Changes in airway dead space in response to methacholine provocation in normal subjects

Correspondence between Capnovolumetric and Conventional Lung Function Parameters in the Diagnosis of Obstructive Airway Diseases

BACKGROUND

Capnovolumetry is of interest as a method for the diagnosis of obstructive airway diseases, requiring little cooperation from the patient.

OBJECTIVE

To help in the interpretation of capnovolumetric parameters, we aimed to identify their correspondence to conventional lung function indices.

METHODS

We studied 978 patients from a diagnostic study with complete functional data and the clinical diagnosis of asthma, chronic obstructive pulmonary disease (COPD), or no respiratory disease. Using path analysis, four capnovolumetric parameters (slope of expiratory phase 3, ratio of slopes of phases 3 and 2, volume of phase 2, and the ratio area/volume of phase 3) previously identified as predictors of airway obstruction in terms of spirometry and body ple-thysmography, were analyzed regarding their relationship to each other and the diagnostic categories of asthma or COPD versus control, or obstruction versus no obstruction. We then identified four lung function parameters showing relationships as much as possible isomorphic to those between capnovolumetric parameters.

RESULTS

The four capnovolumetric parameters were related to COPD and obstruction via both direct and indirect influences, but only two of them to asthma. Regarding the correspondence to lung function parameters, the slope of expiratory phase 3 corresponded best to the ratio of residual volume to total lung capacity, the ratio of slopes of phases 3 and 2 to forced expiratory volume in 1 s, the volume of phase 2 to forced expired flow at 50% of vital capacity, and the ratio area/volume of phase 3 to forced vital capacity.

CONCLUSIONS

Our results indicated an intricate relationship of capnovolumetric parameters to each other and to airway obstruction, asthma, or COPD. The correspondence to conventional lung function measures seemed to reflect the entities lung hyperinflation, overall ventilatory impairment, bronchoconstriction, and ventilated lung volume, in that order. These findings might be helpful for clinicians in the interpretation of capnovolumetry.

Diagnostic accuracy of capnovolumetry for the identification of airway obstruction - results of a diagnostic study in ambulatory care

BACKGROUND

One of the known weaknesses of spirometry is its dependence on patients' cooperation, which can only partially be alleviated by educational efforts. Therefore, procedures less dependent on cooperation might be of value in clinical practice. We investigated the diagnostic accuracy of ultrasound-based capnovolumetry for the identification of airway obstruction.

METHODS

Consecutive patients from a pulmonary outpatient clinic were included in the diagnostic study. As reference standard, the presence of airway obstruction was evaluated via spirometry and bodyplethysmography. Capnovolumetry was performed as index test with an ultrasound spirometer providing a surrogate measure of exhaled carbon dioxide. Receiver operating characteristic (ROC) analysis was performed using the ratio of slopes of expiratory phases 3 and 2 (s3/s2) ≥ 0.10 as primary capnovolumetric parameter for the recognition of airway obstruction. Logistic regression was performed as secondary analysis to identify further useful capnovolumetric parameters. The diagnostic potential of capnovolumetry to identify more severe degrees of airway obstruction was evaluated additionally.

RESULTS

Of 1400 patients recruited, 1287 patients were included into the analysis. Airway obstruction was present in 29% of patients. The area under the ROC-curve (AUC) of s3/s2 was 0.678 (95% CI 0.645, 0.710); sensitivity of s3/s2 ≥ 0.10 was 47.7 (95% CI 42.7, 52.8)%, specificity 79.0 (95% CI 76.3, 81.6)%. When combining this parameter with three other parameters derived from regression analysis (ratio area/volume phase 3, slope phase 3, volume phase 2), an AUC of 0.772 (95% CI 0.743, 0.801) was obtained. For severe airway obstruction (FEV1 ≤ 50% predicted) sensitivity of s3/s2 ≥ 0.10 was 75.9 (95% CI 67.1, 83.0)%, specificity 75.8 (95% CI 73.3, 78.1)%; for very severe airway obstruction (FEV1 ≤ 30% predicted) sensitivity was 86.7 (95% CI 70.3, 94.7)%, specificity 72.8 (95% CI 70.3, 75.2)%. Sensitivities increased and specificities decreased considerably when the combined capnovolumetric score was used as index test.

