Diaphragm activity during breath holding: factors related to its onset

The electrical activity of the diaphragm during breath holding has been studied. After a period of no or slight tonic activity a marked discharge is recorded, this is followed by others at a progressively higher rate up to the breaking point. The action of the diaphragm, not balanced by antagonist muscles, causes a fall of the intrathoracic pressure. The period of breath holding, in most subjects, may be divided into two parts: the first, characterized by voluntary inhibition of respiratory muscles activity; the second, by involuntary respiratory efforts. The standard error (se) of alveolar Pco 2 values determined at the onset of diaphragm activity in groups of three similar tests is <0.5 mm Hg. Arterial Pco 2 in the brain stem at the onset of diaphragm activity has been estimated: breathing O 2 the mean values ±se in three subjects were, respectively, 46.2 ± 0.3, 46.4 ± 0.2, and 50.2 ± 0.2. The onset of diaphragm activity seems not affected by neurogenic factors related to lung volume or respiratory movements. An alveolar Pco 2 – Po 2 curve at the onset of diaphragm activity has been determined.