Physical activity, exercise time and peak oxygen uptake
Interestingly, there were no significant differences between the two groups with regard to the amount of physical activity before the infection with SARS-CoV-2, nor in their school transport habits, or in their subjective exercise tolerance. The notion that premorbid
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) may have been low in the post-COVID-19 group, one possible explanation for the previously lower values observed in the adult population, therefore seems unlikely [
40]. However, after the infection, the participants in the post-COVID-19 group needed a longer rest period from physical activity than the control group (over 4 weeks vs. less than 1 week) and felt subjectively less exercise-tolerant. This is in accordance with previous survey studies investigating the effects of post-COVID-19 in children [
6,
9‐
11,
13,
15].
Almost all participants achieved maximal exercise (defined as an RER > 1.00). There were also no differences with regard to heart rate recovery (HRR) or breathing reserve (BR). In previous studies in adults, the fact that HRR was lower in patients suffering from post-COVID-19 was taken as a sign of earlier termination of exercise due to deconditioning and fatigue [
41]. Raman et al. [
19] who also observed shorter walk distances in a 6-min-walk-test observed that many patients stopped CPET early because of generalized muscle ache rather than breathlessness. Interestingly, the majority of participants who suffered from post-COVID-19 in the current study also reported muscle fatigue as the main cause for stopping the exercise.
The absolute value of peak oxygen uptake was lower in the post-COVID19 group. This is in accordance with numerous studies conducted in the adult population [
23,
24,
40] and reflects observations from survey and chart review studies conducted in the pediatric population [
6,
9,
13‐
15]. However, when comparing the values for the percentage of predicted
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) achieved, there was no significant difference. This could be a consequence of a relatively small sample size. Normal values for children represent an approximation and are gathered according to age. This leads to a smoothing of the results, which could therefore have annihilated the differences observed for
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\). Nor was there any significant difference in the oxygen uptake efficiency slope (OUES). These findings suggest that children affected by post-COVID-19 do not show a measurable impairment of their cardiopulmonary function, a fact that is underlined by values of 90% and more of their predicted values in both groups.
When differentiating the data according to sex, it became apparent that the lower
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) as well as the lower percentage of predicted
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) was limited to the girls. The fact that females seem to experience more pronounced symptoms for post-COVID-19 has been described before, using surveys and 6MWT [
42,
43], although the pathomechanism behind this finding is yet unclear.
Pulmonary function
The fact that there were no significant differences between the pulmonary variables (\({\dot{{\text{V}}}}_{{\text{E}}}{\text{peak}}\), \({\dot{V}}_{{\text{E}}}\) at VT1, breathing reserve BR, breathing frequency BF) recorded during CPET between the two groups underlines the fact that pulmonary function does not seem to be the cause for exertional dyspnea or reduced exercise tolerance in children.
One possible explanation for dyspnea after COVID-19 is dysfunctional breathing [
44]. A marker for ventilatory efficiency is the ventilatory equivalent (
\({\dot{V}}_{{\text{E}}}/{\dot{V}}_{{{\text{CO}}}_{2}}\)-slope) [
40]. Altered pulmonary diffusion capacity, ventilation/perfusion mismatch and hyperventilation-syndrome are possible causes for dysfunctional breathing and have been documented in COVID-19 survivors [
45]. Interestingly the ventilatory equivalent between the two groups in our study did not differ significantly. Thus, ventilatory inefficiency seems an unlikely candidate for the subjective reduced exercise tolerance reported by children suffering from post-COVID-19.
Cardiac function
Parameters for unmasking cardiovascular limitation using CPET are the O
2pulse, the peak HR, or an abnormal increase of the
\(\dot{V}{{\text{O}}}_{2}/P\)-slope. Despite the concerns around cardiac involvement during the SARS-CoV-2 pandemic, most studies showed normal values for the O
2pulse in patients suffering from post-COVID-19 [
23] or recovering from severe illness [
46]. This was also true in this study.
At least a mild chronotropic incompetence has been observed in most studies conducting CPET in adults after infection with SARS-CoV-2 [
46]. Lower peak HR was discussed either being due to chronotropic incompetence or a pharmaceutical betablockade or as a consequence of ceasing exercise early [
41]. The children in this study did not show any significant differences with regard to chronotropic incompetence, and the HRR was well above the pathological 12 beats/minute [
47]. This difference between the adult population and the children studied here may be due to several factors. First of all, the children showed a high willingness to reach peak exertion, reflected in the fact that almost all participants reached RER values above 1, irrespective of their symptoms, whereas the adults had probably ceased exercise before reaching peak exertional capacity, which may have been related to dyspnea unrelated to post-COVID-19 [
41]. On the other hand, the use of beta-blocker was wide-spread in the investigated adult cohorts, also offering an explanation for the chronotropic incompetence. In contrast, none of the children involved in this study was on beta-blockers.
