Measurement of Expired Air During Exercise

Updated:Jun 18,2014

Disclosure: NONE
Pub Date: Wednesday, July 7, 2010
Author: Ray W. Squires, PhD&


Balady GJ, Arena R, Sietsema K, et al. Clinician's guide to cardiopulmonary exercise testing in adults: a scientific statement from the American Heart Association. Circulation 2010. Published online before print, June 28, 2010. 10.1161/CIR.0b013e3181e52e69.

Article Text

"He lives most life whoever breathes most air."
          -- Elizabeth Barrett Browning [1]

Article Text

The direct measurement of oxygen uptake (Vo2) and carbon dioxide production (Vco2) during progressive exercise for the determination of maximal oxygen uptake (Vo2max) has been a standard measurement in exercise physiology for many decades.[2] For clinical medicine, pulmonologists were the first to appreciate the utility of cardiopulmonary exercise testing (CPX) and the classic texts of Jones [3] and Wasserman and colleagues [4] were primarily written for that audience. At my own institution, pulmonologists began clinical CPX in 1975 with collection of expired air in Douglas bags with the measurement of volume with a manual gas meter, chemical analysis by hand of expired air samples, and calculation by hand of Vo2 and Vco2 for each sample.

The cumbersome and time-consuming nature of performing cardiopulmonary measurements during exercise was overcome with commercially available automated "metabolic carts" introduced in the 1970s and 1980s. In 1985, we incorporated CPX into our clinical stress testing laboratory in cardiology. In the first years of offering the service, approximately 500 tests were performed annually. The popularity of CPX among cardiologists has increased progressively, and currently, we perform approximately 3,000 tests each year. The vast majority of CPX performed in our laboratory include standard electrocardiographic and blood pressure measurements during treadmill or cycle ergometer exercise. In selected patients, we have combined cardiopulmonary measurements with echocardiographic or nuclear imaging techniques during exercise to provide additional information.

What do the cardiologists at my institution find clinically useful about CPX? One critically important reason for the increased utilization of CPX was the finding that Vo2max was an important independent predictor of survival. The landmark paper of Mancini and colleagues in 1991 reported that for patients with systolic heart failure, Vo2max was closely related to prognosis.[5] Vo2max became an important factor in determining the appropriateness and timing of heart transplantation. Furthermore, Vo2max has been shown to be a powerful predictor of cardiac events for patients with and without clinical cardiovascular disease.[6] Vo2 is strongly related to cardiac output.[7] By directly measuring Vo2 during exercise, the behavior of cardiac output during exertion may be appreciated. My colleagues find this helpful in their assessment of patients with a variety of cardiovascular diseases, including chronic heart failure, coronary heart disease, valvular disease, hypertrophic cardiomyopathy, arrhythmia, heart transplantation, congenital heart disease, infiltrative diseases of the heart, other cardiomyopathies, and pulmonary hypertension. Additional variables obtained during CPX, such as the rate of increase in Vo2, the oxygen pulse, the respiratory exchange ratio, the ventilatory equivalent for CO2, the anaerobic or ventilatory threshold, the breathing reserve, the Vo2 during recovery from exercise, and arterial oxygen saturation are helpful in more completely understanding the oxygen transport system. Compared with conventional stress testing, cardiopulmonary measurements made during exercise provide, in addition to prognosis, additional clinically important information, such as [8]:

  • Potential reasons for a poor exercise capacity
  • Objective measure of effort (peak exercise respiratory exchange ratio)
  • More precise measure of functional capacity than peak exercise workload
  • The severity of impairment
  • The primary cause of dyspnea on exertion
  • The impact of therapy

CPX requires expertise in the use of specialized equipment and in the interpretation of the data. Unfortunately, experience with CPX is not necessarily a component of fellowship training programs in cardiology, especially in past years. Many clinicians do not adequately understand the clinically important information provided by CPX and the technique remains underutilized. Although there have been useful publications describing the methods of CPX and interpretation [9-12], this latest American Heart Association Scientific Statement by Balady et al. is a welcome and needed addition to the literature.[13] The authors have arrived at an expert consensus document based on an extensive review of the available literature and their considerable clinical experience in CPX.

This statement provides a comprehensive and extremely practical overview of CPX for the clinician. Topics are covered concisely and clearly. Descriptions of CPX technology, including system calibration, maintenance, quality control, exercise test protocol selection, and even reimbursement coding are an important part of the paper. Basic gas exchange physiology is discussed. Standard CPX variables are described and their clinical relevance is explained. Clinical applications of CPX are provided for a variety of disease states, such as chronic heart failure, unexplained dyspnea, myopathy, and stroke. Emerging applications of CPX for congenital heart disease, pulmonary hypertension, ischemic heart disease, evaluation of pacemaker function, arrhythmias, and preoperative evaluation for pulmonary resection and bariatric surgery are discussed. The structure and components of the CPX final report are provided.

This publication should greatly assist in educating and inspiring cardiologists to apply CPX appropriately in exercise testing laboratories. In doing so, CPX will assist clinicians in the diagnosis and management of patients with a wide variety of cardiovascular diseases.


  1. Browning EB. Available at: Accessed April 20, 2010.
  2. Taylor HL, Buskirk ER, Henschel A. Maximal oxygen uptake as an objective measure of cardio-respiratory performance. J Appl Physiol 1955;8:73-80.
  3. Jones NL. Clinical Exercise Testing. Philadelphia: WB Saunders, 1975.
  4. Wasserman K, Hansen JE, Sue DY, et al. Principles of Exercise Testing and Interpretation. Philadelphia: Lea & Febiger, 1987.
  5. Mancini DM, Eisen H, Kussmaul W, et al. Value of peak exercise oxygen consumption for optimal timing of cardiac transplantation in ambulatory patients with heart failure. Circulation 1991;83:778-786.
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  7. Astrand PO, Cuddy TE, Saltin B, et al. Cardiac output during submaximal and maximal work. J Appl Physiol 1964;19:268-274.
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  9. Myers JN. Essentials of Cardiopulmonary Exercise Testing. Ventura, CA: VacuMed, 1996.
  10. American Thoracic Society/American College of Chest Physicians. ATS/ACCP statement on cardiopulmonary exercise testing. Am J Respir Crit Care Med 2003;167:211-277.
  11. Milani RV, Lavie CJ, Mehra MR, et al. Understanding the basics of cardiopulmonary exercise testing. Mayo Clin Proc 2006;81:1603-1611.
  12. Mezzani A, Agostoni P, Cohen-Solal A, et al. Standards for the use of cardiopulmonary exercise testing for the functional evaluation of cardiac patients: a report from the Exercise Physiology Section of the European Association for Cardiovascular Prevention and Rehabilitation. Eur J Cardiovasc Prev Rehabil 2009;16:249-267.
  13. Balady GJ, Arena R, Sietsema K, et al. Clinician's guide to cardiopulmonary exercise testing in adults: a scientific statement from the American Heart Association. Circulation 2010. Published online before print, June 28, 2010. 10.1161/CIR.0b013e3181e52e69. 

-- The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association

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