Pulmonary gas exchange responses to histamine and methacholine challenges in mild asthma

A method for mapping regional oxygen and CO2 transfer in the lung

This paper presents a novel approach to visualizing regional lung function, through quantitative three-dimensional maps of O2 and CO2 transfer rates. These maps describe the contribution of anatomical regions to overall gas exchange and demonstrate how transfer rates of the two gas species' differ regionally. An algorithm for generating such maps is presented, and for illustration, regional gas transfer maps were generated using values of ventilation and perfusion imaged by PET/CT for a healthy subject and an asthmatic patient after bronchoprovocation. In a sensitivity analysis, compartment values of gas transfer showed minor sensitivity to imaging noise in the ventilation and perfusion data, and moderate sensitivity to estimation errors in global lung input values, chiefly global alveolar ventilation, followed by cardiac output and arterial-venous O2 content difference. Gas transfer maps offer an intuitive display of physiologically relevant lung function at a regional level, the potential for an improved understanding of pulmonary gas exchange in health and disease, and potentially a presurgical evaluation tool.

Blood gas tensions in adult asthma: a systematic review and meta-regression analysis

OBJECTIVE

The last half-century has seen substantial changes in asthma treatment and care. We investigated whether arterial blood gas parameters in acute and non-acute asthma have changed historically.

METHODS

We performed a systematic search of the literature for studies reporting P(aO2) , P(aCO2) and forced expiratory volume in 1 s, percentage of predicted (FEV1%). For each of the blood gas parameters, meta-regression analyses examined its association with four background variables: the publication year, mean FEV1%, mean age and female fraction in the respective studies.

RESULTS

After screening, we included 43 articles comprising 61 datasets published between 1967 and 2013. In studies of habitual-state asthma, mean P(aO2) was positively associated with the publication year (p = 0.001) and negatively with mean age (p < 0.01). Mean P(aCO2) showed a positive association with publication year (p = 0.001) and a negative association with female fraction (p < 0.05). In acute asthma studies, blood gas levels were unassociated with publication year and mean age, mean P(aO2) was positively associated with FEV1% (p < 0.05) whereas mean P(aCO2) showed a negative association with FEV1% (p < 0.05) for studies with mean FEV1% <40. In neither acute nor habitual-state studies was mean arterial pH associated with any of the predictor variables.

CONCLUSIONS

In studies of habitual-state asthma, mean reported P(aO2) and P(aCO2) levels were found to have increased since 1967. In acute asthma studies, mean P(aO2) and P(aCO2) were associated with mean FEV1% but not with either publication year or patient age.

The effect of conductive ventilation heterogeneity on diffusing capacity measurement

It has long been assumed that the ventilation heterogeneity associated with lung disease could, in itself, affect the measurement of carbon monoxide transfer factor. The aim of this study was to investigate the potential estimation errors of carbon monoxide diffusing capacity (DlCO) measurement that are specifically due to conductive ventilation heterogeneity, i.e., due to a combination of ventilation heterogeneity and flow asynchrony between lung units larger than acini. We induced conductive airway ventilation heterogeneity in 35 never-smoker normal subjects by histamine provocation and related the resulting changes in conductive ventilation heterogeneity (derived from the multiple-breath washout test) to corresponding changes in diffusing capacity, alveolar volume, and inspired vital capacity (derived from the single-breath DlCO method). Average conductive ventilation heterogeneity doubled ( P < 0.001), whereas DlCO decreased by 6% ( P < 0.001), with no correlation between individual data ( P > 0.1). Average inspired vital capacity and alveolar volume both decreased significantly by, respectively, 6 and 3%, and the individual changes in alveolar volume and in conductive ventilation heterogeneity were correlated ( r = −0.46; P = 0.006). These findings can be brought in agreement with recent modeling work, where specific ventilation heterogeneity resulting from different distributions of either inspired volume or end-expiratory lung volume have been shown to affect DlCO estimation errors in opposite ways. Even in the presence of flow asynchrony, these errors appear to largely cancel out in our experimental situation of histamine-induced conductive ventilation heterogeneity. Finally, we also predicted which alternative combination of specific ventilation heterogeneity and flow asynchrony could affect DlCO estimate in a more substantial fashion in diseased lungs, irrespective of any diffusion-dependent effects.

Treatment of airway inflammation improves exercise pulmonary gas exchange and performance in asthmatic subjects

BACKGROUND

Asthma is an inflammatory disease of the airways that can lead to impaired arterial blood oxygenation during exercise.

OBJECTIVE

We asked whether treatment of airway inflammation in asthmatic subjects would improve arterial blood gases during whole-body exercise.

