasive and noninvasive ventilation. FOT allows the assessment of the respiratory system linearity by measuring resistance and reactance at different lung volumes or end-expiratory pressures. Moreover, FOT allows the physician to track the changes in patient mechanics along the ventilation cycle. Applying FOT at different frequencies may allow the physician to interpret patient mechanics in terms of models with pathophysiological interest. The current methodological and technical experience make possible the implementation of portable and compact computerised FOT systems specifically addressed to its application in the mechanical ventilation setting.

Modulation of airway caliber by deep inhalation in children

To elucidate whether deep inhalation (DI) modulates changes in airway caliber in childhood, we measured the effect of DI on respiratory impedance before and after inhaled methacholine or salbutamol in 4- to 7-yr-old children (n = 15) suffering from recurrent wheezing. In all children, the real part of impedance between 12 and 16 Hz (Re[Z](12-16)) increased after methacholine from 5.6 +/- 1.2 to 8.2 +/- 1.6 cmH(2)O. l(-1). s (P < 0.001) and resonance frequency from 18 +/- 3 to 25 +/- 5 Hz (P < 0.001). These changes were partially reversed by DI: Re[Z](12-16) decreased to 7.2 +/- 1.2 cmH(2)O. l(-1). s (P < 0.01) and resonance frequency to 19 +/- 5 Hz (P < 0.001). In nine children, on a separate occasion, Re[Z](12-16) decreased after salbutamol from 8.3 +/- 1.9 to 5.1 +/- 0.9 cmH(2)O. l(-1). s (P < 0.001) and resonance frequency from 21 +/- 6 to 15 +/- 3 Hz (P < 0.05). The decrease of Re[Z](12-16) was partially reversed by DI (to 6.2 +/- 1.4 cmH(2)O. l(-1). s, P < 0. 01), but resonance frequency did not change significantly (P = 0.75). We conclude that in 4- to 7-yr-old children pharmacologically induced changes in airway caliber are modulated by DI. These findings suggest that airway-to-parenchyma interdependence is operative in this age range.

Respiratory resistive impedance as an index of airway obstruction during nasal continuous positive airway pressure titration

Esophageal pressure amplitude (DeltaPes), inspiratory pulmonary resistance (RLI) and inspiratory flow limitation score (FS) are used as indices of upper airway obstruction for the titration of nasal continuous positive airway pressure (nCPAP) in patients with obstructive sleep apnea syndrome (OSAS). This study was designed to determine whether oscillatory respiratory resistive impedance at 16 Hz (RFO) might be proposed as an alternative index. Eleven OSAS patients were studied during a night of polysomnography-controlled nCPAP titration. Nasal flow (V) and airway opening and esophageal pressures (Pao and Pes, respectively) were continuously measured during nasal breathing, and forced-flow oscillations (FO) were applied for 5 min at each nCPAP level. RLI was calculated by linear regression analysis of resistive pressure versus V over inspiration. R FO was obtained by linear regression analysis of respiratory resistive impedance versus frequency. Application of FO affected neither sleep nor pulmonary mechanics. RFO correlated with RLI in all patients. RFO did not correlate with DeltaPes in two patients, and was not significantly related to FS in five patients. This study demonstrates the applicability of the FO technique in sleeping patients receiving nCPAP, and the reliability of RFO for assessing pulmonary resistance. RFO might therefore be proposed as a quantitative index of airway obstruction for nCPAP titration.

Respiratory oscillation mechanics in infants with bronchiolitis during mechanical ventilation

The aim of the study was to describe the pattern of respiratory oscillation mechanics and responses to positive end-expiratory pressure (PEEP) in bronchiolitis. Six infants were studied during the course of mechanical ventilation. A 20 Hz sinusoidal pressure variation was applied at the endotracheal tube where flow was measured with a pneumotachograph. Resistance and reactance obtained from the complex pressure-flow ratio were separated during inspiration (R(rs,i); X(rs,i)) and expiration (R(rs,e); X(rs,e)), and the differences between R(rs,i) and R(rs,e) (deltaR(rs)) and X(rs,i) and X(rs,e) (deltaX(rs)) were calculated. The data were corrected for the mechanical characteristics of the endotracheal tube. The measurements were repeated while PEEP was varied between 0 and 8 hPa. Two infants were found to have normal R(rs) and near-zero X(rs) and both parameters exhibited little change within the respiratory cycle or with varying PEEP. Four infants had high R(rs) at zero PEEP. In two, R(rs,i) was markedly elevated (108.5 and 85.2 hPa.s/L, respectively), and X(rs,i) was markedly negative (-25.0 and -22.5 hPa.s/L, respectively) at zero PEEP, while deltaR(rs) and deltaX(rs) were small. R(rs,i) and the absolute value of X(rs,i) decreased with increasing PEEP. This pattern of oscillation mechanics was consistent with low lung volumes and atelectasis, being reversed by increasing PEEP. In the remaining two subjects, R(rs,i) was moderately elevated (57.8 and 53.6 hPa.s/L, respectively) and X(rs,i) moderately negative (-12.5 and -7.7 hPa.s/L, respectively) at zero PEEP. DeltaR(rs) (-59.8 and -56.5 hPa.s/L, respectively) and delta(rs) (28.1 and 48.7 hPa.s/L, respectively) were large, but were dramatically reduced by increasing PEEP. These patterns were consistent with expiratory airflow limitation. Measurements of respiratory impedance are, therefore, informative in regard to the pathophysiological mechanisms occurring in bronchiolitis during mechanical ventilation, and they may be helpful in setting the level and assessing the effect of PEEP.