Ventilatory response to head-down-tilt in healthy human subjects.

Content Provider	Europe PMC
Author	Alsharifi, Abdulaziz Carter, Niamh Irampaye, Akbar Stevens, Charlotte Mejia, Elisa Steier, Joerg Rafferty, Gerrard F.
Description	What is the central question of this study? Postural fluid shifts affect respiratory control via complex interactions between baroreceptor and chemoreceptor reflexes, and cerebral blood flow. Few data exist concerning steady state ventilatory responses during head down tilt. What is the main finding and its importance? Acute 50° head‐down tilt augments the ventilatory response to steady state normoxic and hyperoxic hypercapnia possibly via mechanisms involving cerebral blood flow. These findings are relevant clinically during procedures performed in a steep head‐down position and microgravity environments where individuals are exposed to fluid shifts and elevated atmospheric CO2. What is the central question of this study?Postural fluid shifts affect respiratory control via complex interactions between baroreceptor and chemoreceptor reflexes, and cerebral blood flow. Few data exist concerning steady state ventilatory responses during head down tilt.What is the main finding and its importance?Acute 50° head‐down tilt augments the ventilatory response to steady state normoxic and hyperoxic hypercapnia possibly via mechanisms involving cerebral blood flow. These findings are relevant clinically during procedures performed in a steep head‐down position and microgravity environments where individuals are exposed to fluid shifts and elevated atmospheric CO2. AbstractPostural fluid shifts may directly affect respiratory control via a complex interaction of baro‐ and chemo‐reflexes, and cerebral blood flow. Few data exist concerning the steady state ventilatory responses during head‐down tilt. We examined the cardiorespiratory responses during acute 50° head‐down tilt (HDT) in 18 healthy subjects (mean [SD] age 27 [10] years). Protocol 1 (n = 8, two female) was 50° HDT from 60° head‐up posture sustained for 10 min, while exposed to normoxia, normoxic hypercapnia (5% CO2), hypoxia (12% inspired O2) or hyperoxic hypercapnia (95% O2, 5% CO2). Protocol 2 (n = 10, four female) was 50° HDT from supine, sustained for 10 min, while breathing either medical air or normoxic hypercapnic (5% CO2) gas. Ventilation (V˙E, pneumotachograph), end‐tidal O2 and CO2 concentration and blood pressure (Finapres) were measured continuously throughout each protocol. Middle cerebral artery blood flow velocity (MCAv; transcranial Doppler) was also measured during protocol 2. Ventilation increased significantly (P < 0.05) compared to baseline during HDT in both hyperoxic hypercapnia (protocol 1 by mean [SD] 139 [26]%) and normoxic hypercapnia (protocol 1 by mean [SD] 131 [21]% and protocol 2 by 129 [23]%), despite no change in PETCO2 or PETO2 from baseline. No change in V˙E was observed during HDT with medical air or hypoxia, and there was no significant change in MCAv during HDT compared to baseline. The absence of change in cerebral blood flow leads us to postulate that the augmented ventilatory response during steep HDT may involve mechanisms related to cerebral venous pressure and venous outflow.
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Page Count	13
ISSN	09580670
Volume Number	109
PubMed Central reference number	PMC11607625
Issue Number	12
PubMed reference number	39447579
Journal	Experimental Physiology [Exp Physiol]
e-ISSN	1469445X
DOI	10.1113/ep092014
Language	English
Publisher	John Wiley and Sons Inc.
Publisher Date	2024-10-24
Publisher Place	Hoboken
Access Restriction	Open
Rights License	This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2024 The Author(s). Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.
Subject Keyword	baroreceptors cerebral blood flow chemoreceptors head‐down tilt respiratory control Trendelenburg position ventilation
Content Type	Text
Resource Type	Article
Subject	Nutrition and Dietetics Physiology Physiology (medical)