METHODS

By using a double-blind parallel-group design, 19 asthmatic subjects completed treadmill exercise to exhaustion on 2 occasions: (1) before and (2) after 6 weeks' treatment with an inhaled corticosteroid (ICS; n = 9) or placebo (n = 10).

RESULTS

The ICS group had improved resting pulmonary function, decreased exercise-induced bronchospasm, and decreased postexercise sputum histamine during the posttreatment study compared with that during the pretreatment study. In the ICS group exercise Pao(2) was significantly increased after treatment (84.8 to 93.8 mm Hg). Increased alveolar ventilation (arterial Pco(2) decreased from 36.9 to 34.1 mm Hg) accounted for 37% of the increased Pao(2) and improved gas exchange efficiency (alveolar-to-arterial Po(2) difference decreased from 22.5 to 16.3 mm Hg) accounted for the remaining 63% of the increased Pao(2) after treatment. In the ICS group exercise time to exhaustion was increased from 9.9 minutes during the pretreatment study to 14.8 minutes during the posttreatment study.

CONCLUSION

Treatment of airway inflammation in asthmatic subjects can improve arterial blood oxygenation during exercise by (1) improving airway function, thereby allowing increased alveolar ventilation during exercise, and (2) improving the efficiency of alveolar-to-arterial blood O(2) exchange.

CLINICAL IMPLICATIONS

In asthmatic patients ICSs not only attenuate exercise-induced bronchospasm but also improve arterial blood oxygenation during exercise.

Repeat exercise normalizes the gas-exchange impairment induced by a previous exercise bout in asthmatic subjects

Twenty-one subjects with asthma underwent treadmill exercise to exhaustion at a workload that elicited approximately 90% of each subject's maximal O2 uptake (EX1). After EX1, 12 subjects experienced significant exercise-induced bronchospasm [(EIB+), %decrease in forced expiratory volume in 1.0 s = -24.0 +/- 11.5%; pulmonary resistance at rest vs. postexercise = 3.2 +/- 1.5 vs. 8.1 +/- 4.5 cmH2O.l(-1).s(-1)] and nine did not (EIB-). The alveolar-to-arterial Po2 difference (A-aDo2) was widened from rest (9.1 +/- 6.7 Torr) to 23.1 +/- 10.4 and 18.1 +/- 9.1 Torr at 35 min after EX1 in subjects with and without EIB, respectively (P < 0.05). Arterial Po2 (PaO2) was reduced in both groups during recovery (EIB+, -16.0 +/- -13.0 Torr vs. baseline; EIB-, -11.0 +/- 9.4 Torr vs. baseline, P < or = 0.05). Forty minutes after EX1, a second exercise bout was completed at maximal O2 uptake. During the second exercise bout, pulmonary resistance decreased to baseline levels in the EIB+ group and the A-aDo2 and PaO2 returned to match the values seen during EX1 in both groups. Sputum histamine (34.6 +/- 25.9 vs. 61.2 +/- 42.0 ng/ml, pre- vs. postexercise) and urinary 9alpha,11beta-prostaglandin F2 (74.5 +/- 38.6 vs. 164.6 +/- 84.2 ng/mmol creatinine, pre- vs. postexercise) were increased after exercise only in the EIB+ group (P < 0.05), and postexercise sputum histamine was significantly correlated with the exercise PaO2 and A-aDo2 in the EIB+ subjects. Thus exercise causes gas-exchange impairment during the postexercise period in asthmatic subjects independent of decreases in forced expiratory flow rates after the exercise; however, a subsequent exercise bout normalizes this impairment secondary in part to a fast acting, robust exercise-induced bronchodilatory response.

