REGIONAL PULMONARY FUNCTION STUDIED WITH XENON IN PATIENTS WITH BRONCHIAL ASTHMA

Lung functional imaging

Pulmonary functional imaging modalities such as computed tomography, magnetic resonance imaging and nuclear imaging can quantitatively assess regional lung functional parameters and their distributions. These include ventilation, perfusion, gas exchange at the microvascular level and biomechanical properties, among other variables. This review describes the rationale, strengths and limitations of the various imaging modalities employed for lung functional imaging. It also aims to explain some of the most commonly measured parameters of regional lung function. A brief review of evidence on the role and utility of lung functional imaging in early diagnosis, accurate lung functional characterisation, disease phenotyping and advancing the understanding of disease mechanisms in major respiratory disorders is provided.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Pulmonary Functional Imaging: Part 2-State-of-the-Art Clinical Applications and Opportunities for Improved Patient Care

Pulmonary functional imaging may be defined as the regional quantification of lung function by using primarily CT, MRI, and nuclear medicine techniques. The distribution of pulmonary physiologic parameters, including ventilation, perfusion, gas exchange, and biomechanics, can be noninvasively mapped and measured throughout the lungs. This information is not accessible by using conventional pulmonary function tests, which measure total lung function without viewing the regional distribution. The latter is important because of the heterogeneous distribution of virtually all lung disorders. Moreover, techniques such as hyperpolarized xenon 129 and helium 3 MRI can probe lung physiologic structure and microstructure at the level of the alveolar-air and alveolar-red blood cell interface, which is well beyond the spatial resolution of other clinical methods. The opportunities, challenges, and current stage of clinical deployment of pulmonary functional imaging are reviewed, including applications to chronic obstructive pulmonary disease, asthma, interstitial lung disease, pulmonary embolism, and pulmonary hypertension. Among the challenges to the deployment of pulmonary functional imaging in routine clinical practice are the need for further validation, establishment of normal values, standardization of imaging acquisition and analysis, and evidence of patient outcomes benefit. When these challenges are addressed, it is anticipated that pulmonary functional imaging will have an expanding role in the evaluation and management of patients with lung disease.

Correlation of pulmonary 99mTc-DTPA ventilation and 99mTc-MAA perfusion scans with pulmonary function tests in asymptomatic asthmatic children

Our objective was to examine and correlate 99mTc-diethylenetriaminopentaacetic acid (99mTc-DTPA) ventilation and 99mTc-macroaggregated albumin (99mTc-MAA) perfusion (V/Q) lung scans with spirometry in asymptomatic asthmatic children. We evaluated 89 subjects (age range, 6-15 years; mean age, 9.4 years), all with at least 70% predicted forced expiratory volume in 1 s (FEV(1)). There were four V/Q scan patterns: normal in 38 (42.7%), inhomogeneous ventilation in 11 (12.4%), matched defects in 25 (28.3%) and mismatched perfusion defects in 13 (14.6%). The maximal mid-expiratory flow rate (MMEF) of the normal scan group was significantly different from that in the other groups. The MMEF of the inhomogeneous group was significantly different from that in the matched defect group and the mismatched perfusion defect group. No other significant differences in spirometric indices were found. In two children with perfusion defects, pulmonary arteriograms demonstrated no obstructive lesions. In conclusion, lung scans provide diagnostic information in asymptomatic asthmatic children, even when they are uncooperative. Abnormal scans are common in these children and are significantly correlated with reduced MMEF (% predicted), reflecting small airway flow obstruction. The pathophysiology of V/Q defects in asymptomatic asthmatic children warrants further investigation.

The pathobiology of acute asthma

Acute bronchial asthma is a common problem with immense medical and economic impacts. It is estimated that this disease affects 12 to 14 million people in the United States with costs in excess of $6 billion per year. Most of the morbidity and all of the mortality of asthma tends to be associated with acute exacerbations, and treatment of these events accounts for the majority of expenditures in money and health care resources. Unfortunately, the factors that contribute to the destabilization of asthma are rarely studied and much of the pathogenesis and pathobiology of acute asthma remains unknown. This article examines these issues and suggests treatment for acute asthma.

