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Airway Resistance Measurements in the Evaluation of Obstructive Lung Disease by Michael Snow, RPFT

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Airway Resistance Measurements in the Evaluation of Obstructive Lung Disease by Michael Snow, RPFT Clinical Perspectives Airway Resistance Measurements in the Evaluation of Obstructive Lung Disease by Michael Snow, RPFT hronic obstructive Measuring maximal airflow is determined by the size pulmonary disease (COPD) and airflow of the airway. Once maximal air- Casthma are growing health prob- Most commonly, the diagnosis flow has been achieved, continu- lems and are expected to be of airway obstruction is based on ing to increase driving pressure among the leading causes of changes in airflow as assessed by will not increase airflow. In a mortality worldwide as we enter maximal, forced expiratory patient model, the expiratory the next century. Recent studies maneuvers (FEV1 and PEF). airflow proceeds from small, evaluating the role of quality of Although these maneuvers non-rigid, peripheral airways, life indicators with educational reflect changes in airway resis- combining into larger, more interventions in asthma, as well tance, forced expiratory maneu- rigid, central airways. The cumu- as a growing understanding of vers reflect a complex, dynamic lative cross-sectional area of the the role of inflammatory relationship between patient larger airways determines the processes in the progression of effort, muscle strength, compli- maximal airflow. COPD, have highlighted the ance, and elastic recoil, as well as The magnitude of the inspired importance of early diagnosis airway resistance. As a result, volume is determined by the and treatment of airway using forced expiratory maneu- elastic properties of the lung obstruction.1-4 The primary vers to assess changes in airway combined with muscle strength cause of airway obstruction, resistance can frequently be mis- required to overcome these regardless of etiology, is leading or create uncertainty forces. The shape of the maximal increased airway resistance about the diagnosis or the thera- expiratory flow-volume curve is a (Raw). Unfortunately, the rela- peutic value of treatments. result of decreased Raw and tionship of airway resistance to In a simple model of a single increased elastic recoil due to determinants of airflow is less rigid airway, airflow results from lung expansion combining with well understood and, as a result, driving pressure overcoming the patient effort to produce higher underutilized. resistance of the airway. Maximal expiratory flows near total lung 48 AARC Tımes January 2000 Clinical Perspectives capacity, which progressively tion, or airway collapse. Bron- excluded. It is usually more effec- decrease as residual volume is choconstriction can occur as a tive to monitor the parameter approached. result of mucosal buildup with directly rather than infer change Since patients frequently have the airway, as in bronchitis. Bron- from an indirect measurement.9,10 the ability to generate greater dri- choconstriction can result from Multiple approaches to clinical ving pressure than is required to bronchospasm or inflammatory measurement of Raw are available. achieve maximal airflow, signifi- processes that compress the air- Interrupter, forced oscillations, cant increases in Raw can be way, as seen in asthma or COPD. and body plethysmography are missed when forced expiratory Airway collapse can result from the primary options, each of volume in one second (FEV1) or increased compliance or a combi- which offer relative advantages peak expiratory flow (PEF) are nation of occlusion and compres- and disadvantages. the only measured parameters. sion, as seen in emphysema and The interrupter technique is This can be easily demonstrated cystic fibrosis. Typically, collapse simple to implement and only by performing forced expiratory patterns are seen on the expira- requires that airway pressure and maneuvers with and without an tory phase rather than during flow be monitored while flow is external, fixed resistance (see inspiration. abruptly interrupted. Resistance Table 1). Even with as much as 4.0 is obtained by dividing the pres- Airway resistance cm/H2O/L/sec added, the FEV1 sure change after interruption by and PEF do not change in clini- measurement methods the flow obtained immediately cally meaningful ways. Documented correlation before the interruption. The During quiet, non-forced between Raw and FEV1 only interruption may be repeated breathing, the influence of elastic shows that both variables share a rapidly to permit multiple mea- recoil and compliance are mini- common causative factor.5-8 The surements over a single breath. mized except in the case of airway greater sensitivity of airway resis- This method is easy to use and collapse. Increased airway resis- tance should be intuitively obvi- requires minimal