Related Experiment Videos
Inspiratory CO2 increases orthostatic tolerance during repeated tilt
A P Blaber1, R L Bondar, P Moradshahi
1School of Kinesiology, Simon Fraser University, Burnaby, BC, Canada. ablaber@sfu.ca
Aviation, Space, and Environmental Medicine
|November 23, 2001
Summary
Maintaining end-tidal CO2 levels during head-up tilt increases cerebral blood flow and may improve orthostatic tolerance. This study investigated cerebral and cardiovascular responses to repeated tilting under eucapnic and isocapnic conditions.
Area of Science:
- Physiology
- Cardiovascular Science
- Neuroscience
Background:
- Head-up tilt commonly reduces partial pressure of end-tidal CO2 (PetCO2).
- Decreased arterial CO2 lowers cerebral blood flow (CBF) and may precipitate presyncope.
- This study compared cerebral and cardiovascular responses to repeated tilting with spontaneous (eucapnic) versus controlled (isocapnic) PetCO2.
Purpose of the Study:
- To investigate the effects of maintaining PetCO2 on cerebral blood flow and cardiovascular responses during repeated head-up tilt.
- To determine if isocapnic conditions enhance orthostatic tolerance compared to eucapnic conditions.
Main Methods:
- Eight healthy subjects underwent repeated 90-degree head-up tilts.
- Measurements included ventilation (VE), PetCO2, blood pressure (BP), heart rate (HR), and middle cerebral artery mean flow velocities (MFV).
- Subjects were tilted under two conditions: eucapnic (PetCO2 allowed to vary) and isocapnic (PetCO2 clamped at supine levels).
Main Results:
- MFV was higher in the isocapnic condition compared to eucapnic.
- MFV decreased from the start to the end of the first tilt in both conditions.
- While BP and HR responses were similar, isocapnic tilt extended presyncopal time in most subjects, though presyncope values for MFV, BP, and HR were consistent across conditions.
Conclusions:
- Inspiratory CO2 supplementation during tilt increases MFV.
- Maintaining PetCO2 during head-up tilt appears to enhance orthostatic tolerance.