Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 5: Pulmonary Function Measurements

5-1	A low final helium concentration indicates a large functional residual capacity (FRC). The larger the FRC, the more it dilutes the spirometer’s helium concentration as the subject rebreathes the helium test gas.

5-2	For a given total volume of exhaled gas collected over the 7-minute nitrogen washout test, the lower its measured N2%, the smaller the functional residual capacity. To illustrate, once we know that the concentration of nitrogen in the exhaled gas collected at the test’s end is 6% (Figure 5-3, B), we can compute the volume of N2 that was in the lungs at the test’s beginning (A); that is, the volume of nitrogen must be equal to 6% of 35,000 mL, or 2100 mL. When this 2100 mL of nitrogen was in the lung, it represented 80% of the lung volume (air contains 80% nitrogen); thus lung volume can be computed as follows: 2100 = 0.8(x), where x = lung volume. If the N2% in the accumulated exhaled volume at the end of the test in Figure 5-3, B, had been only 3%, for example, then only 3% of 35,000 mL, or 1050 mL, would represent 80% of the lung volume, and the computed lung volume would thus be smaller.

5-3	An increased residual volume to total lung capacity (RV/TLC) ratio means RV represents a greater percentage of the TLC, and therefore the vital capacity (VC) must be decreased: VC = TLC  RV.

5-4	1. Thoracic gas volume (TGV), as measured by plethysmography, includes air trapped in the lung that is not in communication with a patent airway. The N2-washout test can measure only gas volume that communicates with patent airways.
2. Air trapping is exaggerated in forced expiratory maneuvers because of the earlier onset of dynamic bronchiolar compression (see Chapter 3). Thus in patients prone to air trapping, a forced vital capacity test is likely to yield smaller exhaled volumes than a “slow” vital capacity test.

5-5	The forced expiratory volume (FEV1) is low in restrictive disease simply because the forced vital capacity (FVC) is low. The FEV1’s percentage of the FVC (FEV1%) is normal if airway obstruction is absent.

5-6	An increased effort will most likely improve the peak expiratory flow (PEF) results because this test measures the expiratory flow rate beginning from the total lung capacity (TLC), whereas the FEF25%-75% (forced expiratory flow) test measures expiratory flow rate beginning from a lower lung volume. Dynamic airway compression is unlikely to occur at high lung volumes within the first 1200 mL of the TLC because pressures inside the airways never fall below the surrounding intrapleural pressure. As expiration proceeds to lower lung volumes, pressures inside the airways do fall below intrapleural pressure, and airway compression progressively 
increases. Progressive narrowing of the airways occurs after the patient exhales the first 20% of the forced vital capacity (FVC) and continues until many airways are closed at residual volume. This phenomenon limits the maximal expiratory flow that can be achieved after the first 20% of the FVC is exhaled (i.e., maximal expiratory flow rate is effort independent).

5-7	Low lung volumes are associated with lower elastic recoil forces, limiting the maximal attainable expiratory pressure gradient that can be generated to drive the flow rate.

5-8	The maximum ventilation healthy individuals can achieve during exercise is always less than the measured maximum voluntary ventilation (MVV); this implies that some factor other than ventilatory capacity limits exercise in healthy people. In fact, the maximum cardiac output capacity normally limits exercise. Reduced maximum ventilatory capacity clearly limits exercise in patients with severe chronic obstructive pulmonary disease (COPD); these individuals often attain or even exceed their MVVs, implying that their ventilatory capacity is the factor that limits exercise intensity. Patients with severe COPD often fail to attain their predicted maximal heart rates during maximal exercise, which shows that cardiac pumping capacity is not the factor that limits their exercise.