Commercially available metabolic carts use various flow sensing devices to measure the MFVL at rest and FV responses during exercise, typically in conjunction with standard gas-exchange measurements. Two or three MFVLs are produced at rest prior to exercise in addition to several maximal inspiratory maneuvers from functional residual capacity. Exercise tidal FV responses (typically two to five) are obtained toward the end of each work intensity, followed by repeat inspiratory capacity (IC) maneuvers. The IC measurements are used to place the tidal loops within the MFVL by aligning the maximal inspiratory volume points at total lung capacity (TLC). The change in IC, therefore, represents changes in end-expiratory lung volume (EELV), assuming that TLC does not change. Since bronchodilation or constriction may occur with exercise, MFVLs may also be obtained either during or immediately after exer-cise. Previous studies have taken single representative tidal exercise breaths or a mean of two or more breaths to plot within the largest MFVL obtamed.
Information Gained From the extFVL
Varied information has been gleaned from the extFVL plotted within the MFVL, from simple visual information to actual quantification of various indexes of VE constraint. To date, no consensus exists on which variables will prove to be the most useful clinically or how best these may be quantified, The most commonly reported variables are shown in Table 2. Other investigators have used various additional indexes’ such as tidal volume (Vt) relative to IC or vital capacity in conjunction with breathing frequency to help describe the degree of Ve constraint.

Clinical Application
The extFVL has been examined in a number of populations. Varied information has been obtained’ including the specific source and degree of VE constraint, possible mechanisms of associated dyspnea, as well as effectiveness of treatment (eg, surgery or bronchodilator therapy). The advantages over a traditional assessment of VE reserve (Ve/MVV ratio) includes more specific information on the mechanisms of VE constraint and more formal quantification of the degree of constraint.
Table 2—Indices of Ventilatory Constraint Using extFVLs Plotted Within the Maximal FV Envelope

Indices of V/e Constraint How Indices Are Assessed Proposed Role in V/e Constraint
Expiratory flow limitation % Vt meeting or exceeding the expiratory boundary of the MFVL May cause dynamic compression of airways, reflex inhibition of V/e, increases WOB and cost of breathing
Inspiratory flow reserve Peak inspiratory flow during exercise relative to the maximal flow available High WOB and cost of breathing, respiratory muscles working in a fatiguing range
Elastic load EILV/TLC ratio Increased WOB and cost of breathing, inspiratory muscles more easily fatigued
Dynamic rise in EELV Decrease in IC (EELV = TLC — IC) Reduces inspiratory muscle length, increases elastic load, WOB and cost of breathing
Ve capacity Calculated Ve based on the extFVL, EELV and MFVL Overall assessment of breathing reserve, Ve/Ve capacity