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Whether the mechanism is respiratory heat loss1 or increased osmolarity due to respiratory water loss,2exercise is a potent stimulus for provoking asthma symptoms in children. For this reason children with asthma may avoid exercise which may in turn be detrimental to their physical and social conditioning. On this background the efficacy of exercise training in children with asthma has generated continued interest over the years. The use of exercise training programmes in the clinical management of children with asthma is at best controversial. While a number of studies have reported an improvement in lung function, aerobic capacity/conditioning, psychosocial behaviour, and a reduced incidence and severity of exercise induced bronchoconstriction,3-11the findings in these studies have been variable and have not addressed the clinical efficacy of such programmes. The quality of the studies has varied and assessing the efficacy of such programmes depends on which outcomes are assessed and how these outcomes are measured.
It is widely recognised that suitable training regimens increase exercise tolerance and capacity in healthy individuals, with greater improvements usually being seen in the more sedentary subjects.12 This is generally due to a greater capacity for improvement in non-trained individuals. Improvements in exercise capacity are usually accompanied by a number of physiological adaptations including increased oxygen uptake (V˙o 2), reduced ventilatory requirements, reduced cardiac frequency, and a reduction in lactic acid production at any given work load.13 Anatomical and metabolic changes such as increased mitochondrial density, increased capillarisation of trained muscles, and changes in muscle fibre type and density generally accompany these physiological changes.13 Along with these physiological changes in response to training, improved psychosocial outcomes such as improved sense of self-worth and well being as well as improved concentration and decreased stress levels have been reported. These changes in response to appropriate modes, duration, frequency, and intensity of exercise training programmes should apply equally well to children with and without asthma, provided they have no other physical limitations. It therefore comes as no surprise when demonstrable physiological and psychosocial improvements are observed in studies of children with asthma following suitable exercise training regimens. The qualitative and quantitative differences between published studies3-11 is likely to be due, at least in part, to variations in subject selection, both in terms of severity of disease (that is, the amount and type of airway inflammation likely to be present and the degree of bronchial responsiveness), medication usage, age, and training status. Differences in the methodology used and interpretation of the results also contribute to the reported differences in outcome.
At rest, compromised respiratory function has been reported in young asthmatic patients including decreased flows, increased residual volume, increased ratio of physiological dead space to tidal volume (Vd/Vt), increased alveolar–arterial oxygen tension difference (A–aPo 2), and mild arterial hypoxaemia and desaturation.14 During exercise these physiological variables return to normal and exercise tolerance is not limited by these factors. Provided that a child’s asthma is well managed and there is no significant degree of fixed airflow obstruction, there are few physiological reasons why they would not tolerate and, indeed, significantly improve their aerobic capacity following an aerobic training regimen. However, the clinical benefit of such a training regimen to the patient may not be comparable to the physical improvements. There is no consistent evidence that exercise training decreases the incidence of exercise induced bronchoconstriction or improves peak expiratory flows (PEF). It is likely that in patients with airway pathology there is a threshold ventilatory rate (which may be modulated by inspired air conditions) at which exercise induced bronchoconstriction is triggered.15When patients are challenged at or above this ventilatory threshold, exercise induced bronchoconstriction is likely to occur regardless of the fitness level of the subject. This would mean that exercise challenges performed after a period of aerobic training need to be conducted at the same relative loads—that is, the same ventilatory equivalent—rather than at the same absolute load, and with the same inspired air conditions. This would allow valid comparisons of the incidence of exercise induced bronchoconstriction after a period of exercise training. However, if exercise training allows a patient to exercise more before reaching the threshold for triggering exercise induced bronchoconstriction, that patient may report an increased exercise capacity without experiencing exercise induced bronchoconstriction.
Two publications in this issue of Thoraxdescribe similar improvements in aerobic exercise capacity after swimming and cycle ergometry training in children with asthma. The study by Matsumoto and colleagues16 showed that, when asthmatic children were assessed at the same relative work loads before and after an aerobic training programme, the fall in FEV1following exercise was reduced. This occurred despite finding no change in the PC20 to histamine, which suggests minimal change in airway structure/inflammation. The authors did not report medication usage during the training phase and this may also have affected the results. Interestingly, the lack of change in airway responsiveness to histamine suggests that airway structure may be a more important determinant of in vivo airway responsiveness to exercise than inflammatory stimuli.
The study by Neder and colleagues17 reported a short term decrease in the daily use of inhaled and oral steroids in a group of children with severe asthma following a two month cycle ergometry training programme, with no change in the number of positive exercise challenges. The authors attributed the decrease in medication usage to improved psychosocial factors related to exercise training. The interpretation of these data requires a cautious approach. The authors provide no evidence of longer term treatment requirements. Also, it is possible that the reported decrease in medication use was a “trial” effect or that the subjects did not require the dose of inhaled steroids initially prescribed.
Because exercise induced bronchoconstriction may itself limit the maximum aerobic work capacity, adequate asthma control is required during any aerobic training regimens. This may involve an increase in preventative medication usage and/or pre-exercise use of a bronchodilating agent. The observation that the severity of exercise induced bronchoconstriction is reduced but airway responsiveness to inhaled bronchoconstricting stimuli remains the same following aerobic exercise training16 raises an interesting possibility that aerobic conditioning may reduce the airway response to specific stimuli. This hypothesis has not been examined and warrants further investigation. From the point of view that exercise training in children with asthma demonstrates a beneficial effect on aerobic conditioning and psychosocial behaviour, it warrants consideration from a general health perspective. If aerobic conditioning reduces the likelihood of provoking an asthma attack due to decreased ventilatory requirements for any given task, then increased participation in physical activity by children with asthma is desirable. In terms of clinical management of children with asthma, the evidence is still not strong enough to support modifications in conventional therapeutic treatment even in well trained children with asthma. We encourage researchers in this field to conduct well designed studies to address the important issue of whether exercise training programmes can improve asthma control and decrease medication requirements in asthmatic children.