Respiration Physiology
Discontinuous lung sounds and hysteresis in control and Tween 20-rinsed excised rat lungs
Introduction
In a recent publication, we examined the contribution of the opening and closing of lung units to lung pressure–volume (Pl–Vl) hysteresis (Cheng et al., 1995). That study tested the predictions of a simple lung model describing the recruitment–derecruitment process by examining the hysteresis of pressure–volume loops as a function of end-expiratory pressure (EEP). The goal of this study was to correlate the production of discontinuous lung sounds, which are potentially indicative of the recruitment–derecruitment process, with the hysteresis of Pl–Vl loops recorded over a wide range of EEPs. Our model of lung expansion assumes that individual lung units open abruptly when their particular opening pressure has been reached during lung inflation. It also assumes that lung units are capable of generating discontinuous, explosive sounds or crackles during the opening process. In the past, it has been widely accepted that crackles are generated by the opening of lung structures which we broadly define in this study as lung units (Forgacs, 1967, Forgacs, 1974, Forgacs, 1978, Nath and Capel, 1974, Ploy-Song-Sang and Schonfeld, 1982, Fredberg and Holford, 1983, Munakata et al., 1986)
Forgacs, 1967, Forgacs, 1978 and Forgacs et al. (1971) have argued rather convincingly that crackles result from the rapid equalization of gas pressure between upstream and downstream airway sections following their sudden opening during inspiration. He presented as evidence, the often striking repetitive occurrence of crackles during succeeding inspirations, even when the inspiration followed a cough. These crackles rarely occurred during expiration and were often localized over dependent areas of the lung where gravitational stress predisposes the airways to collapse.
Fredberg and Holford (1983) have conceded that the opening of closed lung units is the mechanism likely to be responsible for the generation of lung crackles, but they suggested that the sudden equalization of pressure may not be required to produce the crackle sound pressure wave. Instead, they proposed that when a closed unit opens, the sudden release of tension in the airway wall, as it approaches a new lower energy state, produces sound energy in the form of a crackle.
There are still some questions as to the size of the region of the lung that opens when a crackle is generated. Munakata et al. (1986) have reported that crackles in normal excised dog lungs are produced by the opening of airways and are not generated by the sudden inflation of individual alveoli.
Even though neither the exact mechanism responsible for generating crackles nor the size of the lung structures from which the sound originates has yet been determined, there appears to be general agreement that crackles originate during the discontinuous opening of small lung structures. As a consequence, the sound energy associated with discontinuous lung sounds has potential not only as an index of when lung units open but also as an indicator of the number of lung units that open. In the past it has been suggested that discontinuous events that occur during lung inflation and deflation could have a significant effect on the hysteresis of lung Pl–Vl curves (Frazer et al., 1985). In an attempt to test this hypothesis, lung sound power was compared with the hysteresis for individual Pl–Vl loops which was computed by a previously described method (Cheng et al., 1995). This analysis was based on the work of Bachofen and Hildebrandt (1971) in which the hysteresis area, H, of lung Pl–Vl curves which formed closed loops were found to be related to the change in transpulmonary pressure (ΔP) and tidal volume (Vt) through the relationship H=K(Vt)(ΔPl) where K represents a proportionality constant. Rearrangement of the terms in this expression leads to the hysteresis index (H/Vt) where (H/Vt)=K(ΔPl). It should be noted that the lung tissue hysteresivity, η, as described by Fredberg and Stamenovic (1989) is related to K, by the expression η=[(π/4K)2−1]1/2 while the dynamic elastance, Edyn, can be written in terms of K as Edyn=[1−(K/π)2]1/2ΔPl/ΔVl.
In order to explore the role of surface forces at the air–liquid interface of the lung with regard to sequential lung unit inflation–deflation, we measured both Pl–Vl loop hysteresis and the power of lung sounds generated in control lungs and in lungs rinsed with Tween 20. Tween 20 is a non-ionic detergent that is often used to replace the normal alveolar surface film and to generate an air–liquid interface having a near constant surface tension of approximately 30 dynes/cm (Radford, 1962). It should be noted that rinsing lungs with Tween 20 alters the configuration of lung microstructure indicating that the lung’s peripheral geometry is a function of surface tension (Bachofen et al., 1979).
Section snippets
Methods
Lungs were excised from Long Evans hooded male rats weighing between 300 and 350 g. Animals were exsanguinated via the abdominal aorta following intraperitoneal pentobarbital injection (85 mg/kg). The abdominal cavity was opened and a bilateral pneumothorax was obtained by penetrating the sternal area of the diaphragm. Next, the rib cage was sectioned on both sides of the midline, and the heart, lungs, and diaphragm were removed en bloc. The excised lungs were rinsed in saline and placed in a
Results
The instantaneous sound pressure level and lung volume, recorded as a function of tracheal pressure on an X-Y1-Y2 recorder, are shown in Fig. 2. The acoustical sound power (Leq±SEM), measured at the trachea during lung inflation, following lung deflation to a decreasing sequence of end-expiratory pressures is shown in Fig. 3A. These results illustrate that the average sound power level resulting from the summation of all the discontinuous lung sounds during lung inflation increased as
Discussion
Tracheal lung sound measurements were used in this study to explore the effects of lung unit recruitment–derecruitment on pulmonary hysteresis. Despite the fact that the stethoscope has been used for more than a century and a half, the connection between discontinuous lung sounds and lung unit opening has been difficult to prove. This is due, in part, to the subjectivity of the interpretation of stethoscopic auscultation (Wooten et al., 1978, Thacker and Kraman, 1982, Murphy et al., 1984).
In
Acknowledgements
The authors wish to thank W.T. Goldsmith for his technical assistance, and N. Nehrig and K.C. Weber for their help in preparing the manuscript. Brand names are used for information only and do not constitute endorsement by the National Institute for Occupational Safety and Health.
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