Some Factors Affecting the Relationship of Maximal Expiratory Flow to Lung Volume in Health and Disease

Asthma is a disease characterized by inflammation which affects both proximal and distal airways. We evaluated the prevalence of small airway obstruction (SAO) in a group of clinically stable asthmatics with both normal forced expiratory volume in the first second (FEV₁) and normal FEV₁/forced vital capacity (FVC) and treated with an association of inhaled corticosteroids (ICSs) and long acting β₂-agonists (LABAs). Clinical evaluation included the measurement of dyspnea, asthma control test and drug compliance.

The prevalence of SAO was estimated by spirometry and plethysmography and defined by the presence of one or more of the following criteria: functional residual capacity (FRC) > 120% predicted (pred), residual volume (RV) > pred + 1.64 residual standard deviation (RSD), RV/total lung capacity (TLC) > pred + 1.64 RSD, forced expiratory flow (FEF)_25–75% < pred − 1.64 RSD, FEF_50% < pred − 1.64 RSD, slow vital capacity (SVC) − FVC > 10%.

Among the 441 patients who were included, 222 had normal FEV₁ and FEV₁/FVC. At least one criteria of SAO was found in 115 (52%) mainly lung hyperinflation (39% based on high FRC, RV or RV/TLC) and more rarely distal airflow limitation (15% based on FEF_25–75% or FEF_50%) or expiratory trapping (10% based on increased SVC − FVC). In the patients with only SAO (no PAO), there was no relationship between SAO, asthma history and the scores of dyspnea, asthma control or drug compliance.

These results suggest that in asthmatics with normal FEV₁ and FEV₁/FVC, treated with ICSs and LABAs, SAO is found in more than half of the patients indicating that the routinely used lung function tests can underestimate dysfunctions occurring in the small airways.

Les petites voies aériennes sont définies par un diamètre interne inférieur à 2 mm.

Chez l’adulte, comme chez l’enfant, l’épreuve fonctionnelle respiratoire standard n’est pas un outil très performant pour l’évaluation de l’atteinte des voies aériennes distales. Le DEM 25-75 et le DEM 50 sont classiquement utilisés en pratique clinique comme paramètre d’obstruction « distale ». Mais les critères d’interprétation de ces deux indices limitent leur intérêt.

De nouveaux indices fonctionnels sont donc nécessaires afin de mieux évaluer l’atteinte inflammatoire distale comme la mesure des résistances par oscillation. La mesure du monoxyde d’azote dans les gaz expirés informe sur les processus inflammatoires liés à la réponse immunologique de type Th2 siégeant dans le secteur bronchique et, sous certaines conditions, dans le poumon profond.

L’augmentation de la concentration alvéolaire en NO pourrait refléter la présence d’une inflammation de nature allergique siégeant dans les voies aériennes de petit calibre. Ces examens sont réalisables à partir de l’âge de 7 ans.

G. Garcia, T. Perez, B. Mahut

Small airways are defined (in humans) as those <2mm in diameter.

They were originally described as the “quiet zone” of the lungs contributing less than 10% of the total resistance to airflow. Pulmonary function tests remain the most used method to assess distal airway flow limitation.

However, these tests are limited in adults and also in children because MEF_25–75% and FEF_50% are highly variable spirometric indices and they depend on vital capacity, which increases with expiratory time in obstructed subjects. There is a need for promising non invasive new tools like the forced oscillation technique to measure resistance. The increased availability of the exhaled fraction of nitric oxide (FeNO) measurement means that this method is accessible and attractive.

The production of nitric oxide (NO) can be assessed by measuring the fraction of NO during a prolonged expiration (FENO) or by estimating other parameters of NO exchange including the alveolar NO concentration (CalvNO) and may provide information about small airway inflammation and assist the optimal control of the disease.

La curva flujo volumen constituye la prueba no invasiva más habitual en el diagnóstico de la obstrucción de la vía aérea superior. Con el objetivo de analizar su utilidad en la detección de obstrucciones fijas en la vía aérea superior en presencia de enfermedad pulmonar obstructiva crónica (EPOC), se realizaron curvas flujo volumen con maniobras inspiratorias y espiratorias máximas a 60 pacientes con EPOC y a 15 controles sanos. En todos los casos se hizo un estudio basal, repitiéndose posteriormente de forma aleatoria tras la aplicación de resistencias externas con un diámetro interno de 4, 6, 8 y 10 mm. Aunque se detectaron caídas significativas en el flujo espiratorio pico (PEF) y flujos inspiratorios (FIF₅₀) con resistencias de 10 mm, incluso en el grupo con limitación ventilatoria muy grave, fue necesario reducir el diámetro interno a 6 mm para detectar cambios en el volumen espirado máximo en un segundo (FEV₁). En los pacientes con valores basales inferiores al 50% del teórico, se necesitaron estenosis de 4 mm para provocar cambios en el FEV ₁. La caída en el FEV₁ y PEF se hizo menos evidente a medida que aumentaba la gravedad de la EPOC, correlacionándose en ambos casos con el porcentaje de FEV, basal sólo con diámetros de 6 mm (p < 0,01) y 4 mm (p < 0,001).

La sensibilidad de los índices más habituales utilizados en la detección de la obstrucción de la vía aérea superior, tales como el FEV₁/PEF y el FEV₁/FEV_0,5, fue baja en las formas graves de EPOC, incluso con estenosis de 4 mm, obteniéndose sensibilidades inferiores al 50% en todos los grupos con diámetros superiores a 6 mm. Otros índices como el FEF₅₀/FIF₅₀ y el FEV₁/FIV₁ no mejoraron la rentabilidad diagnóstica.

