Interpretation of Pulmonary Function Tests A Practical Guide, 3. ed

11. Simple Tests of Exercise Capacity

In most instances the clinician has an estimate of a patient's exercise capacity. This is based on the history, results of physical examination, and pertinent data such as chest radiographs, electrocardiogram, blood cell count, and standard pulmonary function tests, possibly including arterial blood gas values.

However, in some situations a quantitative estimate of a patient's exercise capacity is needed. Before formal exercise studies are requested, some relatively simple tests can be performed. These can be done in the office or in a hospital's pulmonary function laboratory. They may obviate more extensive testing by providing a sufficient assessment of a patient's limitation.

11A. EXERCISE OXIMETRY

Pulse oximetry, available in most hospitals, is an inexpensive and noninvasive method of estimating arterial oxygen saturation in the absence of high concentrations of abnormal hemoglobins. After an appropriate site for exercising is selected and the pulse oximetry quality assurance criteria are satisfied, the oxygen saturation at rest is recorded. If the resting saturation is normal or near normal, the patient exercises until he or she is short of breath. In some disease entities, such as pulmonary fibrosis, pulmonary hypertension, and emphysema, values at rest are normal but surprising desaturation is noted with exercise. In this situation, a wise step is to repeat the exercise with the patient breathing oxygen to determine whether the saturation is easily corrected and the dyspnea ameliorated.

If a patient's resting saturation is low, this may be all the information needed. If supplemental oxygen is to be prescribed, however, the flow rate of oxygen that will provide an adequate resting saturation and the flow needed to maintain adequate saturation with mild exertion may need to be determined.

For such studies, it is important to record the distance and time walked. For prescribing oxygen, the levels of exercise (distance walked) can be compared without and with supplemental oxygen. In some patients with chronic obstructive pulmonary disease (COPD) and those with chest wall and neuromuscular limitations in whom carbon dioxide retention may be of concern, resting arterial blood gas values while the patient is breathing the prescribed oxygen concentration should be obtained to rule out progressive hypercapnia. Determining arterial blood gas values during exercise while breathing the prescribed oxygen concentration is generally not necessary.

PEARL: In a few situations, the saturation is falsely low when the pulse oximeter is used on the finger. These situations include thick calluses, excessive ambient light, use of dark shades of nail polish, jaundice, and conditions with poor peripheral circulation such as scleroderma and Raynaud's disease. In cases with a poor pulse signal, the earlobe or forehead is an alternative site. If there is any doubt about the reliability of the oximeter readings, or if the reading does not match the clinical situation, arterial blood gas studies are recommended.

11B. WALKING TESTS: 6- AND 12-MINUTE

These simple walking tests are useful for quantifying and documenting over time a patient's exercise capacity. They can be utilized in both pulmonary and cardiac disease with reasonable precautions. They are also valuable for quantifying the progress of patients in rehabilitation programs [1].

The tests are best performed in a building with unobstructed, level corridors. A distance of 100 ft can be measured and the number of laps counted. Neither test is superior over the other. Because the 6-minute test is less demanding, it is used more often, especially in very sick patients. The subject is instructed to walk back and forth over the course and go as far as possible in 6 minutes. The subject should be encouraged by standardized statements such as ''You're doing well'' or ''Keep up the good work.'' Subjects are allowed to stop and rest during the test but are asked to resume walking as soon as possible. Pulse rate is recorded before and after the test. If the patient is receiving oxygen, the flow rate and mode of transport, such as carried or pulled unit, are recorded.

TABLE 11-1. Relation of 6- and 12-minute walks to speed*

‘Prediction equations for the distance walked during the 6-minute (6MWD) test for adults of ages 40 to 80 years. Results are given in meters (1 meter = 3.28 ft) [2]. Men: 6MWD = (7.57 x height cm) — (5.02 x age yr) — (1.76 x weight kg) — 309 meters. Women: 6MWD = (2.11 x height cm) — (5.78 x age yr) — (2.29 x weight kg) + 667 meters [2].

Table 11-1 relates the distances walked to the average rate of walking in miles per hour. Prediction equations for the 6-minute test are available for average healthy adults of ages 40 to 80 years [2]. These are listed in Table 11-1. The use of the test is twofold. First, by comparing a patient's results with the predicted norm, the patient's degree of impairment can be estimated. Second, the test is most valuable as a measure of the patient's response to therapy or the progression of disease.