CONCLUSIONS

Capnovolumetry by way of an ultrasound spirometer had a statistically significant albeit moderate potential for the recognition of airway obstruction in a heterogeneous population of patients typically found in clinical practice. Diagnostic accuracy of the capnovolumetric device increased with the severity of airway obstruction.

TRIAL REGISTRATION

The study is registered under DRKS00013935 at German Clinical Trials Register (DRKS).

Model-Based Estimation of Respiratory Parameters from Capnography, With Application to Diagnosing Obstructive Lung Disease

OBJECTIVE

We use a single-alveolar-compartment model to describe the partial pressure of carbon dioxide in exhaled breath, as recorded in time-based capnography. Respiratory parameters are estimated using this model, and then related to the clinical status of patients with obstructive lung disease.

METHODS

Given appropriate assumptions, we derive an analytical solution of the model, describing the exhalation segment of the capnogram. This solution is parametrized by alveolar CO₂ concentration, dead-space fraction, and the time constant associated with exhalation. These quantities are estimated from individual capnogram data on a breath-by-breath basis. The model is applied to analyzing datasets from normal (n = 24) and chronic obstructive pulmonary disease (COPD) (n = 22) subjects, as well as from patients undergoing methacholine challenge testing for asthma (n = 22).

RESULTS

A classifier based on linear discriminant analysis in logarithmic coordinates, using estimated dead-space fraction and exhalation time constant as features, and trained on data from five normal and five COPD subjects, yielded an area under the receiver operating characteristic curve (AUC) of 0.99 in classifying the remaining 36 subjects as normal or COPD. Bootstrapping with 50 replicas yielded a 95% confidence interval of AUCs from 0.96 to 1.00. For patients undergoing methacholine challenge testing, qualitatively meaningful trends were observed in the parameter variations over the course of the test.

SIGNIFICANCE

A simple mechanistic model allows estimation of underlying respiratory parameters from the capnogram, and may be applied to diagnosis and monitoring of chronic and reversible obstructive lung disease.

Deposition efficiency of inhaled particles (15-5000 nm) related to breathing pattern and lung function: an experimental study in healthy children and adults

BACKGROUND

Exposure to airborne particles has a major impact on global health. The probability of these particles to deposit in the respiratory tract during breathing is essential for their toxic effects. Observations have shown that there is a substantial variability in deposition between subjects, not only due to respiratory diseases, but also among individuals with healthy lungs. The factors determining this variability are, however, not fully understood.

METHOD

In this study we experimentally investigate factors that determine individual differences in the respiratory tract depositions of inhaled particles for healthy subjects at relaxed breathing. The study covers particles of diameters 15-5000 nm and includes 67 subjects aged 7-70 years. A comprehensive examination of lung function was performed for all subjects. Principal component analyses and multiple regression analyses were used to explore the relationships between subject characteristics and particle deposition.

RESULTS

A large individual variability in respiratory tract deposition efficiency was found. Individuals with high deposition of a certain particle size generally had high deposition for all particles <3500 nm. The individual variability was explained by two factors: breathing pattern, and lung structural and functional properties. The most important predictors were found to be breathing frequency and anatomical airway dead space. We also present a linear regression model describing the deposition based on four variables: tidal volume, breathing frequency, anatomical dead space and resistance of the respiratory system (the latter measured with impulse oscillometry).

CONCLUSIONS

To understand why some individuals are more susceptible to airborne particles we must understand, and take into account, the individual variability in the probability of particles to deposit in the respiratory tract by considering not only breathing patterns but also adequate measures of relevant structural and functional properties.

