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 de termination of breath-holding time (BHT). Earlier studies have shown that c hemical factors can be modified by training. The aim of this study was to e stablish whether BHT can be lengthened or shortened by changes in non-chemi cal factors according to the degree of training of the divers. Hyperoxic hy percapnic rebreathing challenges interrupted by different periods of volunt ary apnoea (20, 40 and 50 s) were performed by divers (n=8) and non-divers (n=6). The relationships between ventilation ((V) over dot (E)) and end-tid al PCO2 and mean inspiratory flow (V-T/T-I) where V-T is the tidal volume a nd T-I the inspiration time) and end-tidal PCO2 were measured before (pre) and after (post) apnoea for each population. The slopes of these linear reg ressions ((V) over dot (E,s) and V-T/T-I,T-s, respectively) reflected respi ratory centre activity. (V) over dot (E,s)-post and V-T/T-I,T-s-post increa sed with BHT and were higher in the non-divers than in divers. There were n o significant differences between V-pre and (V) over dot (E,s)-post or betw een V-T/T-I,T-s-pre and V-T/T-I,T-s-post in divers for all BHT. However, in non-divers, significant differences between pre and post values appeared a t 20, 40 and 50 s for V-T/T-I,T-s and at 40 and 50 s for (V) over dot (E,s) . Non-chemical factors were obtained by calculating the differences between pre and post values and were different at 20 and 40 s for (V) over dot (E, s) (P <0.05) and for all BHT for V-T/T-I,T-s (P <0.05). The non-chemical fa ctors increased significantly with BHT and were higher in the non-divers. T hese adaptations in divers may modify the breath-hold breaking point and co nsequently increase BHT.