[Pulmonary gas transfer in pediatrics (carbon monoxide and dioxide)]

In pediatrics, measurements of gaseous transfer in steady-state (SS) are easily applied. Nevertheless, interpretation of results is uncertain because predicted values depend on numerous metabolic and ventilatory parameters. In healthy subjects, the only invariant parameter of reference, beside the blood gas, is the CO uptake (VCO) when normalized by the reject of CO2 (VCO2). Theoretically, a deficit of VCO/VCO2 (VCOSpecifique = SpVCO) should not be observed on a young asthmatic in remission. Such a deficit should be due to either a circulatory impairment of the "gas-exchanging organ" or an alveolary ventilation failure. However, the respiratory equivalent for CO2 (V/VCO2) increases in case of hyperventilation. When not observed at rest, hyperventilation can occur at exercise, that can again induce a non specific bronchitic hyperreactivity.

AIM

To define links: 1) between the spirometry of the young asthmatic and the value of ERCO2 and CO tests; 2) between a circulatory anomaly and the value of SpVCO.

PATIENTS AND METHOD

The asthmatic adolescents aged between 10 and 20 were separated from infants (age < 10) and classified in three degrees of spirometrical alteration according to the maximum expiratory flow when 25% of the forced vital capacity remains in the lung (V25) and the residual volume (RV); 48 adolescents were examined at rest and 17 were exercising on a cyclo-ergometer. Two adolescents with circulatory anomaly, one by idiopathic pulmonary hemosiderosis (IPH), the other by agenesia of the left lung were examined at rest and exercise.

RESULTS

In asthmatics at rest the three degrees of spirometric alteration differed from one another in ERCO2 and classical CO tests; SpVCO alone was not altered at rest or during exercise. However the deficit of SpVCO, confirmed during the exercise, was significant in HPI.

CONCLUSION

Simple and quick simultaneous measurements of (FICO-FECO) and FECO2 allows one to detect hyperventilation of young asthmatic subjects at rest and during exercise, or can confirm a circulatory anomaly.

Left lung asthma

The chest radiograph of a woman with acute asthma showed signs of obstructive emphysema of the left lung. Fiberoptic bronchoscopy excluded obstruction of a large bronchus. The signs disappeared after antiasthmatic treatment, suggesting that they were caused by airway closure. In usual asthma, airway closure affects both lungs; the reason for the unilateral predominance we observed is unknown.

Abnormalities of lung mechanics in young asthmatic children

Measurements of total compliance of the respiratory system by the weighted spirometer technique and of the functional residual capacity by helium gas dilution were attempted in 86 asthmatic children aged 2.2-7.9 years. In all but six of the 86 children reliable measurements could be obtained. Significantly raised functional residual capacity was detected in children with asthma of all degrees of severity. The compliance of the respiratory system was significantly abnormal (reduced) only in children who had symptoms at the time of measurement or who had chronic persistent asthma. The results indicate that these measurements are well tolerated in young asthmatic children. Further work needs to be undertaken to assess the value of this technique in following the response to treatment.

