© 2003 American Thoracic Society
ATS/ACCP statement on cardiopulmonary exercise testingTo the Editor:The recent ATS/ACCP Statement on Cardiopulmonary Exercise Testing (CPET) (1) discusses when arterial blood gas analysis might be required and points out the need for further decision analysis for invasive versus noninvasive CPET. I would like to comment on this issue.
The cardiopulmonary and metabolic adaptations to a required oxygen uptake can be described mathematically by two equations (2):
Where:
Noninvasive CPET measures As can be seen in Equation 1, which is also discussed in the article, without measuring PaCO2, values for PaCO2 and VD/VT must be assumed in the interpretation of the ventilatory response to exercise. As is pointed out, this assumption is tenuous in persons with significant cardiopulmonary disease.
A review of Equation 2 indicates that only In summary, in seriously ill patients, when detailed cardiopulmonary and metabolic information is required, noninvasive assessment may not be enough. It may be necessary to measure both arterial blood gasses and cardiac output or mixed venous oxygen content.
Baylor College of Medicine Houston, Texas REFERENCES
From the Authors:We appreciate Dr. Ross's comments on the ATS/ACCP Statement on Cardiopulmonary Exercise Testing (CPET) (1). Specifically, Dr. Ross highlights an example where noninvasive CPET may not adequately or completely distinguish physiological abnormalities. Two mass balance equations are used to identify cardiac and pulmonary factors that may impact noninvasive measurement of oxygen consumption ( O2). In Dr. Ross's example, the patient has a high cardiac output related to a reduction in peripheral muscle O2 extraction, and low oxygen carrying capacity (due to anemia?) with an apparent normal/near normal O2. The work rate, which may be helpful, is unreported. However, we feel that his example contains some unrealistic assumptions. The assumed cardiac output of 33 L/minute would be unusual for a patient. This leads to an exaggerated contrast. We question Dr. Ross's statement that shunt fraction can be approximated from pulse oximetry (Section III.1.8 of ATS/ACCP Statement). As gas exchange abnormalities were apparent (due to O2 desaturation) in the example he poses, arterial blood gases (ABGs) would be appropriate. Inspection of other noninvasive resting and CPET variables not mentioned by Dr. Ross might assist in the differential diagnosis of the subject's exercise intolerance.
Dr. Ross correctly states that noninvasive CPET provides limited information on the quantitation of cardiac output and O2 extraction. Noninvasive CPET would, however, suggest possible sources of abnormalities. The cardiovascular support necessary to achieve a given In summary, we agree with Dr. Ross that noninvasive CPET may sometimes be inadequate and that more invasive testing is often helpful/necessary in selected cases. This is clearly noted within the ATS/ACCP Statement and is explicitly recommended when certain patterns of response emerge from noninvasive tests (Sections III.2, VIII.7.2, IV.5.1). Important considerations include: clinical setting, reasons for CPET, available resources, and health care provider judgment. We believe that additional studies are required for optimal decision analysis (Section IX.3).
a University of Iowa Medical Center Iowa City, Iowa REFERENCES
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represents cardiac output (L/minute), 
O2 represents mixed venous oxygen content (ml/L), and Cc'O2 represents oxygen content in the end pulmonary capillary blood exposed to alveolar PO2 (PAO2) (ml/L). 
100 watts in normals. Therefore, if these data were collected at 150 watts, a clear problem in O2 delivery would be indicated, specifically suggesting inadequate cardiovascular response. Finally, anemia would be known by measuring hemoglobin before exercise testing. 