Methacholine-Induced Variations in Airway Volume and the Slope of the Alveolar Capnogram Are Distinctly Associated with Airflow Limitation and Airway Closure

Mechanisms driving alteration of lung function in response to inhalation of a methacholine aerosol are incompletely understood. To explore to what extent large and small airways contribute to airflow limitation and airway closure in this context, volumetric capnography was performed before (n = 93) and after (n = 78) methacholine provocation in subjects with an intermediate clinical probability of asthma. Anatomical dead space (VDaw), reflecting large airway volume, and the slope of the alveolar capnogram (slope3), an index of ventilation heterogeneity linked to small airway dysfunction, were determined. At baseline, VDaw was positively correlated with lung volumes, FEV₁ and peak expiratory flow, while slope3 was not correlated with any lung function index. Variations in VDaw and slope3 following methacholine stimulation were correlated to a small degree (R² = -0.20). Multivariate regression analysis identified independent associations between variation in FEV₁ and variations in both VDaw (Standardized Coefficient-SC = 0.66) and Slope3 (SC = 0.35). By contrast, variation in FVC was strongly associated with variations in VDaw (SC = 0.8) but not Slope3. Thus, alterations in the geometry and/or function of large and small airways were weakly correlated and contributed distinctly to airflow limitation. While both large and small airways contributed to airflow limitation as assessed by FEV1, airway closure as assessed by FVC reduction mostly involved the large airways.

Comparison of capnovolumetry-derived dead space parameters with pulmonary function test in normal adults using histamine provocation

OBJECTIVES

This study in healthy adults was conducted to explore the clinical application of capnovolumetric indices as compared to lung function parameters using histamine provocation.

METHODS

Forty healthy subjects received aerosol histamine or salbutamol in an automatic stimulation system with escalating doses of histamine. Dead space volumes of capnovolumetry and lung function parameters were examined with increased concentrations of histamine at a fixed time interval. The doses of histamine were selected from 0.0562 mg-2.2 mg and 0.1 mg salbutamol was inhaled when a maximal dose of histamine was reached. Baseline values in each group were calculated prior to histamine inhalation.

RESULTS

Fowler dead space (VDF), Wolff dead space (VDW), threshold dead space (VDT), Bohr dead space (VDB), forced expiratory volume in 1 s (FEV1 ) and peak expiratory flow (PEF) showed a dose-dependent reduction following histamine provocation, but there were no statistical differences in the measurements at baseline and post S6 provocation. The value of dC3/DV at the maximal dose was significantly increased over its baseline value (P < 0.05). VDF, VDT and VDW were significantly increased after bronchodilator use (P < 0.05 or <0.01). The changes in capnovolumetry did not correspond with the results of lung function test.

CONCLUSIONS

The dC3/DV and airway dead spaces of capnovolumetry in healthy adults are significantly increased compared to lung function parameters before or after bronchodilator use, suggesting that capnovolumetry is feasible in diagnostic evaluation of airway reactivity, especially for persons who are unable to undertake lung function test.

Comparison of two different methods for physiologic dead space measurements in ventilated dogs in a clinical setting

OBJECTIVE

To compare physiologic dead space (V(D)) and physiologic dead space to tidal volume (V(T)) ratio (V(D)/V(T)) obtained by an automated single breath test for carbon dioxide (CO(2)) (method SBT) and a manual calculation (method MC) in ventilated healthy dogs.

STUDY DESIGN

Prospective clinical study.

ANIMALS

Twenty client-owned dogs, ASA I and II undergoing anaesthesia for clinical purposes.

METHODS

Following pre-medication, induction of anaesthesia, and intubation of the trachea, intermittent positive pressure ventilation was commenced. Mixed expired CO(2) partial pressure (PēCO(2)) was measured by two methods: method MC by analysis, using an infrared capnograph, of the expired gas collected in a mixing box and method SBT which calculated it automatically by a device consisting of a mainstream capnograph and a pneumotachograph. At four time points arterial partial pressure of CO(2) (PaCO(2)) was measured. Physiologic dead space variables (V(D) and V(D)/V(T)) were calculated manually (method MC) or automatically (method SBT) using the Bohr-Enghoff equation. Method MC and SBT were compared using Bland-Altman plots and linear regression. Intra-class correlation coefficient (ICC) was used to measure consistency of each method.

