Detection of Airway Obstruction in Exercise-Induced Asthma Observations

In most studies, the workload is reported as treadmill speed, the angle of inclination (or percentage of grade), and length of the exercise period. It has been shown that the external work performed on a treadmill also depends on body weight. Thus, to obtain more uniform and comparable data in the period after exercise, the workload expressed in watts was standardized per kilogram of body weight. Mild to moderate workloads requiring only a fast walk was selected (1.26 and 1.76 watts/kg of body weight). The workloads were calculated to be far below Wahlund’s physical working capacity, yet still produced significant changes in the airways. Since the patients were receiving daily therapy and due to the time lag between W1 and W2, some of the data may have been affected. The dose of aminophylline per kilogram of body weight did not significantly change during this time in any patient.

Although an attempt was made, it was impossible for most patients to determine CV, especially in the period after exercise. In both workloads, we obtained complete data in only five patients. In these five patients, the CV was clearly the most sensitive parameter of airway obstruction. The expected sudden change in the slope of the “alveolar” plateau (phase 3) of the nitrogen washout curve which signals airway closure, did not appear despite the fact that the test in each instance was repeated three to five times. In addition, no significant differences were seen in the actual slope of phase 3 in any subjects. Some of the factors responsible for the abnormal nitrogen washout tracings in the other 19 patients, which produced no sudden change in the “alveolar” plateau could be as follows: nonuniform distribution of ventilation, abnormal respiratory flow patterns, local changes in lung compliance which can be accentuated even more during exercise, and the inability of the patients (especially after exercise ) to hold their breaths at vital capacity. All the limitations of CV determination (nitrogen bolus technique) are not yet known, but it certainly appears CV is a technique best suited for detection of minimal and early airway obstruction rather than severe or established airway obstruction.

In healthy children neither the total lung capacity (TLC) nor Vtg significantly changed after exercise, but they may decrease slightly as a result of an increase in intrathoracic blood volume. It was reported that both Raw and lung volumes (TLC and Vtg) were increased after EIA. Our data failed to show any significant difference in values between Vtg measured at baseline and after exercise. This difference with previously published results could be partially explained because in this study severe asthmatic patients were tested, and a marked degree of hyperinflation was present in severely asthmatic patients in the period before exercise. In support of this concept, the Vtg values in all patients tested were significantly increased at baseline (mean = 2.5 liters), which is approximately 1 liter above predicted values. Thus, limiting factors for any further increase of V,g in the period after exercise certainly may have been the mechanical properties of the chest and lungs.

The majority of the patients had less than predicted baseline values for Raw. This may be a reflection of their around-the-clock bronchodilator therapy. A statistically significant increase in absolute and relative values of Raw was observed seven minutes after exercise in both workloads. Valid data for Raw was obtained as early as seven minutes after exercise since no patients were still tachypneic at this time and thus properly performed the panting in the body box. Both Raw and SGaw appear to be very sensitive parameters for detection of airway obstruction. It should also be noted that the baseline values of Raw were significantly decreased from predicted values, and although the values measured after exercise were significantly increased, only the level of the predicted values was reached. This too may be a reflection of around-the-clock bronchodilator therapy.

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MMEF is a reproducible and sensitive parameter excluding the effort-dependent initial and terminal portion of the FVC curve. Lloyd and Wright demonstrated that MMEF was more sensitive than FEV1 percent. Similarly, Weng and Levison have shown that during asymptomatic periods in asthmatic patients, the only detectable parameter that was significantly abnormal was the MMEF. In all of our patients at baseline (before exercise) MMEF was significantly below predicted data. Any decreased elasticity and decreased strength of the bronchial wall can effectively increase the collapsibility of the peripheral airways; this might affect the readings of MMEF, but not the readings of Raw and SGaw. Assuming that SGaw changes best reflect large (central ) airway obstruction and MMEF small (peripheral) airway changes, the baseline results also suggest that the obstructive process in our patients was more likely localized in the small airways and not in the large airways.

Many authors suggest FEV1 or FVC as a highly sensitive index for airway obstruction. To utilize data reported in FEV1 percent can be erroneous and misleading since in some cases an increase to a more normal FEV1 percent after exercise could result from a simultaneous but greater decrease in the FVC than in the FEV1. This may be a reflection that the FVC is even more effort-dependent than the FEV1. However, FEV1, besides not being as sensitive as the other parameters, is also quite effort dependent.