Gas exchange during exercise in habitually active asthmatic subjects

We determined the relations among gas exchange, breathing mechanics, and airway inflammation during moderate- to maximum-intensity exercise in asthmatic subjects. Twenty-one habitually active (48.2 +/- 7.0 ml.kg(-1).min(-1) maximal O2 uptake) mildly to moderately asthmatic subjects (94 +/- 13% predicted forced expiratory volume in 1.0 s) performed treadmill exercise to exhaustion (11.2 +/- 0.15 min) at approximately 90% of maximal O2 uptake. Arterial O2 saturation decreased to < or =94% during the exercise in 8 of 21 subjects, in large part as a result of a decrease in arterial Po2 (PaO2): from 93.0 +/- 7.7 to 79.7 +/- 4.0 Torr. A widened alveolar-to-arterial Po2 difference and the magnitude of the ventilatory response contributed approximately equally to the decrease in PaO2 during exercise. Airflow limitation and airway inflammation at baseline did not correlate with exercise gas exchange, but an exercise-induced increase in sputum histamine levels correlated with exercise Pa(O2) (negatively) and alveolar-to-arterial Po2 difference (positively). Mean pulmonary resistance was high during exercise (3.4 +/- 1.2 cmH2O.l(-1).s) and did not increase throughout exercise. Expiratory flow limitation occurred in 19 of 21 subjects, averaging 43 +/- 35% of tidal volume near end exercise, and end-expiratory lung volume rose progressively to 0.25 +/- 0.47 liter greater than resting end-expiratory lung volume at exhaustion. These mechanical constraints to ventilation contributed to a heterogeneous and frequently insufficient ventilatory response; arterial Pco2 was 30-47 Torr at end exercise. Thus pulmonary gas exchange is impaired during high-intensity exercise in a significant number of habitually active asthmatic subjects because of high airway resistance and, possibly, a deleterious effect of exercise-induced airway inflammation on gas exchange efficiency.

Effects of helium-oxygen on respiratory mechanics, gas exchange, and ventilation-perfusion relationships in a porcine model of stable methacholine-induced bronchospasm

OBJECTIVE

To explore the consequences of helium/oxygen (He/O(2)) inhalation on respiratory mechanics, gas exchange, and ventilation-perfusion (VA/Q) relationships in an animal model of severe induced bronchospasm during mechanical ventilation.

DESIGN

Prospective, interventional study.

SETTING

Experimental animal laboratory, university hospital.

INTERVENTIONS

Seven piglets were anesthetized, paralyzed, and mechanically ventilated, with all ventilator settings remaining constant throughout the protocol. Acute stable bronchospasm was obtained through continuous aerosolization of methacholine. Once steady-state was achieved, the animals successively breathed air/O(2) and He/O(2) (FIO(2) 0.3), or inversely, in random order. Measurements were taken at baseline, during bronchospasm, and after 30 min of He/O(2) inhalation.

RESULTS

Bronchospasm increased lung peak inspiratory pressure (49+/-6.9 vs 18+/-1 cm H(2)O, P<0.001), lung resistance (22.7+/-1.5 vs 6.8+/-1.5 cm H(2)O x l(-1).s, P<0.001), dynamic elastance (76+/-11.2 vs 22.8+/-4.1 cm H(2)O x l(-1), P<0.001), and work of breathing (1.51+/-0.26 vs 0.47+/-0.08, P<0.001). Arterial pH decreased (7.47+/-0.06 vs 7.32+/-0.06, P<0.001), PaCO(2) increased, and PaO(2) decreased. Multiple inert gas elimination showed an absence of shunt, substantial increases in perfusion to low VA/Q regions, and dispersion of VA/Q distribution. He/O(2) reduced lung resistance and work of breathing, and worsened hypercapnia and respiratory acidosis.

CONCLUSIONS

In this model, while He/O(2) improved respiratory mechanics and reduced work of breathing, hypercapnia and respiratory acidosis increased. Close attention should be paid to monitoring arterial blood gases when He/O(2) is used in mechanically ventilated acute severe asthma.

Pulmonary gas exchange and sputum cellular responses to inhaled leukotriene D(4) in asthma

Inhalational challenges with inflammatory mediators may provoke lung function disturbances similar to those shown in spontaneous acute asthma. Cysteinyl leukotrienes (CysLTs) have recently been established as mediators of bronchoconstriction in asthma but their effects on pulmonary gas exchange in asthma have not been assessed. We therefore investigated the effects of leukotriene D(4) (LTD(4)) challenge resulting in a significant decrease in FEV(1) (mean +/- SE, by 32 +/- 3%) in 13 nonsmoking, mild asthmatics. Respiratory system resistance (Rrs), and respiratory and inert gases were measured before and immediately after, and at 15 and 45 min after challenge. After bronchoprovocation, Rrs increased (by 106 +/- 12%), Pa(O(2)) decreased (by 25 +/- 4 mm Hg), and ventilation-perfusion distributions moderately to severely deteriorated, as shown by increases in the dispersions of pulmonary blood flow (Log SDQ, by 59 +/- 12%) and alveolar ventilation (Log SDV, by 65 +/- 20%) (p < 0.05 each). Sputum eosinophils (p < 0.05) and urinary LTE(4) (p < 0.005) increased after challenge. Despite the lack of mathematical correlations between spirometric and Rrs changes and gas exchange indices, the pattern of improvement of the functional variables after challenge ran in parallel. These findings support the evidence that CysLTs, in addition to being potent bronchoconstrictors, also provoke profound disturbances of pulmonary gas exchange in asthma.