Beat-to-beat autonomic cardiovascular response to short-term 100%02 breathing: A time-frequency analysis approach

In sixteen healthy volunteers, we assessed the effects produced by short-term 100% O2 breathing on the instantaneous time courses of R-R intervals (RR), arterial pressure (AP), arterial oxygen saturation (SaO
₂
), respiratory volume (Res), and, estimated by a time frequency distribution, the high frequency powers of RR (HF
_rr
) and Res (HF
_Res
), the low frequency powers of RR (LF
_rr
) and systolic pressure (LF
_SP
), the LF
_RR
/HF
_RR
ratio, baroreflex sensitivity (BRS) by alpha index and respiratory sinus arrhythmia sensitivity (RSAS). Short-time hyperoxemia provoked various effects with different latencies: a) RR lengthening, with 15-s latency; b) AP increase, with 75-s latency, c) BRS and RSAS increases with 120-s latency, d) HFrr increase and LF
_SP
and LFrr/HFrr decreases, with 135-s latency, and d) no significant changes on Res. During hyperoxemia, the instantaneous time courses of these variables are fluctuating, with subtle yet significant changes at different latencies. Initially, hyperoxemia lengthens RR by direct inhibition of the sinoatrial node, and later on, it elicits a direct vasoconstriction-dependent increment of AP, which, via baroreflex with augmented sensitivity, increases vagal outflow and reduces sympathetic activity, changes strengthened by the augmented RSAS, but hyperoxemia does not modify the respiratory variables.