Trapped gas and airflow limitation in children with cystic fibrosis and asthma

Trapped gas (TG) has been shown to be present in cystic fibrosis (CF) and asthma, but its relationship with airflow limitation (AL) has not been explored. TG was defined as the difference between the functional residual capacity measured by body plethysmography [FRC(BP)] and that measured by helium dilution [FRC(He)] expressed as a percentage of total lung capacity (TLC). We studied 21 children with CF and 15 with asthma who had a similar degree of AL and hyperinflation, however the children with CF had greater amounts of TG compared with asthma [15% +/- 2 (mean +/- 1 SE) vs 8 +/- 2, P less than 0.05]. Mixing efficiency (ME), an index of the distribution of ventilation, was found to be lower in the CF children than in those with asthma (34 +/- 4 vs 54 +/- 4). In CF, TG correlated with forced expiratory flow between 25 and 75% of vital capacity (FEF25-75), ME, RV/TLC, and maximal expiratory flow rates at 25 and 50% of vital capacity (Vmax25 and 50, respectively; r = -0.66, -0.61, 0.72, -0.71, -0.74). ME also correlated with the above measures. In asthma, TG did not correlate with the FEF25-75, ME, RV/TLC, Vmax25, or Vmax50. Furthermore, TG was frequently found in patients with asthma with moderate AL while it was often absent in patients with severe AL. For both CF and asthma, neither the severity of AL nor the magnitude of the TG could be predicted from the increase in Vmax50 with helium (delta V50).(ABSTRACT TRUNCATED AT 250 WORDS)

Lung scan abnormalities in asthma and their correlation with lung function

We have used asthma as a model of airways disease to test how well an automated, quantitative method of analysis of lung scans correlates with physiological measurements of disturbed lung function and gas exchange. We studies 25 asthmatics (age 16-73) of widely differing severity (forced expiratory volume in 1-s (FEV1) 22%-123% of predicted value), who had airways tests, arterial blood gas analysis, and krypton-technetium lung scans within a short time of each other. In all patients with airways obstruction and in some with normal function during remission, scans showed the typical appearances of multiple defects of ventilation and perfusion. The severity of ventilation defects was computed from the posterior view of the krypton scan compared to an age- and sex-matched normal range to yield an underventilation score. This correlated closely with the severity of airways obstruction as measured by forced expiratory manoeuvres. Ventilation and perfusion defects were usually imperfectly matched; the severity of this was computed using a subtraction method applied to the counts on the posterior krypton and technetium scans. The degree of mismatch was inversely related to the arterial partial pressure of oxygen (r = -0.86). The results suggest that computer scan analysis can provide useful functional information about the lung in airways disease.

The effect of methacholine inhalation challenge on regional residual volume in patients with subclinical asthma

Regional residual volume to total lung capacity (RVr/TLCr) was measured with xenon 133 before and after methacholine challenge in 26 nonsmoking subjects (mean age 34 years). Eleven were normal control subjects and 15 were patients referred for methacholine challenge because of previous asthma-like symptoms. All had normal pulmonary function and normal RVr/TLCr distribution. Following methacholine challenge, RVr/TLCr increased in two control subjects and ten patients who also had decreases in FEV1 of greater than 20 percent. The RVr/TLCr changes were patchy, suggesting that the degree of bronchospasm varied between individual lung regions. The other 14 subjects did not have a 20 percent decrease in FEV1, but two controls and four patients had generalized increases in RVr/TLCr, while seven controls and one patient had no significant changes in RVr/TLCr. In all subjects, FEV1 and RVr/TLCr returned to the baseline level after salbutamol administration. The results indicate that methacholine can cause localized or diffuse effects on lung emptying and that bronchodilator completely reverses the bronchoconstriction induced by methacholine.

Histamine bronchial challenge: effect on regional ventilation and aerosol deposition

We studied regional changes in ventilation and aerosol deposition after histamine challenge in six patients with asthma and two with rhinitis and a history of wheezing. All were known to have bronchial hyperreactivity and all showed an increased response to histamine. Ventilation and aerosol deposition studies, using xenon-133 and an aerosol of sulphur colloid tagged with technetium 99m, were performed while they were sitting. Before administration of histamine radioaerosol scintiscans were abnormal in five of six patients; after histamine challenge all were abnormal and central deposition was significantly greater in all of them. The decrease in aerosol penetration correlated with the percentage decrease in FEV1, indicating that the efficiency of aerodynamic filtration depends on the degree of airway narrowing. In six of the eight subjects the distribution of ventilation changed from predominantly basal to predominantly apical after histamine, which suggests the airways response was greater, at least initially, in the better ventilated regions. This indicates a close relationship between regional ventilation and the site of histamine deposition and has implications for the delivery of aerosolised agents in general.