A further parameter which is typically reduced in patients with cardiovascular disease is the
\(\dot{{\text{V}}}{{\text{O}}}_{2}/P\)-slope reflecting limitations in the supply and/or metabolism of oxygen. None of the children investigated in this study exhibited pathological values, defined as values below 10 ml/min/W [
48]. This stands in contrast to studies investigating this parameter in the adult population, where the fact that it was slightly reduced was taken as a sign for a potential contribution of cardiovascular factors to the observed low
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) [
49].
Peripheral function
The third compartment influencing
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) is the periphery (musculature and mitochondria). Peripheral limitations can be assessed through an abnormal response in RER, abnormal
\(\dot{{\text{V}}}{{\text{CO}}}_{2}\)-kinetics throughout exercise, a shallower
\(\dot{V}{O}_{2}/P\)-slope or a reduced VT1 in relation to
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\). However, all of these parameters are unspecific and are also used to assess the cardiac compartment (see above) [
40]. It is thus difficult to attribute abnormal findings in these parameters to deconditioning [
40]. Some authors have therefore declared the observed reduction in
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) to be a consequence of deconditioning in the absence of ventilatory and cardiac exercise limitations.
None of these parameters showed any significant difference between the group of children suffering from post-COVID-19 and those who did not. After completing the exercise, all children suffering from post-COVID-19 stated that they had to stop the exercise due to muscular fatigue, which was not the case in the comparison group. This observation suggests muscular deconditioning as the possible mechanism for the observed reduction in \(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\).
Another parameter reintroduced by Longobardi et al. [
24], the
\({T}_{1/2}\dot{{\text{V}}}{{\text{O}}}_{2}\), is defined as the time needed for
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) to decrease by half [
24]. This value proved to be significantly longer in adults suffering from post-COVID-19, suggesting a slower replenishment of energy stores in peripheral muscles underlying this defective off-transient
\(\dot{{\text{V}}}{{\text{O}}}_{2}\) kinetic response [
24]. Even though there were no significant differences between the two groups in this study, the children suffering from post-COVID-19 had values above a cut-off value of 90 s, which is generally assumed to be the upper limit of normal [
24]. This supports the idea of a peripheral mechanism as the cause for the reduced exercise tolerance.
Interestingly, the only other significant difference between the two groups was the duration of rest after an infection with SARS-COV-2. The children suffering from post-COVID-19 stated a mean absence from physical activity of 4 weeks compared to 0.3 weeks in the comparison group. Furthermore, the only significant correlation of all the study parameters was between the duration of the rest period and
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\). A study investigating the effects of early deconditioning of human skeletal muscle found several deconditioning processes to be initiated within the first 5 days of hypoactivity [
50]. A rest period of a mean of 4 weeks as observed in our cohort of post-COVID-19 children could explain the observed decrease of
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) and abnormal
\({T}_{1/2}\dot{{\text{V}}}{{\text{O}}}_{2}\). Since neither the pulmonary nor the cardiac compartment seems to be affected by infection with SARS-CoV-2, it seems reasonable to expect the cause in the peripheral compartment, i.e. the muscles. The combination of a significant difference between the two groups regarding the duration of rest with the negative correlation between rest period and
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\) suggests a connection between inactivity and the occurrence of post-COVID-19 symptoms. However, as this is a retrospective study, it is impossible to say whether the inactivity was the cause of the prolonged symptoms after SARS-CoV-2 infection or whether a more serious course of the infection leads to prolonged inactivity and prolonged symptoms at the same time. The fact remains that long rest periods before returning to sports as recommended with myocarditis [
51] are probably not warranted as the heart seems unaffected. Instead we should encourage children after a Coronavirus infection to return to sports within the limits of an infectious disease[
52] in order to improve the peripheral compartment and consequently the
\(\dot{{\text{V}}}{{\text{O}}}_{2}{\text{peak}}\). This recommendation obviously exclusively applies to children with post-COVID-19 syndrome and does not apply to children with paediatric inflammatory multisystem syndrome temporally associated with COVID-19 (PIMSts)/multisystem inflammatory syndrome in children (MIS-C) who need to be followed up as currently recommended [
53].