Determining Functional Capacity
This is an excerpt from Cardiopulmonary Exercise Testing in Children and Adolescents by Thomas Rowland, American College of Sports Medicine & North American Society for Pediatric Exercise Medicine (NASPEM).
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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