Since Wright and McKerrow described the technique of PEFR, it has been utilized by numerous authors in evaluating not only the phenomenon of EIA but also the efficacy of drugs to block EIA. PEFR is a test that is very effort dependent. Since it was introduced late in our study, we have adequate data from only 17 patients. As seen in Figure 3, there appears to be a minimal visible change in the PEFR in the periods after exercise, especially in Wl. In W2 only 7 of 17 patients showed a decrease greater than 20 percent, but 11 of 17 showed a decrease greater than 15 percent. In only one patient was a 20-percent decrease seen in PEFR without corresponding changes in all other parameters. A moderately obstructed asthmatic patient, especially if he is familiar with the technique for determination of PEFR, often achieves a reasonably high PEFR since he performs the test in a most mechanically advantageous respiratory position due to his hyperinflation. In the assessment of early changes in airway flow in our patients, the relative insensitivity of PEFR may also be inherent to the peak flow meter itself. The peak flow meter is less reliable when the maximal expiratory flow-volume curve shows a steeper slope than normal. The asthmatic patients show a maximum expiratory flow-volume curve with a very steep rise and a conspicuous decline. This sharp peak is displaced toward the TLC, thereby resulting in a higher peak flow value than expected (Fig 8,9).

Auscultation based on our data was an insensitive method of determining even mild airway obstruction in the period after exercise and correlated poorly with other more objective measurements of airway obstruction. This observation is consistent with those of Leiner s.

Correlation coefficients were calculated between absolute values of individual pulmonary functions observed at baseline and seven minutes after exercise (Tables 3, 4). Significant relationships exist between the different pulmonary function tests. However, generally, in our study, lower correlation coefficients were obtained between the individual pulmonary function tests as compared to Weng and Levison. This is especially indicated in correlation coefficients between PEFR and the parameters FVC, R,w, FEVi/FVC, SGaw, and CV. Similarly, no significant correlations were found between CV and other pulmonary parameters. The physiologic meaning of this is yet unclear to us. In W2 the correlation coefficients markedly increased between most of the pulmonary functions (Table 4). The higher correlation coefficients in W2 may reflect that a greater workload is a more effective stimulus for airway obstruction in asthmatic patients.

In conclusion, a technique is suggested for determination of workload in exercise testing, which results in a more equivalent workload among patients. The phenomenon of EIA is workload-dependent and not protected by adequate aminophylline therapy if the interval since the last dose is greater than three hours. Results illustrate that reports concerning EIA must be evaluated in terms of which pulmonary function parameter is measured. SGaw and MMEF appeared much more sensitive in detection of airway obstruction than others (CV, FVC, FEVi, FEVi%), whereas PEFR and auscultation might even provide erroneous conclusions.

Figure 8. Relative sensitivity of different pulmonary function tests as measured in Wl at 7 minutes and 30 minutes after exercise expressed as a percentage of baseline values.

Figure 9. Relative sensitivity of different pulmonary function tests as measured in W2 at 7 minutes and 30 minutes after exercise expressed as a percentage of baseline values.

Table 3—Correlation Coefficient*: Workload 1 V» Workload 2 at Bateline

	FVC	FEV,	MMEF	PEFR	R..	FEV, FVC	SG,W	CV
FVC		0.718	0.608	0.295	—0.510		0.112	—0.234
FEV,	0.871		0.759	0.715	—0.776		0.303	0.115
MMEF	0.527	0.828	—	0.450	—0.634	0.403	0.368	—0.009	Po
PEFR	0.507	0.669	0.653	—	—0.863	0.629	0.717	0.399	£.
	—0.546	—0.520	—0.372	0.269	–	—0.420	—0.703	—0.532	1o
FEVi/FVC		—	0.668	0.273	0.003		0.340	0.292	5
SG.W	0.479	0.609	0.544	0.009	—0.769	0.267	—	0.148
CV	0.199	0.222	0.172	—0.237	—0.226	0.076	—0.153

Table 4—Correlation Coefficients: Workload 1 Vs Workload 2 Seven Minutes After Exercise

	FVC	FEV,	MMEF	PEFR	R..	FEV,FVC	SG..	CV
FVC	—	0.956	0.663	0.786	—0.621		0.261	—0.283	35 О
FEVi	0.889	—	0.703	0.892	—0.740		0.417	0.061	Po’
MMEF	0.523	0.816		0.786	—0.519	0.775	0.597	0.487	£
PEFR	0.857	0.768	0.315	–	—0.780	0.817	0.716	0.340	i,
R»w	—0.652	—0.623	0.472	—0.292		—0.842	0.639	—0.470	8.
FEV,/FVC	—	—	0.857	0.377	0.059	—	—0.613	0.559	a •n
SG	0.348	0.381	0.581	—0.018	0.730	0.382	–	0.676	8•n
CV	—0.552	—0.537	—0.298	—0.365	—0.392	—0.297	—0.251		Is
			Workload 1—7 min after exercise