Advances in Pulmonary Laboratory Testing
Many advances in the pulmonary laboratory have occurred over the last 2 decades. The majority of these advances involve automation of routine pulmonary function measurements, refinement of the diffusing capacity test, innovation of fast-response analyzers to obtain “real time” cardiopulmonary exercise data, and progression into the standard use of the body plethysmograph for assessment of lung volumes and airway resistance. Although there are many techniques that are applicable primarily in the research setting, there are several emerging methods that may offer unique clinical insight or may offer advantages over more “traditional” techniques. The following sections offer a brief review of several methods that are gaining popularity or, through advances in technology, are becoming available in the clinical setting. These include the following: (1) the use of the tidal flow-volume (FV) loop measurement during exercise (extFVL) to help distinguish the degree of ventilatory limitation; (2) the use of the negative expiratory pressure (NEP) technique to detect expiratory flow limitation; (3) the use of expired nitric oxide (NO) in the assessment of airway inflammation; and (4) the forced oscillation (FO) technique to assess airway resistance.
Assessment of Ventilatory Limitation Using the extFVL
There has been a growing trend in both research and clinical laboratories to find alternative ways to evaluate ventilatory limitation during exercise.
This stems in part from the realization that patients may discontinue exercise due to ventilatory constraints and dyspnea prior to the achievement of classic indexes associated with ventilatory limitation (ie, minute ventilation [Ve] that reaches the maximum voluntary ventilation [MVV] or a rise in arterial CO2) and that Ve limitation is not an “all or none” phenomenon. Thus, investigators have used techniques such as breathing helium-oxygen mixtures (to increase the maximal FV envelope [MFVL]), inspired CO2, and dead space loading (to stimulate ventilation) to assess whether Ve is truly con-strained. Another approach that has gained popularity includes the measurement of the extFVL and its plotting within the MFVL. This latter technique provides a good visual index of the degree of VE constraint, allows a more detailed approach to defining VE limitation (relative to the Ve/MVV relationship), and has gained popularity due to the ease of measurement using many of the commercially available metabolic carts. Figure 1 shows an example of the rest and peak exercise FV responses in a healthy, average fit adult plotted within the MFVL, Key features of the VE response in the healthy adult are shown in Table 1. In this particular example, at peak exercise, there is only minimal encroachment on the MFVL, which implies that there is little constraint to breathing. More info
Figure 1. Example of rest and peak exercise FV responses in a healthy’ average fit adult plotted within the MFVL. Key features of the VE response in the healthy adult include the following: (1) a drop in the EELV due to the recruitment of expiratory muscles; (2) an increase in Vt through equal encroachment on inspiratory and expiratory reserve volumes (IRV and ERV’ respectively); (3) expiratory flow rates generally well within the maximum available flow rates (especially at the higher lung volumes); (4) avoidance of high EILVs (a high elastic load) or extremely low EELVs (reduced compliance and airway closure); and (5) inspiratory flow rates that are well within the maximal available flows. Exp = expiration; Insp = inspiration.
Table 1—Key Features of the Ve Response to Exercise in a Healthy Adult
|Response During Exercise||Advantage|
|Decrease in lung volume at the end of a normal expiration ( 2 EELV) through recruitment of expiratory muscles||Optimizes inspiratory muscle length|
|Vt increases through equal encroachment on the IRV and ERV||Vt is kept on the linear portion of the pressure-volume relationship of the lung and chest wall|
|Expiratory flow rates remain within the maximum available flow rates (particularly at the higher lung volumes)||Minimal expiratory flow limitation|
|Avoidance of high EILVs||Reduces the elastic load|
|Inspiratory flow rates are well within the maximal available flows||Inspiratory flow reserve|