This is an excerpt from Cardiopulmonary Exercise Testing in Children and Adolescents.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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