title: The effects of head-up and head-down tilt on central respiratory chemoreflex loop gain tested by hyperoxic rebreathing.
abstract: Central respiratory chemosensitivity is mediated via chemoreceptor neurons located throughout brain stem tissue. These receptors detect proximal CO2/[H(+)] (i.e., controller gain) and modulate breathing in a classic negative feedback loop. Loop gain (responsiveness) is the theoretical product of controller (chemoreceptors), mixing/feedback (cardiovascular and cerebrovascular systems), and plant (pulmonary system) gains. The level of chemoreceptor stimulation is determined by interactions between mixing and plant gains. The extent to which steady-state changes in body position may affect central chemoreflex loop gain in response to CO2 is unclear. Because of the potential effects of tilt on pulmonary mechanics, we hypothesized that plant gain would be altered by head-up and head-down tilt (HUT, HDT) during hyperoxic rebreathing, which theoretically isolates plant gain by eliminating systemic arterial-tissue gradients. Sixteen subjects (eight females) underwent hyperoxic rebreathing tests on a tilt table to quantify central chemoreflex loop gain in five steady-state positions: 90° HUT, 45° HUT, supine, 45° HDT, and 90° HDT. Respiratory responses (tidal volume, VT; frequency, fR; minute ventilation, VE) were quantified during steady-state and increases in CO2 during rebreathing by linear regression above the ventilatory recruitment threshold (VRT). Using one-factor analysis of variance, we found that there were no differences in the respiratory responses between the five positions (VRT, P=0.711; VT, P=0.290; fR, P=0.748; VE, P=0.325). Our findings suggest that during steady-state orthostatic stress, the ability of subjects to mount a normal ventilatory response to increased CO2 was unaffected, despite any potential changes in pulmonary mechanics associated with positional challenges.
authors.affiliation: Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta, Canada; Faculty of Science and Technology, Department of Biology, Mount Royal University, Calgary, Alberta, Canada.
authors.forename: Rachel J
authors.initials: RJ
authors.lastname: Skow
authors.affiliation: Faculty of Science and Technology, Department of Biology, Mount Royal University, Calgary, Alberta, Canada; Faculty of Health and Social Development, School of Health and Exercise Sciences, University of British Columbia Okanagan, Kelowna, British Columbia, Canada.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Tymko
authors.affiliation: Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta, Canada; Faculty of Science and Technology, Department of Biology, Mount Royal University, Calgary, Alberta, Canada.
authors.forename: Christina M
authors.initials: CM
authors.lastname: MacKay
authors.affiliation: Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Craig D
authors.initials: CD
authors.lastname: Steinback
authors.affiliation: Faculty of Science and Technology, Department of Biology, Mount Royal University, Calgary, Alberta, Canada. Electronic address: tday@mtroyal.ca.
authors.forename: Trevor A
authors.initials: TA
authors.lastname: Day
mesh_terms: D000328:Adult; D002628:Chemoreceptor Cells / Q000502:physiology*; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D008297:Male; D011187:Posture / Q000502:physiology*; D012143:Respiratory Physiological Phenomena; D018667:Tilt-Table Test
keywords: central respiratory chemoreflex; control of breathing; head-down tilt; head-up tilt; loop gain