Effects of hyperventilation on repeated breath-holding while in a fasting state: do risks outweigh the benefits?

INTRODUCTION:Breath-holding preceded by either an overnight fast or hyperventilation have been shown to potentiate the risk of a hypoxic blackout. However, no study has explored the combined effects of fasting and hyperventilation on apneic performance and associated physiological responses. METHODS:Nine, non-divers (8 males) attended the laboratory on two separate occasions (≥48-h apart), both after a 12-h overnight fast. During each visit a hyperoxic rebreathing trial was performed followed by three repeated maximal static apneas preceded by either normal breathing (NORM) or a 30s hyperventilation (HYPER). Splenic volume, hematology, cardiovascular and respiratory variables were monitored. RESULTS:There were no inter-protocol differences at rest or during the hyperoxic rebreathing for any variable (p≥0.09). On 9 occasions (8 in HYPER), the subjects reached our safety threshold (oxygen saturation 65%) and were asked to abort their apneas, with the preponderance of these incidents (6) occurring during the third repetition. Across the sequential attempts, longer apneas were recorded in HYPER [median(range),220(123-324)s vs. 185(78-296)s,p≤0.001], with involuntary breathing movements occurring later [134(65-234)s vs. 97(42-200)s,p≤0.001] and end-apneic partial end-tidal pressures of oxygen (PETO2) being lower (p≤0.02). During the final repetition, partial end-tidal pressure of carbon dioxide [(PETCO2),6.53±0.46kPa vs. 6.01±0.45kPa,p=0.005] was lower in HYPER. Over the serial attempts, pre-apneic tidal volume was gradually elevated [from apnea 1 to 3, by 0.26±0.24L (HYPER) and 0.28±0.30L (NORM),p≤0.025], with acorrelation noted with pre-apneic PETCO2 (r=-0.57,p<0.001) and PETO2 (r=0.76,p<0.001), respectively. CONCLUSION:In a fasted state, pre-apnea hyperventilation compared to normal breathing leads to longer apneas, but may increase the susceptibility to a hypoxic blackout.