Protocols
The protocol selected for exercise testing will depend on the purpose of the test and the characteristics of the patient. Nevertheless, the main criterion to guarantee obtaining a maximum oxygen uptake and good reproducibility of exercise parameters (e.g., ventilatory anaerobic threshold) with a particular exercise protocol on a particular individual is that the exercise protocol should be designed to have the subject reach his or her limit of tolerance in 10 ± 2 minutes.
In principle, many distinct protocols can be implemented and used on either a treadmill or a cycle ergometer. However, to facilitate comparisons with established normative values, standard exercise protocols have been developed that are suitable for most clinical and research applications. The most commonly used protocols fall into one of the following 3 categories: (1) multistage incremental (every 2 or 3 minutes, with a "pseudo" steady state at each stage); (2) progressive incremental (every minute) or continuously increasing (ramp); (3) constant work rate (5 to 10 minutes). In all cases, the exercise protocol is typically preceded by baseline (3 minutes) and warm-up (3 minutes) measurements as well as followed by recovery measurements (5 to 10 minutes).
Multistage Incremental Protocols
Treadmill Protocols
Bruce Protocol
The Bruce protocol was designed originally for diagnosing coronary artery disease in adults, but it has great popularity among pediatric cardiologists. One of the major advantages of using this protocol is that it can be used on subjects of all ages, and thus it can provide comparative exercise data using the same protocol as a child grows. Other advantages are that exercise responses to submaximal work rates can be measured (e.g., VO2 and cardiac output) and that VO2max can be estimated from determination of endurance time (r = 0.88). However, the Bruce protocol has some practical disadvantages. For younger or more limited children, the work increments between successive stages may be too great, resulting in the tendency for subjects to quit during the first minute of a new 3-minute stage. For subjects who are well trained, the first 4 stages of the Bruce protocol are too slow, leading to boredom. In addition, the most appropriate running speeds for these young athletes occur at very high elevations (>18%). The 3-minute stages are too long and thus boring for young subjects. In general, regardless of degree of fitness of the individual, most exercise is performed at relatively steep grades when the Bruce protocol is used, which encourages subjects to hold onto the handrails, thereby affecting the oxygen cost of exercise significantly. In addition, the large increases in speed and grade may limit the ability to accurately measure submaximal metabolic data such as the anaerobic threshold.
Balke Protocol
The Balke protocol involves increases in slope while the treadmill speed is kept constant (3.5 mph). As in the case of the Bruce protocol, the Balke protocol is rather limited when one attempts to obtain appropriate exercise responses in a reasonable amount of time (8 to 10 minutes) in populations of children ranging from very unfit to highly fit, from 6 to 18 years of age, or from normal healthy to chronically ill children.
In general, the Balke protocol may be modified to tailor it to the subject's age and fitness level by adjusting the constant treadmill speed and by starting at a higher grade. The goal is to keep the exercise time between 8 and 10 minutes.
Cycle Protocols
James Protocol
The James protocol separates subjects into 3 specific exercise protocols consisting of 3 progressive 3-minute stages at predetermined work rates based on gender and body surface area. After completion of these 3 stages, work rate is increased by ~100 or 200 kilopounds-meter per minute (kpm/min) (18 or 36 W/min) until a maximal voluntary effort is reached. This protocol has some limitations when applied to small children or children with moderate to severe exercise intolerance in whom test duration may range between 4.5 and 7 minutes, thus providing limited data for analysis.
McMaster Protocol
The McMaster protocol separates subjects into 3 specific exercise protocols of 2-minute stages at predetermined work rates based on gender and height. The work rate increments in this protocol are linear, and the 2-minute duration of each stage seems to be long enough to achieve a pseudo-steady state for most physiological variables.
Strong Protocol
The Strong protocol separates subjects into 4 specific exercise protocols of 3-minute stages at work rates based on the subject's weight. The goal of the protocol is to determine physical working capacity at a heart rate of 170 beats per minute.
Progressive Incremental Cycle Ergometer Protocols
Included in this category are continuously incremental (ramp) protocols and protocols in which each stage lasts 1 minute. This kind of protocol is very efficient in providing exercise responses in a short amount of time, thus enabling acquisition of diagnostic data within 10 to 12 minutes.
