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Cardiorespiratory Fitness Assessment

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INTRODUCTION

Cardiorespiratory fitness (CRF) is one of the five health-related components of physical fitness (CRF, body composition, muscular strength, muscular endurance, flexibility). It is characterized by the body’s ability to perform moderate- to vigorous-intensity exercise using large muscle groups in a dynamic/rhythmic and continuous manner for prolonged periods of time. Thus, the ability to sustain this level of exertion is dependent on the integration of the respiratory, cardiovascular, and musculoskeletal systems. Higher levels of CRF are often associated with higher levels of physical activity, which are associated with a number of health benefits. This type of association can be characterized as a dose-response relationship. Low levels of CRF are associated with a marked increase in all-cause mortality (specifically from cardiovascular disease [CVD]). Increases in CRF result in a reduction in all-cause mortality (12,13,38,61,64). The assessment of CRF is, therefore, an important part of any primary or secondary prevention and rehabilitative program. The skills and knowledge required to complete the assessment, interpret the results, and write an appropriate exercise prescription (ExR_x) are an important responsibility of the exercise professional.

Measuring Cardiorespiratory Fitness and the Maximal Oxygen Uptake

Measurement (or assessment) of CRF can assist the professional by providing valuable information that can be used to determine the intensity, duration, and mode of exercise recommended as part of an exercise program. Additionally, the measurement of CRF following the initiation of an exercise training program can serve as motivation to the patient as reason for continuing with a regular exercise program and may encourage the addition of other modes of exercise to improve overall fitness. Last, the assessment of CRF can assist in identifying, diagnosing, and prognosis of comorbid conditions.

It is important to choose the test that best fits the patient’s characteristics. The following are some of the factors to consider when choosing the appropriate type of test:

  What will the information be used for (functional capacity, ExR_x)?

  Whether physician supervision is required

  Health status of the participant

  Maximal or submaximal

  Length of the test

  Willingness of the participant

  Cost of the test to administer

  What personnel are needed (i.e., qualifications)?

  What equipment and facilities are needed for the test?

  Whether there are any safety concerns

Maximal Oxygen Uptake (Absolute and Relative)

Maximal volume of oxygen consumed per unit of time (O_2max) is accepted as the criterion measure of CRF. This variable is typically expressed in absolute or relative terms. Absolute O_2max is expressed in liters per minute (L ∙ min⁻¹) or milliliters per minute (mL ∙ min⁻¹) and provides a measure of energy expenditure for both non–weight- and weight-bearing activities such as arm or leg cycle ergometry and the treadmill. Absolute O_2max is directly related to body mass or size and is typically greater in men compared with women. O_2max is most often described relative to an individual’s body weight; thus, relative O_2max is expressed in milliliters per kilogram of body weight per minute (mL ∙ kg⁻¹ ∙ min⁻¹) and is used to classify an individual’s CRF level to allow for meaningful comparisons between/among individuals with differing body weight. Relative O₂ is often used to estimate energy expenditure of weight-bearing activities such as walking, running, and stair climbing. When expressing O_2max simply as a linear function of body mass, CRF may be underestimated for heavier individuals (>75.4 kg) and overestimated for lighter individuals (<67.7 kg) (32).

Maximal Oxygen Consumption: Net and Gross Rates

It has been proposed that expressing O_2max relative to an individual’s fat-free mass (mL ∙ kg FFM⁻¹ ∙ min⁻¹) would provide a more accurate estimate of his or her CRF and is independent of changes in body mass (19). The rate of oxygen consumption can also be expressed as gross O₂ or net O₂. Gross O₂ represents the total rate oxygen consumed (or caloric cost) at rest and during a bout of exercise. Net O₂, on the other hand, represents the rate of oxygen consumption in excess of an individual’s resting O₂ and is used to describe the caloric cost of exercise. Both net and gross O₂ can be expressed in either absolute (L ∙ min⁻¹) or relative terms (mL ∙ kg⁻¹ ∙ min⁻¹).

O_2max is the product of the maximal cardiac output (; L blood ∙ min⁻¹) and arterial–venous oxygen (a-O₂) difference (mL O₂ ∙ L blood⁻¹) or put more simply, delivery () and utilization (a-O₂ difference) and is illustrated in the following equation (Fick equation):

O_2max = _max × (a-O₂ diff_max)

Differences in O_2max across populations and fitness levels result primarily from differences in ; therefore, O_2max is closely related to the functional capacity of the heart (delivery). During exercise, the achievement of O_2max implies that an individual’s true physiological limit has been reached and a plateau in O₂ was observed between the final two work rates of a progressive exercise test. It is important to note that the intrinsic motivation of an individual as well as the test mode may influence their ability to achieve a “true” O_2max. Individuals with CVD or pulmonary disease rarely are able to achieve a plateau in O₂ despite exercising maximally. The term O_2peak may be used instead when an individual is not able to achieve a plateau of O₂ during a maximal effort and is limited by local muscular factors or fatigue rather than central circulatory dynamics (44). O_2peak is commonly used to describe CRF in these and other populations with chronic diseases and health conditions (3).

