Detection of Airway Obstruction in Exercise-Induced Asthma

Increased airway obstruction following exercise has frequently been observed in asthmatic patients. Different parameters such as forced expiratory volume in one second (FEV1), peak expiratory flow rate (PEFR), airway resistance (Raw), and specific airway conductance (SG.w)e have been used as indices of airway obstruction. Some authors have commented on the value of the stethoscope or the patients’ dyspnea in determining airway obstruction. The failure to establish a more accurate definition of exercise-induced asthma (EIA) can be partially explained since most investigators have directed their efforts to the mechanisms involved. The definition of EIA may vary depending on which pulmonary function parameter is observed. Therefore, it seemed appropriate to examine the relationships between several pulmonary function tests and clinical symptom scores in asthmatic patients before and after exercise. To establish which parameters most sensitively reflect airway obstruction, it is critical to use a comparable exercise load in each patient. Using these criteria, the present study compares the relationships between different parameters measuring airway obstruction in asthmatic children after exercise.

Patients

Asthmatic Boy

Twenty-four bronchodilator-dependent asthmatic boys, ranging in age from 8 to 14 years, were tested. Full clinical histories were obtained from both parents and patients, especially with regard to history of EIA. All boys played football, basketball, or baseball recreationally, and all claimed some degree of difficulty (wheezing) during or after these activities. Anthropometric characteristics of these subjects are shown in Table 1. These patients had severe asthma and required around-the-clock aminophylline therapy, but they were not given any bronchodilator therapy for at least three hours prior to testing. Fourteen of the patients were on altemate-day prednisone therapy. Any evidence of wheezing prior to testing disqualified the patient. Every patient was familiar with all techniques used.

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Clinical Scoring

One of us (JMB) has observed clinically that when airway obstruction developed in asthmatic patients, certain objectively defined auscultatory and physical findings (retracting) were noted. Utilizing this experience, the clinical scoring system was established. The clinical score ranges from zero to four, with the highest number denoting the peak of clinical severity, as seen in Table 2. Based on such an evaluation, the number (clinical score) attributed to the auscultatory and physical findings of a given patient thus reflects a certain degree of airway obstruction.

Experimental Protocol

Experimental protocol is shown in Figure 1. Baseline data were established first. The exercise tests followed immediately thereafter and were of ten minutes’ duration, using the treadmill (Quinton) at a 10 percent grade, with the speed varied according to workload desired. Expected workload was calculated just before exercise, using the patients’ body weights, predicted distances run, and percentage of inclination. In such a way all patients, regardless of age or body weight, performed comparable workloads. The first workload (Wl) was equivalent to 1.26 watts Ag of body weight, and the second workload (W2) corresponded to 1.75 watts Ag of body weight. Pulmonary function tests (see below) were again determined at 7 and 30 minutes after exercise. At seven minutes after exercise the majority of subjects were no longer tachypneic and were capable of performing all pulmonary function tests. By 30 minutes after exercise each patient’s pulmonary function parameters had usually returned to the normal range. All pulmonary function tests were accomplished within three minutes, and the order was randomized so as not to bias the results. Figure 1 shows the intervals at which heart rates or electrocardiograms were monitored.

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Spirometry Test

 

Conventional spirometry was used to determine forced vital capacity (FVC), FEV1, and maximal midexpiratory flow (MMEF). Peak expiratory flow rate (PEFR) was ascertained by means of the Wright peak flow meter, and the highest reading of three measurements was recorded. Thoracic gas volume (Vtg) and airway resistance (Raw) were determined using the Collins constant volume body plethysmograph. The method of DuBois et al was applied. Raw was measured during inspiration between flows of 0 to 0.5 liters/sec at a frequency of two pants per second. The measurements in the body box were obtained three times in succession, and the average of the two readings nearest to each other was reported. Specific conductance of the airways (SGaw) was calculated as SGaw = 1/Raw/Vaw. Determination of closing volume (CV) was obtained using the nitrogen bolus technique as previously described.

exercise testFigure 1. Experimental protocol of exercise test and all parameters measured prior, during, and after exercise. (For explanation, see text).

Table 1—Anthropometric Data

N=24 Age, Yrs Body Weight, Kg Ht, Cm BSA, M* Workload 1, Watte/Kg BW Workload 2, Watts/Kg BW
Mean 11.1 34.8 141.7 1.17 1.26 1.75
Standard Deviation 2.4 8.7 10.8 0.10 0.08 0.08
Standard Error 0.4 1.7 2.2 0.03 0.01 0.02

Table 2—Asthma Scoring Syttem

asthma-scoring-syttem