RESULTS

Four measurement pairs were obtained in all 20 dogs for method SBT and MC. The bias was -1.15 mmHg, 7.97 mL and 0.02 for PēCO(2), V(D) and V(D)/V(T), respectively. Linear regression analysis revealed a correlation coefficient (r(2)) of 0.79, 0.94, and 0.83 for PēCO(2), V(D) and V(D)/V(T), respectively. The ICC revealed an excellent consistency for both methods.

CONCLUSIONS

The single breath test (SBT) can be used for clinical evaluation of V(D) and V(D)/V(T) in anaesthetized ventilated dogs.

CLINICAL RELEVANCE

Through measuring V(D) and V(D)/V(T) important information about lung ventilation can be obtained and the SBT is an easy method to use for this purpose.

Capnovolumetry: a new tool for lung function testing in children with asthma

In capnovolumetry, the expiratory CO2 concentration of exhaled air is plotted against the volume and thereby allows to determine functional dead space volumes. This method might offer additional information in lung function testing in children and adolescents with bronchial asthma. We aimed at determining whether a bronchospasmolysis (BSL) effect in the lower airways could also be detected by capnovolumetry as reflected by changes in the functional threshold dead space volumes (VDT). In 47 patients (aged 4-16 years) with a mild persistent bronchial asthma, VDT were determined before and after bronchodilation prior to starting therapy with inhaled steroids and after 6 months of treatment. Additionally, spirometry and body plethysmography were performed in all patients. There were significantly higher VDT values after BSL before and after 6 months of therapy (P<0.0001). VDT values before BSL were tendatively higher after 6 months of therapy compared with baseline values (P=0.07). VDT values correlated with parameters derived from conventional pulmonary function testing, i.e. vital capacity, forced expiratory volume in 1 s (FEV1), and maximum expiratory flow (MEF50). As VDT values particularly reflect the volumes of the lower bronchi this method may provide supplementary information to conventional lung function tests which are based on breathing mechanics. This seems to be especially helpful in situations where body plethysmography is not available or cooperation in forced expiration manoeuvres is insufficient.

Relationship between physiologic deadspace/tidal volume ratio and gas exchange in infants with acute bronchiolitis on invasive mechanical ventilation

OBJECTIVES

To evaluate the association between deadspace/tidal volume ratio (Vd/Vt) and gas exchange variables: Pao2, Paco2, Pao2/Fio2, arterial/alveolar oxygen tension ratio (Pao2/PAo2), alveolar-arterial oxygen tension difference/arterial oxygen tension ratio (P(A-a)o2/Pao2), carbon dioxide production (Vco2), ventilation index ([Paco2 x peak inspiratory pressure x mechanical respiratory rate]/1000), and oxygenation index ([mean airway pressure x Fio2 x 100]/Pao2), all measured at an early stage in children with obstructive acute respiratory failure.

DESIGN

Prospective, cross-sectional, observational study.

SETTING

Pediatric intensive care unit, university hospital.

PATIENTS

Twenty-nine infants with acute bronchiolitis, defined according to clinical and radiologic criteria. Children with chronic pulmonary disease, neuromuscular disease, congenital cardiopathies, or hemodynamic instability were excluded.

INTERVENTIONS

Measurements were made between 24 and 72 hrs of mechanical ventilation using volumetric capnography and arterial blood gas analysis.

MEASUREMENTS AND MAIN RESULTS

The following variables significantly correlated with Vd/Vt, calculated using Spearman's correlation coefficient (rs): Pao2 (rs = -0.63, p < .001), Pao2/Fio2 (rs = -0.56, p = .002), Pao2/PAo2 (rs = -0.46, p = .012), P(A-a)o2/Pao2 (rs = -0.46, p = .012), Paco2 (rs = 0.51, p = .005), Vco2 (rs = -0,62, p < .01), oxygenation index (rs = 0.48, p = .009), and ventilation index (rs = -0.53, p = .003). A statistically significant association was found between an increase in Vd/Vt and severity of lung injury, defined as Pao2/Fio2 <200 (p = .03, Mann-Whitney).