Effects of acute and long-term bronchodilator treatment on regional lung function in asthma assessed with krypton-81m and technetium-99m-labelled macroaggregates

We have investigated the effect of acute and long-term bronchodilator treatment on the distribution of ventilation and perfusion in 15 asthmatics using a gamma camera, krypton-81m (for ventilation) and technetium-99m macroaggregate (for perfusion). Individual peak expiratory flow (PEF) values before bronchodilation were slightly or moderately below the predicted values. The simultaneous ventilation images (analysed visually) showed areas of delayed ventilation in all patients (mean distribution score on 3-point scale 2.1). After isoprenaline inhalation (240 micrograms) the mean PEF increased by 24%, but the distribution of ventilation remained virtually unchanged in all patients (mean score 2.0). Simultaneously defects in perfusion could be seen in all patients (mean score 1.5). After intensive treatment, when the mean PEF increased by a further 29%, the distribution scores of ventilation and perfusion fell to 0.8 and 0.9, respectively. The results indicate that, without intensive and long-term treatment, appreciable inequality of ventilation and perfusion are usual consequences of asthma; and suggest that although larger airways are dilated by isoprenaline inhalations residual bronchial obstruction may still remain in some smaller airways, maintaining uneven distribution. Perfusion disturbances seem to be secondary to changes in regional ventilation.

Clinical ventilation-perfusion scintigraphy

Ventilation-perfusion scintigraphy is an established procedure for the investigation of lung disease. Perfusion scans are commonly obtained with a gamma-camera following injection of 99mTc-labelled microspheres. For the assessment of regional lung ventilation, a number of techniques are now being employed, i.e. 133Xe and 127Xe single breath/washout studies, continuous inhalation of 81mKr and inhalation of radioactive aerosols. The latter two methods are now gaining consideration in clinical practice. Lung perfusion scanning is highly sensitive for detection of regional abnormalities of blood flow; the diagnosis of pulmonary embolism remains the most important clinical application of the technique. In this context, the use of a ventilation scan is required in order to increase the specificity of the procedure. In general, lung ventilation-perfusion scintigraphy is of great value for the management of patients with both primary lung disease and heart disease, by providing pathophysiological information of importance for the diagnosis, follow up and the functional evaluation of the patient.

Pulmonary scintigraphy in pediatrics

Pulmonary radionuclide studies in children provide reliable information on regional lung perfusion and ventilation. They are easily and safely performed and they do not usually require full patient cooperation. Their clinical indications and applications are not as well defined as and frequently differ from those in pulmonary disorders of the adult patient. Radiation exposures resulting from radionuclide pulmonary examinations are well within acceptable limits set up for a variety of radiologic diagnostic procedures. Our lung scanning experience with a spectrum of congenital and acquired cardiopulmonary diseases is presented and the contribution of the radionuclide methods to the specific problems facing the pediatrician or the surgeon is discussed.

Transient unilateral hypoperfusion of the lung following mediastinoscopy

Two cases of pulmonary hypoperfusion occurring after mediastinoscopic examination were demonstrated on lung scans. In one case, this finding on the ninth day required a pulmonary angiographic study that yielded normal findings. Repeat lung scans were normal. We propose that localized mediastinal edema or bleeding after mediastinoscopic examination can produce defects of hypoperfusion, and we urge caution in the interpretation of lung scans up to nine days after mediastinoscopic examination.

