Measurement of functional capacity

Exercise tolerance testing (ETT), or field tests of functional capacity, can produce an estimated METs value to guide risk stratification and exercise prescription. True values can only be obtained through cardio-pulmonary exercise testing using gas analysis. Predicted VO2max or extrapolated MET values have a degree of error, as compared to true VO2max when measured using gas analysis.

Factors Influencing Accuracy in Clinical Practice (METs or Vo2max/peak)

• The use of a sub-maximal symptom limited test: A sub-maximal test is thought to give a measurement within 10-20% of a normal individual's actual VO2max. These tests are often used in pre-discharge evaluation of post-MI patients (ACSM, 2001). In relation to a clinical population, symptom-limited testing is probably the most appropriate, as any exercise prescriptions should be based on what actually limits the patient.

• The most widely used ETT protocol is the Bruce treadmill protocol (ACSM, 2001) with a recent survey showing that 82% of treadmill tests in America used either the Bruce or modified Bruce (Myers, et al., 2000). The METs levels associated with each increment of the test were developed from early research on healthy subjects, of a younger age than the clinical population and on the assumption of reaching steady state exercise within the three-minute, large incremental increases in workload (ACSM, 2001; AHA, 2001).

• The use of age-adjusted predicted maximum heart rates:

There is a +/- 10bpm standard deviation in individual maximum heart rate (Balady and Donald, 1991; AHA, 2001; McArdle, et al, 2001).

• An assumed efficiency or economy of task performance:

This is applicable to the Bruce protocol where METs are estimated from the applied workload or in the shuttle walk test where estimated METs levels are associated with a particular speed of walking. This is also illustrated by evidence that the use of handrails during treadmill testing can reduce the energy cost by 30% and would underestimate an individual's true exercise capacity (McArdle, et al., 2001). Another example is patients with significant co-morbidities, e.g. peripheral vascular disease or osteoarthritis. These patents are potentially working at a higher percentage of their VO2max at a set pace on incremental treadmill tests to archive the workload. This could lead to over-prescription for their CR exercise intensity.

• Psychological and motivational factors may also contribute to the accuracy of functional capacity measurement. This is an important consideration in the CR population, where levels of anxiety may be high. Individuals are less likely to be familiar with either performing strenuous exercise or with the associated sensations and possible discomfort (McArdle, et al., 2001). This may, therefore, result in an underestimation of true exercise capacity potential (ACSM, 2001). Conversely, trained individuals are more likely to be motivated, confident and able to push themselves into anaerobic exercise and closer to true VO2max.

There are, therefore, many factors that can affect the accuracy of determining METs, peak or VO2max; both under-estimation and over-estimation are possible, depending on the patient. We concur with other texts (ACSM, 2001; AHA, 2001; McArdle, et al, 2001) that accept that these errors exist, but for the purposes of risk stratification in a clinical setting such as CR, the most commonly practised methods of estimating METs or VO2 give a sufficiently accurate indication of exercise capacity.

Most clinical guidelines (ACPICR, 1999; Goble and Worcester, 1999; SIGN, 2002) recommend the inclusion of a measure of exercise tolerance (e.g. tread mill or cycle ergometer testing, shuttle walk test or six-minute walking test) prior to programme commencement in order to allow appropriate stratification and prescription of an individualised exercise programme. Repeated postrehabilitation testing by the clinical exercise leader allows an increase in exercise capacity to be quantified, supplying an outcome measure for exercise intervention within local programmes.

One of the most commonly used and cardiac population validated tests is the shuttle walking test (SWT) first described by Singh, et al. (1992) and used for measurement of functional capacity (Tobin and Thow, 1999). A protocol for implementation of the SWT is included in the SIGN guideline 57 (2002) along with information as to how the test can be purchased (see Chapter 3). It is simple to use, requires little equipment and can be undertaken by the majority of cardiac patients. A study by Fowler, et al. (2005) of 39 CABG patients found the SWT was reproducible and sensitive to change. The authors suggest one practice walk for the SWT. None of the 39 subjects in Fowler, et al.'s (2005) study completed the entire 12-level protocol. The SWT does, however, have limitations for a small number of patients, who may have a higher baseline fitness level. These patients will 'top out', i.e. they will reach the end of the test protocol before reaching their termination point. The SWT has an estimated top energy expenditure of 9.1 METs, with a maximum walking/running speed of 8.53 km/hour. This leaves the practitioner unable to measure improvement in this small number of patients post-rehabilitation (Armstrong, 2005).

In addition, the ageing population of cardiac rehabilitation patients, with numerous co-morbidities, may find the SWT less sensitive to change when measurement of improved aerobic capacity cannot be demonstrated by incremental walking. A six-minute walking test (Demers, et al., 2001) or other functional outcome measures currently used in elderly rehabilitation may be more sensitive to change, and also more relevant to patient goals in the older population (e.g. functional reach).These examples highlight the importance, when dealing with such a varied group of patients, of having a variety of outcome measures to suit both the patients' abilities and their goals (see Chapter 3 for more on functional capacity).

Run for Your Life The Health Benefits Of Treadmills

Run for Your Life The Health Benefits Of Treadmills

Improve your hearts health? Lose a few pounds? Or simply become more active? If that is your goal, then maybe its time for you to do some exercise. But where do you start?

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  • amanuel
    What is a mets measurement in cardiac rehab?
    7 months ago
  • aisha
    How to measure functional capacity?
    4 months ago

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