Respiratory Muscle Pressure Development during Breath Holding in Apnea Divers

INTRODUCTION: We sought to characterize the patterns of active pressure development of the inspiratory and expiratory rib cage muscles (P(rcm,i) and P(rcm,e)), the diaphragm (P(di,i)), and the expiratory abdominal muscles (P(abm,e)) during maximal "dry" breath holding in trained apnea divers (n = 8). METHODS: Respiratory contractions were assessed via esophageal and gastric manometry. It was expected that inspiratory/expiratory pressures would progressively increase in both magnitude and frequency during the struggle phase, and that inspiratory rib cage muscle pressures would rise at a rate exceeding that of the diaphragm by the break point. RESULTS: P(rcm,i), P(di,i), P(rcm,e), and P(abm,e) significantly increased from the beginning until the end of the struggle phase (P < 0.05). Moreover, P(di,i)/P(rcm,i) and P(abm,e)/P(rcm,e) ratios had declined by the break point (P < 0.05), indicating that rib cage muscles increased their contribution to net inspiratory/expiratory pressure development by the end of the breath hold, relative to that contributed by the diaphragm and abdominal muscles. The pressure-time indices of the diaphragm and inspiratory rib cage muscles continuously increased over the struggle phase (P < 0.05). CONCLUSIONS: The "extradiaphragmatic" shift in inspiratory muscle recruitment, commensurate with increasing P(rcm,e) and P(abm,e), may reflect an extreme loading response to breathing against a heavy elastance (i.e., closed glottis). In addition, the relative intensity of diaphragmatic and inspiratory rib cage muscle contractions approaches potentially "fatiguing" levels by the break point of maximal breath holding.