Human pulmonary resistance: effect of frequency and gas physical properties

Oscillometry and pulmonary magnetic resonance imaging in asthma and COPD

Developed over six decades ago, pulmonary oscillometry has re-emerged as a noninvasive and effort-independent method for evaluating respiratory-system impedance in patients with obstructive lung disease. Here, we evaluated the relationships between hyperpolarized 3 He ventilation-defect-percent (VDP) and respiratory-system resistance, reactance and reactance area (AX ) measurements in 175 participants including 42 never-smokers without respiratory disease, 56 ex-smokers with chronic-obstructive-pulmonary-disease (COPD), 28 ex-smokers without COPD and 49 asthmatic never-smokers. COPD participants were dichotomized based on x-ray computed-tomography (CT) evidence of emphysema (relative-area CT-density-histogram ≤ 950HU (RA950 ) ≥ 6.8%). In asthma and COPD subgroups, MRI VDP was significantly related to the frequency-dependence of resistance (R5-19 ; asthma: ρ = 0.48, P = 0.0005; COPD: ρ = 0.45, P = 0.0004), reactance at 5 Hz (X5 : asthma, ρ = -0.41, P = 0.004; COPD: ρ = -0.38, P = 0.004) and AX (asthma: ρ = 0.47, P = 0.0007; COPD: ρ = 0.43, P = 0.0009). MRI VDP was also significantly related to R5-19 in COPD participants without emphysema (ρ = 0.54, P = 0.008), and to X5 in COPD participants with emphysema (ρ = -0.36, P = 0.04). AX was weakly related to VDP in asthma (ρ = 0.47, P = 0.0007) and COPD participants with (ρ = 0.39, P = 0.02) and without (ρ = 0.43, P = 0.04) emphysema. AX is sensitive to obstruction but not specific to the type of obstruction, whereas the different relationships for MRI VDP with R5-19 and X5 may reflect the different airway and parenchymal disease-specific biomechanical abnormalities that lead to ventilation defects.

Oscillation mechanics of the respiratory system

The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Electrical circuit models of the human respiratory system reflect small airway impairment measured by impulse oscillation (IOS)

The use of the forced oscillatory input impedance parameter, frequency-dependence of Resistance (fdR), to assess small airway impairment (SAI) has not been widely accepted due to concern about the effects of "upper airway shunt" on oscillometric resistance and low frequency reactance. On the other hand, recent medical studies suggest that low frequency reactance is a very sensitive index of treatment intervention directed at small airways. The present study was undertaken to analyze and compare Impulse Oscillometry (IOS) resistance and reactance data with model-derived indices of small airway function from two models of the respiratory impedance, one with, and the other without an element for upper airway shunt capacitance. Fifty six patients with stable chronic obstructive lung disease of varying severity due to Cystic Fibrosis (CF) and 21 patients with asthma were evaluated by IOS testing. IOS data were input into the augmented RIC (aRIC) model with an upper airway shunt capacitance, and the extended RIC (eRIC) model, without a shunt capacitance element. Model-derived indices were compared between the two models for CF patients separately from asthma patients. We conclude that IOS indices of SAI are modeled equally well with or without upper airway shunt capacitance, and do not seem to be dependent on upper airway shunt capacitance.

Emotional reactivity of the airways in asthma: consistency across emotion-induction techniques and emotional qualities

Considerable individual differences exist in asthma patients' airway responses to emotional stimuli, but little is known about the generalization of such responses across situations or states of airways constriction. Fifty-four asthma patients and 25 healthy controls viewed in two separate sessions, films and blocks of pictures from each of three emotional qualities, pleasant, unpleasant, and neutral. At the beginning of each session, patients received a placebo or anti-cholinergic bronchodilator (ipratropium bromide), respectively, in a randomized double-blind design. Respiratory resistance, reactance and impedance were recorded throughout stimulus presentations with impulse oscillometry. Resistance increases showed a moderate degree of generalization across unpleasant films and pictures, unpleasant and pleasant pictures, as well as cholinergic blockade and placebo. Thus, the intensity of airway responses to unpleasant emotional stimuli is a moderately stable characteristic of asthma patients. In addition to the central airway, peripheral and extrathoracic airways may also contribute the consistency of such responses.