Regional bronchoconstriction in asthma. 133Xenon washout scans following parenteral methacholine

To determine the influence of bronchoconstriction on the distribution of ventilation during an asthma attack, pulmonary clearance of 133xenon was evaluated in four normal and eight asthmatic subjects within three to five minutes after intramuscular injection of methacholine. In asthmatics, administration of 4-10 mg methacholine resulted in a decrease of forced vital capacity of 28.5 +/- 5.1 (SE) percent and a decrease in expiratory flow at 60 percent vital capacity of 44.2 +/- 6.9 percent (P less than 0.001). The cumulative ventilation required to reach 50 percent of the pre-washout radioactivity increased from 3.6 +/- 0.8 to 9.9 +/- 1.6 L after administration of the drug (P less than 0.05). The normal subjects showed no ventilatory effects after receiving 10 mg methacholine. Comparison of clearance of 133xenon from ten areas of lung (each representing approximately 6 percent of the surface area of one lung) showed that all areas were affected to approximately the same extent during drug-induced asthma. These findings suggest that parenteral methacholine is an effective way to demonstrate airway hyperreactivity and that the airway response to methacholine in asthmatics is relatively generalized throughout the lung.

The chronicity of acute attacks of asthma--mechanical and therapeutic implications

Defects in ventilatory function can persist for considerable periods of time following the amelicoration of the signs and symptoms of acute episodes of asthma. Serial spirographic and lung volume determinations in such patients demonstrate that the pattern of resolution of these abnormalities is such that their subtlest manifestations are depressed flow rates in the mid vital capacity range and/or elevations in residual volumes. These changes are believed to represent the effects of residual obstruction that is located in the airways in the periphery of the lung. Recent studies suggest that this residua is capable of influencing the lung's response to asthmogenic stimulis, and imply that it may be beneficial to place asthmatics on continuous therapy for as long as they have alterations in lung function.

Exertional dyspnea and cough as preludes to acute attacks of bronchial asthma

Although wheezing is believed to be a cardinal manifestation of asthma, some patients with this disorder may not present with wheezing, but rather with either exertional dyspnea or cough. In 14 such patients with dyspnea, there was peripheral airway dysfunction with markedly elevated residual volumes, frequency dependence of dynamic compliance and depressed flow rates in the middle-vital-capacity range, whereas specific conductance and one-second forced expiratory volumes were normal. Circumstantial evidence suggests that mucosal edema or mucous secretions may have been responsible. In seven patients with cough, studies revealed a more severe obstructive pattern that appeared to be the result of increased large-airway resistance, and the patients' response to isoproterenol indicated that contraction of bronchial smooth muscle may have been principally responsible. Thus, intermittent episodes of cough or breathlessness may represent variant aspects of asthmatic attacks.

Static and dynamic lung volumes and ventilation-perfusion abnormality in adult asthma

Dynamic lung volumes (forced vital capacity, forced expiratory volume in 0·5 second and in 1·0 second), static lung volumes (total lung capacity, functional residual capacity, residual volume), and ventilation-perfusion relationships (alveolar-arterial oxygen tension difference, alveolar dead space ventilation to tidal volume ratio, arterial oxygen and carbon dioxide tension, and the fractional ventilation and perfusion relationship by the three-compartment lung model) were measured in adult asthmatics during the acute, recovery, and stable or asymptomatic phases of an asthmatic attack. Eighteen patients were studied during 20 separate asthmatic attacks.

The patients behaved in one of three ways with regard to total lung capacity (TLC): group I had an elevated TLC during the acute asthmatic attack which returned to normal, group II had a normal TLC throughout the attack, and group III had an elevated TLC that did not return to normal on recovery from the asthmatic attack. With the patients separated into the three groups, the other pulmonary function measurements, especially the measurements of ventilation-perfusion abnormality, were compared. There were no statistically significant differences of ventilation-perfusion abnormality between groups I, II, or III. There was a tendency for perfusion abnormality to be less during the acute phase of the asthmatic attack in patients with an elevated TLC (group I). The three-compartment lung model revealed the major abnormality in all groups to be an increased fraction of unventilated but perfused lung.

