INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

A consistent feature of tissue inflammation is hyperemia and hyperperfusion. Therefore, asthma which is considered an inflammatory airway disease is likely to be associated with an increased airway blood flow. Several animal models of airway inflammation including allergen challenge have been shown to have an increase in airway blood flow (1, 2). In patients with asthma, measurements of airway blood flow have not been carried out because noninvasive methods have not been available. The expected increase in airway blood flow has therefore not been confirmed.

Most of the blood perfusion in the conducting airways is through the bronchial artery (3), and the majority of blood flow is distributed to the subepithelial tissue (4). Because asthma-associated airway inflammation is generally believed to involve mainly the subepithelial airway tissue, the measurement of subepithelial blood flow is of particular importance.

The purpose of this study was to (1) compare airway mucosal blood flow (_aw) between asthmatics and normal subjects using a recently developed noninvasive technique (6, 7), (2) determine if the magnitude of _aw correlates with the degree of airflow obstruction, and (3) assess _aw responses to an inhaled beta-adrenergic agonist.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Test Population

Thirty-four current nonsmokers participated in the study (Table 1). They denied having cardiovascular disease and none of them had taken cardiovascular medications. They also denied having had a respiratory infection for at least 6 wk before the study. Eleven subjects were normal and 23 had stable asthma. Among the latter, two had a history of hay fever and 10 had a history of skin test positivity but were not receiving immunotherapy. Ten asthmatics were using inhaled beta-adrenergic agonists on-demand and 13 were using inhaled glucocorticosteroids (GS) regularly along with beta-adrenergic agonists on-demand. In the latter patients, the daily dose of inhaled GS ranged between 84 and 336 µg for beclomethasone, 800 and 1,600 µg for triamcinolone, 500 and 2,000 µg for flunisolide, and was 220 µg for fluticasone. None of the subjects were on oral bronchodilators, oral GS, or other anti-inflammatory agents.

Pulmonary function was measured with an Essential Medics Unit (Model 6200 Autobox DL; Yorba Linda, CA). The highest forced expired volume in one second (FEV₁) of three forced vital capacity maneuvers was determined and expressed as percent of predicted (8). Airway resistance and thoracic gas volume were determined plethysmographically for the calculation of specific airway conductance (SG_aw).

Airway Mucosal Blood Flow

A soluble inert gas uptake method was used to measure _aw (7). The subjects were seated in front of a valve system that allowed them to inhale through a mouthpiece (with the nasal passage occluded by a noseclip) room air or a gas mixture from a Teflon bag containing 10% dimethyl ether (DME), 5% helium, balance oxygen, and to exhale into a rolling seal spirometer (Model 842; Ohio Instruments, Houston, TX). The subjects inhaled room air to and then exhaled 500 ml from the TLC position, and then rapidly inhaled the same volume of the gas mixture from the Teflon bag. They held their breath for a predetermined duration and then exhaled into the spirometer through a critical flow orifice to standardize expiratory flow. The maneuver was performed with two breathhold times each of 5, 10, 15, and 20 s in random order. During exhalation, the instantaneous concentrations of DME, nitrogen, and helium were measured at the airway opening with a mass spectrometer (Perkin-Elmer, Pomona, CA) along with the expired gas volume. The mass spectrometer inlet was not heated, and no corrections were made for water pressure. The resulting overestimation of DME concentration by measuring it at the airway opening was considered to be negligible (approximately 0.3%). The mass spectrometer was also used to verify the gas concentrations in the Teflon bag before inhalation of the gas mixture. Anatomic dead space was determined from the expired nitrogen concentration curve as described by Fowler and coworkers (9). The helium-corrected decrease in the DME concentration over time was obtained by least-squares fit, using the two measurements per gas for each of the four breathhold times. This was done in the expired volume fraction corresponding to the anatomic dead space minus the most proximal 50 ml (DS). From the helium-corrected DME slope multiplied by the DS (_DME), the mean DME concentration in the DS (F), and the solubility coefficient for DME in blood and tissue (), _aw was calculated using Fick's principle (_aw = _DME/ · F). _aw was normalized for DS and expressed as µl · min¹ · ml¹.

