Cardiopulmonary exercise testing: relevant but underused

Abstract

Cardiopulmonary exercise testing (CPX) is a relatively old technology, but has sustained relevance for many primary care clinical scenarios in which it is, ironically, rarely considered. Advancing computer technology has made CPX easier to administer and interpret at a time when our aging population is more prone to comorbidities and higher prevalence of nonspecific symptoms of exercise intolerance and dyspnea, for which CPX is particularly useful diagnostically and prognostically. These discrepancies in application are compounded by patterns in which CPX is often administered and interpreted by cardiology, pulmonary, or exercise specialists who limit their assessments to the priorities of their own discipline, thereby missing opportunities to distinguish symptom origins. When used properly, CPX enables the physician to assess fitness and uncover cardiopulmonary issues at earlier phases of work-up, which would therefore be especially useful for primary care physicians. In this article, we provide an overview of CPX principles and testing logistics, as well as some of the clinical contexts in which it can enhance patient care.

Figure 1.

Slope of the relationship between measured and estimated oxygen uptake using the ramp and Bruce treadmill protocols in patients with heart failure. The unity line would be achieved if the predicted value were equal to the estimated value. These data demonstrate that measured maximal oxygen uptake (VO₂) is overpredicted by estimated values in exercise tests performed on patients with heart disease, particularly when using the Bruce protocol. In contrast to the Bruce protocol, exercise estimates achieved using a ramp protocol are much closer to the line of unity, highlighting that the choice of exercise protocol has a substantial impact on the accuracy with which VO₂ is estimated.

Figure 2A–B.

A) O₂ consumption (VO₂) relative to exercise intensity (eg, watts on a bicycle or treadmill speed and incline). VO_2max is identified as the level of greatest oxygen utilization in association with a physiologic plateau of capacity. B) Peak VO₂ is also used to identify oxygen utilization; while it may indicate the same O₂ utilization as VO_2max in some, for others the capacity of achieving and sustaining a plateau of maximum O₂ utilization capacity may not be achievable. Therefore, many regard peak VO₂ is a more accurate characterization of maximum O₂ utilization that may be maximal, but also may be limited by other factors (eg, breathlessness, anxiety, deconditioning).

Figure 3A–B.

A) The ventilatory threshold (VT) is the point at which respirations increase to maintain PaCO₂ equilibrium despite rising lactic acid production in the exercising muscles. Most commonly VT is determined as the departure of VO₂ from a line of identity drawn through a plot of VCO₂ versus VO₂, often called the V-slope method. B) An alternate mechanism to ascertain VT is achieved through assessment of ventilator equivalent for oxygen (VE/VO₂) and ventilatory equivalent for carbon dioxide (VE/VCO₂); VT is the point at which VE/VO₂ increases without an increase in VE/VCO₂.

Figure 4.

Cardiopulmonary parameters associated with different diagnoses.