The predictive value of exhaled nitric oxide measurements in assessing changes in asthma control

How should we quantify asthma control? A proposal

BACKGROUND

Current asthma guidelines suggest a series of criteria to assess if asthma is controlled. However, there is a need to develop a simple and practical method to quantify the degree of such control, both in clinical practice and research.

STUDY OBJECTIVES

This report describes a new method to quantify asthma control based on a percentage score. It also aims at comparing the percentage scores obtained with patient's self-evaluation of asthma control and a current validated Mini Asthma Quality of Life (MAQOL) questionnaire.

SETTING AND SUBJECTS

Forty-two subjects (25 female and 17 male patients) with asthma of different severity recruited from a tertiary center asthma clinic.

METHODS

The asthma scoring method provided a percentage control for symptoms, baseline expiratory flows and, an optional parameter, for airway inflammation assessed from induced-sputum eosinophil count. These control parameters were compared to an overall assessment of asthma control by the patient (also on a 100% scale) and the score obtained from a validated MAQOL questionnaire.

RESULTS

Mean +/- SEM scores for symptoms, expiratory flows, and airway eosinophilia (last 2 weeks) were 87.8 +/- 1.4%, 88.6 +/- 1.8%, and 66.2 +/- 3.9%, respectively. No significant correlation was found between these three parameters (p > 0.05). The mean global asthma control score and the score estimated by the patient were 80.9 +/- 1.5% and 91.7 +/- 1.5%, respectively (not significantly different). There was a significant correlation between asthma control score (percentage) and percentage symptom score (p < 0.001), while it almost achieved significance for FEV(1) (p = 0.05). Only symptom scores correlated with the MAQOL questionnaire.

CONCLUSIONS

We developed a simple easy-to-use asthma control scoring system based on a percentage of optimal control. The percentage symptom score but not the global control score of this new method correlated with patient's global assessment of asthma control. This could be a simple tool that is potentially useful both for the clinician and for research purposes, to quantify global or specific aspects of asthma control.

Mediators of asthma: nitric oxide

Endogenous nitric oxide is an ubiquitous gaseous molecule that regulates many aspects of human airway biology including the modulation of airway and vascular smooth muscle tone. It is generated from the three different enzymes nitric oxide synthases (NOS) -1, -2 and -3 which are all expressed in pulmonary cells. NOS-1 is localised primarily to neuronal structures, where NO is a mediator of the inhibitory Non-Adrenergic Non-Cholinergic System and NOS-3 is present in endothelial cells. While these enzymes are constitutively expressed, NOS-2 is an inducible enzyme independent of calcium and highly induced in inflammatory diseases such as allergic asthma, where NO may act beneficial or deleterious depending on the site of and amount of generation. The use of NO-donor compounds or classical unselective NOS inhibitors did not lead to significant therapeutical effects in asthmatic patients. Insights on the precise role of NO in asthma can only be achieved by targeting NO generation selectively. More potent and selective NOS-2 inhibitors have to clarify a role of NOS-modification based therapy in clinical routine. NO can also be detected in the exhaled air. Increased levels of exhaled NO in asthmatic patients may be useful for a non-invasive determination of airway inflammation.

Exhaled nitric oxide as a diagnostic test for asthma: online versus offline techniques and effect of flow rate

Measurement of the fraction of exhaled nitric oxide (FENO) has been proposed as a noninvasive assessment of asthmatic airway inflammation. The influence of the expiratory flow rate during the collection maneuver on the ability of FENO to discriminate healthy subjects from those with asthma is unknown. We compared online and offline measurement of FENO at different flow rates. FENO was collected with expiratory flows of 50-500 ml/second in 34 patients with asthma (PC(20) of less than 8 mg/ml) and 28 healthy subjects (PC(20) of more than 10 mg/ml) using offline collection techniques. In a subgroup of 18 individuals with asthma and 17 healthy subjects, we additionally measured FENO at multiple expiratory flow rates (47-250 ml/second) using online methods. FENO fell with an increasing expiratory flow rate; FENO was higher in subjects with asthma as compared with healthy subjects at each flow rate studied with both techniques (p < 0.001). Receiver operating characteristic (ROC) curves for the diagnosis of asthma indicated that FENO is a robust discriminator between individuals with asthma and healthy subjects (area under the ROC curves 0.79 +/- 0.06 to 0.86 +/- 0.06, p for significant discrimination < 0.0001). Neither expiratory flow rate nor collection technique (online versus offline) significantly altered this discriminatory capacity (area under the ROC curves = 0.84 +/- 0.07 with the slowest online method versus 0.80 +/- 0.07 with the fastest offline method, p = 0.46). These data indicate that the choice of expiratory flow rate and collection method can be based on practicality and patient comfort without compromising the utility of this test for asthma.

Acute asthma: under attack

The burden of asthma (death, disability, and an increasing prevalence) makes it a major public health problem worldwide. In an effort to decrease this burden, investigators are studying many aspects of this disease. The role of race, ethnicity, infections, and pollutants as triggers, as well as the risk factors are now being defined. Research into methods to decrease acute exacerbations and improve emergency and in-hospital management, using standardized protocols and incentives for follow-up care, has yielded valuable information but has met with limited success. Adherence to the national guidelines has been poor and to some extent can be attributed to the lack of a practical method of measuring the degree of lung inflammation and cumbersome treatment protocols. Exhaled nitric oxide is a noninvasive marker of inflammation and may provide a rational method to titrate corticosteroid and leukotriene receptor antagonist therapy. The best route and dosing regimen for corticosteroid administration (oral vs intramuscular vs nebulized) are the subject of several studies, with no clear-cut winner. The burden of asthma in developing countries with limited financial resources has also triggered a search for simpler, cheaper, and practical methods for beta-agonist delivery using indigenous spacers. Recent research in asthma has unveiled our incomplete knowledge of the disease but has also provided a sense of where efforts should be expended. Research into the genetics and pharmacogenetics of asthma and into the societal factors limiting the delivery of optimal care is likely to yield useful and practical information.

Outpatient monitoring of asthma

Monitoring asthma outcomes is an essential step to the successful implementation of national asthma guidelines. Symptoms, airflow obstruction and exacerbations can be monitored by patients with asthma and by physicians. Patients who practise self-monitoring in conjunction with use of a written action plan and regular medical review have significantly fewer hospitalizations, emergency room visits and lost time from work. Additional monitoring tools are under evaluation, and these include measures of airway responsiveness, airway inflammation, and Internet-based monitoring systems.