CPET example interpretations

Questions 1

Answer

These exercise data are a focused report addressing key elements. This patient has a significant limitation in exercise capacity as evidenced by a maximum Vo2 of only 62% predicted. A common approach to determining the cause of the exercise limitation is to evaluate ventilatory, gas exchange, and cardiovascular factors. The most obvious limiting factor in this patient involves the ventilatory system where we see that his exercise ventilation exceeds his measured MVV. Moreover, this limitation appears to limit his ability to lower his Pco2 to buffer the developing metabolic acidosis from exercise induced lactate production.

The MVV is commonly used as a reference for maximal exercise ventilation with normal subjects generally requiring less than 70-80% of this capability to reach maximal exercise capacity. There are several causes of a reduced MVV: reduced lung volumes from lung disease, neuromuscular impairment, and poor effort. To separate these mechanisms one can compare the measured MVV with a predicted MVV based on the FEV1. Normally the MVV should be 35-40 times the FEV1 with values lower than that suggesting either impaired neuromuscular function or poor effort. This patient has a predicted MVV of at least 35 x 3.0 (the measured FEV1) of 105 L/min, well above the measured 49. While this could reflect poor effort during the MVV maneuver, the lack of an appropriate ventilatory response as evidenced by rising Pco2 in the face of developing acidosis suggests that a neuromuscular problem exists (choice B is correct, and choices A and C are incorrect).

The normal oxygenation response to increasing exercise is to maintain ventilation-perfusion (V/Q) relationships (and thus oxygenation) up to very high levels of exercise (choice D is incorrect). Exercise desaturation (gas exchange limitation) usually reflects impaired V/Q relationships from lung disease. Interestingly, at extremely high levels of exercise in trained athletes, blood transit time through pulmonary capillaries become so short that a diffusion time limitation can produce hypoxemia.

The final mechanism to consider in patients with limited exercise tolerance is cardiovascular. Extensive cardiovascular measurements were not done in this patient but a heart rate % predicted commensurate with the Vo2% predicted and the s