The synergistic interactions of allergic lung inflammation and intratracheal cationic protein

RATIONALE

Airways hyperresponsiveness (AHR) is a hallmark feature of asthma, and can be caused by various disparate mechanisms. Mouse models of AHR have been useful for studying these mechanisms in isolation, but such models still typically do not exhibit the same degree of AHR as seen in severe human asthma. We hypothesized that more severe AHR in mice could be achieved by imbuing them with more than one mechanism of AHR.

OBJECTIVES

We sought to determine if the airway wall thickening accompanying allergic inflammation and the exaggerated smooth muscle shortening induced by intratracheal cationic protein could act together to produce a severe form of AHR.

METHODS

We used the forced oscillation technique to measure methacholine responsiveness in BALB/c mice that had been sensitized and challenged with ovalbumin followed by an intratracheal instillation of poly-l-lysine.

MEASUREMENTS AND MAIN RESULTS

We found that both ovalbumin and poly-l-lysine treatment alone caused moderate levels of AHR. When the two treatments were combined, however, they synergized in terms of their effect on lung stiffness to an extent that could even be fatal, reflecting a significantly enhanced level of airway closure.

CONCLUSIONS

Our results suggest that mechanistic synergy between airway wall thickening and exaggerated smooth muscle shortening produces a more germane mouse model of asthma that may have particular relevance to the pathophysiology of the acute severe asthma exacerbation.

Clinical applications of forced oscillation to assess peripheral airway function

Forced oscillation applies external pressures to the respiratory system to measure respiratory impedance. Impedance of larger central airways may be dissected from that of peripheral airways using multiple oscillation frequencies. Respiratory impedance is calculated by computer-assisted methods that yield separate resistive and reactive components. The reactive component includes respiratory system capacitative and inertive properties, which may be separately visualized for clinical purposes using resonance as a rough dividing line. Low oscillation frequencies comprise those below resonance, and relate most prominently to capacitative properties of peripheral airways. High oscillation frequencies comprise those greater than resonance, which relate most prominently to inertial properties of larger central airways. Measurements of resistance and reactance in patients with peripheral airway disease, before and after therapeutic intervention, manifest characteristic patterns of response in low frequency resistance and reactance measures that appear to be closely correlated with each other. In contrast, changes in large central airways manifest resistance change uniformly over low and high frequencies.

Complexity of factors modulating airway narrowing in vivo: relevance to assessment of airway hyperresponsiveness

In vivo, the airway response to constrictor stimuli is the net result of a complex array of factors, some facilitating and some opposing airway narrowing, which makes the interpretation of bronchial challenges far from being straightforward. This review begins with a short description of the complex mechanisms of airway smooth muscle activation and force generation as the starting events for airway narrowing. It then focuses on gain factors modulating airway smooth muscle shortening and on the geometric factors determining the magnitude of reduction in airway caliber in vivo. Finally, in light of the evidence that mechanical modulation of airway smooth muscle tone and airway narrowing is at least as important as the inflammatory contractile mediators in the pathogenesis of airway hyper-responsiveness, the implications for the interpretation of bronchial challenges in clinical settings are discussed.

Within- and between-day variability of respiratory impedance, using impulse oscillometry in adolescent asthmatics

The objectives of the present study were to: 1) assess spirometric indices and respiratory impedance with forced oscillation (FO), using impulse oscillometry (IOS) in clinically stable asthmatic children over 3 consecutive days; 2) assess FO reactance (X), using an integrated index and resistance (R) separately during inspiration and expiration; and 3) assess effects on FO of hand support of cheeks vs. no hand support. Our hypotheses were: 1) because of increased sensitivity, IOS manifests day-to-day variability not demonstrable by spirometry; 2) IOS R during expiration exceeds that during inspiration; and 3) hand support of cheeks affects IOS R and X only minimally. We obtained triplicate twice-daily measures of IOS R and X in asthmatic adolescents at summer camp, in a convenience sample of children willing, with parental permission, to undergo repeated testing on consecutive days. Subjects received all medications between 6:30-7:30 AM, and were bronchodilated at time of testing. Subjects underwent IOS tests without hand support of cheeks, followed by tests with both hands supporting cheeks. ANOVA and regression analyses were used to discern technique differences.Significant differences in IOS inspiratory R5, R5 - R15 (frequency dependence of R), and low frequency reactance area (AX) occurred across 3 days, but spirometric indices were unchanged. Inspiratory R at 5 Hz (R5) was significantly smaller than expiratory R5 (P < 0.0001). ANOVA revealed no significant differences between hand and facial muscle cheek support for IOS R and X below 15 Hz, but significant differences occurred above 15 Hz. In conclusion, inspiratory R5, R5 - R15, and AX are sensitive measures for detecting changes in bronchomotor tone in adolescent asthmatic subjects, while expiratory R5 may be influenced by additional factors. Manual support of cheeks does not appear to affect IOS indices of peripheral airway obstruction in adolescent asthmatics. IOS is a practical method for quantifying respiratory mechanics, and its potential role in disease management warrants further study.