Open-Circuit Spirometry

Open-circuit spirometry, also known as indirect calorimetry, is the preferred method for the measurement of O_2max and is measured during a graded incremental or ramp exercise test to exhaustion. During this procedure, the subject breathes through a mouthpiece, with the nose occluded (or through a facemask that covers the mouth and nose). This configuration allows pulmonary ventilation and expired fractions of oxygen (O₂) and carbon dioxide (CO₂) to be measured. An accurate assessment of anaerobic/ventilatory threshold and O_2max/O_2peak can be achieved using open-circuit spirometry. Currently, there are a number of automated systems available that provide ease of use as well as mobility. Regardless of the type of automated system that is used, calibration of the unit is essential in order to obtain valid and reliable results (50). In addition, administration and interpretation of the test should be reserved for trained professionals. It is important to note that based on the health status of the patient, equipment costs, space, and required personnel, the direct measurement of O_2max may not always be feasible and is often reserved for research or clinical settings.

Exercise Tests to Estimate O_2max

If O_2max is not able to be directly measured, there are a variety of maximal and submaximal exercise tests that can be used to estimate O_2max. Exercise tests that estimate O_2max have been validated by examining (a) the correlation between directly measured O_2max and the O_2max estimated from physiological responses to submaximal exercise (e.g., heart rate [HR] at a specified power output) or (b) the correlation between directly measured O_2max and field test performance (e.g., time to run 1 or 1.5 mile [1.6 or 2.4 km]) or time to volitional fatigue using a standard graded exercise test protocol. It is important to understand that by estimating O_2max, there is a potential for error. Often, overestimation is more likely to occur with an exercise protocol that is chosen which is too aggressive for a given individual (e.g., Bruce treadmill protocol in patients with heart failure) (3). Every effort should be taken to choose the appropriate exercise protocol given an individual’s characteristics and minimize handrail use during testing on a treadmill (29).

Contraindications to Exercise Testing

Prior to initiating an exercise test, the risk of performing the test must be weighed against the potential benefits. An exercise professional must understand both the relative and absolute contraindications to exercise testing (Box 4.1) (23). This emphasizes the importance of performing a thorough preexercise test evaluation in addition to carefully reviewing the patient’s exercise history (as described in Chapter 3) to assist the exercise professional in identifying any potential contraindications to exercise testing. Individuals who are identified as having any absolute contraindications should not be tested until the condition has been stabilized or adequately treated. Those who have relative contraindications may be tested only after a careful evaluation that has determined that the benefit involved in performing the test outweighs the associated risks.

Maximal versus Submaximal Exercise Testing

The decision to perform a maximal or submaximal exercise test depends largely on the reasons for the test, physical condition of the patient, and availability of appropriate equipment and personnel. Maximal exercise tests require participants to exercise to the point of volitional fatigue, which may be inappropriate for some individuals and may require the need for emergency equipment (23,50).

Exercise professionals often rely on submaximal exercise tests to assess CRF because maximal exercise testing is not always feasible in the health/fitness setting. The foundation of submaximal exercise testing is to determine the HR response to one or more submaximal work rates and to use the data to predict an individual’s O_2max. In addition to predicting O_2max from the HR–work rate relationship, the exercise professional should collect additional important physiological responses from the exercise test. The measurement of HR, blood pressure (BP), work rate, and rating of perceived exertion (RPE) can give valuable information to the exercise professional in regard to the patient’s health and functional response to exercise. Combined with the patient’s estimated O_2max, this information can be used to evaluate and track the patient’s submaximal physiological responses over time and can be used to make modifications to his or her ExR_x.

To ensure an accurate estimation of O_2max from a submaximal exercise test, all of the following assumptions must be met or achieved (34):

  A steady-state HR is obtained for each exercise work rate.

  A linear relationship exists between HR and work rate.

  The difference between actual and predicted maximal heart rate (HR_max) is minimal.

  Mechanical efficiency (i.e., O₂ at a given work rate) is the same for everyone.

  The subject is not on any medications that may alter the HR response to exercise (i.e., β-blockers).

  The subject is not using high quantities of caffeine, ill, or in a high-temperature environment, all of which may alter the HR response.

Guidelines for Exercise Testing

General Guidelines

Prior to any type of CRF testing, pertinent data such as preactivity screening (refer to Chapter 3), demographic, medical, and personal information should be gathered and reviewed to reduce the occurrence of unwanted or potentially harmful events that could occur during the exercise test. Once an individual has been properly screened and it has been determined he or she is safely able to undergo the CRF test, the exercise professional should ensure that the following pretest instructions are given the patient. These instructions should be provided to the patient at least 24 hours before the exercise test to ensure patient adherence as well as maximize patient safety and comfort.

  Review the patient’s completed consent and screening forms.

  Have the appropriate data collection forms (data recording sheets, normative tables) ready prior to the exercise test.

  Calibrate all equipment (e.g., cycle ergometer, treadmill, sphygmomanometer, skinfold calipers) at least monthly or more frequently based on usage.