Ideally, the slope of the ramp should be tailored to have subjects terminate the test within 10 minutes and is based on the child's body size and physical condition. A good estimate of this slope is S (W/min) = (predicted VO2max − VO2 unloaded)/92.5. However, in most cases a slope (i.e., work rate increment per minute) is selected on a case-by-case basis. An appropriate work rate increment for fit adolescents may be 20 to 25 W/min, whereas for unfit patients and young children it may be 10 W/min. For normal children, a work rate increment relative to body weight (3.5 W • min-1 • kg-1) has been suggested.
Even though ramp protocols do not permit a steady state, the ramp-like change in work rate elicits submaximal responses that are equivalent to those derived from incremental protocols with stages lasting 2 to 3 minutes. However, to establish this correspondence and to interpret properly submaximal ramp responses, appropriate analysis is required. Consequently, because (1) most important exercise responses (e.g., cardiac output, oxygen uptake, minute ventilation, heart rate) during exercise in children have response times <1 minute; (2) work rate increment is typically <30 W/min in pediatric subjects; and (3) desired submaximal measurements (e.g., cardiac output) take <15 seconds to be completed, valid submaximal physiological responses to ramp changes in work rate can be obtained if appropriate and careful data analysis is performed.
Constant–Work-Rate Protocols
Submaximal constant–work-rate exercise tests of 5 to 10 minutes' duration are becoming more common in clinical exercise laboratories as an alternative protocol to maximal exercise tests. This is partly because submaximal exercise tests overcome some of the limitations of maximal exercise testing, which include (1) dependence on the patient's effort; (2) low sensitivity for measuring changes induced by therapeutic interventions; and (3) poor correlation with energy expenditure during activities of daily living, patient symptoms, and quality of life. The kinetic responses of oxygen uptake and/or heart rate at the onset of a brief bout of constant–work-rate exercise or during the recovery from the exercise bout can provide valuable information about the patient's ability to cope with the numerous changes in energy demand encountered in everyday life. A simple measurement, such as heart rate taken after a constant–work-rate exercise bout at an intensity that elicits a heart rate that corresponds to 70% to 85% of predicted maximum heart rate for the person's age, can have great predictive value. Indeed, a study conducted in a large adult population strongly suggests that the rate of heart rate recovery after submaximal exercise is associated with the person's risk of death. The longer it takes the heart rate to return to normal values, the greater the risk for death. When the observed patterns of physical activity in children are considered, submaximal exercise bouts of 1-minute duration at a work rate corresponding to ~90% of predicted maximum heart rate are more appropriate in a pediatric population. However, to date, reference values for healthy children and adolescents are not readily available for comparison.
Alternative Protocols
Six-Minute Walk Test
A 6-minute walk test may be more appropriate for assessing exercise tolerance in children with moderate to severe exercise limitation for whom traditional exercise testing may be too stressful. Guidelines established by the American Thoracic Society should be followed. Basically, the patient is encouraged to try to cover as much distance or as many laps on a measured course (often 30 m) as possible in 6 minutes. Patients using supplemental oxygen should perform the test with oxygen. Although many walk tests are done without monitoring, portable oximeters are available that enable continuous monitoring of both oxyhemoglobin saturation and heart rate without negligible additional weight. In the absence of portable equipment, it may be useful to monitor oxyhemoglobin saturation and heart rate before, during, and after the test. The patient is permitted to stop and rest but should resume walking if possible during the 6-minute period. Standard encouragement as outlined by the American Thoracic Society guidelines should be given. The total distance walked is the primary outcome. It has been suggested that distance walked should be multiplied by the patient's weight to reflect the work of walking. At least 2 practice tests performed on a separate day are advisable to minimize a learning effect and avoid fatigue. Because of the self-paced, submaximal nature of this test, the results may be more applicable than maximal exercise testing to everyday activities that the child may encounter. At this time, reference values for healthy children and adolescents are not readily available for comparison. However, the test is useful for following disease progression or measuring the response to medical interventions. The test is not as useful for healthier patients whose distance walked may be limited by leg length or height more than disease.
