Chest
Volume 116, Issue 2, August 1999, Pages 488-503
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Emerging Concepts in the Evaluation of Ventilatory Limitation During Exercise: The Exercise Tidal Flow-Volume Loop

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Traditionally, ventilatory limitation (constraint) during exercise has been determined by measuring the ventilatory reserve or how close the minute ventilation (

e) achieved during exercise (ie, ventilatory demand) approaches the maximal voluntary ventilation (MVV) or some estimate of the MVV (ie, ventilatory capacity). More recently, it has become clear that rarely is the MVV breathing pattern adopted during exercise and that the
e/MVV relationship tells little about the specific reason(s) for ventilatory constraint. Although it is not a new concept, by measuring the tidal exercise flow-volume (FV) loops (extFVLs) obtained during exercise and plotting them according to a measured end-expiratory lung volume (EELV) within the maximal FV envelope (MFVL), more specific information is provided on the sources (and degree) of ventilatory constraint. This includes the extent of expiratory flow limitation, inspiratory flow reserve, alterations in the regulation of EELV (dynamic hyperinflation), end-inspiratory lung volume relative to total lung capacity (or tidal volume/inspiratory capacity), and a proposed estimate of ventilatory capacity based on the shape of the MFVL and the breathing pattern adopted during exercise. By assessing these types of changes, the degree of ventilatory constraint can be quantified and a more thorough interpretation of the cardiopulmonary exercise response is possible. This review will focus on the potential role of plotting the extFVL within the MFVL for determination of ventilatory constraint during exercise in the clinical setting. Important physiologic concepts, measurements, and limitations obtained from this type of analysis will be defined and discussed.

Section snippets

Information Gained From the extFVL/MFVL Analysis

By aligning the extFVL within the MFVL, specific information is provided on the following: (1) the degree of expiratory flow limitation; (2) breathing strategy (ie, changes in EELV); (3) elastic load, as represented by the EILV as a percent of TLC (EILV/TLC) or the tidal volume (Vt) relative to inspiratory capacity (IC); (4) inspiratory flow reserve; and (5) a theoretical estimate of the ventilatory capacity based on the EELV and the maximal expiratory/inspiratory flows available over the range

The Traditional View of Ventilatory Limitation

The traditional use of the MVV or some estimate of the MVV such as the FEV1 multiplied by 35 or 40 relative to the exercise ventilation as an assessment of breathing reserve has several advantages. It provides a general (although variable) approximation of ventilatory capacity, it is readily and widely applied, it is easily understood, and it requires minimal analysis. However, the use of the MVV to estimate the available ventilatory capacity during exercise and to determine whether or not

Assessing Ventilatory Constraint Using the extFVL

As the sophistication of conventional exercise systems improve, it will become easier to measure and to assess the degree of flow limitation as well as changes in EELV and EILV, with minimal changes in the standard clinical exercise protocol. Conventional exercise metabolic systems have evolved using the pneumotachograph, hot wire anemometer, and turbine (bi-directional rotating vane) technology that provide software to evaluate tidal breathing FV loops relative to a MFVL. Although there are

Exercise Patterns in Health And Disease

The following section will review the tidal FV responses to exercise in various representative clinical examples contrasted with responses observed in the healthy young and older adults. It should be emphasized that the degree of ventilatory constraint is indeed a balance between ventilatory demands and the available capacities. Thus, even the healthy young adult may approach severe ventilatory limitations, albeit at a metabolic and ventilatory demand that far exceeds the patient populations.

Current and Future Aims

Over the last 30 years, little progress has been made in the typical clinical setting in trying to better define mechanisms of exercise intolerance and dyspnea, particularly when associated with the possibility of mechanical constraints imposed by the respiratory system. The classic MVV, while easily applied, is variable and has proved to be limited in its overall usefulness of advancing our understanding of ventilatory limitation during exercise. As such, investigational studies have attempted

Measurement of the extFVL

Most clinical exercise testing laboratories today employ automated systems for measurement of gas exchange during exercise (eg,

e,
o2, and carbon dioxide production). The majority of these systems use a device for measuring flow (eg, pneumotachograph, mass-flow anemometer, turbine) and integrate a flow signal to obtain volume. Few of these automated systems, until recently, have offered continuous output of flow and volume in order to obtain the tidal FV data necessary for plotting the tidal

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