Protocol

The subjects were asked to come to the research laboratory in the morning of the study day without having had any coffee or other caffeinated drinks. They were asked to abstain from ingesting alcoholic beverages the night before. The asthmatics were asked not to use their inhaled medications for 12 h prior to the study. On arrival, subjects performed spirometry and proceeded with the measurement of SG_aw, immediately followed by the measurement of _aw. The subjects then inhaled two puffs of albuterol (180 µg) from a metered dose inhaler, and 10 min later the above measurements were repeated in the same order.

Statistical Analysis

The unpaired and paired variates of Student's t test were used to make statistical comparisons between groups and to assess the effect of albuterol within groups. The Spearman rank order correlation test was used to determine the significance of possible relations between _aw and other parameters. A value of p < 0.05 was considered significant.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Baseline mean SG_aw was significantly lower in GS-naive and GS-treated asthmatics than in normal subjects, whereas baseline mean _aw was significantly higher in GS-naive asthmatics than normal subjects (Table 2). Baseline mean _aw in GS-treated asthmatics fell between and was not significantly different from the corresponding values in normals and GS-naive asthmatics. Albuterol increased mean _aw significantly while having no effect on mean SG_aw in normal subjects (Table 2, Figure 1). In contrast, albuterol had no significant effect on mean _aw in the two asthmatic groups while significantly increasing mean SG_aw. For all subjects, there was an inverse relationship between the albuterol-induced change in _aw and baseline _aw values (Figure 2). Baseline mean DS ranged between 308 and 347 ml in the three groups (p = NS), and albuterol had no significant effect on it. There was no correlation between _aw and FEV₁ or baseline SG_aw in the asthmatics. Likewise, there was no correlation between _aw and age or gender in any of the three groups.

View larger version (17K): [in this window] [in a new window]   Figure 1.   Effect of inhaled albuterol on aw and SGaw in normals (n = 11) and asthmatic patients using (n = 13) or not using (n = 10) long-term inhaled GS. *p < 0.05 versus corresponding parameter in normals.

View larger version (16K): [in this window] [in a new window]   Figure 2.   Relationship between the albuterol-induced change in aw and baseline aw in all subjects (four data points are overlapping).

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

We conclude from these results that stable bronchial asthma is associated with an increased _aw in comparison to normal control subjects, that despite a higher degree of airflow obstruction asthmatics who use inhaled GS seem to have a lower _aw than GS-naive asthmatics, and that inhaled albuterol increases _aw in normal subjects but not in asthmatics. A correlation between _aw and the degree of airflow obstruction was not seen in the asthmatics.

Airway blood flow comprises 0.5 to 1% of cardiac output in mammals including humans (10). Although this seems to be a trivial flow in absolute terms, it assumes a significant value when normalized for the relatively small airway tissue volume. The majority of airway blood flow is distributed to the subepithelial tissue. For example, the microvasculature has been found to occupy 10 to 15% of the subepithelial tissue as assessed by morphometry and to accommodate approximately 85% of airway blood flow in the trachea and bronchi of sheep (4). The depth of the subepithelial tissue is difficult to define. In the sheep trachea, the DME method seems to assess blood flow in the mucosa that can be mechanically stripped from the trachea as demonstrated with the microsphere technique (6). The same may apply in human airways, but this has not been verified. Asthma-associated airway inflammation is believed to involve mainly the subepithelial tissue, and the measurement of subepithelial (mucosal) blood flow is therefore of particular importance in this condition. _aw is a reflection of mucosal blood flow.

Previous studies in allergic sheep have shown that inhalation challenge with antigen causes transient increases in both total bronchial blood flow and _aw, and that the increases are related to inflammatory mediators (1, 11, 12). The demonstration of an increased _aw in stable asthmatics in the present study is in keeping with the results of the animal experiments.

