Breath-holding time: effects of non-chemical factors in divers and non-divers.

Two main factors (chemical and non-chemical factors) are involved in the determination of breath-holding time (BHT). Earlier studies have shown that chemical factors can be modified by training. The aim of this study was to establish whether BHT can be lengthened or shortened by changes in non-chemical factors according to the degree of training of the divers. Hyperoxic hypercapnic rebreathing challenges interrupted by different periods of voluntary apnoea (20, 40 and 50 s) were performed by divers (n=8) and non-divers (n=6). The relationships between ventilation (VE) and end-tidal PCO2 and mean inspiratory flow (VT/TI) where VT is the tidal volume and TI the inspiration time) and end-tidal PCO2 were measured before (pre) and after (post) apnoea for each population. The slopes of these linear regressions (VEs and VT/TI,s, respectively) reflected respiratory centre activity. VFs-post and VT/TIs-post increased with BHT and were higher in the non-divers than in divers. There were no significant differences between VFs-pre and VFs-post or between VT/TIs,-pre and VT/TIs-post in divers for all BHT. However, in non-divers, significant differences between pre and post values appeared at 20, 40 and 50 s for VT/TI,s and at 40 and 50 s for VEs. Non-chemical factorswere obtained by calculating the differences between pre and post values and were different at 20 and 40 s for VEs (P<0.05) and for all BHT for VT/TI,s (P<0.05). The non-chemical factors increased significantly with BHT and were higher in the non-divers. These adaptations in divers may modify the breath-hold breaking point and consequently increase BHT.