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 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.

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 diseases 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” and “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 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.