It cannot be determined whether the observed increase in _aw in our patients with mild to moderate asthma was related to new vessel formation, vasodilation or both. In patients with severe asthma, postmortem examination of the bronchi has revealed an increased number of vessels in the airway wall, consistent with angiogenesis (13, 14). The increase in vascularity is considerably less in mild to moderate asthma (15). The failure of inhaled albuterol to increase _aw significantly in asthmatics would favor inflammatory vasodilation over angiogenesis in these patients. An anatomic increase in the subepithelial microvasculature would not be expected to be resistant to beta-adrenergic vasodilation while a microvasculature dilated by inflammatory mediators might be less responsive to incremental vasodilation. As further support for vasodilation as opposed to increased vascularity in asthma, we also found a negative correlation between the albuterol-induced increase in _aw and baseline _aw values. Thus, there might be a maximum increase in _aw which is reached in some asthmatics and prevents a further increase with inhaled beta-agonists. Another explanation for the resistance to beta-adrenergic vasodilation in the asthmatics might be a downregulation of beta-adrenergic receptor function (16). The observation that GS-treated asthmatics exhibited a tendency toward albuterol-induced vasodilation while GS-naive asthmatics did not, would be consistent with but certainly not a proof of this possibility. Glucocorticosteroids have been reported to restore beta-adrenergic responsiveness under certain conditions (16).

It is tempting to attribute the lower mean _aw in GS-treated than in GS-naive asthmatics to the pharmacologic effects of GS. This assumption is supported by the fact that despite having more severe asthma as reflected by FEV₁ and SG_aw, GS-treated asthmatics had a lower mean _aw than GS-naive asthmatics. However, a prospective study would be required to demonstrate that GS decrease _aw in asthmatics.

Airway caliber determines the relationship between mucosal surface and air volume in the bronchial tree. In a previous study of normal subjects in which we determined _aw in a 50-ml segment of the anatomic DS, we found an inverse relationship between _aw and lung volume (17). At TLC, _aw per ml volume segment was similar to the values reported for normal subjects in the current study in which _aw was determined in the entire DS at TLC.

A given volume segment extends deeper into a narrowed bronchial tree and therefore "sees" a greater mucosal surface. This would lead to an overestimation of mucosal blood flow. By measuring _aw in the DS rather than a predetermined segment within it, the possibility that airway caliber dependent redistribution of DS to mucosal surface ratio influenced the measurements was minimized. Another indication that airway caliber had minor effects on _aw when measured in the entire DS was the observation that albuterol failed to change _aw in GS-naive asthmatics despite a significant increase in SG_aw. The DS values in our study were unexpectedly high. In contrast to the technique described by Fowler (9), our subjects inhaled about 85% oxygen and from TLC minus 500 ml rather than from functional residual capacity. Possibly, this methodological difference was responsible for the high DS values in our study.

We have previously validated the DME uptake technique for the measurement of _aw in sheep using color-coded microspheres (12). In that study, a strong correlation was found between tracheal _aw as determined by the DME uptake method and _aw measured with microspheres in the mucosa removed from the trachea. This indicates that the steady-state uptake of DME from the airway lumen is governed predominantly by mucosal blood flow and that DME does not diffuse significantly deeper into the airway wall.

Theoretically, the increase in _aw in patients with asthma has several physiologic and clinical consequences. First, the increased _aw could attenuate the airway cooling during exercise. Second, the increased _aw could accelerate the clearance of locally released spasmogens. Third, the increased _aw could promote edema formation through the hyperpermeable microvasculature in the airway wall. Fourth, an increase in _aw would favor the distribution of systemically administered antiasthma medications to the airway while at the same time enhancing the clearance of inhaled antiasthma medications from the airway. Thus, the increased _aw in asthma could have both detrimental and beneficial effects on the pathogenesis and treatment of the disease. It is therefore difficult to determine whether vasoconstrictors or vasodilators should be explored as potential therapeutic agents in asthma. Furthermore, the responsiveness to vasoactive agents may be altered in asthmatics as shown in the present study. Inhaled albuterol at the clinically recommended dosage was an ineffective vasodilator in asthmatics while having a potent vasodilator effect in the airway of normal subjects.