Analysis of lung ventilation in the aestivating lungfish Protopterus aethiopicus.

During aestivation, the breathing pattern of the lungfish changed from the usual aquatic pattern of a single breath followed by prolonged apnea to the pattern of alternating tachypnea and apnea that characterizes Cheyne-Stokes breathing. As aestivation continued, the number of breaths per tachypneic period increased gradually to reach a steady level at about the 3rd mo. During the bouts of tachypnea, minute ventilation increased because of the increase in respiratory frequency even though tidal volumes decreased. Ventilation of the lungs during aestivation appeared to involve the same mechanism as during life in water, i.e., a buccal force pump. The breathing cycle began with the aspiration of air into the mouth as the buccal cavity enlarged. Lung deflation then occurred abetted by contraction of the abdominal muscles. Lung inflation followed and involved the "swallowing" of air form the posterior buccal region into the lungs. Cycles of deflation and partial inflation of the lungs were repeated 8-40 times (tachypneic period). At the end of the tachypneic cycle, after the last deflation, a series of larger buccal force-pump maneuvers reinflated the lungs before the start of the apneic period.