Role of lung inflammatory mediators as a cause of exercise-induced arterial hypoxemia in young athletes

We examined whether lung inflammatory mediators are increased during exercise and whether pharmacological blockade can prevent exercise-induced arterial hypoxemia (EIAH) in young athletes. Seventeen healthy athletes (9 men, 8 women; age 23 +/- 3 yr) with varying degrees of EIAH completed maximal incremental treadmill exercise tests after administration of fexofenadine, zileuton, and nedocromil sodium or placebo in a randomized double-blind crossover study. Lung function, arterial blood gases, and inflammatory metabolites in plasma, urine, and induced sputum were assessed. Drug administration did not improve EIAH or gas exchange during exercise. At maximal exercise, oxygen saturation fell to 91.4 +/- 2.6% (drug trial) and 91.9 +/- 2.1% (placebo trial) and alveolar-arterial oxygen difference widened to 28.1 +/- 6.3 Torr (drug trial) and 29.3 +/- 5.7 Torr (placebo trial). Oxygen consumption, ventilation, and other exercise variables were similarly unaffected by drug treatment. Although plasma histamine increased with exercise, values did not differ between trials, and urinary leukotriene E(4) and 11beta-prostaglandin F(2alpha) levels were unchanged after exercise. Postexercise sputum revealed no significant changes in markers of inflammation. These results demonstrate that EIAH in young athletes is not attenuated with acute administration of drugs targeting histamine and bioactive lipids. We conclude that airway inflammation is of insufficient magnitude to cause impairments in gas exchange and does not appear to be linked to EIAH in healthy young athletes.

Clinical application of forced oscillation

This review summarizes current clinical use of the forced oscillation technique (FOT) for analysis of lung function. It presents an intuitive approach to FOT pattern recognition for interpretation of results in human subjects, and the view that FOT is now well established and, clinically, eminently useful in patients with airflow obstruction. The focus of this review is on findings that relate directly to clinical utility, with less emphasis on theoretical mechanisms. The major thrust for clinical application of FOT derives from a number of European clinical research centers. Farre and Navajas and their colleagues in Barcelona, Harf and the Lorinos and their coworkers in Paris, Peslin and Duvivier and their coworkers in Vandoeuvre-les-Nancy, Pride and coworkers in London, and Van de Woestijne, Clement, Demedts, Landser, Van Noord, and their colleagues in Leuven have essentially been responsible for clinical development of FOT over the past 25 years. Publishing space does not permit an exhaustive listing of the many contributions of these investigators, but it is intended that the present review will provide a useful infrastructure from which the reader may progress to other research citations as desired.

Effects of exhaustive endurance exercise on pulmonary gas exchange and airway function in women