Concluimos que, en presencia de EPOC, la curva flujo volumen puede no detectar adecuadamente la existencia de una obstrucción asociada en la vía aérea superior, por lo que, especialmente en los casos más graves, debe considerarse la necesidad de utilizar métodos diagnósticos alternativos.

The flow-volume curve is the usual noninvasive diagnostic test for upper airways obstruction. In order to assess its usefulnes for the detection of fixed upper airwais obstruction in chronic obstructive pulmonary disease (COPD), we plotted flow-volume curves using maximum inspiratory and expiratory maneuvers in 60 COPD patients and in 15 healthy Controls. Baseline readings were taken, followed by readings after random application of fixed external resistances with diameters of 4, 6, 8 and 10 mm in all cases. Although PEF and FIF₅₀ decreased signiflcantly with resistance of 10 mm, even in the group with the most severe ventilatory limitation, it was necessary to reduce the internal diameter to 6 mm to detect changes in FEV₁. In patients with baseline values under 50% of theoretical values, 4 mm stenosis was required to provoke changes in FEV₁. The fall in FEV, and PEF was less evident as the severity of COPD increased, with both parameters correlating with percent baseline FEV, only at diameters of 6 mm (p < 0.01) and 4 mm (p < 0.001).

The sensitivities of the usual indices for detecting upper airways obstruction, such as FEV₁/PEF and FEV₁/FEV_0,5, were low (below 50%) in all groups at diameters over 6 mm, and in the most severe cases of COPD, even with stenosis of 4 mm. Ñor did other indices, such as FEF₅₀/FIF₅₀ and FEV,/FIV₁, give better diagnostic yield.

We conclude that the flow-volume curve may not detect the existence of upper airways obstruction in COPD, and that in the most severely affected patients alternative diagnostic methods should therefore be considered.

A reduction in lung volume is used to diagnose physiologic restriction in the pulmonary function tests of patients with lung disease. Airflow obstruction is commonly associated with hyperinflation of static lung volume. Because restriction and obstruction exert opposite effects on lung volumes, we questioned whether the lack of hyperinflation of static lung volumes could indicate the presence of concomitant restriction in patients with airflow obstructive ventilatory defects. To assess this, we evaluated by pulmonary function tests and chest roentgenograms of 58 patients with airflow obstruction (group 1), 18 of whom then sustained various types of resection for lung cancer (group 2) as a type of superimposed restriction. We selected 80 percent of predicted as the lower limit of “normal” frequently used by clinical pulmonary function laboratories. Despite a statistically significant decrease in total lung capacity (p<0.05) for the postpneumonectomy patients, when the static lung volume measurements of the patients with resection were evaluated, no one lung volume showed a consistent reduction sufficient to detect the superimposed restriction in all these patients. Using 80 percent of predicted as “normal,” 61 percent of our patients with airflow obstruction and superimposed restriction would have been missed. We conclude that it is clinically difficult, based on only static lung volume measurements alone, to detect restriction superimposed on the hyperinflation of airflow obstruction unless these lung volumes are reduced to below accepted “normal” limits.

We compared the dynamic lung function indices in patients with asthma (n = 27), emphysema (n = 20), and UAO (n = 18), with the purpose being to examine whether different patterns of abnormalities could be found and which tests were most discriminative among the three groups. Forced expiratory indices were measured (FEV₁; PEF; MEF50%), as well as indices obtained during quiet breathing (Raw; Gaw; Gaw/V_L). The three groups were comparable as far as PEF was concerned (about 60 ± 20 percent of predicted, yet the group with UAO showed significantly larger FEV₁ (84 percent of predicted vs 55 percent and 57 percent of predicted in asthma and emphysema respectively), and larger MEF50% (71 percent of predicted vs 25 percent and 23 percent of predicted in the other groups), and the group with asthma had the largest Raw (0.37 ± 0.18 kPa·s·L⁻¹ vs 0.24 ± 0.13 in UAO and 0.22 ± 0.10 in emphysema). From these functional tests, several ratios were derived which were discriminative among the three groups. Upper airway obstruction could be recognized by a significantly lower PEF/MEF50% ratio and higher FEV₁/PEF ratio than in the other conditions. Furthermore, a distinction between asthma and emphysema could be made by comparing airway patency during forced and quiet breathing, ie, the MEF50%/Gaw ratio. This ratio was, in fact, significantly different for all three groups, having the lowest value in emphysema (0.19 ± 0.08 kPa vs 0.44 ± 0.23 kPa and 0.63 ± 0.34 kPa in asthma and UAO, respectively). Within the group with UAO, those with variable intrathoracic obstruction showed the least difference from asthma and emphysema for the measured indices.

We evaluated application of a P_th device for testing inspiratory muscle endurance among patients with severe but stable COPD. Endurance time in five patients was reproducible. Magnitude of variability was ± 1.26 minutes with a range of ±0.19 to ±2.28 minutes. Eleven inpatients completed inspiratory muscle training twice daily for four weeks in addition to their usual program of respiratory rehabilitation. The mean age of our experimental cohort was 65 years; FEV₁, 33 ± 12 percent predicted; and Dsb, 42 ±7 percent predicted. Baseline measurements showed no significant differences in pulmonary function, exercise tolerance, inspiratory muscle strength or inspiratory muscle endurance between control and study groups. Following training, the study group significantly improved inspiratory muscle endurance as evidenced by an increase in endurance time while breathing against the same absolute external P_th load used during baseline assessments. There were no associated changes in lung mechanics, muscle strength or exercise tolerance.