11C. STAIR-CLIMBING TEST

For many years, physicians have used stair climbing to estimate a patient's cardiopulmonary reserve. The empirical nature of stair climbing has been a drawback. However, in one study, subjects with COPD climbed stairs until they became limited by symptoms and stopped [3]. A significant correlation was found between the number of steps climbed and (1) peak oxygen consumption and (2) maximal exercise ventilation. This test is another way to estimate operative risk in patients with COPD who are to undergo thoracic operation. The study found that, on average, the ability to climb 83 steps was equivalent to a maximal oxygen consumption (Vo₂ max) of 20 mL/kg per minute. The ability to reach a maximal oxygen consumption of 20 mL/kg per minute has been reported to be associated with fewer complications after lung resection or thoracotomy.

Stair climbing is more cumbersome than the 6- or 12-minute walk. However, it does push most patients closer to their maximal oxygen consumption, Vo₂ max, an end point of greater physiologic significance.

11D. VENTILATORY RESERVE

Measuring a subject's ventilation during a given task or exercise provides an estimate of the demand of that task. The definition of ventilatory reserve (VR) is given by this relationship:

Given a maximal voluntary ventilation (MVV) of 60 L/min and an exercise ventilation (Ve) of 30 L/min during a given task, the VR is 50%The greater the Ve, the lower the reserve and the more likely it is that the patient will become dyspneic. A VR of less than 50% is usually associated with dyspnea. Another approach is to subtract Ve from the MVV. A value of MVV — Ve less than 20 L/min indicates severe ventilatory limitation.

11E. RATING OF RESPIRATORY IMPAIRMENT

Another approach to estimating respiratory impairment is based on the percentage reduction in various pulmonary function tests. One recommendation presented by the American Thoracic Society is summarized in Table 11-2. It provides useful guidelines. If a patient complains of severe dyspnea but the tests show only mild to moderate impairment, muscle weakness, upper airway obstruction, or causes other than respiratory should be considered. If none are found, cardiopulmonary exercise testing might be appropriate.

TABLE 11-2. Estimations of respiratory impairment based on results of pulmonary function tests

Test*

* AH tests relate to the percentage of the normal predicted value for an individual.

Dlco, diffusing capacity of carbon monoxide; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; Vo₂ max, maximal oxygen consumption.

Data from Ad Hoc Committee on Impairment/Disability Evaluation: Evaluation of impair- ment/disability secondary to respiratory disorders. Am Rev Respir Dis 133:1205-1209, 1986.

11F. CARDIOPULMONARY EXERCISE TESTING

Cardiopulmonary exercise testing requires sophisticated equipment and should be performed only by laboratories with strict quality control, experienced physiologic direction, appropriate medical supervision, and considerable experience in doing such tests [4]. Numerous variables of gas exchange and cardiac function, often requiring an indwelling arterial catheter for repeated blood gas determinations, are measured. The measurements include ventilation (V), oxygen consumption (Vo₂), carbon dioxide production (Vco₂), dead space ventilation, and alveolar- arterial oxygen gradients. In some laboratories, Vo₂ and Vco₂ are measured breath by breath. Also measured are the heart rate, blood pressure, and lactate levels, and electrocardiography is performed.

Some of the indications for cardiopulmonary exercise testing are as follows:

1. To distinguish between cardiac and pulmonary causes of dyspnea in complex cases

2. To determine whether the patient's symptoms are due to deconditioning

3. To detect the malingering patient

4. To provide disability evaluation in problem cases

5. To determine level of fitness, including whether a subject can meet the work requirements of a given work assignment

REFERENCES

1. Crapo RO, Casaburi R, Coates AL, et al. ATS statement: Guidelines for the six-minute walk test. Am J Respir Crit Care Med 166:111-117, 2002.

2. Enright PL, Sherrill DL. Reference equations for the six-minute walk in healthy adults. Am J Respir Crit Care Med 158:1384-1387,1998.

3. Pollock M, Roa J, Benditt J, Celli B. Estimation of ventilatory reserve by stair climbing: A study in patients with chronic airflow obstruction. Chest 104:1378-1383,1993.

4. Jones NL, Killian KJ. Exercise limitation in health and disease. N Engl J Med 343:632-641,2000.