DISCUSSION
The present study further
explored physiological responses of aerobic fitness in children and
adolescents with CRD. The main findings have shown that CF patients
presented lower ventilatory efficiency, lower BR and reduced lung
function. In spite of that, a good exercise capacity was achieved,
meaning no difference in VO2peak when compared with
healthy controls. On the other hand, the asthma group was not able to
reach a cardiorespiratory capacity comparable to the healthy group.
These data may contribute to a better understanding of different factors
influencing aerobic fitness, helping to develop more efficient
strategies for monitoring and treatment of patients with CRD.
The effects of asthma on the exercise capacity of children and
adolescents are still controversial. Patients in the asthma group
reported BR and ventilatory efficiency at peak exercise comparable to
the healthy controls but failed to achieve good exercise capacity. The
results presented here agree with previous studies showing a decrease in
VO2peak 8–10, although there is also
evidence reporting no differences 6,7. For children
and adolescents with CF, we have described maintenance of exercise
capacity, contrary to evidence reporting reduced levels compared to
healthy controls 11. Interestingly, this effect was
seen even though CF patients presented lower BR and poor ventilatory
efficiency both at VT1 and peak exercise. Although the
reasons for these differences are not fully comprehended, we hypothesize
that physical conditioning may play a role to explain the maintenance of
VO2peak in patients with CF. A recent systematic review
and meta-analysis concluded that CF children and adolescents have
similar moderate-to-vigorous physical activity and sedentary time as
healthy controls 29. European Cystic Fibrosis Society
states that physical activity and exercise must be integral to the
overall physiotherapy management suggested for every individual with CF,
irrespective of age and disease severity 30. As an
active lifestyle is considered part of standard care, CF patients
participate in a wide range of physical activities and sports. One study
demonstrates that 22,7% of school children with CF reported
participating in three or more (un)structured physical activities or
sports compared to 4,4% of healthy children 31. On
the other hand, although physical activity and exercise should be
encouraged in children and adolescents with asthma 32,
a lower active lifestyle compared to their peers has been reported33. The decrease in physical activity reduces the
stimuli to improve muscular and cardiorespiratory fitness, producing a
progressive and sustained deconditioning 13. A recent
study reported that physical deconditioning is the only significant
determinant of reduced exercise capacity in asthma, irrespective of
asthma diagnosis, BMI, ventilatory limitation or presence of EIB in
children and adolescents with controlled mild-to-moderate asthma34. Taken together, we believe that the most likely
hypothesis for the reduced exercise capacity in asthmatics compared to
patients with CF is physical deconditioning.
The influence of lung
function on exercise capacity in children and adolescents with asthma
and CF is also still a matter of debate. Although FEV1is an important clinical parameter, according to our results, it does
not influence the VO2peak achieved, at least for
patients with mild-to-moderate impairments. In asthmatic children and
adolescents, previous studies reported no significant correlations
between FEV1 and exercise capacity 13,
while others found a positive correlation 14. For
children and adolescents with CF, some studies found a positive
correlation between FEV1 and exercise capacity, while
others reported that VO2peak could be preserved until
FEV1 falls below the predicted 60%35.
In our study, comparisons between asthmatic and healthy controls
revealed no differences in BR, which seems to be in accordance with
previous evidence 6,7. Santuz et al. reported that BR
was comparable among asthmatic and healthy individuals6, as well as Moraes et al. described no significant
differences between children and adolescents with both mild-to-moderate
and mild-persistent asthma as compared to healthy peers7. On the other hand, our results have shown the
patients with CF presented lower BR than the healthy and asthmatic
groups. The reduced BR found for the CF group indicates that these
patients require higher ventilatory demands during exertion, confirming
evidence that reported reduced BR at peak exercise11,22.
Ronen
Bar-Yoseph et al. observed low BR in 49% of patients with CF11, while Borel et al. found a reduced BR for patients
with CF when compared to healthy children 22. It is
also important to highlight that MVV was estimated using the
FEV1 36. Although this is a widely
used method, it is also subjected to underestimation of true ventilatory
capacity in obstructive diseases with low FEV1, which
may have influenced the present results 37.
Ventilatory efficiency has also been considered an important component
of aerobic fitness in CRD 7,38. Our results have shown
that, both at VT1 and peak exercise, an increase in
VE/VO2 and
VE/VCO2 was found for the CF group,
which was previously described 38. Moorcroft et al.
have also described differences in the
VE/VO2 between patients with CF who
survived or not 39. Several factors may explain lower
ventilatory efficiency in patients with CF. As exercise ventilatory
demand increases, progressive expiratory airflow obstruction and
increasing flow resistance occur, leading to dynamic hyperinflation. In
addition, ventilatory efficiency is also reduced by increased dead space
ventilation, even in mildly affected CF patients 16.
Regarding the asthma group, patients have shown an increase in
VE/VO2 and
VE/VCO2 at VT1, but not
at peak exercise. These results are consistent with those reported by a
previous study 7. There is scarce evidence on possible
factors explaining lower ventilatory efficiency at VT1for asthmatics, although an obstructive origin may be the most
likely. In addition, the
role
of inflammatory
mediators could also be important, as there is evidence correlating
exercise-induced sputum histamine levels with low arterial oxygen
partial pressure 40.
The present study presents limitations, including the lack of measures
of the degree of airway inflammation, such as exhaled nitric oxide
fraction, sputum analysis, or exhaled breath condensate, as these
measures could correlate with the outcome measures and help us to
understand the main mechanisms involved in exercise intolerance. In
addition, our study did not evaluate participants’ daily levels of
physical activity, which prevented us from further discussion on the
topic. On the other hand, although indirect estimation of MVV is likely
the optimal test in pediatric patients 36, it may
underestimate the true ventilatory capacity in obstructive diseases
where a low FEV1 is present.
In conclusion, the findings of the present study provide evidence on
aerobic fitness and its related determinants in children and adolescents
with CRD. Patients with CF achieved good exercise capacity despite low
ventilatory efficiency, low BR, and reduced lung function. However,
asthmatics presented reduced cardiorespiratory capacity and normal
ventilatory efficiency at peak exercise, although there were differences
in the ventilatory threshold, when compared to healthy peers,
highlighting the different mechanisms implicated in determining aerobic
fitness in CRD. These results may contribute to a better understanding
of the influence of CRD on exercise capacity, providing data to support
exercise practice aiming to improve physical conditioning, and
emphasizing the importance of routine evaluation of BR and ventilatory
efficiency as part of CPET outcomes.