Going to extremes of lung volume VOLUME CHANGES IN THE LUNG

VOLUME CHANGES IN THE LUNG are the basis for life-sustaining respiration, yet in many regards, the lung is overengineered and the mechanical limits are most often not approached (11). For some activities, however, the entire volume range of the thorax and lung is necessary; for example, consider childbirth. But what determines the extreme limits of lung volume? Common wisdom (14, 15, 17, 20) holds that total lung capacity (TLC) is achieved because the respiratory muscles distort the chest wall to a maximal volume that is resisted by lung recoil. On the other hand, residual volume (RV) is achieved by the respiratory muscles distorting the chest wall toward a minimal volume that, in adults, is limited by airway closure. Is the conventional wisdom regarding the determinants of maximal and minimal lung volume correct; or are there exceptions? In the current issue of the Journal of Applied Physiology, Loring and colleagues (19) explore the mechanical effects of glossopharyngeal insufflation (GI) and exsufflation (GE) on the respiratory system. They studied four competitive breath-hold divers measuring lung volumes (TLC and RV) before and after GI and GE. Additionally, they measured pressure-volume characteristics of the respiratory system during these maneuvers. The results of the present study are provocative in several respects but most importantly further substantiate similar finding made by Lindholm and Nyren (18) and Seccombe et al. (24) that challenge conventional wisdom regarding the lungs’ ability to tolerate extremes of volume and pressure. Why are GI and GE, and thereby alterations in TLC and RV, advantageous to the breath-hold diver? Glossopharyngeal breathing was first described in the 1950s in postpoliomyelitic patients with severe respiratory muscle weakness (9). These patients were observed swallowing air into their lungs, allowing them to significantly increase their vital capacity. The technique of GI entails filling the lungs to TLC and taking a mouthful of air with a closed glottis. The air is compressed within the oropharynx and then forced into the lungs. This process is then repeated several times to achieve a new, maximal TLC. GE represents the opposite extreme. While at maximal end expiration (RV), air is forced from the lungs into the oropharynx followed by glottic closure. The breath is then exhaled (or used to help equalize ear canal pressures), further decreasing RV (18). Duration of breath-hold time is one important factor that may be improved by glossopharyngeal breathing. GI allows for a greater volume of oxygen to be stored in the lungs under greater pressure and in turn allows a longer breath hold (19, 23). In addition, physiologists have long believed that the maximal breath-hold depth is determined by compression of thoracic gas as dictated by Boyle’s law (P1V1 P2V2, where P is pressure and V is volume). If a diver attempted to descend to a depth that resulted in TLC compression below RV, “thoracic squeeze” would result with the risk of lung rupture (27). Yet current world records in breath-hold diving greatly exceed predictions based on this model (13). One explanation may be glossopharyngeal breathing. GE may allow breath-hold divers to increase their diving depths by repeatedly deforming the chest to low volumes, thereby reducing the outward recoil of the chest at very low volumes. This would reduce thoracic squeeze and make the divers more like dolphins, whose chest collapses impressively at depth, without developing high transthoracic pressure (4). The authors of the present study (19) suggest that TLC can be increased beyond normal limits with GI to an extent limited by a “sensation of fullness” rather than by a summation of mechanical forces. The magnitude of TLC achieved after GI is impressive but variable, with one diver increasing their calculated dimensional TLC by 2.85 liters to a new maximal gas volume of an additional 4.16 liters and a second diver able to increase TLC by only several hundred milliliters (19). Interestingly, all divers had greater than predicted baseline TLC, which in the past has been attributed to training or more likely selection of mechanical characteristics important for performance of these feats (1, 6, 8, 10, 25). However, it is also possible that repeatedly increasing the lung beyond TLC alters “fullness sensation” and thereby establishes a new larger endpoint for TLC. Whereas TLC is felt to be determined by a summation of opposing forces, RV is said to be determined by airway closure in adults (17, 20) and can also be altered by glossopharyngeal breathing (18, 19). As was seen with the alterations in TLC after GI, changes in RV after GE were variable between divers. Three of the four divers were able to dramatically decrease their RV (0.31 to 0.45 liter), while the fourth diver was able to make only modest changes (0.09 liter). These reductions in RV were confirmed by chest computed tomography. The ability to decrease RV may result from physiological differences inherent in the individuals’ lung or may result from training; it is equally likely that both factors play a role. Increasing TLC and decreasing RV may be advantageous maneuvers for breath-hold divers, but altering lung volumes