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

Area of Science:

• Physiology
• Respiratory Control
• Cardiovascular Regulation

Background:

• Postural fluid shifts influence respiratory control through baro- and chemo-reflexes.
• Limited data exist on steady-state ventilatory responses during head-down tilt (HDT).

Purpose of the Study:

• To investigate cardiorespiratory responses during acute 50° head-down tilt.
• To examine ventilatory adjustments under various gas conditions during HDT.

Main Methods:

• 18 healthy subjects underwent 50° head-down tilt from head-up or supine positions.
• Measurements included ventilation, end-tidal gases, blood pressure, and middle cerebral artery blood flow velocity (MCAv).
• Subjects breathed normoxia, hypoxia, or hypercapnia (5% CO2) with or without hyperoxia.

Main Results:

• Ventilation significantly increased during HDT in both hyperoxic hypercapnia (139%) and normoxic hypercapnia (131-129%).
• No significant changes in ventilation were observed during HDT with medical air or hypoxia.
• Middle cerebral artery blood flow velocity (MCAv) remained unchanged during HDT.

Conclusions:

• Augmented ventilation during steep head-down tilt in hypercapnia may involve mechanisms beyond cerebral blood flow regulation.
• Cerebral venous pressure and outflow alterations are postulated as potential contributors to these ventilatory changes.