equipment. tance is predominantly due to air- ous when other influences, such Resistance measured by inter- way occlusion, bronchoconstric- as effort and muscle strength, are rupter protocols provides a rea- Table 1: Effect of Fixed Resistance on Forced Expiratory Maneuvers Baseline + 4.0 cm H2O Percent Resistance Change FVC 4.33 4.36 1 FEV1 3.20 3.12 -2 FEF25-75% 2.54 2.32 -9 FEFmax 7.90 5.55 -30 Raw 2.96 7.04 138 sGaw 0.40 0.14 -64 AARC Tımes January 2000 49 Clinical Perspectives sonably good indication of Raw as fulness of the measurement for decreasing lung volumes on air- long as proper methodology is fol- young children and older patients. way caliber. lowed for supporting the cheeks The approach is widely used in A common compensatory during measurement and negligible Europe but has been slow to mechanism to chronic increases compliance differences exist within achieve acceptance in the United in airway obstruction is to the airways. Significant compliance States. This is probably primarily increase the functional residual differences would cause more com- due to lack of familiarity with the capacity (FRC), and ultimately pliant airways to buffer less compli- measurement. This also creates dif- TLC, in order to expand the air- ant compartments, resulting in the ficulty in interpreting the results. ways and maintain Raw within measurement of proximal airways One of the drawbacks is the lack of acceptable ranges. This adjust- resistance only. This frequency- standardization between the vari- ment of FRC is one reason why dependent effect is the result of ous commercially available sys- some patients with significant insufficient time to permit pressure tems, although this should change reduction in FEV1 may have rela- equilibration within the airways and with the adoption of standards. tively normal Raw. After bron- can make interpretation of the Body plethysmographic mea- chodilation, the reduced airway results more complex. Agood analy- surement of Raw is similar to the obstruction allows the overinfla- sis of the interrupter technique is interrupter method but with an tion to decrease, in turn causing presented by Bates et al.11 important difference. As in the the resistance to settle back into a Forced oscillation technique interrupter technique, flow and air- level appropriate for the reduced (FOT) measures resistance by way pressure are continuously mea- lung volume. In this case, there is imposing flow oscillations on the sured. A shutter is closed during a significant bronchodilation airways during spontaneous breathing efforts, and the pressure effect despite the lack of an appar- breathing. This can be accom- changes are measured. The pressure ent change in Raw. It is this plished in several ways, but most changes are compared with the flow dynamic interaction between air- commonly an external loudspeaker changes immediately prior to inter- way resistance and lung volume is used to generate cyclical waves ruption. Unlike the interrupter that makes Raw difficult to evalu- that are applied to the airway. These method, however, the occlusion is ate as an isolated parameter. waves are, in fact, slight changes in complete and sustained, permitting One of the primary advantages airflow. Airway pressure changes pressure equilibration to take place of body plethysmography, com- occur in response to the changes in within the respiratory circuit. The pared to other techniques for airflow. Since the frequency of the flow can be measured during quiet assessing Raw, is the ability to cor- airflow changes is known, it is pos- breathing or with gentle panting. relate the lung volume with the sible to compare the pressure The panting technique tends to measured Raw. Compensatory changes with the flow changes and minimize oropharyngeal compo- changes in lung volume can be calculate airway resistance and nents and minimize the effects of observed by measuring specific other parameters. The analysis of temperature and humidity differ- conductance (sGaw) or specific the resulting pressure responses ences between inspired and expired resistance (sRaw). Essentially, can provide useful information not gas. Descriptions of plethysmo- sGaw and sRaw evaluate resistance 17-19 only regarding Raw, but also can graphic techniques are available. and conductance at the lung vol- potentially help evaluate central ume at which they were mea- 12,13 Lung volume and versus peripheral resistances. sured. For sGaw, this means divid- Numerous studies have evalu- Raw relations ing the measured airway ated various approaches to Raw is strongly correlated with conductance (Gaw) by the FOT.14-16 Current research is lung volume because as the lungs observed thoracic gas volume focused on the dynamic informa- expand, the airways also dilate. As (TGV). For sRaw, the measured tion that can be obtained by scan- you approach total lung capacity Raw is multiplied by the
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