Exercise-Induced Bronchospasm Provocation
The exercise-induced bronchospasm (EIB) provocation allows for quantification of bronchial reactivity as measured by spirometry that is induced while a subject exercises for 5 to 8 minutes on a treadmill at an intensity of 80% of maximum capacity. The treadmill is preferable to cycle testing because it is more prone to induce bronchospasm. Additionally, the exercise room should be as cool (temperature 20 degrees C to 25 degrees C) and as dry as possible (relative humidity <50%) to elicit the best response. Both of these parameters should be recorded for each test. If feasible, some evidence indicates that having the child breathe very cold air (-20 degrees C) can increase the sensitivity of the exercise challenge. The exercise protocol should quickly increase the intensity to 80% of maximal capacity (using predicted heart rate maximum as a surrogate) within 2 minutes. If the intensity is not reached quickly, the likelihood of refractoriness to the development of bronchospasm will greatly increase. An incremental work rate used in many cardiopulmonary exercise tests (i.e., Bruce treadmill or Godfrey cycle protocols) is less likely to be effective in evaluating EIB because of its short duration of high ventilation and thus should be avoided in the evaluation of EIB. Additionally, use of prolonged warm-up periods may also induce refractoriness to EIB.
Exercise is preceded by baseline spirometry. Spirometry is repeated immediately after exercise and again at minutes 5, 10, and 15 of recovery. Most pulmonary function test nadirs occur within 5 to 10 minutes after exercise. If the child becomes symptomatic during or after testing even in the absence of a significant forced expiratory volume in 1 second (FEV1) decline, a bronchodilator may need to be administered. Trained respiratory personnel should be available during and after exercise. Accepted criteria for a significant decline in FEV1 after exercise are variable. Declines of 12% to 15% in FEV1 are typically diagnostic.
Use of medications before testing should be considered in part on the basis of the clinical question being asked. Consultation with the child's primary care physician or asthma specialist will help to optimize the testing procedure.
Pharmacological Stress Protocols
Pharmacological stress testing is generally used when conventional exercise testing is unsuitable or impractical. These circumstances may include patients who are too young or are unable to perform exercise testing or in cases in which the motion of exercise may interfere with data collection. Such cases may include certain types of echocardiographic studies (see echocardiography protocols below).
Pharmacological stress testing is usually performed at the site where additional studies will occur, such as the echocardiography laboratory or the nuclear imaging suite. The patients require a peripheral intravenous line for the infusion of the pharmacological stress agent. Additional equipment will include appropriate infusion pumps, 12-lead ECG monitoring system, and blood pressure–monitoring equipment. Sedation is rarely needed but may be required for young patients or those patients with limited ability to cooperate with the testing protocol.
Two basic types of pharmacological agents exist: those that increase myocardial oxygen consumption and those that cause coronary vasodilatation. Dobutamine and isoproterenol are examples of the former and, to an extent, simulate the effects of exercise. Adenosine causes dilation of normal coronary artery segments, resulting in a shunting of myocardial blood flow away from diseased segments. Dipyridamole inhibits adenosine reuptake, resulting in the same physiology.
See original guideline document for specific dosing instructions of dobutamine, atropine, esmolol, dipyridamole, and adenosine.
The occurrence rate of significant adverse reactions to pharmacological stress in the pediatric population is unknown. However, reports in recent literature suggest that the rate is quite low. Nevertheless, care must be taken to ensure patient safety. Heart rate, rhythm, and ST-segment changes should be closely monitored throughout the study and in the immediate post-study period. Patients should be observed for any complaints or signs of chest pain, hypotension, or bronchospasm. Prompt termination of the infusion and reversal of the stress agent should be undertaken in any of these circumstances.
Echocardiography
Two basic types of exercise are used with echocardiography: treadmill and cycle ergometry (upright or supine). With treadmill and upright cycle testing, echocardiography is usually performed before exercise and immediately after exercise termination (within 60 to 90 seconds). In the case of supine cycle ergometry, echocardiography is performed before and during all stages of exercise (including peak). Oxygen consumption and cardiac output determinations can also be obtained. When pharmacological stress agents are used, imaging is performed as outlined in the section on pharmacological stress protocols.