Seventeen fit women ran to exhaustion (14 +/- 4 min) at a constant speed and grade, reaching 95 +/- 3% of maximal O(2) consumption. Pre- and postexercise lung function, including airway resistance [total respiratory resistance (Rrs)] across a range of oscillation frequencies, was measured, and, on a separate day, airway reactivity was assessed via methacholine challenge. Arterial O(2) saturation decreased from 97.6 +/- 0.5% at rest to 95.1 +/- 1.9% at 1 min and to 92.5 +/- 2.6% at exhaustion. Alveolar-arterial O(2) difference (A-aDO(2)) widened to 27 +/- 7 Torr after 1 min and was maintained at this level until exhaustion. Arterial PO(2) (Pa(O(2))) fell to 80 +/- 8 Torr at 1 min and then increased to 86 +/- 9 Torr at exhaustion. This increase in Pa(O(2)) over the exercise duration occurred due to a hyperventilation-induced increase in alveolar PO(2) in the presence of a constant A-aDO(2). Arterial O(2) saturation fell with time because of increasing temperature (+2.6 +/- 0.5 degrees C) and progressive metabolic acidosis (arterial pH: 7.39 +/- 0.04 at 1 min to 7.26 +/- 0.07 at exhaustion). Plasma histamine increased throughout exercise but was inversely correlated with the fall in Pa(O(2)) at end exercise. Neither pre- nor postexercise Rrs, frequency dependence of Rrs, nor diffusing capacity for CO correlated with the exercise A-aDO(2) or Pa(O(2)). Although several subjects had a positive or borderline hyperresponsiveness to methacholine, this reactivity did not correlate with exercise-induced changes in Rrs or exercise-induced arterial hypoxemia. In conclusion, regardless of the degree of exercise-induced arterial hypoxemia at the onset of high-intensity exercise, prolonging exercise to exhaustion had no further deleterious effects on A-aDO(2), and the degree of gas exchange impairment was not related to individual differences in small or large airway function or reactivity.

Frequency dependence of compliance in the evaluation of patients with unexplained respiratory symptoms

Frequency dependence of compliance (FDC) reflects non-homogeneous ventilatory distribution and, in the presence of a normal measured airway resistance, suggests peripheral airways dysfunction. This study evaluated peripheral airway function and bronchial reactivity in irritant exposed or non-exposed individuals with normal routine pulmonary function tests (PFTs) who had persistent unexplained lower respiratory symptoms. Twenty-two patients were identified with persistent respiratory symptoms and with normal chest X-ray and PFTs. Twenty were non-smokers; two had stopped smoking more than 10 years before evaluation. Twelve patients had been exposed to irritants in their workplaces or at home. Non-specific bronchial hyper-reactivity (nsBHR) and FDC, pre- and post-bronchodilator, were measured in all patients. Studies were repeated in 6/12 irritant-exposed subjects after exposure removal and inhaled corticosteroid treatment. Whereas 12/22 patients had nsBHR, all 22 subjects demonstrated FDC [dynamic lung compliance/static lung compliance Cdyn,1 / Cst,1 at respiratory frequency 60 min(-1) (f60), mean 46%, range 27-67%]. After bronchodilator administration, a 15% improvement Cdyn,1 was observed most consistently at f60 (mean% improvement 26%, 95% CI 14-38%) and in subjects without nsBHR. However, Cdyn,1 at f60 did not return to normal after inhaled bronchodilator. Irritant-exposed and unexposed individuals appeared similar in results of testing for FDC and nsBHR. FDC and its response to bronchodilators provide objective physiological measures of an airway abnormality which may provide a basis for clinical symptoms in patients with normal routine pulmonary function studies. The presence of persistently abnormal FDC after bronchodilator (BD) and on follow up studies may reflect chronic inflammatory and/or structural changes in the airways in addition to bronchoconstriction.

Effect of gas density variations on respiratory input impedance in humans

The forced oscillation technique is a widely-used non-invasive method of characterizing the dynamic behaviour of the respiratory system. We used the forced oscillation technique to investigate respiratory mechanics in healthy subjects during simulated dives in dry hyperbaric chambers. We observed frequency dependence of input impedance, which was mainly density-dependent. To explain this result, we propose a model of the respiratory system, based on flow redistribution in a two-pathway circuit. This model, using the electrical analogue, is composed of two Resistance-Self Inductance-Capacitance (R-I-C) pathways set up in parallel. It allowed us to explain the dynamic behaviour of respiratory impedance under hyperbaric conditions in healthy subjects. Changes in respiratory impedance according to frequency vary with the relative importance of the inequalities of the two time constants RC and I/R between the two pathways. With low values of density, RC inequality predominates, whereas I/R inequality tends to predominate with high values of density.