Breath sounds

Breathing is silent in health but audible at a distance in chronic bronchitis and asthma. This clinical observation was tested by measurements of the intensity of the inspiratory breath sounds at the mouth at flow rates up to 60 1./min. There was a higher rate of increase in the intensity of the sound for equal increments in flow rate in these two diseases than in healthy subjects. Bronchodilator drugs reduced the intensity of abnormally loud inspiratory sounds.

The noise is generated by turbulent flow of air in the upper respiratory tract, the trachea, and the central bronchi. Reduction of bronchial calibre increases flow velocity and intensifies turbulence. The intensity of the inspiratory sound in chronic bronchitis and asthma reflects calibre changes in the trachea, the principal bronchi, and their lobar and segmental branches.

An inverse correlation exists between the intensity of the inspiratory sound, the forced expiratory volume in one second, and the peak expiratory flow rate in chronic bronchitis and asthma, but not in primary emphysema. Discrepancies between sound, airway resistance, and forced expiratory measurements may indicate the site and mechanism of airway obstruction in individual patients.

The sound of inspiration heard with the unaided ear close to the patient's mouth is an important clinical sign. Noisy inspiration is common in chronic bronchitis and asthma, and the degree of narrowing of the central bronchi can be inferred from the loudness of the noise. Silent inspiration in the presence of severe expiratory obstruction is a sign of primary emphysema.

Frequency dependence of compliance as a test for obstruction in the small airways

We selected five bronchitics and four asthmatics in remission, whose routine lung function tests were not significantly abnormal. Dynamic compliance was measured at different respiratory frequencies and the results compared with those obtained from a normal control group. In all patients compliance was frequency dependent and remained so after the administration of bronchodilator aerosols. Compliance was frequency dependent in only one normal subject, and this was completely reversed by bronchodilators. Because the elastic properties of the patients' lungs were normal, and because pulmonary resistance was normal or only minimally increased, we interpret these results as indicating obstruction in peripheral airways.

Quantitated scintillation scanning for the measurement of lung perfusion

A method using quantitated lung scans is described for the accurate proportioning of lung perfusion. The results obtained by this method have been compared directly with those obtained from bronchospirometry, and a good correlation has been obtained (r = + 0·93).

Regional lung function in patients with bronchial asthma

The regional distribution of pulmonary ventilation and perfusion and regional alveolar ventilation/perfusion ratios were measured with radioactive xenon ((133)xenon) in 10 patients with asthma in remission. Four subjects had normal ventilation distribution, four had hypoventilation in some regions and normal ventilation in others, and two patients had abnormal ventilation in almost all lung regions. The lung bases were involved most frequently and the middle zones least frequently. Correlation was good between the degree of over-all ventilatory impairment calculated from (133)xenon values and measurement of the maximal midexpiratory flow rate the same day. Regions which were hypoventilated had low ventilation/perfusion ratios and also tended to be hypoperfused. In the eight subjects who had been studied similarly 5 yr previously, changes in regional function correlated in general with changes in over-all function.

A physiological study of Macleod's syndrome

Seven patients with Macleod's syndrome of abnormal transradiancy of one lung have been investigated with special reference to regional lung function. The principal abnormality was found to be obstruction of the airways. This was severe in the affected lung but was often present in the other lung as well. Five of the seven patients suffered from chronic bronchitis, as judged by the M.R.C. questionnaire. Although functional impairment of the affected lung was severe, it was by no means uniformly distributed there. There was evidence of defective gas transfer in all the patients who complained of breathlessness on exertion; the blood gases were only slightly abnormal at rest, and hypercapnia, in particular, was not a feature. During exercise arterial oxygen tensions tended to fall. Clinically it was found that the stethoscope gave a rough guide to the extent of regional underventilation and that radiographs were useful for judging the distribution of blood within the lung. We conclude that treatment should be conservative, directed towards avoidance of environments and habits likely to cause or exaggerate airways obstruction or bronchitis.