  Assure a room temperature between 68°F and 72°F (20°C and 22°C) and a humidity of less than 60% with adequate ventilation (37). The testing environment can play a very important role in test validity and reliability.

  To minimize subject anxiety, the test procedures should be explained adequately and should not be rushed, and the test environment should be quiet and private.

  The room should be equipped with a comfortable seat and/or examination table to be used for resting BP and HR.

  The demeanor of personnel should be one of relaxed confidence to put the subject at ease.

  Finally, the exercise professional should be familiar with the emergency response plan.

When performing multiple assessments in one session, the sequence of testing is very important. Prior to any exertional assessments, resting measurements such as HR, BP, height, and body weight and body composition should be obtained. Once resting measurements have been taken, the following order can be followed for testing: cardiorespiratory, muscular fitness, and flexibility. Although an optimal order for testing multiple health-related components of fitness has not been determined, sufficient time should be allowed for HR and BP to return as close to baseline as possible between tests. For example, assessing CRF after a muscular fitness assessment (which can elevate HR) can influence the CRF results. Furthermore, the tester should be aware of and note any medications the participant is taking because some, such as β-blockers, can alter the HR response to exercise. In addition, the test sequence should be organized so that the same muscle groups will not be stressed repeatedly.

Pretest Instructions for Cardiorespiratory Fitness Assessment

To ensure the predictive accuracy for measuring CRF, reproducibility of the test, and ensure the safety of the patient, they should be presented with the following general instructions to standardize the test (23):

  The purpose for conducting the test should be clear to ensure diagnostic accuracy and patient safety.

  Patients should abstain from ingesting food, caffeine, alcohol, or tobacco products within 3 hours of testing (routine medications may be taken with small amounts of water).

  Appropriate, comfortable clothing and footwear should be worn.

  Strenuous exercise should not be performed at least 24 hours prior the test.

  If the exercise test is on an outpatient basis, the individual should be made aware that the fitness assessment is maximal and may cause fatigue. They may wish to have someone accompany them to drive home afterward.

  If the exercise test is performed for the diagnosis of ischemia, routine medications may be discontinued because some (β-blockers) can attenuate the HR and BP response to exercise as well as alter the hemodynamic response and reduce the sensitivity of an electrocardiogram (ECG, antianginal agents). No formal guidelines for medication tapering exist, but 24 hours or more could be required.

  If the exercise test is for functional or ExR_x purposes, individuals should continue their medication regimen so the exercise response will be consistent with responses expected during exercise (3).

  Participants should bring a list of their current medications that include dosage and frequency of administration and report when the last dose was taken.

  Ample fluid consumption 24 hours prior to the assessment is encouraged to ensure normal hydration.

Cardiorespiratory Test Sequence and Measurements

Following the appropriate screening, measurements specific for CRF testing should be obtained prior to the start of the exercise test.

Position and Technique

At a minimum, preexercise HR and BP should be measured in the testing position. A preexercise HR should be obtained at the radial artery for 60 seconds. BP should be obtained following standardized procedures (see Chapter 3). During the exercise test, a minimum of HR, BP, RPE, and ECG should be measured at defined intervals while constant subjective measurements of signs or symptoms of cardiovascular or pulmonary disease are also recorded. These measurements should be obtained routinely during the exercise test and through recovery. Table 4.1 provides the recommended sequence for the measurement of HR, BP, RPE, and ECG during an exercise test.

HR can be measured either by palpitation, auscultation, or via HR monitors. The pulse palpation technique involves “feeling” the pulse by placing the second and third digits (i.e., index and middle fingers) over the radial artery which is located on the thumb side of the wrist. The pulse is commonly counted for a 15-second time interval and then multiplied by 4 to determine the HR for 1 minute. During exercise, this 15-second method should be used to ensure that HR has reached a steady state (two measurements that are within four beats). To auscultate the HR, the bell of the stethoscope should be placed to the left of the sternum and just above the nipple. The exercise HR response should be a linear increase in work at a rate of approximately 10 ± 2 beats ∙ MET⁻¹. HR_max decreases with age (64) and is decreased in patients on β-adrenergic receptor blockers along with the submaximal HR response. For an adult population, the most common equations to predict a patient’s HR_max are as follows:

Age-predicted HR_max = 220 − age (year)

Age predicted HR_max = 208 − (0.7 × age)

BP, both preexercise and exercising, should be measured at heart level with the subject’s arm supported and relaxed and not grasping the handrail (treadmill) or handlebar (cycle ergometer). Both systolic blood pressure (SBP) and diastolic blood pressure (DBP) measurements can be used to ensure that there is an appropriate exercise response and can be used as indicators for stopping an exercise test (Boxes 4.2 and 4.3). The normal SBP response to exercise should be to increase with increasing workloads of approximately 10 ± 2 mm Hg ∙ MET⁻¹ (23). On average, there is a greater response in men, increased with age, and in patients taking vasodilators, calcium channel blockers, angiotensin-converting enzyme inhibitors, and α- and β-adrenergic blockers. It is important for the exercise professional to understand what the appropriate response to exercise is, so he or she can correctly interpret what an inappropriate BP response to exercise is (Box 4.4).

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