Impedance measurement during air and helium-oxygen breathing before and after salbutamol in COPD patients

1. The forced oscillation technique is an effort-independent method used to characterize the mechanical impedance of the respiratory system. To support the hypothesis that non-invasive partitioning of total pulmonary resistance is possible by this technique, impedance was measured during air breathing and after equilibration with a mixture of 80% helium (He) and 20% oxygen (O2) in 21 chronic obstructive pulmonary disease (COPD) patients by means of a forced pseudo-random noise pressure signal over a frequency spectrum from 4 to 52 Hz. Furthermore, during inhalation of both gas mixtures impedance was determined before and after inhalation of 0.400 mg Salbutamol. 2. He + O2 breathing caused less negative frequency dependence of resistance and a significant decrease in reactance over the range 16-52 Hz. Inhalation of Salbutamol caused a marked increase in reactance values over the range 8-40 Hz. However after equilibration with the He + O2 mixture, Salbutamol caused a significant decrease in resistance and a significant increase in reactance at all frequencies. 3. The results during He + O2 breathing are in accordance with a partitioning of airways resistance into central and peripheral components. The decrease in reactance during He + O2 can be explained by a density dependent decrease in inductive reactance. By comparing the impedance data during air and He + O2 breathing, it can be concluded that a distribution of pulmonary resistance with minimal losses in the larger airways is more sensitive for detecting changes in the peripheral airways in COPD patients.

Density dependence of respiratory input impedance in normal subjects

1. The forced oscillation technique is a non-invasive and effort-independent test used to characterize the mechanical impedance of the respiratory system. Total respiratory impedance was assessed from 4 to 52 Hz in 15 normal subjects breathing air and a helium-oxygen mixture. 2. Breathing helium-oxygen reduced respiratory resistance and its frequency dependence as well as respiratory reactance very significantly. Resonant frequency during He-O2 breathing was 1.88 times higher than during air breathing. 3. It is concluded that by impedance measurement of the respiratory system in normal subjects a density-dependent decrease of respiratory resistance due to decreased turbulence in the larger airways and a density-dependent decrease in the inductive reactance can be found during breathing of low density gas mixtures.

Response localization of the pharmacological agents histamine and salbutamol along the respiratory system by forced oscillations in asthmatic subjects

The bronchodilating effect of 1 mg and 0.4 mg salbutamol on the impedance of the respiratory system was studied in 25 asthmatic subjects after histamine-induced bronchoconstriction. Histamine caused an increase of respiratory resistance (Rrs) at lower frequencies and a frequency dependence of Rrs. Respiratory reactance (Xrs) decreased at all frequencies after histamine challenge. These changes can be explained by peripheral airway obstruction. Impedance measurements performed 5 min after inhalation of 1 mg and 0.4 mg salbutamol showed a decrease of Rrs values at lower frequencies, a disappearance of the frequency dependence of Rrs, and a significant increase of Xrs values. No significant differences in absolute changes of Rrs and Xrs are observed between the salbutamol regimens. These changes after inhalation of salbutamol can be explained by supposing a predominant action on the peripheral airways.

Respiratory system impedance in patients with acute left ventricular failure: pathophysiology and clinical interest

To investigate the relationship between alterations in lung mechanics and acute pulmonary vascular congestion, repeated measurements of the respiratory system impedance (Zrs) were performed in 11 patients with and in seven without acute left ventricular failure. Indexes of Zrs were obtained by calculating the average and slope of the resistance and reactance in low (10 to 20 Hz) and high (20 to 50 Hz) frequency intervals. Zrs indexes in patients with ventricular failure differ significantly from those in patients without failure. Pulmonary vascular congestion is regularly associated with an abnormal frequency dependence of resistance at low frequencies and with an increased resonant frequency. Discriminant analysis of Zrs indexes allows 92% correct classification of pulmonary capillary wedge pressures lower than and those equal to or higher than 18 mm Hg. Zrs differences between patients with and without left ventricular failure are consistent with the presence of a small airways obstruction even in patients with mild left ventricular failure. Furthermore, use of Zrs indexes permits moderate and severe pulmonary vascular congestion to be distinguished from one another and this is probably due to a significant narrowing of the large airways during severe left ventricular failure.