Exercise in patients with emphysema
Normally, the respiratory system does not limit exercise tolerance. In healthy individuals, arterial blood saturation with oxygen, which averages 98% at rest, is maintained at or near 98% in even the most strenuous dynamic or isometric exercise. The healthy response includes the ability to augment ventilation more than cardiac output; the resulting rise in the ventilation-perfusion ratio counterbalances the falling oxygen content of mixed venous blood.
In patients with emphysema, ventilatory limitations to exercise occur long before ceilings are imposed by either skeletal muscle oxidative capacity or by the ability of the cardiovascular system to deliver oxygen to exercising muscle. These limitations are manifest during a stress test on the basis of three primary measurements. First, patients with ventilatory limitations typically cease exercise at relatively low heart rate, indicating that exhaustion is due to factors unrelated to cardiovascular limitations. Second, their primary complaint is usually shortness of breath, or dyspnea. In fact, patients with chronic obstructive pulmonary disease often first seek medical evaluation because of dyspnea experienced during such routine activities as climbing a flight of stairs. In healthy people, exhaustion is rarely associated solely with dyspnea. In emphysematous patients, exercise-induced dyspnea results, in part, from respiratory muscle fatigue exacerbated by diaphragmatic flattening brought on by loss of lung elastic recoil. Third, in emphysematous patients, arterial oxygen saturation will characteristically fall steeply and progressively with increasing exercise, sometimes reaching dangerously low levels. In emphysema, the inability to fully oxygenate blood ate rest is compounded during exercise by increased pulmonary blood flow, and by increased exercise oxygen extraction that more fully desaturates blood returning to the lungs.
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