CONCLUSIONS

In the study population, Vd/Vt not only reflected ventilatory disorders, as is well recognized, but also was associated with disturbances of oxygenation. These results warrant further evaluation of the usefulness of serial measurement of Vd/Vt as a marker of disease severity in severe acute bronchiolitis and other causes of respiratory failure.

Volumetric capnography: reliability and reproducibility in spontaneously breathing patients

Volumetric capnography provides a breath-by-breath analysis of ventilation-perfusion imbalances and deadspace volumes. The technique has been best described in intubated and ventilated patients, but promising clinical applications also concern spontaneously breathing patients. The objective of the study was to verify the reliability and reproducibility of a new capnographic program in various types of clinical conditions. In a first step, 56 patients, either healthy or with acute respiratory disorders, were connected to a sidestream gas sampler and flow sensor through a mouthpiece. An acquisition software synchronized expired CO2 and flow data to create volumetric capnographic curves. Mixed expired CO2 partial pressure, corresponding to the exhaled CO2 of the effective tidal volume, was simultaneously collected in a neoprene bag for comparison. In a second step, changes in airway deadspace before and after the adjunction of known spacer volumes were compared in six healthy volunteers. The mean difference between both methods in measuring mixed expired CO2 partial pressure was -0.9 mmHg (SE 0.2 mmHg, P<0.001). The limits of agreement extended from -4.4 to 2.5 mmHg. The interobserver correlation coefficient for reproducibility was 0.98. Airway deadspace volume, after correction for extra volumes, was not statistically different than the basic value (P=0.89). In conclusion, volumetric capnography can be compared with references when used in spontaneously breathing patients. Future developments and clinical applications should clarify its role as a non-invasive method for deadspace and ventilation-perfusion imbalances analysis.

The influence of physiotherapy and suction on respiratory deadspace in ventilated children

OBJECTIVE

To assess and compare the effects of respiratory physiotherapy and suction on deadspace volumes, carbon dioxide elimination (VCO(2)), end tidal CO(2) (ETCO(2)), and arterial partial pressure of carbon dioxide (PaCO(2)) in ventilated infants and children.

DESIGN

Randomised crossover study. Participants received both treatments with a washout interval of more than 90 min.

SETTING

Intensive tertiary care units, Great Ormond Street Hospital, London.

PATIENTS

Eighty-seven fully ventilated children, requiring physiotherapy, with arterial lines in situ. Paired measurements were obtained in 81 patients, of whom 6 were excluded because of tracheal tube leak greater than 20%.

INTERVENTIONS

Respiratory physiotherapy and suction.

MEASUREMENTS AND RESULTS

Data were collected April 1998-March 2000. The "CO(2)SMO Plus" respiratory monitor was used to calculate parameters before and 30 min after both interventions. Physiotherapy lasted longer and required more saline and catheters per treatment ( p<0.005). There were significant increases in physiological deadspace (VD(phys))/kg ( p<0.0001), alveolar deadspace (VD(alv))/kg ( p<0.0001) and VD(phys)/tidal volume (V(T)) ( p<0.05) following physiotherapy that were not observed following suction. There were no significant changes following either treatment with respect to airway deadspace (VD(airway)), VCO(2) or PaCO(2). Comparison of the mean differences following treatments indicated significant differences between physiotherapy and suction in terms of VD(phys)/kg ( p<0.005), VD(alv)/kg ( p<0.005), expired tidal volumes (V(TE)) ( p<0.05), mixed expired CO(2) (PeCO(2)) ( p<0.04) and ETCO(2) ( p<0.03).

CONCLUSIONS

Differences between physiotherapy and suction techniques probably accounted for their statistically distinguishable effects on deadspace. VD(phys) and VD(alv) may be more sensitive indicators of subtle changes in gas exchange and regional ventilation than VCO(2) or PaCO(2). However, interpretation of these outcomes is dependent on concurrent examination of the parameters from which they are derived.